Showing papers on "Quantum channel published in 2012"
TL;DR: This review focuses on continuous-variable quantum information processes that rely on any combination of Gaussian states, Gaussian operations, and Gaussian measurements, including quantum communication, quantum cryptography, quantum computation, quantum teleportation, and quantum state and channel discrimination.
Abstract: 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.
2,781 citations
TL;DR: The results show that long-distance quantum cryptography over say 200 km will remain secure even with seriously flawed detectors, and the key generation rate is many orders of magnitude higher than that based on full device independent QKD.
Abstract: 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.
1,699 citations
TL;DR: Different methods for quantifying the quantum and classical parts of correlations are among the more actively studied topics of quantum-information theory over the past decade as mentioned in this paper and different notions of classical and quantum correlations quantified by quantum discord and other related measures are reviewed.
Abstract: One of the best signatures of nonclassicality in a quantum system is the existence of correlations that have no classical counterpart. Different methods for quantifying the quantum and classical parts of correlations are among the more actively studied topics of quantum-information theory over the past decade. Entanglement is the most prominent of these correlations, but in many cases unentangled states exhibit nonclassical behavior too. Thus distinguishing quantum correlations other than entanglement provides a better division between the quantum and classical worlds, especially when considering mixed states. Here different notions of classical and quantum correlations quantified by quantum discord and other related measures are reviewed. In the first half, the mathematical properties of the measures of quantum correlations are reviewed, related to each other, and the classical-quantum division that is common among them is discussed. In the second half, it is shown that the measures identify and quantify the deviation from classicality in various quantum-information-processing tasks, quantum thermodynamics, open-system dynamics, and many-body physics. It is shown that in many cases quantum correlations indicate an advantage of quantum methods over classical ones.
1,504 citations
01 May 2012
TL;DR: In this article, a review of the state of the art in continuous-variable quantum information processing can be found, ranging from the basic theoretical tools and landmark experimental realizations to the most recent successful developments.
Abstract: 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.
1,374 citations
TL;DR: A review of the progress in photonic quantum information processing can be found in this article, where the emphasis is given to the creation of photonic entanglement of various forms, tests of the completeness of quantum mechanics (in particular, violations of local realism), quantum information protocols for quantum communication, and quantum computation with linear optics.
Abstract: Multiphoton interference reveals strictly nonclassical phenomena. Its applications range from fundamental tests of quantum mechanics to photonic quantum information processing, where a significant fraction of key experiments achieved so far comes from multiphoton state manipulation. The progress, both theoretical and experimental, of this rapidly advancing research is reviewed. The emphasis is given to the creation of photonic entanglement of various forms, tests of the completeness of quantum mechanics (in particular, violations of local realism), quantum information protocols for quantum communication (e.g., quantum teleportation, entanglement purification, and quantum repeater), and quantum computation with linear optics. The scope of the review is limited to ``few-photon'' phenomena involving measurements of discrete observables.
1,156 citations
TL;DR: The quality of the quantum teleportation procedure without feed-forward is confirmed, well beyond the classical limit of two-thirds, and the maturity and applicability of such technologies in real-world scenarios are verified, in particular for future satellite-based quantum teleportation.
