Transmit Classical and Quantum Information Secretly

Quantum states cannot be transmitted efficiently classically

We show that any classical two-way communication protocol with shared randomness that can approximately simulate the result of applying an arbitrary measurement (held by one party) to a quantum state of $n$ qubits (held by another), up to constant accuracy, must transmit at least $Ω(2^n)$ bits. This lower bound is optimal and matches the complexity of a simple protocol based on discretisation using an $ε$-net. The proof is based on a lower bound on the classical communication complexity of a distributed variant of the Fourier sampling problem. We obtain two optimal quantum-classical separation…

非局域性辅助增强噪声经典信道中的无误通信

Nonlocality-Assisted Enhancement of Error-Free Communication in Noisy Classical Channels

本文研究贝尔场景中非局域关联对噪声经典信道零错误通信的增强作用，构造出零错误容量为零的信道，借助极值非局域关联可实现1比特或log m比特无误传输，强调其多功能效用。

Transmitting One Qubit Can Increase One Ebit Between Two Parties at Most

Entanglement shared between two seperated parties could not be increased without transmitting quantum system. We suggest the project to gain entanglement shared between Alice and Bob by transmitting quantum system and a nem scheme to achieve efficient rate of entanglement. It is proven that under any local operations and communicating classically, to transmit one qubit through an ideal or a noisy quantum channel can increase no more than one ebit between two parties. Furthermore, the prior nonmaximally transmitted entanglement could not be improved by subsequently transmitted qubit. Although o…

Classicalization of Nonclassical Quantum States in Loss and Noise -- Some No-Go Theorems

The general problem of performance advantage obtainable by the use of nonclassical transmitted states over classical ones is considered. Attention is focused on the situation where system loss is significant and additive Gaussian noise may be present at the receiver. Under the assumption that the total received state is classical, rigorous output density operator representations and their trace distance bounds are developed for classical and nonclassical transmitted states. For applications with high loss in all modes, a practical No-Go theorem is enunciated that rules out the possibility of s…

Remote State Preparation

Quantum teleportation uses prior entanglement and forward classical communication to transmit one instance of an unknown quantum state. Remote state preparation (RSP) has the same goal, but the sender knows classically what state is to be transmitted. We show that the asymptotic classical communication cost of RSP is one bit per qubit - half that of teleportation - and becomes even less when transmitting part of a known entangled state. We explore the tradeoff between entanglement and classical communication required for RSP, and discuss RSP capacities of general quantum channels.

Coding for quantum channels with side information at the transmitter

We consider the problem of coding for quantum channels with side information that is available ahead of time at the transmitter but not at the receiver. We find a single-letter expression for the entanglement-assisted quantum capacity of such channels which closely parallels Gel'fand and Pinsker's solution to the classical version of the same problem. This theorem can also be used to find a lower bound on the unassisted quantum capacity of these channels.

Super-Activation of Zero-Error Capacity of Noisy Quantum Channels

We study various super-activation effects in the following zero-error communication scenario: One sender wants to send classical or quantum information through a noisy quantum channel to one receiver with zero probability of error. First we show that there are quantum channels of which a single use is not able to transmit classical information perfectly yet two uses can. This is achieved by employing entangled input states between different uses of the given channel and thus cannot happen for classical channels. Second we exhibit a class of quantum channel with vanishing zero-error classical c…

通过光学经典通信信道实现量子通信的同步

Synchronization of quantum communication over an optical classical communication channel

针对高损耗量子信道同步难题，本文提出利用共用主时钟的同向经典光通信链路进行时钟数据恢复，以在无需额外硬件的情况下实现QKD同步。

Efficiency in Quantum Key Distribution Protocols with Entangled Gaussian States

Quantum key distribution (QKD) refers to specific quantum strategies which permit the secure distribution of a secret key between two parties that wish to communicate secretly. Quantum cryptography has proven unconditionally secure in ideal scenarios and has been successfully implemented using quantum states with finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we analyze the efficiency of QKD protocols that use as a resource entangled gaussian states and gaussian operations only. In this framework, it has already been shown that QKD is possible (M. Navascués et al.…

