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Showing papers on "Quantum channel published in 2021"


Journal ArticleDOI
TL;DR: In this paper, a dual-band phase stabilization scheme was proposed to reduce the phase fluctuations on optical fiber by more than four orders of magnitude, which can be adapted to other phase sensitive single-photon applications.
Abstract: Twin-field (TF) quantum key distribution (QKD) fundamentally alters the rate-distance relationship of QKD, offering the scaling of a single-node quantum repeater. Although recent experiments have demonstrated the new opportunities for secure long-distance communications allowed by TF-QKD, formidable challenges remain to unlock its true potential. Previous demonstrations have required intense stabilization signals at the same wavelength as the quantum signals, thereby unavoidably generating Rayleigh scattering noise that limits the distance and bit rate. Here, we introduce a dual-band stabilization scheme that overcomes past limitations and can be adapted to other phase-sensitive single-photon applications. Using two different optical wavelengths multiplexed together for channel stabilization and protocol encoding, we develop a setup that provides repeater-like key rates over communication distances of 555 km and 605 km in the finite-size and asymptotic regimes respectively and increases the secure key rate at long distance by two orders of magnitude to values of practical relevance. Twin-field quantum key distribution over 600 km is demonstrated. The key ingredient for success is the dual-band phase stabilization that dramatically reduce the phase fluctuations on optical fibre by more than four orders of magnitude.

126 citations


Journal ArticleDOI
TL;DR: In this paper, a twin-field QKD protocol was proposed for a 511 km long-haul fiber-optic network without a trusted relaying node, achieving a secure key rate around three orders of magnitude greater than that expected if the previous protocol was applied over the same length.
Abstract: The basic principle of quantum mechanics1 guarantees the unconditional security of quantum key distribution (QKD)2–6 at the cost of forbidding the amplification of a quantum state. As a result, and despite remarkable progress in worldwide metropolitan QKD networks7,8 over the past decades, a long-haul fibre QKD network without a trusted relay has not yet been achieved. Here, through the sending-or-not-sending protocol9, we achieve twin-field QKD10 and distribute secure keys without any trusted repeater over a 511 km long-haul fibre trunk that links two distant metropolitan areas. The fibre trunk contains 12 fibres in the cable, three of which are used for the quantum channel, optical synchronization and frequency locking, respectively. The remaining nine are used for classical fibre communication. Our secure key rate is around three orders of magnitude greater than that expected if the previous QKD field-test system was applied over the same length. Efficient quantum-state transmission and stable single-photon interference over such a long-haul deployed fibre pave the way to large-scale fibre quantum networks. A field test of twin-field quantum key distribution was implemented through a 511 km optical fibre. To this end, precise wavelength control of remote independent laser sources and fast time- and phase-compensation systems are developed.

111 citations


Journal ArticleDOI
01 Dec 2021
TL;DR: In this paper, a review of the recent advances in quantum key distribution is presented, including protocols, photon sources, and photon detectors, as well as a discussion of the current state of the art in this area.
Abstract: Quantum key distribution is a matured quantum science and technology. Over the last 20 years, there has been substantial research and development in this area. Recently, silicon technology has offered tremendous promise in the field for improved miniaturization of quantum key distribution through integrated photonic chips. We expect further progress in this area both in terms of protocols, photon sources, and photon detectors. This review captures some of the recent advances in this area.

109 citations


Journal ArticleDOI
25 Mar 2021
TL;DR: This work bound the ultimate rates for key and entanglement distribution through a free-space link, where the propagation of quantum systems is generally affected by diffraction, atmospheric extinction, turbulence, pointing errors, and background noise.
Abstract: This work establishes the limits for free-space quantum communications under the effects of diffraction, atmospheric extinction, pointing error, turbulence, and background noise.