Abstract: The benchmark for a global quantum internet — quantum teleportation of independent qubits using active feed-forward over a free-space link whose attenuation corresponds to the path between a satellite and a ground station — has now been successfully achieved over a distance of 143 km, between the Canary Islands of La Palma and Tenerife. Since the first experimental demonstration of quantum teleportation — a central component of quantum communication and quantum computation — physicists have gradually extended the distance over which the quantum information is distributed. This paper reports long-distance quantum teleportation involving active feed-forward in real time, a feature that will be essential for future applications such as communication between quantum computers. Using two optical links, quantum and classical, quantum teleportation of an independent state was achieved across a 'record' 143 km of free space between the Canary Islands of La Palma and Tenerife. Significantly, this route bears comparison with the path between a satellite and a ground station. The quantum internet1 is predicted to be the next-generation information processing platform, promising secure communication2,3 and an exponential speed-up in distributed computation2,4. The distribution of single qubits over large distances via quantum teleportation5 is a key ingredient for realizing such a global platform. By using quantum teleportation, unknown quantum states can be transferred over arbitrary distances to a party whose location is unknown. Since the first experimental demonstrations of quantum teleportation of independent external qubits6, an internal qubit7 and squeezed states8, researchers have progressively extended the communication distance. Usually this occurs without active feed-forward of the classical Bell-state measurement result, which is an essential ingredient in future applications such as communication between quantum computers. The benchmark for a global quantum internet is quantum teleportation of independent qubits over a free-space link whose attenuation corresponds to the path between a satellite and a ground station. Here we report such an experiment, using active feed-forward in real time. The experiment uses two free-space optical links, quantum and classical, over 143 kilometres between the two Canary Islands of La Palma and Tenerife. To achieve this, we combine advanced techniques involving a frequency-uncorrelated polarization-entangled photon pair source, ultra-low-noise single-photon detectors and entanglement-assisted clock synchronization. The average teleported state fidelity is well beyond the classical limit9 of two-thirds. Furthermore, we confirm the quality of the quantum teleportation procedure without feed-forward by complete quantum process tomography. Our experiment verifies the maturity and applicability of such technologies in real-world scenarios, in particular for future satellite-based quantum teleportation.
520 citations
TL;DR: The high-frequency and high-accuracy acquiring, pointing and tracking technique developed in this experiment can be directly used for future satellite-based quantum communication and large-scale tests of quantum foundations.
Abstract: Quantum teleportation of independent qubits and entanglement distribution have been demonstrated over free-space channels of about 100 kilometres, representing an important step towards a global quantum network. Free-space channels, in which light propagates freely through the open air, offer advantages over optical fibres for quantum communication because they suffer fewer photon losses and less decoherence. Here, Juan Yin et al. report quantum teleportation of independent qubits over a 97-kilometre one-link free-space channel with multiphoton entanglement. Using a two-link channel, they also demonstrate entanglement distribution over 101.8 kilometres. The results represent an important step towards a global quantum network. In particular, the high-frequency and high-accuracy acquiring, pointing and tracking techniques developed in the experiment could be used directly for future satellite-based quantum communication and large-scale tests of quantum foundations. Transferring an unknown quantum state over arbitrary distances is essential for large-scale quantum communication and distributed quantum networks. It can be achieved with the help of long-distance quantum teleportation1,2 and entanglement distribution. The latter is also important for fundamental tests of the laws of quantum mechanics3,4. Although quantum teleportation5,6 and entanglement distribution7,8,9 over moderate distances have been realized using optical fibre links, the huge photon loss and decoherence in fibres necessitate the use of quantum repeaters10 for larger distances. However, the practical realization of quantum repeaters remains experimentally challenging11. Free-space channels, first used for quantum key distribution12,13, offer a more promising approach because photon loss and decoherence are almost negligible in the atmosphere. Furthermore, by using satellites, ultra-long-distance quantum communication and tests of quantum foundations could be achieved on a global scale. Previous experiments have achieved free-space distribution of entangled photon pairs over distances of 600 metres (ref. 14) and 13 kilometres (ref. 15), and transfer of triggered single photons over a 144-kilometre one-link free-space channel16. Most recently, following a modified scheme17, free-space quantum teleportation over 16 kilometres was demonstrated18 with a single pair of entangled photons. Here we report quantum teleportation of independent qubits over a 97-kilometre one-link free-space channel with multi-photon entanglement. An average fidelity of 80.4 ± 0.9 per cent is achieved for six distinct states. Furthermore, we demonstrate entanglement distribution over a two-link channel, in which the entangled photons are separated by 101.8 kilometres. Violation of the Clauser–Horne–Shimony–Holt inequality4 is observed without the locality loophole. Besides being of fundamental interest, our results represent an important step towards a global quantum network. Moreover, the high-frequency and high-accuracy acquiring, pointing and tracking technique developed in our experiment can be directly used for future satellite-based quantum communication and large-scale tests of quantum foundations.
