Research

10 pages, 336 KiB

Open AccessArticle

How Secure Are Two-Way Ping-Pong and LM05 QKD Protocols under a Man-in-the-Middle Attack?

by Mladen Pavičić

Entropy 2021, 23(2), 163; https://0-doi-org.brum.beds.ac.uk/10.3390/e23020163 - 29 Jan 2021

Cited by 2 | Viewed by 1871

We consider a man-in-the-middle attack on two-way quantum key distribution ping-pong and LM05 protocols in which an eavesdropper copies all messages in the message mode, while being undetectable in the mode. Under the attack there is therefore no disturbance in the message mode [...] Read more.

We consider a man-in-the-middle attack on two-way quantum key distribution ping-pong and LM05 protocols in which an eavesdropper copies all messages in the message mode, while being undetectable in the mode. Under the attack there is therefore no disturbance in the message mode and the mutual information between the sender and the receiver is always constant and equal to one and messages copied by the eavesdropper are always genuine. An attack can only be detected in the control mode but the level of detection at which the protocol should be aborted is not defined. We examine steps of the protocol to evaluate its security and find that the protocol should be redesigned. We also compare it with the security of a one-way asymmetric BB84-like protocol in which one basis serves as the message mode and the other as the control mode but which does have the level of detection at which the protocol should be aborted defined. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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13 pages, 414 KiB

Open AccessArticle

Improved Resource State for Verifiable Blind Quantum Computation

by Qingshan Xu, Xiaoqing Tan and Rui Huang

Entropy 2020, 22(9), 996; https://0-doi-org.brum.beds.ac.uk/10.3390/e22090996 - 07 Sep 2020

Cited by 8 | Viewed by 2142

Abstract

Recent advances in theoretical and experimental quantum computing raise the problem of verifying the outcome of these quantum computations. The recent verification protocols using blind quantum computing are fruitful for addressing this problem. Unfortunately, all known schemes have relatively high overhead. Here we [...] Read more.

Recent advances in theoretical and experimental quantum computing raise the problem of verifying the outcome of these quantum computations. The recent verification protocols using blind quantum computing are fruitful for addressing this problem. Unfortunately, all known schemes have relatively high overhead. Here we present a novel construction for the resource state of verifiable blind quantum computation. This approach achieves a better verifiability of 0.866 in the case of classical output. In addition, the number of required qubits is

2 N + 4 c N

, where N and c are the number of vertices and the maximal degree in the original computation graph, respectively. In other words, our overhead is less linear in the size of the computational scale. Finally, we utilize the method of repetition and fault-tolerant code to optimise the verifiability. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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9 pages, 723 KiB

Open AccessArticle

Understanding of Collective Atom Phase Control in Modified Photon Echoes for a Near-Perfect Storage Time-Extended Quantum Memory

by Rahmat Ullah and Byoung S. Ham

Entropy 2020, 22(8), 900; https://0-doi-org.brum.beds.ac.uk/10.3390/e22080900 - 15 Aug 2020

Cited by 3 | Viewed by 2518

Abstract

A near-perfect storage time-extended photon echo-based quantum memory protocol has been analyzed by solving the Maxwell–Bloch equations for a backward scheme in a three-level system. The backward photon echo scheme is combined with a controlled coherence conversion process via controlled Rabi flopping to [...] Read more.

A near-perfect storage time-extended photon echo-based quantum memory protocol has been analyzed by solving the Maxwell–Bloch equations for a backward scheme in a three-level system. The backward photon echo scheme is combined with a controlled coherence conversion process via controlled Rabi flopping to a third state, where the control Rabi flopping collectively shifts the phase of the ensemble coherence. The propagation direction of photon echoes is coherently determined by the phase-matching condition between the data (quantum) and the control (classical) pulses. Herein, we discuss the classical controllability of a quantum state for both phase and propagation direction by manipulating the control pulses in both single and double rephasing photon echo schemes of a three-level system. Compared with the well-understood uses of two-level photon echoes, the Maxwell–Bloch equations for a three-level system have a critical limitation regarding the phase change when interacting with an arbitrary control pulse area. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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17 pages, 967 KiB

Open AccessArticle

Efficient Quantum Simulation of an Anti-P-Pseudo-Hermitian Two-Level System

by Chao Zheng, Jin Tian, Daili Li, Jingwei Wen, Shijie Wei and Yansong Li

Entropy 2020, 22(8), 812; https://0-doi-org.brum.beds.ac.uk/10.3390/e22080812 - 24 Jul 2020

Cited by 10 | Viewed by 2825

Abstract

Besides Hermitian systems, quantum simulation has become a strong tool to investigate non-Hermitian systems, such as PT-symmetric, anti-PT-symmetric, and pseudo-Hermitian systems. In this work, we theoretically investigate quantum simulation of an anti-P-pseudo-Hermitian two-level system in different dimensional Hilbert spaces. In an [...] Read more.