75 citations


Journal ArticleDOI
TL;DR: In this paper, a brief overview of state-of-the-art quantum network experiments employing color centers in diamond and discuss future research directions, focusing on the control and coherence of qubits that distribute and store entangled states, and on efficient spin-photon interfaces.
Abstract: With the ability to transfer and process quantum information, large-scale quantum networks will enable a suite of fundamentally new applications, from quantum communications to distributed sensing, metrology, and computing. This Perspective reviews requirements for quantum network nodes and color centers in diamond as suitable node candidates. We give a brief overview of state-of-the-art quantum network experiments employing color centers in diamond and discuss future research directions, focusing, in particular, on the control and coherence of qubits that distribute and store entangled states, and on efficient spin–photon interfaces. We discuss a route toward large-scale integrated devices combining color centers in diamond with other photonic materials and give an outlook toward realistic future quantum network protocol implementations and applications.

74 citations


Journal ArticleDOI
TL;DR: In this article, a brief overview of state-of-the-art quantum network experiments employing color centers in diamond is given, focusing on the control and coherence of qubits that distribute and store entangled states, and on efficient spin-photon interfaces.
Abstract: With the ability to transfer and process quantum information, large-scale quantum networks will enable a suite of fundamentally new applications, from quantum communications to distributed sensing, metrology, and computing. This perspective reviews requirements for quantum network nodes and color centers in diamond as suitable node candidates. We give a brief overview of state-of-the-art quantum network experiments employing color centers in diamond, and discuss future research directions, focusing in particular on the control and coherence of qubits that distribute and store entangled states, and on efficient spin-photon interfaces. We discuss a route towards large-scale integrated devices combining color centers in diamond with other photonic materials and give an outlook towards realistic future quantum network protocol implementations and applications.

59 citations


Journal ArticleDOI
TL;DR: In this article, a coherently driven quantum dot was used to experimentally demonstrate a modified Ekert quantum key distribution protocol with two quantum channel approaches: both a 250m-long singlemode fiber and in free space, connecting two buildings within the campus of Sapienza University in Rome.
Abstract: Quantum key distribution—exchanging a random secret key relying on a quantum mechanical resource—is the core feature of secure quantum networks. Entanglement-based protocols offer additional layers of security and scale favorably with quantum repeaters, but the stringent requirements set on the photon source have made their use situational so far. Semiconductor-based quantum emitters are a promising solution in this scenario, ensuring on-demand generation of near-unity-fidelity entangled photons with record-low multiphoton emission, the latter feature countering some of the best eavesdropping attacks. Here, we use a coherently driven quantum dot to experimentally demonstrate a modified Ekert quantum key distribution protocol with two quantum channel approaches: both a 250-m-long single-mode fiber and in free space, connecting two buildings within the campus of Sapienza University in Rome. Our field study highlights that quantum-dot entangled photon sources are ready to go beyond laboratory experiments, thus opening the way to real-life quantum communication.

58 citations


Journal ArticleDOI
TL;DR: In this article, the authors demonstrate an entirely standalone secure communication system based on photonic integrated circuits for quantum random number generation and quantum key distribution at gigahertz clock rates and demonstrate longterm continuous operation of the quantum secured communication system using feedback controls to stabilize the qubit phase and propagation delay over metropolitan fibre lengths.
Abstract: Photonic integrated circuits hold great promise in enabling the practical wide-scale deployment of quantum communications; however, despite impressive experiments of component functionality, a fully operational quantum communication system using photonic chips is yet to be demonstrated. Here we demonstrate an entirely standalone secure communication system based on photonic integrated circuits—assembled into compact modules—for quantum random number generation and quantum key distribution at gigahertz clock rates. The bit values, basis selection and decoy pulse intensities used for quantum key distribution are chosen at random, and are based on the output of a chip-based quantum random number generator operating at 4 Gb s–1. Error correction and privacy amplification are performed in real time to produce information-theoretic secure keys for a 100 Gb s–1 line speed data encryption system. We demonstrate long-term continuous operation of the quantum secured communication system using feedback controls to stabilize the qubit phase and propagation delay over metropolitan fibre lengths. These results mark an important milestone for the realistic deployment of quantum communications based on quantum photonic chips. Quantum photonic integrated circuits for a standalone quantum secure communication system are developed and packaged into pluggable interconnects. The system is interfaced with 100 Gb s–1 data encryptors and its performance is evaluated over 10 km to 50 km fibre links.

