Computer Science › Artificial Intelligence

Quantum Information and Cryptography

Works Count: 89080

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Description

This cluster of papers covers a wide range of topics in the field of quantum information, including quantum entanglement, cryptography, computation, metrology, superconducting circuits, photon manipulation, measurement, security, and communication. It reflects the ongoing advancements and diverse applications of quantum technologies in various domains.

Keywords

Quantum; Entanglement; Cryptography; Computation; Metrology; Superconducting Circuits; Photon; Measurement; Security; Communication

Linear optical quantum computing with photonic qubits

2007-01-24

Pieter Kok , William J. Munro , Kae Nemoto +3 more

Linear optics with photon counting is a prominent candidate for practical quantum computing. The protocol by Knill, Laflamme, and Milburn [Nature 409, 46 (2001)] explicitly demonstrates that efficient scalable quantum … Linear optics with photon counting is a prominent candidate for practical quantum computing. The protocol by Knill, Laflamme, and Milburn [Nature 409, 46 (2001)] explicitly demonstrates that efficient scalable quantum computing with single photons, linear optical elements, and projective measurements is possible. Subsequently, several improvements on this protocol have started to bridge the gap between theoretical scalability and practical implementation. We review the original theory and its improvements, and we give a few examples of experimental two-qubit gates. We discuss the use of realistic components, the errors they induce in the computation, and how these errors can be corrected.

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Quantum Measurement and Control

2009-11-12

Howard M. Wiseman , G. J. Milburn

The control of individual quantum systems promises a new technology for the 21st century - quantum technology. This book is the first comprehensive treatment of modern quantum measurement and measurement-based … The control of individual quantum systems promises a new technology for the 21st century - quantum technology. This book is the first comprehensive treatment of modern quantum measurement and measurement-based quantum control, which are vital elements for realizing quantum technology. Readers are introduced to key experiments and technologies through dozens of recent experiments in cavity QED, quantum optics, mesoscopic electronics, and trapped particles several of which are analysed in detail. Nearly 300 exercises help build understanding, and prepare readers for research in these exciting areas. This important book will interest graduate students and researchers in quantum information, quantum metrology, quantum control and related fields. Novel topics covered include adaptive measurement; realistic detector models; mesoscopic current detection; Markovian, state-based and optimal feedback; and applications to quantum information processing.

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Quantum Discord: A Measure of the Quantumness of Correlations

2001-12-14

H Ollivier , Wojciech H. Zurek

Two classically identical expressions for the mutual information generally differ when the systems involved are quantum. This difference defines the quantum discord. It can be used as a measure of … Two classically identical expressions for the mutual information generally differ when the systems involved are quantum. This difference defines the quantum discord. It can be used as a measure of the quantumness of correlations. Separability of the density matrix describing a pair of systems does not guarantee vanishing of the discord, thus showing that absence of entanglement does not imply classicality. We relate this to the quantum superposition principle, and consider the vanishing of discord as a criterion for the preferred effectively classical states of a system, i.e., the pointer states.

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Experimental quantum teleportation

1997-12-01

Dik Bouwmeester , Jian-Wei Pan , Klaus Mattle +3 more

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Charge-insensitive qubit design derived from the Cooper pair box

2007-10-12

Jens Koch , Terri M. Yu , Jay Gambetta +7 more

Short dephasing times pose one of the main challenges in realizing a quantum computer. Different approaches have been devised to cure this problem for superconducting qubits, a prime example being … Short dephasing times pose one of the main challenges in realizing a quantum computer. Different approaches have been devised to cure this problem for superconducting qubits, a prime example being the operation of such devices at optimal working points, so-called ``sweet spots.'' This latter approach led to significant improvement of ${T}_{2}$ times in Cooper pair box qubits [D. Vion et al., Science 296, 886 (2002)]. Here, we introduce a new type of superconducting qubit called the ``transmon.'' Unlike the charge qubit, the transmon is designed to operate in a regime of significantly increased ratio of Josephson energy and charging energy ${E}_{J}∕{E}_{C}$. The transmon benefits from the fact that its charge dispersion decreases exponentially with ${E}_{J}∕{E}_{C}$, while its loss in anharmonicity is described by a weak power law. As a result, we predict a drastic reduction in sensitivity to charge noise relative to the Cooper pair box and an increase in the qubit-photon coupling, while maintaining sufficient anharmonicity for selective qubit control. Our detailed analysis of the full system shows that this gain is not compromised by increased noise in other known channels.

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Photonic quantum technologies

2009-11-30

Jeremy L. O’Brien , Akira Furusawa , Jelena Vučković

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Gaussian quantum information

2012-05-01

Christian Weedbrook , Stefano Pirandola , Raúl García−Patrón +4 more

The science of quantum information has arisen over the last two decades centered on the manipulation of individual quanta of information, known as quantum bits or qubits. Quantum computers, quantum … The science of quantum information has arisen over the last two decades centered on the manipulation of individual quanta of information, known as quantum bits or qubits. Quantum computers, quantum cryptography, and quantum teleportation are among the most celebrated ideas that have emerged from this new field. It was realized later on that using continuous-variable quantum information carriers, instead of qubits, constitutes an extremely powerful alternative approach to quantum information processing. This review focuses on continuous-variable quantum information processes that rely on any combination of Gaussian states, Gaussian operations, and Gaussian measurements. Interestingly, such a restriction to the Gaussian realm comes with various benefits, since on the theoretical side, simple analytical tools are available and, on the experimental side, optical components effecting Gaussian processes are readily available in the laboratory. Yet, Gaussian quantum information processing opens the way to a wide variety of tasks and applications, including quantum communication, quantum cryptography, quantum computation, quantum teleportation, and quantum state and channel discrimination. This review reports on the state of the art in this field, ranging from the basic theoretical tools and landmark experimental realizations to the most recent successful developments.

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Three qubits can be entangled in two inequivalent ways

2000-11-14

Wolfgang Dür , Guifré Vidal , J. I. Cirac

Invertible local transformations of a multipartite system are used to define equivalence classes in the set of entangled states. This classification concerns the entanglement properties of a single copy of … Invertible local transformations of a multipartite system are used to define equivalence classes in the set of entangled states. This classification concerns the entanglement properties of a single copy of the state. Accordingly, we say that two states have the same kind of entanglement if both of them can be obtained from the other by means of local operations and classical communcication (LOCC) with nonzero probability. When applied to pure states of a three-qubit system, this approach reveals the existence of two inequivalent kinds of genuine tripartite entanglement, for which the GHZ state and a W state appear as remarkable representatives. In particular, we show that the W state retains maximally bipartite entanglement when any one of the three qubits is traced out. We generalize our results both to the case of higher dimensional subsystems and also to more than three subsystems, for all of which we show that, typically, two randomly chosen pure states cannot be converted into each other by means of LOCC, not even with a small probability of success.

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Nonlinear optical processes using electromagnetically induced transparency

1990-03-05

Stephen Harris , J. E. Field , Ataç İmamoğlu

We show that by applying a strong-coupling field between a metastable state and the upper state of an allowed transition to ground one may obtain a resonantly enhanced third-order susceptibility … We show that by applying a strong-coupling field between a metastable state and the upper state of an allowed transition to ground one may obtain a resonantly enhanced third-order susceptibility while at the same time inducing transparency of the media. An improvement in conversion efficiency and parametric gain, as compared to weak-coupling field behavior, of many orders of magnitude is predicted.

Advances in quantum metrology

2011-03-31

Vittorio Giovannetti , Seth Lloyd , Lorenzo Maccone

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Superconducting quantum bits

2008-06-01

John Clarke , Frank K. Wilhelm

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Computable measure of entanglement

2002-02-22

Guifré Vidal , Reinhard F. Werner

We present a measure of entanglement that can be computed effectively for any mixed state of an arbitrary bipartite system. We show that it does not increase under local manipulations … We present a measure of entanglement that can be computed effectively for any mixed state of an arbitrary bipartite system. We show that it does not increase under local manipulations of the system, and use it to obtain a bound on the teleportation capacity and on the distillable entanglement of mixed states.

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Entanglement in Quantum Critical Phenomena

2003-06-02

Guifré Vidal , José I. Latorre , E. Rico +1 more

Entanglement, one of the most intriguing features of quantum theory and a main resource in quantum information science, is expected to play a crucial role also in the study of … Entanglement, one of the most intriguing features of quantum theory and a main resource in quantum information science, is expected to play a crucial role also in the study of quantum phase transitions, where it is responsible for the appearance of long-range correlations. We investigate, through a microscopic calculation, the scaling properties of entanglement in spin chain systems, both near and at a quantum critical point. Our results establish a precise connection between concepts of quantum information, condensed matter physics, and quantum field theory, by showing that the behavior of critical entanglement in spin systems is analogous to that of entropy in conformal field theories. We explore some of the implications of this connection.