469 citations
TL;DR: Quantum discord is the total non-classical correlation between two systems as discussed by the authors, which includes entanglement, and is defined as a measure of non-zero quantum discord.
Abstract: Quantum discord is the total non-classical correlation between two systems This includes, but is not limited to, entanglement Photonic experiments now demonstrate that separable states with non-zero quantum discord are a useful resource for quantum information processing and can even outperform entangled states
428 citations
TL;DR: In this paper, a superconducting quantum bit (qubit) coupled to a microwave cavity is used to stabilize the oscillation phase of a driven quantum bit, which can be used to continuously track and steer the quantum state using feedback.
Abstract: Real-time quantum feedback based on weak measurement of the quantum state is used to stabilize the oscillation phase of a driven quantum bit. By performing weak measurements of a quantum state, it is possible to slow the rate of collapse of its wavefunction, so that information about the quantum state can be gradually acquired. Such information can be used to continuously track and steer the quantum state using feedback. This paper reports quantum feedback control of a superconducting quantum bit (qubit) coupled to a microwave cavity. The qubit undergoes coherent oscillations that can be made to speed up, slow down or persist indefinitely. This ability to actively suppress decoherence could find many applications in quantum error correction, quantum-state stabilization and purification, entanglement generation and adaptive measurements. The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single (probabilistic) outcome. The timescale of this ‘instantaneous’ process can be stretched using weak measurements1,2, such that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback3,4,5,6,7,8. Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity9. A weak measurement of the qubit is implemented by probing the cavity with microwave photons, maintaining its average occupation at less than one photon. These photons are then directed to a high-bandwidth, quantum-noise-limited amplifier10,11, which allows real-time monitoring of the state of the cavity (and, hence, that of the qubit) with high fidelity. We demonstrate quantum feedback control by inhibiting the decay of Rabi oscillations, allowing them to persist indefinitely12. Such an ability permits the active suppression of decoherence and enables a method of quantum error correction based on weak continuous measurements13,14. Other applications include quantum state stabilization4,7,15, entanglement generation using measurement16, state purification17 and adaptive measurements18,19.
418 citations
TL;DR: In this paper, the authors compared Bose-Einstein condensates in optical traps with a Bose condensate-based protocol for high fidelity transformation of a quantum system with high fidelity.
Abstract: Transforming a quantum system with high fidelity is usually a trade-off between an increase in speed—thereby minimizing decoherence—and robustness against fluctuating control parameters. Protocols at these two extreme limits are now demonstrated and compared using Bose–Einstein condensates in optical traps.
398 citations
TL;DR: This work shows that the one-shot classical capacity of a quantum channel is well approximated by a relative-entropy-type measure defined via hypothesis testing, and gives a conceptually simple proof of the well-known Holevo-Schumacher-Westmoreland theorem for the capacity of memoryless channels.
Abstract: The one-shot classical capacity of a quantum channel quantifies the amount of classical information that can be transmitted through a single use of the channel such that the error probability is below a certain threshold. In this work, we show that this capacity is well approximated by a relative-entropy-type measure defined via hypothesis testing. Combined with a quantum version of Stein's lemma, our results give a conceptually simple proof of the well-known Holevo-Schumacher-Westmoreland theorem for the capacity of memoryless channels. More generally, we obtain tight capacity formulas for arbitrary (not necessarily memoryless) channels.
TL;DR: In this article, a reconfigurable integrated quantum photonic circuit consisting of a two-qubit entangling gate, several Hadamard-like gates and eight variable phase shifters is presented.