49 citations


Journal ArticleDOI
TL;DR: In this paper, the authors provide a roadmap of key milestones towards a complete, global quantum networked landscape and summarise important challenges in space quantum technologies that must be overcome and recent efforts to mitigate their effects.
Abstract: Concerted efforts are underway to establish an infrastructure for a global quantum internet to realise a spectrum of quantum technologies. This will enable more precise sensors, secure communications, and faster data processing. Quantum communications are a front-runner with quantum networks already implemented in several metropolitan areas. A number of recent proposals have modelled the use of space segments to overcome range limitations of purely terrestrial networks. Rapid progress in the design of quantum devices have enabled their deployment in space for in-orbit demonstrations. We review developments in this emerging area of space-based quantum technologies and provide a roadmap of key milestones towards a complete, global quantum networked landscape. Small satellites hold increasing promise to provide a cost effective coverage required to realised the quantum internet. We review the state of art in small satellite missions and collate the most current in-field demonstrations of quantum cryptography. We summarise important challenges in space quantum technologies that must be overcome and recent efforts to mitigate their effects. A perspective on future developments that would improve the performance of space quantum communications is included. We conclude with a discussion on fundamental physics experiments that could take advantage of a global, space-based quantum network.

48 citations


Journal ArticleDOI
TL;DR: In this article, the geometric Renyi divergence (GRD) has been studied from the point of view of quantum information theory and it has many appealing structural properties, which are not satisfied by other quantum Renyi divergences.
Abstract: Having a distance measure between quantum states satisfying the right properties is of fundamental importance in all areas of quantum information. In this work, we present a systematic study of the geometric Renyi divergence (GRD), also known as the maximal Renyi divergence, from the point of view of quantum information theory. We show that this divergence, together with its extension to channels, has many appealing structural properties, which are not satisfied by other quantum Renyi divergences. For example we prove a chain rule inequality that immediately implies the “amortization collapse” for the geometric Renyi divergence, addressing an open question by Berta et al. [Letters in Mathematical Physics 110:2277–2336, 2020, Equation (55)] in the area of quantum channel discrimination. As applications, we explore various channel capacity problems and construct new channel information measures based on the geometric Renyi divergence, sharpening the previously best-known bounds based on the max-relative entropy while still keeping the new bounds single-letter and efficiently computable. A plethora of examples are investigated and the improvements are evident for almost all cases.

47 citations


Journal ArticleDOI
TL;DR: This work proposes a global-scale quantum internet consisting of a constellation of orbiting satellites that provides a continuous, on-demand entanglement distribution service to ground stations that can also function as untrusted nodes for the purpose of long-distance quantum-key distribution.
Abstract: Recent experimental breakthroughs in satellite quantum communications have opened up the possibility of creating a global quantum internet using satellite links. This approach appears to be particularly viable in the near term, due to the lower attenuation of optical signals from satellite to ground, and due to the currently short coherence times of quantum memories. The latter prevents ground-based entanglement distribution using atmospheric or optical-fiber links at high rates over long distances. In this work, we propose a global-scale quantum internet consisting of a constellation of orbiting satellites that provides a continuous, on-demand entanglement distribution service to ground stations. The satellites can also function as untrusted nodes for the purpose of long-distance quantum-key distribution. We develop a technique for determining optimal satellite configurations with continuous coverage that balances both the total number of satellites and entanglement-distribution rates. Using this technique, we determine various optimal satellite configurations for a polar-orbit constellation, and we analyze the resulting satellite-to-ground loss and achievable entanglement-distribution rates for multiple ground station configurations. We also provide a comparison between these entanglement-distribution rates and the rates of ground-based quantum repeater schemes. Overall, our work provides the theoretical tools and the experimental guidance needed to make a satellite-based global quantum internet a reality.