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Quantum dynamics of single trapped ions

2003-03-10

D. Leibfried , R. Blatt , C. Monroe +1 more

Single trapped ions represent elementary quantum systems that are well isolated from the environment. They can be brought nearly to rest by laser cooling, and both their internal electronic states … Single trapped ions represent elementary quantum systems that are well isolated from the environment. They can be brought nearly to rest by laser cooling, and both their internal electronic states and external motion can be coupled to and manipulated by light fields. This makes them ideally suited for quantum-optical and quantum-dynamical studies under well-controlled conditions. Theoretical and experimental work on these topics is reviewed in the paper, with a focus on ions trapped in radio-frequency (Paul) traps.

Persistent Entanglement in Arrays of Interacting Particles

2001-01-29

Hans J. Briegel , Robert Raussendorf

We study the entanglement properties of a class of $N$ qubit quantum states that are generated in arrays of qubits with an Ising-type interaction. These states contain a large amount … We study the entanglement properties of a class of $N$ qubit quantum states that are generated in arrays of qubits with an Ising-type interaction. These states contain a large amount of entanglement as given by their Schmidt measure. They have also a high {\em persistency of entanglement} which means that $\sim N/2$ qubits have to be measured to disentangle the state. These states can be regarded as an entanglement resource since one can generate a family of other multi-particle entangled states such as the generalized GHZ states of $<N/2$ qubits by simple measurements and classical communication (LOCC).

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Quantum-Enhanced Measurements: Beating the Standard Quantum Limit

2004-11-18

Vittorio Giovannetti , Seth Lloyd , Lorenzo Maccone

Quantum mechanics, through the Heisenberg uncertainty principle, imposes limits on the precision of measurement. Conventional measurement techniques typically fail to reach these limits. Conventional bounds to the precision of measurements … Quantum mechanics, through the Heisenberg uncertainty principle, imposes limits on the precision of measurement. Conventional measurement techniques typically fail to reach these limits. Conventional bounds to the precision of measurements such as the shot noise limit or the standard quantum limit are not as fundamental as the Heisenberg limits and can be beaten using quantum strategies that employ "quantum tricks" such as squeezing and entanglement.

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Quantum State Transfer and Entanglement Distribution among Distant Nodes in a Quantum Network

1997-04-21

J. I. Cirac , P. Zoller , H. J. Kimble +1 more

We propose a scheme to utilize photons for ideal quantum transmission between atoms located at spatially separated nodes of a quantum network. The transmission protocol employs special laser pulses that … We propose a scheme to utilize photons for ideal quantum transmission between atoms located at spatially separated nodes of a quantum network. The transmission protocol employs special laser pulses that excite an atom inside an optical cavity at the sending node so that its state is mapped into a time-symmetric photon wave packet that will enter a cavity at the receiving node and be absorbed by an atom there with unit probability. Implementation of our scheme would enable reliable transfer or sharing of entanglement among spatially distant atoms.

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Measurement-Device-Independent Quantum Key Distribution

2012-03-30

Hoi‐Kwong Lo , Marcos Curty , Bing Qi

How to remove detector side channel attacks has been a notoriously hard problem in quantum cryptography. Here, we propose a simple solution to this problem--measurement-device-independent quantum key distribution (QKD). It … How to remove detector side channel attacks has been a notoriously hard problem in quantum cryptography. Here, we propose a simple solution to this problem--measurement-device-independent quantum key distribution (QKD). It not only removes all detector side channels, but also doubles the secure distance with conventional lasers. Our proposal can be implemented with standard optical components with low detection efficiency and highly lossy channels. In contrast to the previous solution of full device independent QKD, the realization of our idea does not require detectors of near unity detection efficiency in combination with a qubit amplifier (based on teleportation) or a quantum nondemolition measurement of the number of photons in a pulse. Furthermore, its key generation rate is many orders of magnitude higher than that based on full device independent QKD. The results show that long-distance quantum cryptography over say 200 km will remain secure even with seriously flawed detectors.

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Communication via one- and two-particle operators on Einstein-Podolsky-Rosen states

1992-11-16

Charles H. Bennett , Stephen Wiesner

As is well known, operations on one particle of an Einstein-Podolsky-Rosen (EPR) pair cannot influence the marginal statistics of measurements on the other particle. We characterize the set of states … As is well known, operations on one particle of an Einstein-Podolsky-Rosen (EPR) pair cannot influence the marginal statistics of measurements on the other particle. We characterize the set of states accessible from an initial EPR state by one-particle operations and show that in a sense they allow two bits to be encoded reliably in one spin-1/2 particle: One party, ``Alice,'' prepares an EPR pair and sends one of the particles to another party, ``Bob,'' who applies one of four unitary operators to the particle, and then returns it to Alice. By measuring the two particles jointly, Alice can now reliably learn which operator Bob used.

Concentrating partial entanglement by local operations

1996-04-01

Charles H. Bennett , H. J. Bernstein , Sandu Popescu +1 more

If two separated observers are supplied with entanglement, in the form of n pairs of particles in identical partly entangled pure states, one member of each pair being given to … If two separated observers are supplied with entanglement, in the form of n pairs of particles in identical partly entangled pure states, one member of each pair being given to each observer, they can, by local actions of each observer, concentrate this entanglement into a smaller number of maximally entangled pairs of particles, for example, Einstein-Podolsky-Rosen singlets, similarly shared between the two observers. The concentration process asymptotically conserves entropy of entanglement---the von Neumann entropy of the partial density matrix seen by either observer---with the yield of singlets approaching, for large n, the base-2 entropy of entanglement of the initial partly entangled pure state. Conversely, any pure or mixed entangled state of two systems can be produced by two classically communicating separated observers, drawing on a supply of singlets as their sole source of entanglement. \textcopyright{} 1996 The American Physical Society.

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Separability Criterion for Density Matrices

1996-08-19

Asher Peres

A quantum system consisting of two subsystems is separable if its density matrix can be written as $\ensuremath{\rho}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\Sigma}{A}^{}{w}_{A}{\ensuremath{\rho}}_{A}^{\ensuremath{'}}\ensuremath{\bigotimes}{\ensuremath{\rho}}_{A}^{\ensuremath{'}\ensuremath{'}},$ where ${\ensuremath{\rho}}_{A}^{\ensuremath{'}}$ and ${\ensuremath{\rho}}_{A}^{\ensuremath{'}\ensuremath{'}}$ are density matrices for the two subsystems, and … A quantum system consisting of two subsystems is separable if its density matrix can be written as $\ensuremath{\rho}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\Sigma}{A}^{}{w}_{A}{\ensuremath{\rho}}_{A}^{\ensuremath{'}}\ensuremath{\bigotimes}{\ensuremath{\rho}}_{A}^{\ensuremath{'}\ensuremath{'}},$ where ${\ensuremath{\rho}}_{A}^{\ensuremath{'}}$ and ${\ensuremath{\rho}}_{A}^{\ensuremath{'}\ensuremath{'}}$ are density matrices for the two subsystems, and the positive weights ${w}_{A}$ satisfy $\ensuremath{\Sigma}{w}_{A}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$. In this Letter, it is proved that a necessary condition for separability is that a matrix, obtained by partial transposition of \ensuremath{\rho}, has only non-negative eigenvalues. Some examples show that this criterion is more sensitive than Bell's inequality for detecting quantum inseparability.

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Decoy State Quantum Key Distribution

2005-06-16

Hoi‐Kwong Lo , Xiongfeng Ma , Kai Chen

There has been much interest in quantum key distribution. Experimentally, quantum key distribution over 150 km of commercial Telecom fibers has been successfully performed. The crucial issue in quantum key … There has been much interest in quantum key distribution. Experimentally, quantum key distribution over 150 km of commercial Telecom fibers has been successfully performed. The crucial issue in quantum key distribution is its security. Unfortunately, all recent experiments are, in principle, insecure due to real-life imperfections. Here, we propose a method that can for the first time make most of those experiments secure by using essentially the same hardware. Our method is to use decoy states to detect eavesdropping attacks. As a consequence, we have the best of both worlds--enjoying unconditional security guaranteed by the fundamental laws of physics and yet dramatically surpassing even some of the best experimental performances reported in the literature.