Observation of quantum fingerprinting beating the classical limit

Quantum communication has historically been at the forefront of advancements, from fundamental tests of quantum physics to utilizing the quantum-mechanical properties of physical systems for practical applications. In the field of communication complexity, quantum communication allows the advantage of an exponential reduction in the information transmitted over classical communication to accomplish distributed computational tasks. However, to date, demonstrating this advantage in a practical setting continues to be a central challenge. Here, we report an experimental demonstration of a quantum…

Energy-Information Coupling From Classical To Quantum

It is known that there is no possibility of transmitting information without a certain amount of energy. This is arbitrarily small in Classical Physics, due to the continuous nature of the energy parameter, while one cannot reduce that amount below Planck's energy quanta in Quantum Physics. In short, one cannot send less than a photon from a place to another when transmitting a minimum of information. However, as single photons are never completely defined simultaneously in all their parameters as position and momentum, their exact contribution to the information transmitted cannot be known in…

A classical attack on the coherent one way protocol for quantum key distribution

We propose a way to retrieve the secure key generated by the coherent one way protocol without reading the information transmitted on the quantum channel.

Complementarity of quantum discord and classically accessible information

The sum of the Holevo quantity (that bounds the capacity of quantum channels to transmit classical information about an observable) and the quantum discord (a measure of the quantumness of correlations of that observable) yields an observable-independent total given by the quantum mutual information. This split naturally delineates information about quantum systems accessible to observers -- information that is redundantly transmitted by the environment -- while showing that it is maximized for the quasi-classical pointer observable. Other observables are accessible only via correlations with …

Capacities of classical compound quantum wiretap and classical quantum compound wiretap channels

We determine the capacity of the classical compound quantum wiretapper channel with channel state information at the transmitter. Moreover we derive a lower bound on the capacity of this channel without channel state information and determine the capacity of the classical quantum compound wiretap channel with channel state information at the transmitter.

Classical Communications with Indefinite Causal Order for $N$ completely depolarizing channels

If two identical copies of a completely depolarizing channel are put into a superposition of their possible causal orders, they can transmit non-zero classical information. Here, we study how well we can transmit classical information with $N$ depolarizing channels put in superposition of $M$ causal orders via quantum SWITCH. We calculate Holevo quantity if the superposition uses only cyclic permutations of channels and find that it increases with $M$ and it is independent of $N$. For a qubit it never reaches $1$ if we are increasing $M$. On the other hand, the classical capacity decreases wit…

动力学朗道尔原理：传输经典信息的热力学判据

Dynamical Landauer principle: Thermodynamic criteria of transmitting classical information

本文通过能量传输任务界定多种单次经典容量，证明传输 n 比特经典信息等价于传输 n×kBTln2 的能量，并由此揭示动力学朗道尔原理，为 Holevo-Schumacher-Westmoreland 定理及强逆性质赋予热力学意义。

Real-time operation of a multi-rate, multi-protocol quantum key distribution transmitter

Quantum key distribution (QKD) is the best candidate for securing communications against attackers, who may in the future exploit quantum-enhanced computational powers to break classical encryption. As such, new challenges are arising from our need for large-scale deployment of QKD systems. In a realistic scenario, transmitting and receiving devices from different vendors should be able to communicate with each other without the need for matching hardware. Therefore, practical deployment of QKD would require hardware capable of adapting to different protocols and clock rates. Here, we address …

Quantum Communication Complexity (A Survey)

Can quantum communication be more efficient than its classical counterpart? Holevo's theorem rules out the possibility of communicating more than n bits of classical information by the transmission of n quantum bits --- unless the two parties are entangled, in which case twice as many classical bits can be communicated but no more. In apparent contradiction, there are distributed computational tasks for which quantum communication cannot be simulated efficiently by classical means. In extreme cases, the effect of transmitting quantum bits cannot be achieved classically short of transmitting an…

