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EPJ Web of Conferences

Towards the fiber-optic Kennedy quantum receiver

Mikhail S. Elezov, Mikhail Scherbatenko, Сыч Денис Васильевич, Gregory N Gol'tsman

Published Jan 1, 2019• 6 citations
Publisher

Abstract

We consider practical aspects of using standard fiber-optic elements and superconducting nanowire single-photon detectors for the development of a practical quantum receiver based on the Kennedy scheme. Our receiver allows to discriminate two phase-modulated coherent states of light at a wavelength of 1.5 microns in continuous mode with bit rate 200 Kbit/s and error rate about two times below the standard quantum limit.

DOI
10.1051/epjconf/201922003011

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We consider practical aspects of using standard fiber-optic elements and superconducting nanowire single-photon detectors for the development of a practical quantum receiver based on the Kennedy scheme. Our receiver allows to discriminate two phase-modulated coherent states of light at a wavelength of 1.5 microns in continuous mode with bit rate 200 Kbit/s and error rate about two times below the standard quantum limit.
We experimentally demonstrate a quantum receiver based on the Kennedy scheme for discrimination between two phase-modulated weak coherent states. The receiver is assembled entirely from standard fiber-optic elements and operates at a conventional telecom wavelength of 1.55 $\ensuremath{\mu}\text{m}$. The local oscillator and the signal are transmitted through different optical fibers, and the displaced signal is measured with a high-efficiency superconducting nanowire single-photon detector. We show the discrimination error rate is two times below that of a shot-noise-limited receiver with the same system detection efficiency.
Optimal and near-optimal quantum receivers can beat the homodyne limit of discriminating two nonorthogonal coherent states. In particular, the near-optimal receiver proposed by Kennedy is promising for optical communication because of its simple setup. Moreover, Takeoka and Sasaki showed one can further improve the performance of the Kennedy receiver by the optimized displacement operation which was experimentally examined by Wittmann et al. for binary phase shift keyed signals. However, the ideal homodyne limit was not beaten in this experiment because they used a commercially available photon detector based on a silicon avalanche photodiode, which does not have sufficient photon detection efficiency.
Abstract Coherent detection (homodyne detection/heterodyne detection) and single photon detection are commonly applied in classical communication and quantum communication/receiver respectively. In this paper, we firstly propose a quantum-enhanced receiver based on conjugated homodyne detection, which is consisted of two classical balanced homodyne detectors, for discrimination among quadrature-phase-modulated weak coherent states. Our detection part works as a photon counter in every single-shot measurement during the process of detection and feedback, which could help our quantum-enhanced receiver surpass the standard quantum limit when an optimized detection threshold τ=0.3\tau = 0.3τ=0.3 is selected. Moreover, our proposed quantum-enhanced receiver with conjugated homodyne detectors can obtain the same low error probability as conventional quantum-enhanced receivers (with its superconducting nanowire single photon detectors' detection efficiency 75%\%% for pulse separation M=8M=8M=8 and 85%\%% for M=12M=12M=12 under the same adaptive feedback countermeasures.) Meanwhile, our scheme of the quantum-enhanced receiver gains its incomparable advantages on room-temperature working condition and low cost due to the application of conjugated homodyne detectors. As far as we concerned, this is the first time to explore the performance of the quantum-enhanced receiver with commercial homodyne detectors. And the analysis in this paper may pave the way to reduce the cost of the quantum-enhanced receiver and make it more adaptable for long-distance optical communication systems.
At present superconducting detectors become increasingly attractive for various practical applications. In this paper we present results on the depelopment of fiber coupled receiver systems for the registration of IR single photons, optimized for telecommunication and quantum-cryptography. These receiver systems were developed on the basis of superconducting single photon detectors (SSPD) of VIS and IR wavelength ranges. The core of the SSPD is a narrow (~100 nm) and long (~0,5 mm) strip in the form of a meander which is patterned from a 4-nm-thick NbN film (TC=10-11 K, jC=~5-7•106 A/cm2); the sensitive area dimensions are 10×10 μm2. The main problem to be solved while the receiver system development was optical coupling of a single-mode fiber (9 microns in diameter) with the SSPD sensitive area. Characteristics of the developed system at the optical input are as follows: quantum efficiency >10 % (at 1.3 μm), >4 % (at 1.55 μm); dark counts rate ≤1 s-1; duration of voltage pulse ≤5 ns; jitter ≤40 ps. The receiver systems have either one or two identical channels (for the case of carrying out correlation measurements) and are made as an insert in a helium storage Dewar.