Quantum Communications Systems with Thermal Noise

Alessia Allevi, Maria Bondani

Yaoyao Wang, Rong Zhang, Bin Feng Lu +1 more

Pulse position modulation (PPM) is widely adopted in free space optical transmission, and is a candidate for deep space transmission, also in quantum form. The change of signal photon number probability distribution is caused by thermal noise in quantum PPM format. We prove that squeezing improves state discrimination at fixed channel energy, and also in the presence of thermal noise. Then we compare the difference between coherent state and squeezed state in thermal noise and reveal that the squeezed state has better robustness. It is shown that for the fixed channel energy, the larger the squeezed parameter is not the better. For some given total channel energies, we further study and get the optimal squeezed parameter. Finally, combined with the actual situation, the simulation results show that, as a resource for noisy quantum communication channels, the squeezing operation can improve the quality of deep space quantum communication system with PPM.

J. P. Gordon

Renzhi Yuan, Julian Cheng

An optical quantum communication system with receiver diversity is proposed for free-space communication using the optical combining technique and a generalized Kennedy receiver. The quantum channel model of the proposed system is established using the P -representation in the presence of both turbulence and thermal noise. A conditional dynamics based Kennedy receiver with threshold detection is proposed to mitigate the influence of both turbulence and thermal noise. The error probabilities for different types of generalized Kennedy receiver, i.e., the Kennedy receiver, the Type-II receiver, and the conditional dynamics based Kennedy receiver, are analytically studied and the lower bounds for these error probabilities are obtained. Numerical results show that both the Kennedy receiver and the Type-II receiver fail in the presence of either turbulence or thermal noise. The performance of the proposed conditional dynamics based Kennedy receiver with threshold detection can surpass the standard quantum limit given by the homodyne receiver in either weak turbulence or small thermal noise.

Nicolas Gillard, Étienne Belin, François Chapeau‐Blondeau

Laszlo B. Kish, Robert Mingesz, Zoltán Gingl

Very recently, it has been shown that Gaussian thermal noise and its artificial versions (Johnson-like noises) can be utilized as an information carrier with peculiar properties therefore it may be proper to call this topic Thermal Noise Informatics. Zero Power (Stealth) Communication, Thermal Noise Driven Computing, and Totally Secure Classical Communication are relevant examples. In this paper, while we will briefly describe the first and the second subjects, we shall focus on the third subject, the secure classical communication via wire. This way of secure telecommunication utilizes the properties of Johnson(-like) noise and those of a simple Kirchhoff's loop. The communicator is unconditionally secure at the conceptual (circuit theoretical) level and this property is (so far) unique in communication systems based on classical physics. The communicator is superior to quantum alternatives in all known aspects, except the need of using a wire. In the idealized system, the eavesdropper can extract zero bit of information without getting uncovered. The scheme is naturally protected against the man-in-the-middle attack. The communication can take place also via currently used power lines or phone (wire) lines and it is not only a point-to-point communication like quantum channels but network-ready. We report that a pair of Kirchhoff-Loop-Johnson(-like)-Noise communicators, which is able to work over variable ranges, was designed and built. Tests have been carried out on a model-line with ranges beyond the ranges of any known direct quantum communication channel and they indicate unrivalled signal fidelity and security performance. This simple device has single-wire secure key generation/sharing rates of 0.1, 1, 10, and 100 bit/second for copper wires with diameters/ranges of 21 mm / 2000 km, 7 mm / 200 km, 2.3 mm / 20 km, and 0.7 mm / 2 km, respectively and it performs with 0.02% raw-bit error rate (99.98 % fidelity). The raw-bit security of this practical system significantly outperforms raw-bit quantum security. Current injection breaking tests show zero bit eavesdropping ability without setting on the alarm signal, therefore no multiple measurements are needed to build an error statistics to detect the eavesdropping as in quantum communication. Wire resistance based breaking tests of Bergou-Scheuer-Yariv type give an upper limit of eavesdropped raw bit ratio is 0.19 % and this limit is inversely proportional to the sixth power of cable diameter. Hao's breaking method yields zero (below measurement resolution) eavesdropping information.