Maximal Privacy Without Coherence

Privacy lies at the fundament of quantum mechanics. A coherently transmitted quantum state is inherently private. Remarkably, coherent quantum communication is not a prerequisite for privacy: there are quantum channels that are too noisy to transmit any quantum information reliably that can nevertheless send private classical information. Here, we ask how much private classical information a channel can transmit if it has little quantum capacity. We present a class of channels N_d with input dimension d^2, quantum capacity Q(N_d) <= 1, and private capacity P(N_d) = log d. These channels asympt…

An Evolutionary Pathway for the Quantum Internet Relying on Secure Classical Repeaters

Until quantum repeaters become mature, quantum networks remain restricted either to limited areas of directly connected nodes or to nodes connected to a common node. We circumvent this limitation by conceiving quantum networks using secure classical repeaters combined with the quantum secure direct communication (QSDC) principle, which is a compelling form of quantum communication that directly transmits information over quantum channel. The final component of this promising solution is our classical quantum-resistant algorithm. Explicitly, in these networks, the ciphertext gleaned from a quan…

Authenticated Semi-quantum Direct Communication Protocols using Bell States

This study presents the first two authenticated semi-quantum direct communication (ASQDC) protocols without using any classical channel. By pre-sharing the master secret key between two communicants, a sender with advanced quantum devices can transmit a secret message to a receiver who can only perform classical operations without any information leakage. The receiver is then capable of verifying the message up to the single qubit level, i.e., a one-qubit modification of the transmitted quantum sequence can be detected with a probability close to 1. Moreover, the proposed protocols are resista…

Entanlement-Assisted Classical Capacity of Quantum Channels with Correlated Noise

We calculate the entanglement-assisted classical capacity of symmetric and asymmetric Pauli channels where two consecutive uses of the channels are correlated. It is evident from our study that in the presence of memory, a higher amount of classical information is transmitted over quantum channels if there exists prior entanglement as compared to product and entangled state coding.

经典引力理论产生纠缠

Classical theories of gravity produce entanglement

本文论证经典引力可通过局域过程传递量子信息并产生纠缠，为区分经典与量子引力效应提供实验设计依据。

一般物理理论中的单次与渐近经典容量

One-shot and asymptotic classical capacity in general physical theories

本文在一般物理理论中研究单次经典容量，利用假设检验相对熵给出上下界，并证明其与经典容量的渐近等价性。

Classical feedback for quantum channels

We investigate whether the use of a noiseless, classical feedback channel will increase the capacity of a quantum discrete memoryless channel to transmit classical information. This problem has been previously analyzed by Bowen and Nagarajan for the case of protocols restricted to product input states. They showed that feedback did not increase the information capacity. In this paper we introduce a quantum analogue of classical causality and prove a capacity theorem (in regularized form) for the transmission of classical information.

Classical and quantum communication without a shared reference frame

We show that communication without a shared reference frame is possible using entangled states. Both classical and quantum information can be communicated with perfect fidelity without a shared reference frame at a rate that asymptotically approaches one classical bit or one encoded qubit per transmitted qubit. We present an optical scheme to communicate classical bits without a shared reference frame using entangled photon pairs and linear optical Bell state measurements.

