Quantum Communications using Coherent Detection Schemes

Ivan B. Djordjević

Qi, Bi

An improved coherent communication scheme is provided. The coherent communication scheme encodes both classical and quantum information simultaneously using isolated groups of states: classical information is represented by different groups and can be decoded deterministically; and quantum information is represented by highly overlapped states within the same group, thus guaranteeing security. Decoding includes projecting the detection results at the receiver to one of the distinguishable encoding groups first, which allows the classical information to be read out, and then generating a quantum key from the residual randomness. This communications scheme enables simultaneous classical communication and QKD over the same communication channel using the same transmitter and receiver, opening the door to operate QKD in the background of classical communication and at negligible costs.

Zhichao Dong, Xiaolin Zhou, Wei Ni +1 more

Coherent states are increasingly being adopted for transmitting information in underwater quantum communications (UQCs). This paper presents what we believe to be a novel quantum multiple-symbol detection (QMSD) scheme designed to alleviate the performance degradation present in classical quantum symbol-by-symbol detection schemes, while operating without the need for channel state information (CSI) at the receiver. We model the communication system under the influence of an underwater channel and thermal noise at the receiver, and propose a novel QMSD metric based on positive operator-valued measure (POVM) quantum measurement results. Furthermore, we derive a closed-form expression for the QMSD metric for facilitating easy evaluation under marine turbulence conditions and thereby reducing implementation complexity. Using the simplified QMSD metric, we employ a sliding window algorithm to efficiently process the received quantum symbol sequence. Simulation results demonstrate that our proposed QMSD scheme enhances the quality of UQCs and exhibits robustness under marine turbulence conditions. Specifically, as the length of the detection sequence increases, the error probability of our scheme decreases. Moreover, as the average number of transmitted photons increases, the proposed QMSD scheme demonstrates superior performance, reducing error probability by 3 orders of magnitude compared to the quantum maximum likelihood detection (QMLD) scheme and by 5 orders of magnitude relative to the traditional homodyne detection (HD) scheme.

Ivan B. Djordjević

Siyu Song, Chuan Xi Wang

Shri Ramtej Kondamuri, Neel Kanth Kundu, Amit Ghanshyala

In this letter, we analyze the average probability of error of quantum modulation schemes in weakly turbulent quantum communication channels utilizing coherent states. Atmospheric turbulence-induced fading is a significant challenge that degrades the performance of free-space quantum communication systems. We derive and present closed-form expressions for the average probability of error for various detection methods: classical (shot-noise limited) single-photon detection, coherent detection, and the optimal quantum hypothesis-testing (Helstrom) detection for binary quantum modulation schemes. Our analysis focuses on the transmission of classical data modulated by quantum coherent states over a log-normal turbulent channel. Additionally, we discuss the lower bounds of the average capacities for Helstrom detection, providing comprehensive insights into the achievable information rates in realistic atmospheric conditions. The derived expressions show excellent agreement with numerical Monte Carlo simulations across a broad range of average transmitted photon numbers, offering valuable guidelines for the design and optimization of quantum communication systems.