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Quantum detection and estimation theory

1969-01-01

Carl W. Helstrom

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Purification of Noisy Entanglement and Faithful Teleportation via Noisy Channels

1996-01-29

Charles H. Bennett , Gilles Brassard , Sandu Popescu +3 more

Two separated observers, by applying local operations to a supply of not-too-impure entangled states ({\em e.g.} singlets shared through a noisy channel), can prepare a smaller number of entangled pairs … Two separated observers, by applying local operations to a supply of not-too-impure entangled states ({\em e.g.} singlets shared through a noisy channel), can prepare a smaller number of entangled pairs of arbitrarily high purity ({\em e.g.} near-perfect singlets). These can then be used to faithfully teleport unknown quantum states from one observer to the other, thereby achieving faithful transfrom one observer to the other, thereby achieving faithful transmission of quantum information through a noisy channel. We give upper and lower bounds on the yield $D(M)$ of pure singlets ($\ket{Ψ^-}$) distillable from mixed states $M$, showing $D(M)>0$ if $\bra{Ψ^-}M\ket{Ψ^-}>\half$.

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‘‘Event-ready-detectors’’ Bell experiment via entanglement swapping

1993-12-27

Marek Żukowski , Anton Zeilinger , M. A. Horne +1 more

Using independent sources one can realize an ``event-ready'' Bell--Einstein-Podolsky-Rosen experiment in which one can measure directly the probabilities of the various outcomes including nondetection of both particles. Our proposal involves … Using independent sources one can realize an ``event-ready'' Bell--Einstein-Podolsky-Rosen experiment in which one can measure directly the probabilities of the various outcomes including nondetection of both particles. Our proposal involves two parametric down-converters. Subcoherence-time monitoring of the idlers provides a noninteractive quantum measurement entangling and preselecting the independent signals without touching them. We give the conditions for high fringe visibility and particle collection efficiency as required for a Bell test.

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Quantum Computations with Cold Trapped Ions

1995-05-15

J. I. Cirac , P. Zoller

A quantum computer can be implemented with cold ions confined in a linear trap and interacting with laser beams. Quantum gates involving any pair, triplet, or subset of ions can … A quantum computer can be implemented with cold ions confined in a linear trap and interacting with laser beams. Quantum gates involving any pair, triplet, or subset of ions can be realized by coupling the ions through the collective quantized motion. In this system decoherence is negligible, and the measurement (readout of the quantum register) can be carried out with a high efficiency.

Strong coupling of a single photon to a superconducting qubit using circuit quantum electrodynamics

2004-09-01

Andreas Wallraff , David Schuster , Alexandre Blais +6 more

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Simple Proof of Security of the BB84 Quantum Key Distribution Protocol

2000-07-10

Peter W. Shor , John Preskill

We prove that the 1984 protocol of Bennett and Brassard (BB84) for quantum key distribution is secure. We first give a key distribution protocol based on entanglement purification, which can … We prove that the 1984 protocol of Bennett and Brassard (BB84) for quantum key distribution is secure. We first give a key distribution protocol based on entanglement purification, which can be proven secure using methods from Lo and Chau's proof of security for a similar protocol. We then show that the security of this protocol implies the security of BB84. The entanglement purification based protocol uses Calderbank-Shor-Steane codes, and properties of these codes are used to remove the use of quantum computation from the Lo-Chau protocol.

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Entanglement of a Pair of Quantum Bits

1997-06-30

Scott Hill , William K. Wootters

The ``entanglement of formation'' of a mixed state \ensuremath{\rho} of a bipartite quantum system can be defined as the minimum number of singlets needed to create an ensemble of pure … The ``entanglement of formation'' of a mixed state \ensuremath{\rho} of a bipartite quantum system can be defined as the minimum number of singlets needed to create an ensemble of pure states that represents \ensuremath{\rho}. We find an exact formula for the entanglement of formation for all mixed states of two qubits having no more than two nonzero eigenvalues, and we report evidence suggesting that the formula is valid for all states of this system.

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Quantum Repeaters: The Role of Imperfect Local Operations in Quantum Communication

1998-12-28

H. J. Briegel , Wolfgang Dür , J. I. Cirac +1 more

In quantum communication via noisy channels, the error probability scales exponentially with the length of the channel. We present a scheme of a quantum repeater that overcomes this limitation. The … In quantum communication via noisy channels, the error probability scales exponentially with the length of the channel. We present a scheme of a quantum repeater that overcomes this limitation. The central idea is to connect a string of (imperfect) entangled pairs of particles by using a novel nested purification protocol, thereby creating a single distant pair of high fidelity. Our scheme tolerates general errors on the percent level, it works with a polynomial overhead in time and a logarithmic overhead in the number of particles that need to be controlled locally.

Unconditional Quantum Teleportation

1998-10-23

Akira Furusawa , J. L. Sørensen , Samuel L. Braunstein +3 more

Quantum teleportation of optical coherent states was demonstrated experimentally using squeezed-state entanglement. The quantum nature of the achieved teleportation was verified by the experimentally determined fidelity Fexp = 0.58 +/- … Quantum teleportation of optical coherent states was demonstrated experimentally using squeezed-state entanglement. The quantum nature of the achieved teleportation was verified by the experimentally determined fidelity Fexp = 0.58 +/- 0.02, which describes the match between input and output states. A fidelity greater than 0.5 is not possible for coherent states without the use of entanglement. This is the first realization of unconditional quantum teleportation where every state entering the device is actually teleported.

Quantum Information Processing Using Quantum Dot Spins and Cavity QED

1999-11-15

A. Imamog ̄lu , D. D. Awschalom , Guido Burkard +4 more

The electronic spin degrees of freedom in semiconductors typically have decoherence times that are several orders of magnitude longer than other relevant time scales. A solid-state quantum computer based on … The electronic spin degrees of freedom in semiconductors typically have decoherence times that are several orders of magnitude longer than other relevant time scales. A solid-state quantum computer based on localized electron spins as qubits is therefore of potential interest. Here, a scheme that realizes controlled interactions between two distant quantum dot spins is proposed. The effective long-range interaction is mediated by the vacuum field of a high finesse microcavity. By using conduction-band-hole Raman transitions induced by classical laser fields and the cavity-mode, parallel controlled-not operations, and arbitrary single qubit rotations can be realized.

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Inseparability Criterion for Continuous Variable Systems

2000-03-20

L.-M. Duan , G. Giedke , J. I. Cirac +1 more

An inseparability criterion based on the total variance of a pair of Einstein-Podolsky-Rosen type operators is proposed for continuous variable systems. The criterion provides a sufficient condition for entanglement of … An inseparability criterion based on the total variance of a pair of Einstein-Podolsky-Rosen type operators is proposed for continuous variable systems. The criterion provides a sufficient condition for entanglement of any two-party continuous variable states. Furthermore, for all Gaussian states, this criterion turns out to be a necessary and sufficient condition for inseparability.

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Quantum-mechanical noise in an interferometer

1981-04-15

Carlton M. Caves

The interferometers now being developed to detect gravitational waves work by measuring the relative positions of widely separated masses. Two fundamental sources of quantum-mechanical noise determine the sensitivity of such … The interferometers now being developed to detect gravitational waves work by measuring the relative positions of widely separated masses. Two fundamental sources of quantum-mechanical noise determine the sensitivity of such an interferometer: (i) fluctuations in number of output photons (photon-counting error) and (ii) fluctuations in radiation pressure on the masses (radiation-pressure error). Because of the low power of available continuous-wave lasers, the sensitivity of currently planned interferometers will be limited by photon-counting error. This paper presents an analysis of the two types of quantum-mechanical noise, and it proposes a new technique---the "squeezed-state" technique---that allows one to decrease the photon-counting error while increasing the radiation-pressure error, or vice versa. The key requirement of the squeezed-state technique is that the state of the light entering the interferometer's normally unused input port must be not the vacuum, as in a standard interferometer, but rather a "squeezed state"---a state whose uncertainties in the two quadrature phases are unequal. Squeezed states can be generated by a variety of nonlinear optical processes, including degenerate parametric amplification.

The security of practical quantum key distribution

2009-09-29

Valerio Scarani , H. Bechmann-Pasquinucci , Nicolas J. Cerf +3 more

Quantum key distribution (QKD) is the first quantum information task to reach the level of mature technology, already fit for commercialization. It aims at the creation of a secret key … Quantum key distribution (QKD) is the first quantum information task to reach the level of mature technology, already fit for commercialization. It aims at the creation of a secret key between authorized partners connected by a quantum channel and a classical authenticated channel. The security of the key can in principle be guaranteed without putting any restriction on the eavesdropper's power. The first two sections provide a concise up-to-date review of QKD, biased toward the practical side. The rest of the paper presents the essential theoretical tools that have been developed to assess the security of the main experimental platforms (discrete variables, continuous variables and distributed-phase-reference protocols).