Abstract: Researchers demonstrate a reconfigurable integrated quantum photonic circuit. The device comprises a two-qubit entangling gate, several Hadamard-like gates and eight variable phase shifters. The set-up is used to generate entangled states, violate a Bell-type inequality with a continuum of partially entangled states and demonstrate the generation of arbitrary one-qubit mixed states.
TL;DR: In this paper, the authors proposed two realistic entanglement concentration protocols (ECPs) for pure partially entangled photons, which are implemented with linear optics and with cross-Kerr-nonlinearities.
Abstract: We present two realistic entanglement concentration protocols (ECPs) for pure partially entangled photons. A partially entangled photon pair can be concentrated to a maximally entangled pair with only an ancillary single photon with a certain probability, while the conventional ECPs require two copies of partially entangled pairs at least. Our first protocol is implemented with linear optics and the second protocol is implemented with cross-Kerr-nonlinearities. Compared with other ECPs, they do not need to know the accurate coefficients of the initial state. With linear optics, it is feasible with current experiments. With cross-Kerr-nonlinearities, it does not require sophisticated single-photon detectors and can be repeated to get a higher success probability. Moreover, the second protocol can get the higher entanglement transformation efficiency and it may be the most economical protocol by far. Meanwhile, both protocols are more suitable for multiphoton system concentration because they need less operations and classical communications. All these advantages make the two protocols useful in current long-distance quantum communications.
TL;DR: A demonstration of quantum key distribution (QKD) over a standard telecom fiber exceeding 50 dB in loss and 250 km in length is reported, with careful optimization of the 1 bit delayed Faraday-Michelson interferometer and the use of the superconducting single photon detector (SSPD).
Abstract: We report a demonstration of quantum key distribution (QKD) over a standard telecom fiber exceeding 50 dB in loss and 250 km in length. The differential phase shift QKD protocol was chosen and implemented with a 2 GHz system clock rate. By careful optimization of the 1 bit delayed Faraday–Michelson interferometer and the use of the superconducting single photon detector (SSPD), we achieved a quantum bit error rate below 2% when the fiber length was no more than 205 km, and of 3.45% for a 260 km fiber with 52.9 dB loss. We also improved the quantum efficiency of SSPD to obtain a high key rate for 50 km length.
TL;DR: In this paper, the authors proposed a design of quantum communication based on directly transmitting quantum information in encoded form across a network, which does not require entangled links between nodes and long-lived quantum memories.
Abstract: Researchers propose a design of quantum communication based on directly transmitting quantum information in encoded form across a network. Involving no teleportation, the scheme does not require entangled links between nodes and long-lived quantum memories. It potentially provides higher communication rates than existing entanglement-based schemes.
10 Dec 2012
TL;DR: The authors present the fundamental theoretical results of quantum information theory, while also presenting the details of advanced quantum ccommunication protocols with clear mathematical and information theoretical background.
Abstract: The book provides an overview of the most advanced quantum informational geometric techniques, which can help quantum communication theorists analyze quantum channels, such as security or additivity properties. Each section addresses an area of major research of quantum information theory and quantum communication networks. The authors present the fundamental theoretical results of quantum information theory, while also presenting the details of advanced quantum ccommunication protocols with clear mathematical and information theoretical background. This book bridges the gap between quantum physics, quantum information theory, and practical engineering.
TL;DR: It is shown that, by employing state-of-the-art optoelectromechanical devices, one can realize an effective source of (bright) two-mode squeezing with an optical idler and a microwave signal, which can be used for high-fidelity transfer of quantum states between optical and microwave fields by means of continuous variable teleportation.
Abstract: We describe a reversible quantum interface between an optical and a microwave field using a hybrid device based on their common interaction with a micromechanical resonator in a superconducting circuit. We show that, by employing state-of-the-art optoelectromechanical devices, one can realize an effective source of (bright) two-mode squeezing with an optical idler (signal) and a microwave signal, which can be used for high-fidelity transfer of quantum states between optical and microwave fields by means of continuous variable teleportation.
