Communication rates for phase noise channels

Gerhard G. Kramer, Hassan Ghozlan

Gerhard G. Kramer, Hassan Ghozlan

M. T. DiMario, L. Kunz, K. Banaszek +1 more

Abstract The achievable rate of information transfer in optical communications is determined by the physical properties of the communication channel, such as the intrinsic channel noise. Bosonic phase noise channels, a class of non-Gaussian channels, have emerged as a relevant noise model in quantum information and optical communication. However, while the fundamental limits for communication over Gaussian channels have been extensively studied, the properties of communication over Bosonic phase noise channels are not well understood. Here we propose and demonstrate experimentally the concept of optimized communication strategies for communication over phase noise channels to enhance information transfer beyond what is possible with conventional methods of modulation and detection. Two key ingredients are generalized constellations of coherent states that interpolate between standard on-off keying and binary phase-shift keying formats, and non-Gaussian measurements based on photon number resolving detection of the coherently displaced signal. For a given power constraint and channel noise strength, these novel strategies rely on joint optimization of the input alphabet and the measurement to provide enhanced communication capability over a non-Gaussian channel characterized in terms of the error rate as well as mutual information.

Shinji Koyama, Tsuyoshi Sasaki Usuda

In previous research on quantum information the- ory, channel capacity of the lossy bosonic channel that is a model of free space or an ideal optical fiber was shown. However, in the optical space communication, the influence of temporal and spatial change in the propagation path on the quantum communication has not been clarified. In order to clarify them, we have studied properties of quantum communications via channels with various fluctuations. We consider phase noise since phase fluctuates by a slight drift of the receiver and it has not been considered well compared to amplitude fluctuation. In this paper, we compute the error probability of the optimum quantum receiver in the presence of phase noise and compare with that of homodyne receiver when BPSK coherent-state signals are used. Then we consider the robustness of the optimum receiver against phase fluctuations. I. INTRODUCTION In order to increase the communication rate, higher fre- quency signals should be used. Light wave has a very high frequency in particular, optical communication systems using this were realized by the invention of the laser. Currently, optical fiber communication systems which consist of laser light source, optical fiber, and photodetector have come to be generally used. Moreover, light is also a promising carrier for wireless or space communications with high capacity. However, the performance of communication systems of this scheme has been approached to the theoretical limit. The factor is due to quantum mechanical effects of light, called quantum noise. Quantum noise increases in proportion to the frequency of the electromagnetic wave. Therefore, it cannot be ignored in the high frequency band, such as optical frequency. In addition, quantum noise is essentially different from the classical noise such as thermal noise. Therefore, it is not possible to suppress quantum noise using information theory which is the basis of current communication technology. That is, in order to realize higher communication rate, a theory which can describe accurately the quantum mechanical nature of light is required. For this reason, we should study quantum information theory which had been introduced axioms of quantum mechanics into the conventional information theory (2), (3), (4). As a previous study, the channel capacity of the lossy bosonic channel which is a model of free space and optical fiber communications was shown (5). That is, performance of ideal quantum communication was clarified. Therefore, as the next step, a study aiming at the actual use is required. As an example, the study on communication model that takes account of the propagation path environment will be needed. In classical communication, such models have been studied as a theory for realizing mobile communications, etc. However, in quantum communication, there are few such results of study now. This study is intended to clarify the influence of temporal and spatial change of the propagation path environment for the quantum communication. We consider phase noise since phase fluctuates by a slight drift of the receiver and it has not been considered well compared to amplitude fluctuation.

Luca Barletta

Bhaskar Roy Bardhan

Noise present in an environment has significant impacts on a quantum system affecting properties like coherence, entanglement and other metrological features of a quantum state. In this dissertation, we address the effects of different types of noise that are present in a communication channel (or medium) and an interferometric setup, and analyze their effects in the contexts of preserving coherence and entanglement, phase sensitivity, and limits on rate of communication through noisy channels. We first consider quantum optical phase estimation in quantum metrology when phase fluctuations are introduced in the system by its interaction with a noisy environment. By considering path-entangled dual-mode photon Fock states in a Mach-Zehnder optical interferometric configuration, we show that such phase fluctuations affect phase sensitivity and visibility by adding noise to the phase to be estimated. We also demonstrate that the optimal detection strategy for estimating a phase in the presence of such phase noise is provided by the parity detection scheme. We then investigate the random birefringent noise present in an optical fiber affecting the coherence properties of a single photon polarization qubit propagating through it. We show that a simple but effective control technique, called dynamical decoupling, can be used to suppress the effects of the dephasing noise, thereby preserving its ability to carry the encoded quantum information in a long-distance optical fiber communication system. Optical amplifiers and attenuators can also add noise to an entangled quantum system, deteriorating the non-classical properties of the state. We show this by considering a two-mode squeezed vacuum state, which is a Gaussian entangled state, propagating through a noisy medium, and characterizing the loss of entanglement in the covariance matrix and the symplectic formalism for this state. Finally, we discuss limits on the rate of communication in the context of sending messages through noisy optical quantum communication channels. In particular, we prove that a strong converse theorem holds under a maximum photon number constraint for these channels, guaranteeing that the success probability in decoding the message vanishes in the asymptotic limit for the rate exceeding the capacity of the channels.