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Classical, quantum and total correlations

2001-08-24

Leah Henderson , Vlatko Vedral

We discuss the problem of separating consistently the total correlations in a bipartite quantum state into a quantum and a purely classical part. A measure of classical correlations is proposed … We discuss the problem of separating consistently the total correlations in a bipartite quantum state into a quantum and a purely classical part. A measure of classical correlations is proposed and its properties are explored.

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The quantum internet

2008-06-01

H. J. Kimble

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Long-distance quantum communication with atomic ensembles and linear optics

2001-11-01

L.-M. Duan , Mikhail D. Lukin , J. I. Cirac +1 more

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Quantum information with continuous variables

2005-06-29

Samuel L. Braunstein , Peter van Loock

Quantum information is a rapidly advancing area of interdisciplinary research. It may lead to real-world applications for communication and computation unavailable without the exploitation of quantum properties such as nonorthogonality … Quantum information is a rapidly advancing area of interdisciplinary research. It may lead to real-world applications for communication and computation unavailable without the exploitation of quantum properties such as nonorthogonality or entanglement. We review the progress in quantum information based on continuous quantum variables, with emphasis on quantum optical implementations in terms of the quadrature amplitudes of the electromagnetic field.

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Peres-Horodecki Separability Criterion for Continuous Variable Systems

2000-03-20

R. Simon

The Peres-Horodecki criterion of positivity under partial transpose is studied in the context of separability of bipartite continuous variable states. The partial transpose operation admits, in the continuous case, a … The Peres-Horodecki criterion of positivity under partial transpose is studied in the context of separability of bipartite continuous variable states. The partial transpose operation admits, in the continuous case, a geometric interpretation as mirror reflection in phase space. This recognition leads to uncertainty principles, stronger than the traditional ones, to be obeyed by all separable states. For all bipartite Gaussian states, the Peres-Horodecki criterion turns out to be a necessary and sufficient condition for separability.

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A scheme for efficient quantum computation with linear optics

2001-01-01

E. Knill , Raymond Laflamme , G. J. Milburn

Two-photon coherent states of the radiation field

1976-06-01

Horace P. Yuen

The concept of a two-photon coherent state is introduced for applications in quantum optics. It is a simple generalization of the well-known minimum-uncertainty wave packets. The detailed properties of two-photon … The concept of a two-photon coherent state is introduced for applications in quantum optics. It is a simple generalization of the well-known minimum-uncertainty wave packets. The detailed properties of two-photon coherent states are developed and distinguished from ordinary coherent states. These two-photon coherent states are mathematically generated from coherent states through unitary operators associated with quadratic Hamiltonians. Physically they are the radiation states of ideal two-photon lasers operating far above threshold, according to the self-consistent-field approximation. The mean-square quantum noise behavior of these states, which is basically the same as those of minimum-uncertainty states, leads to applications not obtainable from coherent states or one-photon lasers. The essential behavior of two-photon coherent states is unchanged by small losses in the system. The counting rates or distributions these states generate in photocount experiments also reveal their difference from coherent states.

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Measurement of qubits

2001-10-16

Daniel F. V. James , Paul G. Kwiat , William J. Munro +1 more

We describe in detail the theory underpinning the measurement of density matrices of a pair of quantum two-level systems (``qubits''). Our particular emphasis is on qubits realized by the two … We describe in detail the theory underpinning the measurement of density matrices of a pair of quantum two-level systems (``qubits''). Our particular emphasis is on qubits realized by the two polarization degrees of freedom of a pair of entangled photons generated in a down-conversion experiment; however, the discussion applies in general, regardless of the actual physical realization. Two techniques are discussed, namely, a tomographic reconstruction (in which the density matrix is linearly related to a set of measured quantities) and a maximum likelihood technique which requires numerical optimization (but has the advantage of producing density matrices that are always non-negative definite). In addition, a detailed error analysis is presented, allowing errors in quantities derived from the density matrix, such as the entropy or entanglement of formation, to be estimated. Examples based on down-conversion experiments are used to illustrate our results.

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Quantum Metrology

2006-01-03

Vittorio Giovannetti , Seth Lloyd , Lorenzo Maccone

We point out a general framework that encompasses most cases in which quantum effects enable an increase in precision when estimating a parameter (quantum metrology). The typical quantum precision enhancement … We point out a general framework that encompasses most cases in which quantum effects enable an increase in precision when estimating a parameter (quantum metrology). The typical quantum precision enhancement is of the order of the square root of the number of times the system is sampled. We prove that this is optimal, and we point out the different strategies (classical and quantum) that permit one to attain this bound.

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Quantifying Entanglement

1997-03-24

Vlatko Vedral , Martin B. Plenio , M. A. Rippin +1 more

We present conditions every measure of entanglement has to satisfy, and construct a whole class of ``good'' entanglement measures. The generalization of our class of entanglement measures to more than … We present conditions every measure of entanglement has to satisfy, and construct a whole class of ``good'' entanglement measures. The generalization of our class of entanglement measures to more than two particles is straightforward. We present a measure which has a statistical operational basis that might enable experimental determination of the quantitative degree of entanglement.

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Quantum communication

2007-03-01

Nicolas Gisin , Rob Thew

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Entanglement detection

2009-03-15

Otfried Gühne , G. Tóth

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Quantum cryptography

2002-03-08

Nicolas Gisin , G. Ribordy , Wolfgang Tittel +1 more

Quantum cryptography could well be the first application of quantum mechanics at the single-quantum level. The rapid progress in both theory and experiment in recent years is reviewed, with emphasis … Quantum cryptography could well be the first application of quantum mechanics at the single-quantum level. The rapid progress in both theory and experiment in recent years is reviewed, with emphasis on open questions and technological issues.

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Separability of mixed states: necessary and sufficient conditions

1996-11-01

Michał Horodecki , Paweł Horodecki , Ryszard Horodecki

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Quantum Optics

1994-01-01

D. F. Walls , G. J. Milburn

Wigner-negative states in the steady-state emission of a two-level system driven by squeezed light

2025-06-24

NULL AUTHOR_ID | Physical review. A/Physical review, A

Field mixing in a thermal medium: a quantum master equation approach

2025-06-24

Shuyang Cao | Journal of High Energy Physics

A bstract We studied the nonequilibrium dynamics of the indirect mixing of two (pseudo-)scalar fields induced by their couplings to common decay channels in a medium. The effective non-Markovian quantum … A bstract We studied the nonequilibrium dynamics of the indirect mixing of two (pseudo-)scalar fields induced by their couplings to common decay channels in a medium. The effective non-Markovian quantum master equation (QME) for the two fields’ reduced density matrix is derived to leading order in the couplings of the two fields with the medium, but to all orders of the couplings among degrees of freedom in the medium. The self-energy and noise-kernel in the QME satisfy a fluctuation-dissipation relation. The solutions show that an initial expectation value (condensate) of one field induces a condensate of the other field through the indirect mixing and that the populations and coherence of the two fields thermalize and approach to non-vanishing values asymptotically. The nearly-degenerate field masses and coupling strengths resonantly enhance the quantum beats and asymptotic coherence, and induce a prominent dynamics of the vacuum after the switch-on of the couplings. We argue that time-dependent definitions of particles due to the changing vacuum must be introduced so as to obtain results consistent with the calculations of equilibrium states in the asymptotic limit. A coupling strength hierarchy breaks down the resonant enhancement in the nearly-degenerate case but leads to different power countings of the coupling strengths in the magnitudes of the observables and time-scales in the evolution, suggesting the possibility of detecting extremely long-lived particles using prepared short-lived particles within a practical experimental period.