TL;DR: A different quantum private comparison protocol enabling two parties to compare the equality of their information without revealing the information content is proposed, which provides easier implementation as well as better qubit efficiency than the other QPCs.
Abstract: Based on EPR pairs, this paper proposes a different quantum private comparison (QPC) protocol enabling two parties to compare the equality of their information without revealing the information content. Due to the use of quantum entanglement of Bell state as well as one-way quantum transmission, the new protocol provides easier implementation as well as better qubit efficiency (near 50%) than the other QPCs. It is secure against Trojan horse attack and other well-known attacks.
Book•
01 Jan 2012
TL;DR: This book gives an accessible, albeit mathematically rigorous and self-contained introduction to quantum information theory, starting from primary structures and leading to fundamental results and to exiting open problems.
Abstract: The subject of this book is theory of quantum system presented from information science perspective. The central role is played by the concept of quantum channel and its entropic and information characteristics. Quantum information theory gives a key to understanding elusive phenomena of quantum world and provides a background for development of experimental techniques that enable measuring and manipulation of individual quantum systems. This is important for the new efficient applications such as quantum computing, communication and cryptography. Research in the field of quantum informatics, including quantum information theory, is in progress in leading scientific centers throughout the world. This book gives an accessible, albeit mathematically rigorous and self-contained introduction to quantum information theory, starting from primary structures and leading to fundamental results and to exiting open problems.
TL;DR: A survey of the main properties of quantum channels and of their entropic characterization, with a variety of examples for finite-dimensional quantum systems, and the remarkable role of specific quantum correlations-entanglement-as a novel communication resource is stressed.
Abstract: One of the major achievements of the recently emerged quantum information theory is the introduction and thorough investigation of the notion of a quantum channel which is a basic building block of any data-transmitting or data-processing system. This development resulted in an elaborated structural theory and was accompanied by the discovery of a whole spectrum of entropic quantities, notably the channel capacities, characterizing information-processing performance of the channels. This paper gives a survey of the main properties of quantum channels and of their entropic characterization, with a variety of examples for finite-dimensional quantum systems. We also touch upon the 'continuous-variables' case, which provides an arena for quantum Gaussian systems. Most of the practical realizations of quantum information processing were implemented in such systems, in particular based on principles of quantum optics. Several important entropic quantities are introduced and used to describe the basic channel capacity formulae. The remarkable role of specific quantum correlations?entanglement?as a novel communication resource is stressed.
TL;DR: This work proposes and experimentally addresses a continuous variable quantum key distribution protocol that uses modulated fragile entangled states of light to greatly enhance the robustness to channel noise and demonstrates that the resulting protocol can tolerate more noise than the benchmark set by the ideal continuous variable coherent state protocol.
Abstract: Quantum key distribution enables two remote parties to grow a shared key, which they can use for unconditionally secure communication over a certain distance. The maximal distance depends on the loss and the excess noise of the connecting quantum channel. Several quantum key distribution schemes based on coherent states and continuous variable measurements are resilient to high loss in the channel, but are strongly affected by small amounts of channel excess noise. Here we propose and experimentally address a continuous variable quantum key distribution protocol that uses modulated fragile entangled states of light to greatly enhance the robustness to channel noise. We experimentally demonstrate that the resulting quantum key distribution protocol can tolerate more noise than the benchmark set by the ideal continuous variable coherent state protocol. Our scheme represents a very promising avenue for extending the distance for which secure communication is possible.
TL;DR: In this article, the problem of non-vanishing thermal occupation of the phononic quantum channel can be overcome by implementing optomechanical multi-and continuous mode cooling schemes to create a?cold? frequency window for transmitting quantum states.