Journal ArticleDOI
TL;DR: The aim is the development of scalability on the platform, to point the way to ever-closer integration, toward silicon quantum photonic systems-on-a-chip.
Abstract: Quantum technology is poised to enable a step change in human capability for computing, communications and sensing. Photons are indispensable as carriers of quantum information—they travel at the fastest possible speed and readily protected from decoherence. However, the system requires thousands of near-transparent components with ultra-low-latency control. To be implemented, a new paradigm photonic system is required: one with in-built coherence, stability, the ability to define arbitrary circuits, and a path to manufacturability. Silicon photonics has unparalleled density and component performance, which, with CMOS compatible fabrication, place it in a strong position for a scalable quantum photonics platform. This paper is a progress report on silicon quantum photonics, focused on developments in the past five years. We provide an introduction on silicon quantum photonic component and the challenges in the field, summarise the current state-of-the-art and identify outstanding technical challenges, as well as promising avenues of future research. We also resolve a conflict in the definition of Hong-Ou-Mandel interference visibility in integrated quantum photonic experiments, needed for fair comparison of photon quality across different platforms. Our aim is the development of scalability on the platform, to which end we point the way to ever-closer integration, toward silicon quantum photonic systems-on-a-chip.

Journal ArticleDOI
Qin Liao1, Qin Liao2, Haijie Liu1, Lingjin Zhu, Ying Guo2 
TL;DR: In this paper, a discretely modulated coherent states (DMCS) based QSS protocol is proposed for point-to-point quantum privacy communication over a standard telecommunication fiber link, which allows a legitimate user to share a secret key with multiple remote users through an insecure quantum channel.
Abstract: Point-to-point quantum privacy communication over a standard telecommunication fiber link can be implemented by continuous-variable quantum key distribution (CV QKD). However, as communication networks develop, the two-party CV QKD system may hardly meet the requirements of secret key sharing of multiple users (at least three users). In this paper, we consider a protocol called quantum secret sharing (QSS) which allows a legitimate user, a so-called dealer, to share a secret key with multiple remote users through an insecure quantum channel. These users can correctly recover the dealer's secret key only when they work cooperatively. We carry out QSS with discretely modulated coherent states (DMCSs) because they are easy to prepare and resilient to losses. An asymptotic security proof for the proposed DMCS-based QSS protocol against both eavesdroppers and dishonest users is presented. Numerical simulation based on a linear bosonic channel shows that the maximal transmission distance of the DMCS-based QSS protocol reaches more than 100 km, and it can be further lengthened by exploiting a higher-dimensional discrete modulation strategy. Moreover, the composable security of the DMCS-based QSS protocol is also presented.

Proceedings ArticleDOI
09 May 2021
TL;DR: In this paper, the authors demonstrate quantum teleportation of a polarization-encoded optical input state onto the joint state of a pair of nanomechanical resonators, which is a key component in long distance quantum communication protocols.
Abstract: Quantum teleportation is a key component in long distance quantum communication protocols. Here we demonstrate quantum teleportation of a polarization-encoded optical input state onto the joint state of a pair of nanomechanical resonators.

Journal ArticleDOI
TL;DR: In this paper, the authors presented a practical QSS protocol among three participants based on the differential phase shift scheme and twin field ideas for the solution of high-efficiency multiparty communication task.
Abstract: Quantum secret sharing (QSS) is essential for multiparty quantum communication, which is one of cornerstones in the future quantum internet. However, a linear rate-distance limitation severely constrains the secure key rate and transmission distance of QSS. Here, we present a practical QSS protocol among three participants based on the differential phase shift scheme and twin field ideas for the solution of high-efficiency multiparty communication task. In contrast to a formerly proposed differential phase shift QSS protocol, our protocol can break the linear rate-distance bound, theoretically improving the secret key rate by three orders of magnitude in a 300-km-long fiber. Furthermore, the new protocol is secure against Trojan horse attacks that cannot be resisted by previous differential phase shift QSS.