Higher-dimensional-entanglement detection and quantum-channel characterization using moments of generalized positive maps

2025-06-24

NULL AUTHOR_ID | Physical review. A/Physical review, A

Tunneling control of dipolar bosons in triple-well circuits via dipole polarization orientation and quantum sensing application

2025-06-24

NULL AUTHOR_ID | Physical review. A/Physical review, A

Sub-Harmonic Control of a Fluxonium Qubit via a Purcell-Protected Flux Line

2025-06-24

NULL AUTHOR_ID | PRX Quantum

High-rate self-referenced continuous-variable quantum key distribution over high-loss free-space channel

2025-06-24

Xiaojuan Liao , Yuehan Xu , Qijun Zhang +4 more | Photonics Research

Quantum Tricritical Point Induced by Staggered Qubit Biases in a Two-Qubit Quantum Rabi Model

2025-06-24

Yanzhi Wang , Ye Tian , Qing‐Hu Chen | Communications in Theoretical Physics

Abstract We investigate the quantum phase transitions (QPTs) of the two-qubit quantum Rabi model with staggered qubit biases. In the limit of an infinite qubit-to-cavity frequency ratio, we analytically derive … Abstract We investigate the quantum phase transitions (QPTs) of the two-qubit quantum Rabi model with staggered qubit biases. In the limit of an infinite qubit-to-cavity frequency ratio, we analytically derive the mean-field Hamiltonian and the order-parameter-dependent energy density functional, which yields the ground-state energy and order parameter. The rich superradiant phase transitions (SRPTs), including both second- and first-order QPTs and a tricritical point (TCP), are analytically derived. Specifically, we derive the analytical expressions for all phase transition points, including the nonperturbative point of the first-order SRPT. The analytical findings are further corroborated by numerical finite-size scaling analysis. It is found that both the critical correlation-length and order-parameter exponents at the TCP differ from those of the original second-order SRPTs, implying that the TCP belongs to a new universality class. This work provides a reliable theoretical framework for designing new, simple experimental platforms to explore the rich QPTs.

Manipulating Entanglement Dynamics in Dephased Interacting Qubits Using a Radiation Field

2025-06-24

Omar Qisieh , Rahma Abdelmagid , Gehad Sadiek | Entropy

We study the entanglement dynamics of a pair of non-identical interacting atoms (qubits) coupled off-resonance to a single-mode cavity radiation field and exposed to dephasing environments. The qubits are studied … We study the entanglement dynamics of a pair of non-identical interacting atoms (qubits) coupled off-resonance to a single-mode cavity radiation field and exposed to dephasing environments. The qubits are studied starting from various initial states that are disentangled from an initially coherent field. The system models the basic building units of quantum information processing (QIP) platforms under the realistic considerations of asymmetry and external environmental influences. We investigate how introducing a radiation field alters the system’s entanglement dynamics in the presence of dephasing environments, and how it impacts the effects of the dephasing environments themselves. The work examines the problem under various settings of inter-qubit interactions, which are now experimentally controllable in some of the newly engineered artificial qubit systems. We illustrate that only upon introducing the radiation field, the system suffers a terminal disentanglement (followed by no revivals) in a finite time. This behavior is exacerbated when the atoms’ interaction with the field is stronger. Moreover, the effects of the field’s intensity and the atoms’ detunings are vastly sensitive to the choice of the initial state. We also demonstrate that the closer the atoms’ transition frequencies are to resonance with the field, the more pronounced are the effects of strengthening the independent dephasing environments corresponding to some initial states. Those states also suffered a greater reduction in entanglement content when the qubits with stronger atom–field interaction strength were influenced by a stronger independent dephasing environment. In addition, we examined the ability of the correlated dephasing environment to induce a noise-enhanced efficiency in the presence of an external radiation field. We showed that the radiation field could play a decisive role in enabling or restricting noise-enhanced efficiency, but one that is also highly sensitive to the system’s initial state.

Enhancing dynamic range of sub-standard-quantum-limit measurements via quantum deamplification

2025-06-23

NULL AUTHOR_ID | Physical Review Letters

The glare blinding attack strategy in quantum secret sharing system

2025-06-23

Wen-Da Zhang , Z. D. Nie , Jia‐Wei Ying +4 more | Optics Letters

Research on the convergence rate of online quantum state estimation algorithms considering disturbance and noise

2025-06-23

Shuang Cong , Weiyi Qin | Natural Computing

Quantum coherence and distinguishability as complementary resources: A resource-theoretic perspective from wave-particle duality

2025-06-23

Zhiping Liu , Chengkai Zhu , Hua‐Lei Yin +1 more | Physical review. A/Physical review, A

Efficiency Comparison of Two Key Relaying Architectures in Quantum Key Distribution Networks

2025-06-23

María Álvarez Roa , Catalina Stan , Sebastian R. Verschoor +2 more | Journal of Optical Communications and Networking

The quantromon: A qubit-resonator system with orthogonal qubit and readout modes

2025-06-23

Kishor V. Salunkhe , S. Kundu , Srijita Das +3 more | Applied Physics Letters

The measurement of a superconducting qubit is implemented by coupling it to a resonator. The common choice is transverse coupling, which, in the dispersive approximation, introduces an interaction term that … The measurement of a superconducting qubit is implemented by coupling it to a resonator. The common choice is transverse coupling, which, in the dispersive approximation, introduces an interaction term that enables the measurement. This cross-Kerr term provides a qubit-state dependent dispersive shift in the resonator frequency with the device parameters chosen carefully to get sufficient signal while minimizing Purcell decay of the qubit. We introduce a two-mode circuit, nicknamed quantromon, with two orthogonal modes implementing a qubit and a resonator. Unlike before, where the coupling term emerges as a perturbative expansion, the quantromon has intrinsic cross-Kerr coupling by design. Our experiments implemented in a hybrid 2D–3D circuit QED architecture demonstrate some unique features of the quantromon like weak dependence of the dispersive shift on the qubit-resonator detuning and intrinsic Purcell protection. In a tunable qubit-frequency device, we show that the dispersive shift (2χ/2π) changes by only 0.8 MHz, while the qubit-resonator detuning (Δ/2π) is varied between 0.398 and 3.288 GHz. We also demonstrate Purcell protection in a second device where we tune the orthogonality between the two modes. Finally, we demonstrate a single-shot readout fidelity of 98.3%, which is comparable to the state-of-the-art measurements without the use of a parametric amplifier and suggests a potential simplification of the measurement circuitry for scaling up quantum processors.

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Quantifying the quantumness of mixed-state ensembles via fidelity

2025-06-23

Meng Zhang , Yajing Fan | Quantum Information Processing

Phase estimation in lossy optical interferometry without a reference beam

2025-06-23

Jun Tang , Dongqing Wang , Wei Zhong +2 more | Communications in Theoretical Physics

Abstract We investigate phase estimation in a lossy interferometer using entangled coherent states (ECSs), with particular focus on a scenario where no reference beam is employed. By calculating the quantum … Abstract We investigate phase estimation in a lossy interferometer using entangled coherent states (ECSs), with particular focus on a scenario where no reference beam is employed. By calculating the quantum Fisher information (QFI), we reveal two key results: (1) the metrological equivalence between scenarios with and without a reference beam, established under ideal lossless conditions for the two-phase-shifting configuration, breaks down in the presence of photon loss, and (2) the pronounced inferior performance of ECSs relative to NOON states, observed in the presence of a reference beam, disappears in its absence.

Correlations in a quantum-gravitationally coupled masses

2025-06-22

Mohamed El Harrar , M. El Falaki , Rachid Ahl Laamara | Modern Physics Letters A

This study investigates the thermal and magnetic field dependencies of quantum coherence, quantum negativity, Local Quantum Uncertainty ([Formula: see text]), and Local Quantum Fisher Information ([Formula: see text]) in quantum-gravitationally … This study investigates the thermal and magnetic field dependencies of quantum coherence, quantum negativity, Local Quantum Uncertainty ([Formula: see text]), and Local Quantum Fisher Information ([Formula: see text]) in quantum-gravitationally coupled masses. Quantum negativity exhibits distinct peak behavior with shifts in temperature and decreases in magnitude as the magnetic field strength increases, highlighting the influence of stronger fields on the optimal temperature range, while quantum coherence displays a peak that shifts to higher temperatures, with its value decreasing due to thermal effects. The analysis reveals that thermal effects reduce coherence but moderate the suppression caused by the magnetic field, particularly at higher temperatures, in addition the dependence of quantum negativity on magnetic field strength, gravitational parameter, and coupling strength shows non-monotonic behavior with respect to the magnetic field and a monotonic increase with the gravitational parameter and stronger magnetic coupling suppresses quantum negativity. Moreover, [Formula: see text] and [Formula: see text] show significant dependence on temperature and energy gap due to the magnetic field strength, [Formula: see text] demonstrates a peak with broadening and reduced sensitivity to variations in the energy gap at higher temperatures, and shows a peak at low magnetic field strengths and decays as the field increases, with the decay more pronounced at lower temperatures while [Formula: see text] exhibits a more pronounced sensitivity to the energy gap at lower temperatures, with decreasing sensitivity as the temperature rises and exhibits a greater sensitivity to the magnetic field at lower temperatures, with a peak at a specific magnetic field and more gradual decay at higher temperatures.