Besides Hermitian systems, quantum simulation has become a strong tool to investigate non-Hermitian systems, such as PT-symmetric, anti-PT-symmetric, and pseudo-Hermitian systems. In this work, we theoretically investigate quantum simulation of an anti-P-pseudo-Hermitian two-level system in different dimensional Hilbert spaces. In an arbitrary phase, we find that six dimensions are the minimum to construct the anti-P-pseudo-Hermitian two-level subsystem, and it has a higher success probability than using eight dimensions. We find that the dimensions can be reduced further to four or two when the system is in the anti-PT-symmetric or Hermitian phase, respectively. Both qubit-qudit hybrid and pure-qubit systems are able to realize the simulation, enabling experimental implementations in the near future. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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11 pages, 284 KiB

Open AccessArticle

A New Limit Theorem for Quantum Walk in Terms of Quantum Bernoulli Noises

by Caishi Wang, Suling Ren and Yuling Tang

Entropy 2020, 22(4), 486; https://0-doi-org.brum.beds.ac.uk/10.3390/e22040486 - 24 Apr 2020

Cited by 2 | Viewed by 2302

Abstract

In this paper, we consider limit probability distributions of the quantum walk recently introduced by Wang and Ye (C.S. Wang and X.J. Ye, Quantum walk in terms of quantum Bernoulli noises, Quantum Inf. Process. 15 (2016), no. 5, 1897–1908). We first establish several [...] Read more.

In this paper, we consider limit probability distributions of the quantum walk recently introduced by Wang and Ye (C.S. Wang and X.J. Ye, Quantum walk in terms of quantum Bernoulli noises, Quantum Inf. Process. 15 (2016), no. 5, 1897–1908). We first establish several technical theorems, which themselves are also interesting. Then, by using these theorems, we prove that, for a wide range of choices of the initial state, the above-mentioned quantum walk has a limit probability distribution of standard Gauss type, which actually gives a new limit theorem for the walk. Full article

(This article belongs to the Special Issue Quantum Information Processing)

10 pages, 273 KiB

Open AccessArticle

Symmetry-Like Relation of Relative Entropy Measure of Quantum Coherence

by Chengyang Zhang, Zhihua Guo and Huaixin Cao

Entropy 2020, 22(3), 297; https://0-doi-org.brum.beds.ac.uk/10.3390/e22030297 - 05 Mar 2020

Cited by 5 | Viewed by 2350

Abstract

Quantum coherence is an important physical resource in quantum information science, and also as one of the most fundamental and striking features in quantum physics. To quantify coherence, two proper measures were introduced in the literature, the one is the relative entropy of [...] Read more.

Quantum coherence is an important physical resource in quantum information science, and also as one of the most fundamental and striking features in quantum physics. To quantify coherence, two proper measures were introduced in the literature, the one is the relative entropy of coherence

C_{r} (ρ) = S (ρ_{diag}) - S (ρ)

and the other is the

ℓ_{1}

-norm of coherence

C_{ℓ_{1}} (ρ) = \sum_{i \neq j} | ρ_{i j} |

. In this paper, we obtain a symmetry-like relation of relative entropy measure

C_{r} (ρ^{A_{1} A_{2} \dots A_{n}})

of coherence for an n-partite quantum states

ρ^{A_{1} A_{2} \dots A_{n}}

, which gives lower and upper bounds for

C_{r} (ρ)

. As application of our inequalities, we conclude that when each reduced states

ρ^{A_{i}}

is pure,

ρ^{A_{1} \dots A_{n}}

is incoherent if and only if the reduced states

ρ^{A_{i}}

and

{tr}_{A_{i}} ρ^{A_{1} \dots A_{n}} (i = 1, 2, \dots, n)

are all incoherent. Meanwhile, we discuss the conjecture that

C_{r} (ρ) \leq C_{ℓ_{1}} (ρ)

for any state

ρ

, which was proved to be valid for any mixed qubit state and any pure state, and open for a general state. We observe that every mixture

η

of a state

ρ

satisfying the conjecture with any incoherent state

σ

also satisfies the conjecture. We also observe that when the von Neumann entropy is defined by the natural logarithm ln instead of