Resource requirements of private quantum channels and consequence for oblivious remote state preparation

Shannon in celebrated works had shown that n bits of shared key is necessary and sufficient to transmit n-bit classical information in an information-theoretically secure way. Ambainis, Mosca, Tapp and de Wolf in quant-ph/0003101 considered a more general setting, referred to as Private quantum channels, in which instead of classical information, quantum states are required to be transmitted and only one-way communication is allowed. They show that in this case 2n bits of shared key is necessary and sufficient to transmit an n-qubit state. We consider the most general setting in which we allow…

纠缠辅助的认证 BB84 协议

Entanglement-assisted authenticated BB84 protocol

提出一种利用最大纠缠量子比特对的认证 BB84 QKD 协议，支持预共享秘密复用与密钥增长，并通过统计方法和 DNN 在噪声光子-AFC 存储场景下实现伪造识别。

Secret Keys Assisted Private Classical Communication Capacity over Quantum Channels

We prove a regularized formula for the secret key-assisted capacity region of a quantum channel for transmitting private classical information. This result parallels the work of Devetak on entanglement assisted quantum communication capacity \cite{DHW05RI}. This formula provides a new family protocol, the private father protocol, under the resource inequality framework that includes private classical communication \it{without} secret key assistance as a child protocol.

Quantum cost of dense coding and teleportation

The quantum cost is a key ingredient to evaluate the quality of quantum protocols from a practical viewpoint. We show that the quantum cost of d-dimensional dense coding protocol is equal to d+3 when transmitting the classical message (0,0), and that is equal to d+4 when transmitting other classical message. It appears linear growth with the dimension and thus makes sense for implementation. In contrast, the quantum cost of high-dimensional teleportation protocols is equal to 13, which is the maximum value of the cost for the two-dimensional case. As an application, we establish the relation b…

A direct GHz-clocked phase and intensity modulated transmitter applied to quantum key distribution

Quantum key distribution (QKD), a technology that enables perfectly secure communication, has evolved to the stage where many different protocols are being used in real-world implementations. Each protocol has its own advantages, meaning that users can choose the one best-suited to their application, however each often requires different hardware. This complicates multi-user networks, in which users may need multiple transmitters to communicate with one another. Here, we demonstrate a direct-modulation based transmitter that can be used to implement most weak coherent pulse based QKD protocols…

Classical Teleportation of a Quantum Bit

Classical teleportation is defined as a scenario where the sender is given the classical description of an arbitrary quantum state while the receiver simulates any measurement on it. This scenario is shown to be achievable by transmitting only a few classical bits if the sender and receiver initially share local hidden variables. Specifically, a communication of 2.19 bits is sufficient on average for the classical teleportation of a qubit, when restricted to von Neumann measurements. The generalization to positive-operator-valued measurements is also discussed.

Classical information and distillable entanglement

We establish a quantitative connection between the amount of lost classical information about a quantum state and the concomitant loss of entanglement. Using methods that have been developed for the optimal purification of mixed states we find a class of mixed states with known distillable entanglement. These results can be used to determine the quantum capacity of a quantum channel which randomizes the order of transmitted signals.

Quantum and Classical Data Transmission through Completely Depolarising Channels in a Superposition of Cyclic Orders

Completely depolarising channels are often regarded as the prototype of physical processes that are useless for communication: any message that passes through them along a well-defined trajectory is completely erased. When two such channels are used in a quantum superposition of two alternative orders, they become able to transmit some amount of classical information, but still no quantum information can pass through them. Here we show that the ability to place N completely depolarising channels in a superposition of N alternative causal orders enables a high-fidelity, heralded transmission of…

Classical capacity per unit cost for quantum channels

In most communication scenarios, sending a symbol encoded in a quantum state requires spending resources such as energy, which can be quantified by a cost of communication. A standard approach in this context is to quantify the performance of communication protocol by classical capacity, quantifying the maximal amount of information that can be transmitted through a quantum channel per single use of the channel. However, different figures of merit are also possible, and a particularly well-suited one is the classical capacity per unit cost, which quantifies the maximal amount of information th…

Entanglement-enhanced classical capacity of two-qubit quantum channels with memory: the exact solution

The maximal amount of information which is reliably transmitted over two uses of general Pauli channels with memory is proven to be achieved by maximally entangled states beyond some memory threshold. In particular, this proves a conjecture on the depolarizing channel by Macchiavello and Palma [Phys. Rev. A {\bf 65}, 050301(R) (2002)]. Below the memory threshold, for arbitrary Pauli channels, the two-use classical capacity is only achieved by a particular type of product states.