Lindblad Dephasing Relaxation and Quantum Entanglement in Two-Spin Systems in Multiple-Quantum NMR Experiments

2025-06-21

É. B. Fel’dman , E. I. Kuznetsova , E. I. Shipulya +2 more | Applied Magnetic Resonance

Performance of free-space quantum key distribution over Malaga turbulence channel and pointing errors

2025-06-20

Ha Duyen Trung | Journal of Optical Communications

Abstract In this paper, we analyze the performance of free-space optical quantum key distribution (FSO-QKD) using the BB84 protocol with BPSK modulation under realistic channel conditions. The Málaga distribution is … Abstract In this paper, we analyze the performance of free-space optical quantum key distribution (FSO-QKD) using the BB84 protocol with BPSK modulation under realistic channel conditions. The Málaga distribution is employed to model the composite atmospheric turbulence, considering pointing errors. We derive closed-form expressions for the average quantum bit error rate (QBER) and the secure key rate (SKR), incorporating both turbulence and misalignment effects. Numerical results demonstrate the impact of turbulence parameters and beam misalignment on system performance. The findings provide insights for the robust design of FSO-QKD systems under practical deployment conditions.

Roadmap on Atomically-Engineered Quantum Platforms

2025-06-20

Soo‐hyon Phark , Bent Weber , Yasuo Yoshida +7 more | Nano Futures

Abstract Matter at the atomic-scale is inherently governed by the laws of quantum mechanics. This makes charges and spins confined to individual atoms – and interactions among them – an … Abstract Matter at the atomic-scale is inherently governed by the laws of quantum mechanics. This makes charges and spins confined to individual atoms – and interactions among them – an invaluable resource for fundamental research and quantum technologies alike. However, harnessing the inherent “quantumness” of atomic-scale objects requires that they can be precisely engineered and addressed at the individual atomic level. Since its invention in the 1980s, scanning tunneling microscopy (STM) has repeatedly demonstrated the unrivalled ability to not only resolve but manipulate matter at atomic length scales. Over the past decades, this has enabled the design and investigation of bottom-up tailored nanostructures as reliable and reproducible platforms to study designer quantum physics and chemistry, band topology, and collective phenomena. The vast range of STM-based techniques and modes of operation, as well as their combination with electromagnetic fields from the infrared to microwave spectral range, has even allowed for the precise control of individual charge and spin degrees of freedom. This roadmap reviews the most recent developments in the field of atomically-engineered quantum platforms and explores their potential in future fundamental research and quantum technologies.

Verifiable dynamic (t, n) threshold quantum secret sharing

2025-06-20

J. Li , Ming‐Ming Wang | Physica Scripta

Abstract Numerous high-quality quantum secret sharing (QSS) protocols have been proposed as integral components of quantum cryptography. However, only a limited number effectively integrate both dynamic and verifiable characteristics. This … Abstract Numerous high-quality quantum secret sharing (QSS) protocols have been proposed as integral components of quantum cryptography. However, only a limited number effectively integrate both dynamic and verifiable characteristics. This paper proposes a verifiable dynamic (t, n) QSS protocol based on the asymmetric binary polynomial and the d-dimensional Bell states. In the protocol, the dealer can find out the internal fraudsters by verifying whether an agent has used a correct share. The agents can also verify the correctness of the shares provided by the dealer. Besides, the dealer has the capability to independently adding or revoking of agents without requiring assistance from other agents. Furthermore, the protocol requires at least t agents to recover the secret, which makes the proposed protocol more flexible and practical than existing protocols. Our protocol is secure against the intercept-and-resend, the entangle-and-measure, the internal, and the dishonest revoked agent attacks.

Quantum noise spectroscopy by qudit spectators

2025-06-20

NULL AUTHOR_ID | Physical review. A/Physical review, A

A Passive and Self-Characterizing Cross-Encoded Receiver for Reference-Frame-Independent Quantum Key Distribution

2025-06-20

Massimo Giacomin , Francesco B. L. Santagiustina , Giuseppe Vallone +2 more | Optica Quantum

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An accessible planar charged particle trap for experiential learning in quantum technologies

2025-06-20

Robert E. Thomas , Cole E. Wolfram , Noah B. Warren +3 more | American Journal of Physics

We describe an inexpensive and accessible instructional setup that explores particle trapping with a planar linear ion trap. The planar trap is constructed using standard printed circuit board manufacturing and … We describe an inexpensive and accessible instructional setup that explores particle trapping with a planar linear ion trap. The planar trap is constructed using standard printed circuit board manufacturing and is designed to trap macroscopic charged particles in air. Trapping, shuttling, and splitting are demonstrated to students using these particles, which are visible to the naked eye. Students control trap voltages and can compare properties of particle motion with an analytic model of the trap using a computer vision program for particle tracking. Learning outcomes include understanding the design considerations for planar AC traps, mechanisms underpinning particle ejection, the physics of micromotion, and methods of data analysis using standard computer vision libraries.

Integration of Quantum Key Distribution with Classical Cryptographic Schemes: Enhancing Security in the Face of Post-Quantum Challenges

2025-06-20

Elizar Filaretovich Kustov , Altana Felixovna Khutsaeva , Anastasia Pavlovna Kiryanova +3 more | The Herald of the Siberian State University of Telecommunications and Information Science

Deterministic Generation of Frequency-Bin-Encoded Microwave Photons

2025-06-20

Jiaying Yang , Maryam Khanahmadi , Ingrid Strandberg +7 more | Physical Review Letters

A distributed quantum computing network requires a quantum communication channel between spatially separated processing units. In superconducting circuits, such a channel can be implemented based on propagating microwave photons to … A distributed quantum computing network requires a quantum communication channel between spatially separated processing units. In superconducting circuits, such a channel can be implemented based on propagating microwave photons to encode and transfer quantum information between an emitter and a receiver. However, traveling microwave photons can be lost during the transmission, leading to the failure of information transfer. Heralding protocols can be used to detect such photon losses. In this Letter, we propose such a protocol and experimentally demonstrate a frequency-bin encoding method of microwave photonic modes using superconducting circuits. We deterministically encode the quantum information from a superconducting qubit by simultaneously emitting its information into two photonic modes at different frequencies, with a process fidelity of 94.9%. The frequency-bin-encoded photonic modes can be used, at the receiver processor, to detect the occurrence of photon loss. Our Letter thus provides a reliable method to implement high-fidelity quantum state transfer in a distributed quantum computing network, incorporating error detection to enhance performance and accuracy. Published by the American Physical Society 2025

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Ultrafast High-Fidelity State Readout of Single Neutral Atom

2025-06-20

Jiannong Wang , Dong-Yu Huang , Xiao-Long Zhou +6 more | Physical Review Letters

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Arbitrary Nondemolition Measurements through Hamiltonian Stabilization for Quantum Error Correction

2025-06-20

Rodrigo G. Cortiñas | PRX Quantum

Quantum measurement is performed by coupling a quantity <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mrow><a:mrow><a:mover><a:mrow><a:mi>A</a:mi><a:mspace width="0.278em"/></a:mrow><a:mo stretchy="false">^</a:mo></a:mover></a:mrow><a:mspace width="-0.278em"/></a:mrow><a:mrow/></a:mrow></a:math> of a system to a “meter” and reading the meter. In the regime of strong measurement, … Quantum measurement is performed by coupling a quantity <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mrow><a:mrow><a:mover><a:mrow><a:mi>A</a:mi><a:mspace width="0.278em"/></a:mrow><a:mo stretchy="false">^</a:mo></a:mover></a:mrow><a:mspace width="-0.278em"/></a:mrow><a:mrow/></a:mrow></a:math> of a system to a “meter” and reading the meter. In the regime of strong measurement, the possible outcomes and the final state are entirely defined by <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:mrow><f:mrow><f:mover><f:mrow><f:mi>A</f:mi><f:mspace width="0.278em"/></f:mrow><f:mo stretchy="false">^</f:mo></f:mover></f:mrow><f:mspace width="-0.278em"/></f:mrow><f:mrow/></f:mrow></f:math>, with probabilities dictated by the initial state. In this work, we will be interested in the regime in which a strong self-Hamiltonian, and not the coupling, defines the measurement outcomes and the final state. In this regime, the expectation value of any arbitrary set of observables can be measured in a quantum nondemolition (QND) way without reset or repreparation of the state. We discuss applications that include single-shot measurement of the expectation value of all noncommuting observables from a single copy of the state, without any statistics or ensembles, and the decoding of dephasing errors in a Schrödinger-cat qubit quantum error-correction (QEC) code. In the latter, the error syndrome is obtained from single-shot QND measurements of the mean photon number of the cat, even if neither the cat code words nor the stabilizing Hamiltonian commute with the photon-number operator. Published by the American Physical Society 2025

Minimum-error state discrimination and Fano's inequality

2025-06-20

Georgios M. Nikolopoulos | American Journal of Physics

The discrimination between non-orthogonal quantum states plays a pivotal role in quantum information processing and quantum technology. Strategies that minimize the error probability are of particular importance, but they are … The discrimination between non-orthogonal quantum states plays a pivotal role in quantum information processing and quantum technology. Strategies that minimize the error probability are of particular importance, but they are only known for special classes of problems. Certain forms of Fano's inequality yield a bound on the error probability, but it is not known how close this bound is to the minimum-error probability achieved by means of optimal measurements. In this work, we discuss how the minimum-error probability compares to the error bound obtained through the Fano's inequality for several scenarios, some of which are amenable to analytic treatments.

