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Photonics Research

Experimental demonstration of phase-sensitive multimode continuous variable quantum key distribution with improved secure key rate

Zikang Su, Jintao Wang, Dajian Cai, Xiaojie Guo, Dawei Wang, Zhaohui Li

Published Sep 5, 2023• 5 citations
Publisher

Abstract

We develop and experimentally demonstrate a phase-sensitive continuous variable quantum key distribution system with improved secure key rate. This is achieved using multimode coherent states with phase-conjugated subcarrier modulation and phase-sensitive detection. The local oscillator for phase-sensitive detection is regenerated from a polarization-multiplexed carrier wave via optical injection locking. The proposed scheme has a higher classical information capacity at a given number of received photons and exhibits a higher secure key rate when applying the security analysis of the GG02 protocol. Experimental results confirm the higher secret key rate and better excess noise tolerance of the new scheme compared to the typical implementation of GG02.

DOI
10.1364/prj.485654

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We develop and experimentally demonstrate a phase-sensitive continuous variable quantum key distribution system with improved secure key rate. This is achieved using multimode coherent states with phase-conjugated subcarrier modulation and phase-sensitive detection. The local oscillator for phase-sensitive detection is regenerated from a polarization-multiplexed carrier wave via optical injection locking. The proposed scheme has a higher classical information capacity at a given number of received photons and exhibits a higher secure key rate when applying the security analysis of the GG02 protocol. Experimental results confirm the higher secret key rate and better excess noise tolerance of the new scheme compared to the typical implementation of GG02.
In this paper we report a continuous-variable quantum key distribution protocol using multimode coherent states generated on subcarrier frequencies of the optical spectrum. We propose a coherent detection scheme where power from a carrier wave is used as a local oscillator. We compose a mathematical model of the proposed scheme and perform its security analysis in the finite-size regime using fully quantum asymptotic equipartition property technique. We calculate a lower bound on the secret key rate for the system under the assumption that the quantum channel noise is negligible compared to detector dark counts, and an eavesdropper is restricted to collective attacks. Our calculation shows that the current realistic system implementation would allow distributing secret keys over channels with losses up to 9 dB.
We theoretically investigate and experimentally demonstrate a RF-assisted four-state continuous-variable quantum key distribution (CV-QKD) system. Classical coherent detection is implemented with a simple digital phase noise cancelation scheme. In the proposed system, there is no need for frequency and phase locking between the quantum signals and the local oscillator laser. Moreover, in principle, there is no residual phase noise, and a mean excess noise of 0.0115 (in shot-noise units) can be acquired experimentally. In addition, the minimum transmittance of 0.45 is reached experimentally for secure transmission with commercial photodetectors, and the maximum secret key rate (SKR) of >12 Mbit/s can be obtained. The proposed RF-assisted CV-QKD system opens the door of incorporating microwave photonics into a CV-QKD system and improving the SKR significantly.
In this Letter, we propose a novel implementation of continuous variable quantum key distribution that operates with a real local oscillator placed at the receiver site. In addition, pulsing of the continuous wave laser sources is not required, leading to an extraordinary practical and secure setup. It is suitable for arbitrary schemes based on modulated coherent states and heterodyne detection. The shown results include transmission experiments, as well as an excess noise analysis applying a discrete 8-state phase modulation. Achievable key rates under collective attacks are estimated. The results demonstrate the high potential of the approach to achieve high secret key rates at relatively low effort and cost.
We develop an optical injection locking (OIL) based local oscillator (LO) regeneration for continuous variable quantum key distribution (CVQKD) by sending a weak polarization multiplexed pilot carrier from the transmitter. The OIL at the receiver has superior performance in terms of minimum input power and noise level at offset frequencies to the erbium-doped fiber amplifier (EDFA)-based scheme. The weak pilot carrier is recovered both in power and phase via the OIL while incurring little excess noise to the CVQKD system. The phase-locked LO enables heterodyne detection of a Gaussian modulated quantum signal with a simple data-aided phase recovery without pilot tone. The obtained parameters are compatible with a raw key rate of 0.83 Mbit/s in the asymptotic regime over a 22-km fiber transmission. The technique is expected to be used in more phase-sensitive quantum optical applications.
Abstract The type of system employing continuous variables, such as light field quadrature for signal modulation, is an important branch for quantum key distribution. Achieving a high key rate using quantum key distribution technology is beneficial for realizing more frequent key updates or enabling one-time pad encryption for high-speed communication tasks. However, the deficiencies of typical large-bandwidth balanced photodetectors prevent the continuous variable quantum key distribution from matching the speed of state-of-the-art classical optical communications. Previous attempts to improve the key rate have focused on designing and manufacturing high-speed balanced photodetectors. It is well known that phase-sensitive amplifiers also improve the performance of balanced photodetectors and thus the performance of key distribution, but the concept remains purely theoretical. Here, for the first time, we experimentally demonstrate a high-speed continuous-variable quantum key distribution operating with a 10 GHz balanced photodetector enhanced by phase-sensitive amplification and detection. A conjugated multi-mode continuous variable protocol is used to accommodate the phase-sensitive operations. The optical amplifier offers a 10 dB increase in the photodetector’s clearance (quantum noise over electrical noise) and improves the detection efficiency from 72% to 96%, thus enabling an overall 248.9 Mb/s key distribution rate through a 16.7 km field-deployed optical fiber.
We propose and experimentally demonstrate a continuous-variable quantum key distribution (CV-QKD) protocol using dual-phase-modulated coherent states. We show that the modulation scheme of our protocol works equivalently to that of the Gaussian-modulated coherent-states (GMCS) protocol, but shows better experimental feasibility in the plug-and-play configuration. Besides, it waives the necessity of propagation of a local oscillator (LO) between legitimate users and generates a real local LO for quantum measurement. Our protocol is proposed independent of the one-way GMCS QKD without sending a LO [Opt. Lett. 40, 3695 (2015); Phys. Rev. X 5, 041009 (2015); Phys. Rev. X 5, 041010 (2015)]. In those recent works, the system stability will suffer the impact of polarization drifts induced by environmental perturbations, and two independent frequency-locked laser sources are necessary to achieve reliable coherent detection. In the proposed protocol, these previous problems can be resolved. We derive the security bounds for our protocol against collective attacks, and we also perform a proof-of-principle experiment to confirm the utility of our proposal in real-life applications. Such an efficient scheme provides a way of removing the security loopholes associated with the transmitting LO, which have been a notoriously hard problem in continuous-variable quantum communication.