通过带助手的量子多址接入信道进行隐蔽通信

Covert Communication Over a Quantum MAC with a Helper

研究带助手的量子多址接入信道上的隐蔽经典通信，考虑三个发射机，其中一个帮助另两个向接收机隐蔽通信，证明此信道实现正隐蔽速率的可行性并建立可达速率区域。

Optimization of Quantum Information Processing Maximizing Mutual Information

A model of quantum noisy channel with input encoding by a classical random vector is described. An equation of optimality is derived to determine a complete set of wave functions describing quantum decodings based on quasi-measurements maximizing the classical amount of transmitted information. A solution of this equation is found for the Gaussian multimode case with input Gaussian distribution. It is described by the overcomplete family of coherent vectors describing an optimal quasimeasurement of the canonical annihilation amplitudes in the output Hilbert space. It is found that the optimal …

The capacity of the noisy quantum channel

An upper limit is given to the amount of quantum information that can be transmitted reliably down a noisy, decoherent quantum channel. A class of quantum error-correcting codes is presented that allow the information transmitted to attain this limit. The result is the quantum analog of Shannon's bound and code for the noisy classical channel.

通过量子传输线解复用广义信息

Demultiplexing Generalized Information via Quantum Transmission Lines

提出量子解复用器（Q-DEMUX）实现经典和量子信息的完美路由，揭示其与量子仪器不相容性的联系，并扩展到发送者不知数据类型的更强变体。

Device-independent quantum reading and noise-assisted quantum transmitters

In quantum reading, a quantum state of light (transmitter) is applied to read classical information. In the presence of noise or for sufficiently weak signals, quantum reading can outperform classical reading by enhanced state distinguishability. Here we show that the enhanced quantum efficiency depends on the presence in the transmitter of a particular type of quantum correlations, the discord of response. Different encodings and transmitters give rise to different levels of efficiency. Considering noisy quantum probes we show that squeezed thermal transmitters with non-symmetrically distribu…

动力学朗道尔原理：用热力学量化信息传输

Dynamical Landauer Principle: Quantifying Information Transmission by Thermodynamics

本文证明量子动力学传输 n 比特经典信息的能力等价于其在热力学任务中传输 n 单位能量的能力，从而建立信息传输与能量传输的解析对应，并揭示动力学版朗道尔原理，为经典通信提供热力学量化解释。

The hidden cost of quantum teleportation and remote state preparation

The amount of information transferred during standard quantum teleportation or remote state preparation is equal to the preparation information of the transmitted state, rather than the classical communication required by respective protocol. This is shown by noting that the information required to specify the operation that verifies the transmitted state is identical to the preparation information, at the given level of precision m (in bits). Depending on the resolution of the projective Hilbert space, the preparation information can be made arbitrarily precise and hence indefinitely larger t…

Aggregating Quantum Networks

Quantum networking allows the transmission of information in ways unavailable in the classical world. Single packets of information can now be split and transmitted in a coherent way over different routes. This aggregation allows information to be transmitted in a fault tolerant way between different parts of the quantum network (or the future internet) - even when that is not achievable with a single path approach. It is a quantum phenomenon not available in conventional telecommunication networks either. We show how the multiplexing of independent quantum channels allows a distributed form o…

On Feedback and the Classical Capacity of a Noisy Quantum Channel

In Shannon information theory the capacity of a memoryless communication channel cannot be increased by the use of feedback from receiver to sender. In this paper the use of classical feedback is shown to provide no increase in the unassisted classical capacity of a memoryless quantum channel when feedback is used across non-entangled input states, or when the channel is an entanglement--breaking channel. This gives a generalization of the Shannon theory for certain classes of feedback protocols when transmitting through noisy quantum communication channels.