Journal ArticleDOI
TL;DR: In this paper, the authors employ the right logarithmic derivative Fisher information and the geometric Renyi relative entropy for channel estimation and channel discrimination, respectively, and identify connections between these distinguishability measures.
Abstract: Quantum channel estimation and discrimination are fundamentally related information processing tasks of interest in quantum information science. In this paper, we analyze these tasks by employing the right logarithmic derivative Fisher information and the geometric Renyi relative entropy, respectively, and we also identify connections between these distinguishability measures. A key result of our paper is that a chain-rule property holds for the right logarithmic derivative Fisher information and the geometric Renyi relative entropy for the interval $$\alpha \in (0,1) $$ of the Renyi parameter $$\alpha $$ . In channel estimation, these results imply a condition for the unattainability of Heisenberg scaling, while in channel discrimination, they lead to improved bounds on error rates in the Chernoff and Hoeffding error exponent settings. More generally, we introduce the amortized quantum Fisher information as a conceptual framework for analyzing general sequential protocols that estimate a parameter encoded in a quantum channel. We then use this framework, beyond the aforementioned application, to show that Heisenberg scaling is not possible when a parameter is encoded in a classical–quantum channel. We then identify a number of other conceptual and technical connections between the tasks of estimation and discrimination and the distinguishability measures involved in analyzing each. As part of this work, we present a detailed overview of the geometric Renyi relative entropy of quantum states and channels, as well as its properties, which may be of independent interest.

Journal ArticleDOI
09 May 2021
TL;DR: In this article, a proof-of-principle demonstration of four-party quantum conference agreement using photonic GHZ (Greenberger-Horne-Zeilinger) states transmitted in fiber is presented.
Abstract: We report a proof-of-principle demonstration of four-party quantum conference agreement using photonic GHZ (Greenberger-Horne-Zeilinger) states transmitted in fiber. Our results demonstrate the viability of multi-user entangled-based quantum key distribution beyond the two-party paradigm.

Journal ArticleDOI
16 Mar 2021
TL;DR: A fundamental limit in quantum metrology is obtained: the achievable sensitivity enhancement allowed by quantum strategies is determined, and protocols to reach such levels are proposed as mentioned in this paper. But this limit is not applicable to quantum cryptography.
Abstract: A fundamental limit in quantum metrology is obtained: The achievable sensitivity enhancement allowed by quantum strategies is determined, and protocols to reach such levels are proposed.

Posted Content
TL;DR: This work provides a systematic analysis of the finite block size effects on secret key length generation for low latency operation using BB84 weak coherent pulse decoy state protocols and finds that optimisation of basis bias, pulse probabilities and intensities, and data selection is crucial for extending the range of satellite trajectories and link efficiencies for which finite-block size keys can be extracted.
Abstract: Global quantum communications will enable long-distance secure data transfer, networked distributed quantum information processing, and other entanglement-enabled technologies. Satellite quantum communication overcomes optical fibre range limitations, with the first realisations of satellite quantum key distribution (SatQKD) being rapidly developed. However, limited transmission times between satellite and ground station severely constrains the amount of secret key due to finite-block size effects. Here, we analyse these effects and the implications for system design and operation, utilising published results from the Micius satellite to construct an empirically-derived channel and system model for a trusted-node downlink employing efficient BB84 weak coherent pulse decoy states with optimised parameters. We quantify practical SatQKD performance limits and examine the effects of link efficiency, background light, source quality, and overpass geometries to estimate long-term key generation capacity. Our results may guide design and analysis of future missions, and establish performance benchmarks for both sources and detectors.

Journal ArticleDOI
Zhengwen Cao1, Lei Wang1, Kexin Liang1, Geng Chai1, Jinye Peng1 
TL;DR: The system performance analysis shows that the proposed QSDC protocol based on Gaussian mapping can realize inerrant Gaussian modulation of secret messages, and secure transmission of messages under information theory.
Abstract: Quantum secure direct communication (QSDC) realizes the transmission of secret messages directly in a quantum channel. A continuous-variable- (CV) based quantum-communication system allows high-speed, large-capacity information transmission in optical telecommunication systems. In this work, we propose a QSDC protocol based on Gaussian mapping. As for Gaussian modulation, the designed Gaussian-mapping scheme effectively solves the problem, which results from the nonuniformity of the secret-message bitstream, and is applicable to different modulation variances. The system performance analysis shows that the proposed scheme can realize inerrant Gaussian modulation of secret messages, and secure transmission of messages under information theory. Moreover, our work hopes to stimulate discussion on experimental realization and practical advance of CV-QSDC protocol.