Abstract: We study the implementation of quantum state transfer protocols in phonon networks, where, in analogy to optical networks, quantum information is transmitted through propagating phonons in extended mechanical resonator arrays or phonon waveguides. We describe how the problem of a non-vanishing thermal occupation of the phononic quantum channel can be overcome by implementing optomechanical multi- and continuous mode cooling schemes to create a ?cold? frequency window for transmitting quantum states. In addition, we discuss the implementation of phonon circulators and switchable phonon routers, which rely only on strong coherent optomechanical interactions and do not require strong magnetic fields or specific materials. Both techniques can be applied and adapted to various physical implementations, where phonons coupled to spin- or charge-based qubits are used for on-chip networking applications.
TL;DR: In this article, an ultra-fast quantum random number generator (QRNG) was proposed to generate true randomness by exploiting the fundamental indeterminism of quantum mechanics, which is based on the quantum phase fluctuations of a laser operating near threshold.
Abstract: A quantum random number generator (QRNG) can generate true randomness by exploiting the fundamental indeterminism of quantum mechanics. Most approaches to QRNG employ single-photon detection technologies and are limited in speed. Here, we experimentally demonstrate an ultrafast QRNG at a rate over 6 Gbits/s based on the quantum phase fluctuations of a laser operating near threshold. Moreover, we consider a potential adversary who has partial knowledge on the raw data and discuss how one can rigorously remove such partial knowledge with postprocessing. We quantify the quantum randomness through min-entropy by modeling our system and employ two randomness extractors--Trevisan's extractor and Toeplitz-hashing--to distill the randomness, which is information-theoretically provable. The simplicity and high-speed of our experimental setup show the feasibility of a robust, low-cost, high-speed QRNG.
TL;DR: In this paper, the authors demonstrate frequency-degenerate entanglement between continuous-variable quantum microwaves propagating along two spatially separated paths using a microwave beam splitter.
Abstract: Path entanglement constitutes an essential resource in quantum information and communication protocols. Here, we demonstrate frequency-degenerate entanglement between continuous-variable quantum microwaves propagating along two spatially separated paths. We combine a squeezed and a vacuum state using a microwave beam splitter. Via correlation measurements, we detect and quantify the path entanglement contained in the beam splitter output state. Our experiments open the avenue to quantum teleportation, quantum communication, or quantum radar with continuous variables at microwave frequencies.
Proceedings Article•
01 Oct 2012TL;DR: It is shown that the quantum version of the Even-Mansour cipher is insecure, that is, a key can be found in polynomial time in the key length, an example that the Quantum version of a secure classical cipher is not always secure.
Abstract: Quantum cryptography such as BB84 is a quantum protocol for sharing classical information, but is not a scheme for encrypting quantum information itself. This paper considers that quantum information is encrypted with the quantum circuit of the Even-Mansour cipher. It has been proved that breaking the Even-Mansour cipher requires exponential time in the key length using any classical algorithm. This paper shows that the quantum version of the Even-Mansour cipher is insecure, that is, a key can be found in polynomial time in the key length. This is an example that the quantum version of a secure classical cipher is not always secure.
TL;DR: In this paper, the authors proposed a scheme to distinguish 16 hyperentangled Bell states in both the polarization and the spatial-mode DOF of two-photon systems, by using the giant nonlinear optics in quantum dot-cavity systems.
Abstract: Bell-state analysis (BSA) is essential in quantum communication, but it is impossible to distinguish unambiguously the four Bell states in the polarization degree of freedom (DOF) of two-photon systems with only linear optical elements, except for the case in which the BSA is assisted with hyperentangled states, the simultaneous entanglement in more than one DOF. Here, we propose a scheme to distinguish completely the 16 hyperentangled Bell states in both the polarization and the spatial-mode DOFs of two-photon systems, by using the giant nonlinear optics in quantum dot-cavity systems. This scheme can be applied to increase the channel capacity of long-distance quantum communication based on hyperentanglement, such as entanglement swapping, teleportation, and superdense coding. We use hyperentanglement swapping as an example to show the application of this HBSA.
TL;DR: It is shown that a 2⊗2 maximally entangled state can be used to locally distinguish this set of states without being consumed, thus demonstrating a novel phenomenon of entanglement discrimination catalysis.
