Information-theoretic results for phase noise channels

Gerhard G. Kramer, Hassan Ghozlan

Gerhard G. Kramer, Hassan Ghozlan

Jacopo Trapani, Berihu Teklu, Stefano Olivares +1 more

We address quantum communication channels based on phase modulation of coherent states and analyze in detail the effects of static and dynamical (stochastic) phase diffusion. We evaluate mutual information for an ideal phase receiver and for a covariant phase-space-based receiver, and compare their performances by varying the number of symbols in the alphabet and/or the overall energy of the channel. Our results show that phase communication channels are generally robust against phase noise, especially for large alphabets in the low-energy regime. In the presence of dynamical (non-Markovian) noise the mutual information is preserved by the time correlation of the environment, and when the noise spectra are detuned with respect to the information carrier, revivals of mutual information appear.

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.

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.

Gerhard G. Kramer