Journal ArticleDOI
07 Oct 2021
TL;DR: In this article, a general framework for studying the composable finite-size security of continuous variable QKD with Gaussian-modulated coherent-state protocols under various levels of trust for the loss and noise experienced by the parties is provided.
Abstract: Continuous variable (CV) quantum key distribution (QKD) provides a powerful setting for secure quantum communications, thanks to the use of room-temperature off-the-shelf optical devices and the potential to reach much higher rates than the standard discrete-variable counterpart. In this paper, we provide a general framework for studying the composable finite-size security of CV-QKD with Gaussian-modulated coherent-state protocols under various levels of trust for the loss and noise experienced by the parties. Our paper considers both wired (i.e., fiber-based) and wireless (i.e., free-space) quantum communications. In the latter case, we show that high key rates are achievable for short-range optical wireless (LiFi) in secure quantum networks with both fixed and mobile devices. Finally, we extend our investigation to microwave wireless (WiFi) discussing security and feasibility of CV-QKD for very short-range applications.

Journal ArticleDOI
TL;DR: In this paper, the authors established universal limitations on the processing of both quantum states and channels, expressed in the form of no-go theorems and quantitative bounds for the manipulation of general quantum channel resources under the most general transformation protocols.
Abstract: Quantum channels underlie the dynamics of quantum systems, but in many practical settings it is the channels themselves that require processing. We establish universal limitations on the processing of both quantum states and channels, expressed in the form of no-go theorems and quantitative bounds for the manipulation of general quantum channel resources under the most general transformation protocols. Focusing on the class of distillation tasks — which can be understood either as the purification of noisy channels into unitary ones, or the extraction of state-based resources from channels — we develop fundamental restrictions on the error incurred in such transformations, and comprehensive lower bounds for the overhead of any distillation protocol. In the asymptotic setting, our results yield broadly applicable bounds for rates of distillation. We demonstrate our results through applications to fault-tolerant quantum computation, where we obtain state-of-the-art lower bounds for the overhead cost of magic state distillation, as well as to quantum communication, where we recover a number of strong converse bounds for quantum channel capacity. Several key tasks in quantum information processing can be regarded as channel manipulation. Here, focusing on the class of distillation protocols, the authors derive general bounds on resource overhead and incurred errors, showing application to magic state distillation and quantum channel capacities.

Journal ArticleDOI
TL;DR: In this article, an adaptive compensation method using deep learning technology is developed to improve the performance of OAM-encoded QKD schemes, where a convolutional neural network model is trained to learn the mapping relationship of intensity profiles of inputs and the turbulent phase, and such mapping is used as feedback to control a spatial light modulator to generate a phase screen to correct the distorted vortex beam.
Abstract: A high-dimensional quantum key distribution (QKD), which adopts degrees of freedom of the orbital angular momentum (OAM) states, is beneficial to realize secure and high-speed QKD. However, the helical phase of a vortex beam that carries OAM is sensitive to the atmospheric turbulence and easily distorted. In this paper, an adaptive compensation method using deep learning technology is developed to improve the performance of OAM-encoded QKD schemes. A convolutional neural network model is first trained to learn the mapping relationship of intensity profiles of inputs and the turbulent phase, and such mapping is used as feedback to control a spatial light modulator to generate a phase screen to correct the distorted vortex beam. Then an OAM-encoded QKD scheme with the capability of real-time phase correction is designed, in which the compensation module only needs to extract the intensity distributions of the Gaussian probe beam and thus ensures that the information encoded on OAM states would not be eavesdropped. The results show that our method can efficiently improve the mode purity of the encoded OAM states and extend the secure distance for the involved QKD protocols in the free-space channel, which is not limited to any specific QKD protocol.