{log}_{2}

, the reduced relative entropy measure of coherence

{\bar{C}}_{r} (ρ) = - ρ_{diag} ln ρ_{diag} + ρ ln ρ

satisfies the inequality

{\bar{C}}_{r} (ρ) \leq C_{ℓ_{1}} (ρ)

for any state

ρ

. Full article

(This article belongs to the Special Issue Quantum Information Processing)

12 pages, 382 KiB

Open AccessArticle

Analytic Expression of Quantum Discords in Werner States under LQCC

by Chuanmei Xie, Zhanjun Zhang, Jianlan Chen and Xiaofeng Yin

Entropy 2020, 22(2), 147; https://0-doi-org.brum.beds.ac.uk/10.3390/e22020147 - 26 Jan 2020

Cited by 3 | Viewed by 1934

Abstract

In this paper, quantum discords in a special kind of states, i.e., Werner states by local quantum operations and classical communication (LQCC) protocols (WLQCC states), are studied. Nineteen parameters to quantify the quantum discords are reduced to four parameters in terms of properties [...] Read more.

In this paper, quantum discords in a special kind of states, i.e., Werner states by local quantum operations and classical communication (LQCC) protocols (WLQCC states), are studied. Nineteen parameters to quantify the quantum discords are reduced to four parameters in terms of properties of Werner states and quantum discord. In the case of orthogonal projective measures, analytic expression of quantum discords in WLQCC states is analytically worked out. Some properties of the quantum discord in the WLQCC states are obtained, especially the variation relations between the quantum discords and the parameters characterizing the WLQCC states. By virtue of numerical computations, quantum discords in a Werner state before and after LQCC protocols are compared. It is found that quantum discord in any WLQCC state cannot exceed that in the original Werner state. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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11 pages, 876 KiB

Open AccessArticle

Sending-or-Not-Sending Twin-Field Quantum Key Distribution with Light Source Monitoring

by Yucheng Qiao, Ziyang Chen, Yichen Zhang, Bingjie Xu and Hong Guo

Entropy 2020, 22(1), 36; https://0-doi-org.brum.beds.ac.uk/10.3390/e22010036 - 26 Dec 2019

Cited by 8 | Viewed by 2819

Abstract

Twin-field quantum key distribution (TF-QKD) is proposed to achieve a remote key distribution with a maximum secure transmission distance up to over 500 km. Although the security of TF-QKD in its detection part is guaranteed, there are some remaining problems in the source [...] Read more.

Twin-field quantum key distribution (TF-QKD) is proposed to achieve a remote key distribution with a maximum secure transmission distance up to over 500 km. Although the security of TF-QKD in its detection part is guaranteed, there are some remaining problems in the source part. The sending-or-not-sending (SNS) protocol is proposed to solve the security problem in the phase post-selection process; however, the light source is still assumed to be an ideal coherent state. This assumption is not satisfied in real-life QKD systems, leading to practical secure issues. In this paper, we discuss the condition that the photon number distribution (PND) of the source is unknown for the SNS protocol, demonstrate that the security analysis is still valid under a source with unknown PND, and show that with light source monitoring, the performance of the SNS protocol can remain almost unchanged. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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19 pages, 1108 KiB

Open AccessArticle

A Quantum Cellular Automata Type Architecture with Quantum Teleportation for Quantum Computing

by Dimitrios Ntalaperas, Konstantinos Giannakis and Nikos Konofaos

Entropy 2019, 21(12), 1235; https://0-doi-org.brum.beds.ac.uk/10.3390/e21121235 - 17 Dec 2019

Viewed by 3126

Abstract

We propose an architecture based on Quantum Cellular Automata which allows the use of only one type of quantum gate per computational step, using nearest neighbor interactions. The model is built in partial steps, each one of them analyzed using nearest neighbor interactions, [...] Read more.

We propose an architecture based on Quantum Cellular Automata which allows the use of only one type of quantum gate per computational step, using nearest neighbor interactions. The model is built in partial steps, each one of them analyzed using nearest neighbor interactions, starting with single-qubit operations and continuing with two-qubit ones. A demonstration of the model is given, by analyzing how the techniques can be used to design a circuit implementing the Quantum Fourier Transform. Since the model uses only one type of quantum gate at each phase of the computation, physical implementation can be easier since at each step only one kind of input pulse needs to be applied to the apparatus. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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16 pages, 454 KiB

Open AccessArticle

Unidimensional Continuous-Variable Quantum Key Distribution with Untrusted Detection under Realistic Conditions

by Luyu Huang, Yichen Zhang, Ziyang Chen and Song Yu

Entropy 2019, 21(11), 1100; https://0-doi-org.brum.beds.ac.uk/10.3390/e21111100 - 11 Nov 2019

Cited by 7 | Viewed by 2534

Abstract

A unidimensional continuous-variable quantum key distribution protocol with untrusted detection is proposed, where the two legitimate partners send unidimensional modulated or Gaussian-modulated coherent states to an untrusted third party, i.e., Charlie, to realize the measurement. Compared with the Gaussian-modulated coherent-state protocols, the unidimensional [...] Read more.