Global variational quantum circuits for arbitrary symmetric state preparation

2025-06-20

Liam J. Bond , Matthew J. Davis , Jiří Minář +3 more | Physical Review Research

Quantum states that are symmetric under particle exchange play a crucial role in fields such as quantum metrology and quantum error correction. We use a variational circuit composed of global … Quantum states that are symmetric under particle exchange play a crucial role in fields such as quantum metrology and quantum error correction. We use a variational circuit composed of global one-axis twisting and global rotations to efficiently prepare arbitrary symmetric states, i.e., any superposition of Dicke states. The circuit does not require local addressability or ancilla qubits and thus can be readily implemented in a variety of experimental platforms including trapped-ion quantum simulators and cavity QED systems. We provide analytic and numerical evidence that any <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>N</a:mi></a:math>-qubit symmetric state can be prepared in <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mn>2</b:mn><b:mi>N</b:mi><b:mo>/</b:mo><b:mn>3</b:mn></b:mrow></b:math> steps. We demonstrate the utility of our protocol by preparing (i) metrologically useful <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mi>N</c:mi></c:math>-qubit Dicke states of up to <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>N</d:mi><d:mo>=</d:mo><d:mn>300</d:mn></d:mrow></d:math> qubits in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi mathvariant="script">O</e:mi><e:mo>(</e:mo><e:mn>1</e:mn><e:mo>)</e:mo></e:mrow></e:math> gate steps with theoretical infidelities <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mn>1</g:mn><g:mo>−</g:mo><g:mi mathvariant="script">F</g:mi><g:mo><</g:mo><g:msup><g:mn>10</g:mn><g:mrow><g:mo>−</g:mo><g:mn>3</g:mn></g:mrow></g:msup></g:mrow></g:math>, (ii) the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mi>N</i:mi><i:mo>=</i:mo><i:mn>9</i:mn></i:mrow></i:math> Ruskai codewords in <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mrow><j:mi>P</j:mi><j:mo>=</j:mo><j:mn>4</j:mn></j:mrow></j:math> gate steps with <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mrow><k:mn>1</k:mn><k:mo>−</k:mo><k:mi mathvariant="script">F</k:mi><k:mo><</k:mo><k:msup><k:mn>10</k:mn><k:mrow><k:mo>−</k:mo><k:mn>4</k:mn></k:mrow></k:msup></k:mrow></k:math>, and (iii) the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:mi>N</m:mi><m:mo>=</m:mo><m:mn>13</m:mn></m:mrow></m:math> Gross codewords in <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"><n:mrow><n:mi>P</n:mi><n:mo>=</n:mo><n:mn>7</n:mn></n:mrow></n:math> gate steps with <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:mn>1</o:mn><o:mo>−</o:mo><o:mi mathvariant="script">F</o:mi><o:mo><</o:mo><o:msup><o:mn>10</o:mn><o:mrow><o:mo>−</o:mo><o:mn>4</o:mn></o:mrow></o:msup></o:mrow></o:math>. Focusing on trapped-ion platforms, for the <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:mrow><q:mi>N</q:mi><q:mo>=</q:mo><q:mn>9</q:mn></q:mrow></q:math> Ruskai and <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"><r:mrow><r:mi>N</r:mi><r:mo>=</r:mo><r:mn>13</r:mn></r:mrow></r:math> Gross codewords we estimate that the protocol achieves fidelities <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"><s:mrow><s:mo>≳</s:mo><s:mn>95</s:mn><s:mo>%</s:mo></s:mrow></s:math> in the presence of typical experimental noise levels, thus providing a pathway to the preparation of a wide range of useful highly entangled quantum states. Published by the American Physical Society 2025

State-multiplexing approach for optimized expansion of entanglement-based quantum networks

2025-06-20

Anahita Khodadad Kashi , Michael Kues | Light Science & Applications

Abstract An optimized quantum network design is demonstrated by realizing a state-multiplexing quantum light source via a dual-excitation configuration technique. This approach optimizes the usage of the finite wavelength spectrum, … Abstract An optimized quantum network design is demonstrated by realizing a state-multiplexing quantum light source via a dual-excitation configuration technique. This approach optimizes the usage of the finite wavelength spectrum, facilitating the efficient expansion of entanglement-based fully-connected quantum networks across multiple users.

Influence of initial atomic coherence and direct coupling on qubits entanglement in two-qubit Tavis — Cummings model

2025-06-19

A. Othman , A. R. Farhutdinova , Eugene K. Bashkirov | Vestnik of Samara University Natural Science Series

The exact dynamics of a model consisting of two identical qubits interacting non-resonantly with the electromagnetic field mode of an ideal resonator via single-photon transitions is found in the presence … The exact dynamics of a model consisting of two identical qubits interacting non-resonantly with the electromagnetic field mode of an ideal resonator via single-photon transitions is found in the presence of direct Ising interaction of qubits. The exact solution is used to calculate the two-qubit negativity of qubits in the case of a coherent initial state of qubits and a thermal state of the resonator field. It is shown that in the case of resonant interaction of qubits with the field, the initial atomic coherence leads to a significant increase in the maximum degree of entanglement. The inclusion of direct Ising interaction of qubits can significantly enhance the entanglement of qubits in both coherent and non-coherent initial states of qubits. In the case of non-resonant interaction of qubits and the resonator field, the detuning leads to a weakening of the effect of atomic coherence on the dynamics of qubit entanglement.

A Computational Framework for Simulations of Dissipative Nonadiabatic Dynamics on Hybrid Oscillator-Qubit Quantum Devices

2025-06-18

Nam P. Vu , D. Dong , Xiaohan Dan +3 more | Journal of Chemical Theory and Computation

We introduce a computational framework for simulating nonadiabatic vibronic dynamics on circuit quantum electrodynamics (cQED) platforms. Our approach leverages hybrid oscillator-qubit quantum hardware with midcircuit measurements and resets, enabling the … We introduce a computational framework for simulating nonadiabatic vibronic dynamics on circuit quantum electrodynamics (cQED) platforms. Our approach leverages hybrid oscillator-qubit quantum hardware with midcircuit measurements and resets, enabling the incorporation of environmental effects such as dissipation and dephasing. To demonstrate its capabilities, we simulate energy transfer dynamics in a triad model of photosynthetic chromophores inspired by natural antenna systems. We specifically investigate the role of dissipation during the relaxation dynamics following photoexcitation, where electronic transitions are coupled to the evolution of quantum vibrational modes. Our results indicate that hybrid oscillator-qubit devices, operating with noise levels below the intrinsic dissipation rates of typical molecular antenna systems, can achieve the simulation fidelity required for practical computations on near-term and early fault-tolerant quantum computing platforms.

Authenticated Multi-Party Quantum Private Set Intersection with Single Particles

2025-06-18

Gongde Guo , Li Zheng , Kai Yu +1 more | Mathematics

As an important branch of secure multi-party computation, privacy set intersection enables multiple parties to input their private sets and jointly compute the intersection of these sets without revealing any … As an important branch of secure multi-party computation, privacy set intersection enables multiple parties to input their private sets and jointly compute the intersection of these sets without revealing any information other than the intersection itself. With the increasing demand for privacy protection of user data, privacy set intersection has been widely used in privacy computing and other fields. In this paper, we utilize the properties of mutually unbiased bases to propose a multi-party quantum private set intersection protocol that incorporates identity authentication mechanisms. A semi-honest third party (TP) is introduced to facilitate the secure execution of this task among the multiple participating parties. The TP establishes a shared master key with each party, which serves as the basis for authenticating the identity of each participant throughout the protocol. Single-particle quantum states, prepared by the TP, act as the information carriers and are sequentially transmitted among the participating parties. Each party performs a local unitary operation on the circulating particle, thereby encoding their private data within the quantum state. At the end of the protocol, the TP announces his measurement result, by which all participants can concurrently ascertain the intersection of their private data sets. Notably, the proposed protocol eliminates the need for long-term storage of single-particle quantum states, thereby rendering it feasible with existing quantum technological capabilities. Furthermore, a comprehensive security analysis demonstrates that the protocol effectively resists some common external and internal attacks, thereby ensuring its theoretical security.