Journal ArticleDOI
TL;DR: In this article, the influence of purity of quantum states and excess noise on Gaussian EPR steering by distributing a two-mode squeezed state through lossy and noisy channels, respectively, is analyzed and experimentally demonstrated.
Abstract: Einstein–Podolsky–Rosen (EPR) steering is a useful resource for secure quantum information tasks. It is crucial to investigate the effect of inevitable loss and noise in quantum channels on EPR steering. We analyze and experimentally demonstrate the influence of purity of quantum states and excess noise on Gaussian EPR steering by distributing a two-mode squeezed state through lossy and noisy channels, respectively. We show that the impurity of state never leads to sudden death of Gaussian EPR steering, but the noise in quantum channel can. Then we revive the disappeared Gaussian EPR steering by establishing a correlated noisy channel. Different from entanglement, the sudden death and revival of Gaussian EPR steering are directional. Our result confirms that EPR steering criteria proposed by Reid and I. Kogias et al. are equivalent in our case. The presented results pave way for asymmetric quantum information processing exploiting Gaussian EPR steering in noisy environment.

Journal ArticleDOI
05 May 2021
TL;DR: In this article, the authors generalize the von Neumann entropy from quantum states to quantum channels, which model evolutions of quantum physical systems, and provide a physical interpretation of a quantum channel's entropy in terms of an information-processing task called quantum channel merging.
Abstract: The authors generalize the von Neumann entropy from quantum states to quantum channels, which model evolutions of quantum physical systems, and they provide a physical interpretation of a quantum channel's entropy in terms of an information-processing task called quantum channel merging.

Journal ArticleDOI
TL;DR: In this article, the use of quantum memory (QM)-equipped satellites for quantum communication focussing on global range repeaters and memory-assisted (MA-) QKD, where QMs help increase the key rate by synchronizing otherwise probabilistic detection events.
Abstract: Global-scale quantum communication links will form the backbone of the quantum internet. However, exponential loss in optical fibres precludes any realistic application beyond few hundred kilometres. Quantum repeaters and space-based systems offer solutions to overcome this limitation. Here, we analyse the use of quantum memory (QM)-equipped satellites for quantum communication focussing on global range repeaters and memory-assisted (MA-) QKD, where QMs help increase the key rate by synchronising otherwise probabilistic detection events. We demonstrate that satellites equipped with QMs provide three orders of magnitude faster entanglement distribution rates than existing protocols based on fibre-based repeaters or space systems without QMs. We analyse how entanglement distribution performance depends on memory characteristics, determine benchmarks to assess the performance of different tasks and propose various architectures for light-matter interfaces. Our work provides a roadmap to realise unconditionally secure quantum communications over global distances with near-term technologies.

Journal ArticleDOI
TL;DR: In this article, an idealization of a cmos device where the hole is strongly confined in one direction (thin film geometry), while it is allowed to move more extensively along a one-dimensional channel is studied.
Abstract: Holes confined in semiconductor nanostructures realize qubits where the quantum-mechanical spin is strongly mixed with the quantum orbital angular momentum. The remarkable spin-orbit coupling allows for fast all electrical manipulation of such qubits. We study an idealization of a cmos device where the hole is strongly confined in one direction (thin film geometry), while it is allowed to move more extensively along a one-dimensional channel. Static electric bias and ac electrical driving are applied by metallic gates arranged along the channel. In quantum devices based on materials with a bulk inversion symmetry, such as silicon or germanium, there exist different possible spin-orbit coupling based mechanisms for qubit manipulation. One of them, the $g$-tensor magnetic resonance, relies on the dependence of the effective $g$-factors on the electrical confinement. In this configuration, the hole is driven by an ac field parallel to the static electric field and perpendicular to the channel (transverse driving). Another mechanism, which we refer to here as iso-Zeeman electric dipole spin resonance, is due to the Rashba spin-orbit coupling that leads to an effective time-dependent magnetic field experienced by the pseudospin oscillating along the quantum channel (longitudinal driving). We compare these two modes of operation, and we describe the conditions in which the magnitudes of the Rabi frequencies are the largest. Different regimes can be attained by electrical tuning where the coupling to the ac electric field is made either weak or strong. Spin-orbit coupling can also be tuned by strains, with, in particular, a transition from a mostly heavy- to a mostly light-hole ground state for in-plane tensile strains. Although large strains always reduce the Rabi frequency, they may increase the qubit lifetimes even faster, which calls for a careful optimization of strains and electric fields in the devices. We also discuss the choice of channel material and orientation. The study is relevant to the interpretation of the current experiments on the manipulation of hole qubits and as a guide to the development of quantum devices based on silicon and germanium.