A unidimensional continuous-variable quantum key distribution protocol with untrusted detection is proposed, where the two legitimate partners send unidimensional modulated or Gaussian-modulated coherent states to an untrusted third party, i.e., Charlie, to realize the measurement. Compared with the Gaussian-modulated coherent-state protocols, the unidimensional modulated protocols take the advantage of easy modulation, low cost, and only a small number of random numbers required. Security analysis shows that the proposed protocol cannot just defend all detectors side channels, but also achieve great performance under certain conditions. Specifically, three cases are discussed in detail, including using unidimensional modulated coherent states in Alice’s side, in Bob’s side, and in both sides under realistic conditions, respectively. Under the three conditions, we derive the expressions of the secret key rate and give the optimal gain parameters. It is found that the optimal performance of the protocol is achieved by using unidimensional modulated coherent states in both Alice’s and Bob’s side. The resulting protocol shows the potential for long-distance secure communication using the unidimensional quantum key distribution protocol with simple modulation method and untrusted detection under realistic conditions. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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9 pages, 680 KiB

Open AccessArticle

Bandwidth-Limited and Noisy Pulse Sequences for Single Qubit Operations in Semiconductor Spin Qubits

by Elena Ferraro and Marco De Michielis

Entropy 2019, 21(11), 1042; https://0-doi-org.brum.beds.ac.uk/10.3390/e21111042 - 26 Oct 2019

Cited by 1 | Viewed by 2080

Abstract

Spin qubits are very valuable and scalable candidates in the area of quantum computation and simulation applications. In the last decades, they have been deeply investigated from a theoretical point of view and realized on the scale of few devices in the laboratories. [...] Read more.

Spin qubits are very valuable and scalable candidates in the area of quantum computation and simulation applications. In the last decades, they have been deeply investigated from a theoretical point of view and realized on the scale of few devices in the laboratories. In semiconductors, spin qubits can be built confining the spin of electrons in electrostatically defined quantum dots. Through this approach, it is possible to create different implementations: single electron spin qubit, singlet–triplet spin qubit, or a three-electron architecture, e.g., the hybrid qubit. For each qubit type, we study the single qubit rotations along the principal axis of Bloch sphere including the mandatory non-idealities of the control signals that realize the gate operations. The realistic transient of the control signal pulses are obtained by adopting an appropriate low-pass filter function. In addition. the effect of disturbances on the input signals is taken into account by using a Gaussian noise model. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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17 pages, 909 KiB

Open AccessArticle

Parameter Optimization Based BPNN of Atmosphere Continuous-Variable Quantum Key Distribution

by Yu Su, Ying Guo and Duan Huang

Entropy 2019, 21(9), 908; https://0-doi-org.brum.beds.ac.uk/10.3390/e21090908 - 18 Sep 2019

Cited by 7 | Viewed by 2475

Abstract

The goal of continuous variable quantum key distribution (CVQKD) is to be diffusely used and adopted in diverse scenarios, so the adhibition of atmospheric channel will play a crucial part in constituting global secure quantum communications. Atmospheric channel transmittance is affected by many [...] Read more.

The goal of continuous variable quantum key distribution (CVQKD) is to be diffusely used and adopted in diverse scenarios, so the adhibition of atmospheric channel will play a crucial part in constituting global secure quantum communications. Atmospheric channel transmittance is affected by many factors and does not vary linearly, leading to great changes in signal-to-noise ratio. It is crucial to choose the appropriate modulation variance under different turbulence intensities to acquire the optimal secret key rate. In this paper, the four-state protocol, back-propagation neural network (BPNN) algorithm was discussed in the proposed scheme. We employ BPNN to CVQKD, which could adjust the modulation variance to an optimum value for ensuring the system security and making the system performance optimal. The numerical results show that the proposed scheme is equipped to improve the secret key rate efficiently. Full article

(This article belongs to the Special Issue Quantum Information Processing)

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