刻画具有经典记忆的多时间量子过程层级结构

Characterising the Hierarchy of Multi-time Quantum Processes with Classical Memory

本文系统刻画了多时间量子过程中仅传递经典信息的记忆机制，补充分析了两类结构上可视为经典记忆的过程集合，并建立了量子力学记忆效应的完整层级结构，揭示了许多在两时间场景中无法显现的多时间现象。

量子通信和贝尔非定域性需要无限的经典通信才能模拟

Quantum communication and Bell nonlocality require infinite classical communication to simulate

本文针对所有量子维度解决量子通信与量子关联的模拟问题，发现维度4存在意外定性转变，构建协议用357个经典比特精确模拟量子三重组通信及关联，而量子四元组无法用有限经典通信精确模拟。

Bounds on classical information capacities for a class of quantum memory channels

The maximum rates for information transmission through noisy quantum channels has primarily been developed for memoryless channels, where the noise on each transmitted state is treated as independent. Many real world communication channels experience noise which is modelled better by errors that are correlated between separate channel uses. In this paper, upper bounds on the classical information capacities of a class of quantum memory channels are derived. The class of channels consists of indecomposable quantum memory channels, a generalization of classical indecomposable finite-state channe…

用于端到端无线通信的量子-经典自编码器架构

Quantum-Classical Autoencoder Architectures for End-to-End Radio Communication

本文系统研究了用于端到端无线通信的混合量子-经典自编码器架构，涵盖量子编码器、量子解码器及全量子收发系统，并通过4-QAM和16-QAM仿真验证了模型在噪声与衰落信道下的鲁棒性，为无线通信中的量子机器学习应用提供了通用框架。

Gaussian-state quantum-illumination receivers for target detection

The signal half of an entangled twin-beam, generated using spontaneous parametric downconversion, interrogates a region of space that is suspected of containing a target, and has high loss and high (entanglement-breaking) background noise. A joint measurement is performed on the returned light and the idler beam that was retained at the transmitter. An optimal quantum receiver, whose implementation is not yet known, was shown to achieve 6 dB gain in the error-probability exponent relative to that achieved with a single coherent-state (classical) laser transmitter and the optimum receiver. We p…

多址窃听信道中纠缠发射机的量子优势

Quantum Advantage in Multiple Access Wiretap Channels with Entangled Transmitters

本文研究多址窃听信道中发射机共享纠缠的安全通信，推导可达速率区域、保密容量正则化表达式及单字母上界，证明纠缠严格提升保密容量，并建立新的强软覆盖引理。

Phase Information in Quantum Oracle Computing

Computational devices may be supplied with external sources of information (oracles). Quantum oracles may transmit phase information which is available to a quantum computer but not a classical computer. One consequence of this observation is that there is an oracle which is of no assistance to a classical computer but which allows a quantum computer to solve undecidable problems. Thus useful relativized separations between quantum and classical complexity classes must exclude the transmission of phase information from oracle to computer.

Ghost imaging as loss estimation: Quantum versus classical schemes

Frequency correlations are a versatile and powerful tool which can be exploited to perform spectral analysis of objects whose direct measurement might be unfeasible. This is achieved through a so-called ghost spectrometer, that can be implemented with quantum and classical resources alike. While there are some known advantages associated to either choice, an analysis of their metrological capabilities has not yet been performed. Here we report on the metrological comparison between a quantum and a classical ghost spectrometer. We perform the estimation of the transmittivity of a bandpass filte…

Creation of non-classical states of light in a chiral waveguide

Creating non-classical states of light from simple quantum systems together with classical resources is a challenging problem. We show how chiral emitters under a coherent drive can generate non-classical photon states. For our analysis, we select a specific temporal mode in the transmitted light field, resulting in a coupled master equation for the relevant mode and the chiral emitters. We characterise the mode's state by its Wigner function and show that the emission from the system predominantly produces mixtures of few-photon-added coherent states. We argue that these non-classical states …