Journal ArticleDOI
Jie Gu1, Y. L. Xie1, Wen-Bo Liu1, Y. Fu1, Hua-Lei Yin1, Zeng-Bing Chen1 
TL;DR: In this article, a three-user QSS protocol without monitoring signal disturbance is proposed, which is capable of ensuring the unconditional security in a large-scale QSS network, and the final key rate of the protocol can be demonstrated to break the Pirandola-Laurenza-Ottaviani-Banchi bound of the quantum channel.
Abstract: Quantum secret sharing (QSS) is an essential primitive for the future quantum internet, which promises secure multiparty communication. However, developing a large-scale QSS network is a huge challenge due to the channel loss and the requirement of multiphoton interference or high-fidelity multipartite entanglement distribution. Here, we propose a three-user QSS protocol without monitoring signal disturbance, which is capable of ensuring the unconditional security. The final key rate of our protocol can be demonstrated to break the Pirandola-Laurenza-Ottaviani-Banchi bound of quantum channel and its simulated transmission distance can approach over 600 km using current techniques. Our results pave the way to realizing high-rate and large-scale QSS networks.

Journal ArticleDOI
19 May 2021
TL;DR: In this paper, the authors apply and extend recent results in free-space quantum communications to determine the ultimate limits at which secret (and entanglement) bits can be distributed via satellites, encompassing both downlink and uplink configurations, with satellites at different altitudes and zenith angles.
Abstract: Satellite quantum communications are emerging within the panorama of quantum technologies as a more effective strategy to distribute completely-secure keys at very long distances, therefore playing an important role in the architecture of a large-scale quantum network. In this work, we apply and extend recent results in free-space quantum communications to determine the ultimate limits at which secret (and entanglement) bits can be distributed via satellites. Our study is comprehensive of the various practical scenarios, encompassing both downlink and uplink configurations, with satellites at different altitudes and zenith angles. It includes effects of diffraction, extinction, background noise and fading, due to pointing errors and atmospheric turbulence (appropriately developed for slant distances). Besides identifying upper bounds, we also discuss lower bounds, i.e., achievable rates for key generation and entanglement distribution. In particular, we study the composable finite-size secret key rates that are achievable by protocols of continuous variable quantum key distribution, for both downlink and uplink, showing the feasibility of this approach for all configurations. Finally, we present a study with a sun-synchronous satellite, showing that its key distribution rate is able to outperform a ground chain of ideal quantum repeaters.

Posted ContentDOI
18 Jun 2021
TL;DR: This paper aims to provide a technology-driven and visionary description and exploration on how QIT can be leveraged for future 6G wireless networks.
Abstract: With the increasing number of commercial 5G deployments, research on Beyond 5G (B5G) and 6G has started in earnest. Although it is too early to clearly identify what 6G systems will look like or how they will be designed, it is certain that 6G systems will support novel use cases with challenging Key Performance Indicators (KPIs), which will be empowered by new enabling technologies and network architectures. In parallel with the evolution of cellular systems from 5G towards 6G, Quantum Information Technology (QIT) has been evolving rapidly in recent years in terms of quantum communications and quantum computing. It is envisioned that QIT will enable and boost future 6G systems from both communication and computing perspectives. For example, secure quantum communications such as Quantum Key Distribution (QKD) can be leveraged to improve 6G security. This paper aims to provide a technology-driven and visionary description and exploration on how QIT can be leveraged for future 6G wireless networks.