Abstract: We explicitly exhibit a set of four ququad-ququad orthogonal maximally entangled states that cannot be perfectly distinguished by means of local operations and classical communication. Before our work, it was unknown whether there is a set of d locally indistinguishable d⊗d orthogonal maximally entangled states for some positive integer d. We further show that a 2⊗2 maximally entangled state can be used to locally distinguish this set of states without being consumed, thus demonstrate a novel phenomenon of entanglement discrimination catalysis. Based on this set of states, we construct a new set K consisting of four locally indistinguishable states such that K(⊗m) (with 4(m) members) is locally distinguishable for some m greater than one. As an immediate application, we construct a noisy quantum channel with one sender and two receivers whose local zero-error classical capacity can achieve the full dimension of the input space but only with a multi-shot protocol.
TL;DR: A flexible quantum-key-distribution-based protocol for quantum private queries is presented that, by adjusting the value of θ, the average number of the key bits Alice obtains can be located on any fixed value the users wanted for any database size.
Abstract: By adding a parameter θ in M. Jakobi et al’s protocol [Phys. Rev. A 83, 022301 (2011)], we present a flexible quantum-key-distribution-based protocol for quantum private queries. We show that, by adjusting the value of θ, the average number of the key bits Alice obtains can be located on any fixed value the users wanted for any database size. And the parameter k is generally smaller (even k = 1 can be achieved) when θ < π/4, which implies lower complexity of both quantum and classical communications. Furthermore, the users can choose a smaller θ to get better database security, or a larger θ to obtain a lower probability with which Bob can correctly guess the address of Alice’s query.
TL;DR: The concept of ϵ blindness for UBQC is introduced, in analogy to the concept ofπ security developed for other cryptographic protocols, allowing us to characterize the robustness and security properties of the protocol under possible imperfections.
Abstract: The universal blind quantum computation (UBQC) protocol [A. Broadbent, J. Fitzsimons, and E. Kashefi, in Proceedings of the 50th Annual IEEE Symposiumon Foundations of Computer Science (IEEE Computer Society, Los Alamitos, CA, USA, 2009), pp. 517--526.] allows a client to perform quantum computation on a remote server. In an ideal setting, perfect privacy is guaranteed if the client is capable of producing specific, randomly chosen single qubit states. While from a theoretical point of view, this may constitute the lowest possible quantum requirement, from a pragmatic point of view, generation of such states to be sent along long distances can never be achieved perfectly. We introduce the concept of $ϵ$ blindness for UBQC, in analogy to the concept of $ϵ$ security developed for other cryptographic protocols, allowing us to characterize the robustness and security properties of the protocol under possible imperfections. We also present a remote blind single qubit preparation protocol with weak coherent pulses for the client to prepare, in a delegated fashion, quantum states arbitrarily close to perfect random single qubit states. This allows us to efficiently achieve $ϵ$-blind UBQC for any $ϵg0$, even if the channel between the client and the server is arbitrarily lossy.
TL;DR: The storage capabilities of solid state quantum light matter interfaces to polarization encoding are extended, and the average conditional fidelity of the retrieved qubits exceeds 95% for a mean photon number μ=0.4.
Abstract: We report on the quantum storage and retrieval of photonic polarization quantum bits onto and out of a solid state storage device. The qubits are implemented with weak coherent states at the single photon level, and are stored for a predetermined time of 500 ns in a praseodymium doped crystal with a storage and retrieval efficiency of 10%, using the atomic frequency comb scheme. We characterize the storage by using quantum state tomography, and find that the average conditional fidelity of the retrieved qubits exceeds 95% for a mean photon number $\ensuremath{\mu}=0.4$. This is significantly higher than a classical benchmark, taking into account the Poissonian statistics and finite memory efficiency, which proves that our crystal functions as a quantum storage device for polarization qubits. These results extend the storage capabilities of solid state quantum light matter interfaces to polarization encoding, which is widely used in quantum information science.