Intraparticle entanglement in noisy quantum channels: degradation and revival through amplitude damping

2025-06-18

Animesh Sinha Roy , C. V. Namitha , Subroto Mukerjee +2 more | Frontiers in Quantum Science and Technology

Quantum correlations between two or more different degrees of freedom of the same particle are sometimes referred to as intraparticle entanglement. In this work, we study these intraparticle correlations between … Quantum correlations between two or more different degrees of freedom of the same particle are sometimes referred to as intraparticle entanglement. In this work, we study these intraparticle correlations between two different degrees of freedom under various decoherence channels, viz. amplitude damping, depolarizing, and phase damping channels. We mainly focus on the amplitude-damping channel for which we obtain an exact analytical expression for the concurrence of an arbitrary initial pure state. In this channel, we observe the unique feature of entanglement arising from a separable initial state. We show that this channel allows for a revival of entanglement with an increasing damping parameter, including from a zero value of the concurrence. We also consider the amplitude-damping channel for interparticle entanglement and show that it does not display any of the above-mentioned interesting features. Further, for comparable parameters, the decay of entanglement in the interparticle system is much greater than in the intraparticle system, which we also find to be true for the phase damping and depolarizing channels. Thus, intraparticle entanglement subjected to damping is much more robust than interparticle entanglement.

Fast preparation of three-dimensional entangled states by designing the evolution operators

2025-06-18

Chi Wang , Siming He , Jian Wang +4 more | Chinese Journal of Physics

Quantum fisher information reveals UV-IR mixing in the strange metal

2025-06-18

David Bałut , Xuefei Guo , N de Vries +4 more | Physica C Superconductivity

Bounding fidelity in quantum feedback control: theory and applications to Dicke state preparation

2025-06-17

Eoin O’Connor , Hailan Ma , Marco G. Genoni | Quantum Science and Technology

Abstract Achieving unit fidelity in quantum state preparation is often impossible in the presence of environmental decoherence. While continuous monitoring and feedback control can improve fidelity, perfect state preparation remains … Abstract Achieving unit fidelity in quantum state preparation is often impossible in the presence of environmental decoherence. While continuous monitoring and feedback control can improve fidelity, perfect state preparation remains elusive in many scenarios. Inspired by quantum speed limits, we derive a fundamental bound on the steady-state average fidelity achievable via continuous monitoring and feedback control. This bound depends only on the unconditional Lindblad dynamics, the Hamiltonian variance, and the target state. We also adapt the bound to the case of Markovian feedback strategies. We then focus on preparing Dicke states in an atomic ensemble subject to collective damping and dispersive coupling. By imposing additional constraints on control Hamiltonians and monitoring strategies, we derive tighter fidelity bounds. Finally, we propose specific control strategies and validate them using reinforcement learning. Benchmarking their performance against our theoretical bounds highlights the relevance and usefulness of these bounds in characterizing quantum feedback control strategies.

Experimental demonstration of high-speed continuous variable quantum key distribution enhanced by phase-sensitive amplifier

2025-06-17

Jinpeng Liao , Zhirong Chen , Jintao Wang +4 more | npj Quantum Information

Arbitrary state transition of open qubit system based on switching control

2025-06-17

Guangpu Wu , Shibei Xue , Shan Ma +3 more | Automatica

Multi-Mode Global Driving of Trapped Ions for Quantum Circuit Synthesis

2025-06-17

Philip Richerme | Quantum Science and Technology

Abstract We study the use of global drives with multiple frequency components to improve the efficiency of trapped ion quantum simulations and computations. We show that such `multi-mode' global drives, … Abstract We study the use of global drives with multiple frequency components to improve the efficiency of trapped ion quantum simulations and computations. We show that such `multi-mode' global drives, when combined with a linear number of single-qubit rotations, generate universal Ising-type interactions with shorter overall runtimes than corresponding two-qubit gate implementations. Further, we show how this framework may be extended to efficiently generate $n-$body interactions between any subset $n$ of the ion qubits. Finally, we apply these techniques to encode the Quantum Fourier Transform using quadratically-fewer entangling operations, with quadratically smaller runtime, compared with traditional approaches.&#xD;

Effect of correlated errors on quantum memory

2025-06-17

NULL AUTHOR_ID | Physical review. A/Physical review, A

Family of attainable geometric quantum speed limits

2025-06-17

Z.H. Mai , Zheng Liu , Chang‐shui Yu | Quantum

We propose a quantum state distance and develop a family of geometrical quantum speed limits (QSLs) for open and closed systems. The QSL time includes an alternative function by which … We propose a quantum state distance and develop a family of geometrical quantum speed limits (QSLs) for open and closed systems. The QSL time includes an alternative function by which we derive three QSL times with particularly chosen functions. It indicates that two QSL times are exactly the ones presented in Ref. \cite{Campaioli2019tightrobust} and \cite{mai2023tight}, respectively, and the third one can provide a unified QSL time for both open and closed systems. The three QSL times are attainable for any given initial state in the sense that there exists a dynamics driving the initial state to evolve along the geodesic. We numerically compare the tightness of the three QSL times, which typically promises a tighter QSL time if optimizing the alternative function.

Linear quantum coupler for clean bosonic control

2025-06-17

NULL AUTHOR_ID | PRX Quantum

Peer-to-Peer Distribution of Graph States Across Spacetime Quantum Networks of Arbitrary Topology

2025-06-16

Yuexun Huang , Xiangyu Ren , Bikun Li +3 more | ACM SIGMETRICS Performance Evaluation Review

Graph states are a class of important multiparty entangled quantum states, of which Bell pairs are the special case. Realizing a robust and fast distribution of arbitrary graph states in … Graph states are a class of important multiparty entangled quantum states, of which Bell pairs are the special case. Realizing a robust and fast distribution of arbitrary graph states in the downstream layer of the quantum network is essential for enabling large-scale quantum networks. To address this, we propose a novel quantum network protocol, called P2PGSD, inspired by the classical Peer-to-Peer network. This protocol efficiently implements general graph state distribution in the network layer, demonstrating significant advantages in resource efficiency and scalability, particularly for sparse graph states. An explicit mathematical model for the general graph state distribution problem has also been constructed, above which the intractability for a wide class of resource minimization problems is proved and the optimality of the existing algorithms is discussed. Moreover, we leverage the space-time quantum network for memory management in network challenges, drawing inspiration from special relativity. We suggest a universal quantum distributed computation framework to exploit the strengths of our protocols, as confirmed by numerical simulations that reveal up to a 50% enhancement for general sparse graph states. This work marks a significant step toward resource-efficient multiparty entanglement distribution for diverse network topologies.

Markov chain Monte Carlo detector tomography applied to a NbTiN nanobridge

2025-06-16

F. B. Baalbergen , Iman Esmaeil Zadeh , M. P. van Exter +1 more | Applied Physics Letters

We demonstrate the use of a flexible and highly accurate Markov chain Monte Carlo quantum detector tomography method as a minimization algorithm to best describe the response of an efficient … We demonstrate the use of a flexible and highly accurate Markov chain Monte Carlo quantum detector tomography method as a minimization algorithm to best describe the response of an efficient 120 nm wide NbTiN superconducting nanobridge single photon detector. Separation of the internal quantum efficiency and external quantum efficiency is possible due to the difference in saturation behavior of an ideal one-photon threshold detector as compared to a detector with non-unity one-photon internal quantum efficiency. From a statistical analysis of our measurements (at T=(4.23 ± 0.01) K, I=(29.4 ± 0.1) μA, I/Iswitch=(0.90 ± 0.01)), we find an external quantum efficiency of η=(1.60 ± 0.05)×10−6, a one-photon internal quantum efficiency of p1=0.568 ± 0.008, and a unity multi-photon (two or more) internal quantum efficiency.

The temporal resolution limit in quantum sensing

2025-06-16

Cen Song , Qing-yu Cai | EPJ Quantum Technology