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Journal of the Optical Society of America B

Influence of guided acoustic wave Brillouin scattering on excess noise in fiber-based continuous variable quantum key distribution

Yongmin Li, Ning Wang, Xuyang Wang, Zengliang Bai

Published Sep 17, 2014• 19 citations
Publisher

Abstract

We experimentally study the depolarized guided acoustic wave Brillouin scattering in a long-distance telecom single-mode fiber (SMF-28e). It is shown that such a scattering process can introduce excess noise and further degrade the performance of the fiber-based continuous variable quantum key distribution (CVQKD) system where a fiber is utilized as the quantum channel. Based on the experimental results we deduce a lower bound for the extinction ratio of the pulsed light in the time-multiplexing CVQKD protocol.

DOI
10.1364/josab.31.002379

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We experimentally study the depolarized guided acoustic wave Brillouin scattering in a long-distance telecom single-mode fiber (SMF-28e). It is shown that such a scattering process can introduce excess noise and further degrade the performance of the fiber-based continuous variable quantum key distribution (CVQKD) system where a fiber is utilized as the quantum channel. Based on the experimental results we deduce a lower bound for the extinction ratio of the pulsed light in the time-multiplexing CVQKD protocol.
Using an all-fiber source of pulsed twin beams, with the record intensity difference squeezing of 6.1±0.15 dB for fiber optical parametric amplifiers, we experimentally distribute the pulsed continuous variable (CV) quantum state over a long distance. After separating the signal and idler twin beams by 26 km single-mode fibers, the measured intensity difference noise is still below the shot noise limit by 2 dB. The result shows that the distribution process is only affected by the vacuum noise originated from the fiber transmission loss. The extra noise contributed by the acoustic wave Brillouin scattering and Raman scattering in 26 km transmission fibers has not been observed. The investigation is beneficial for studying long-distance CV quantum communication.
Continuous-variable quantum key distribution (CVQKD) holds an advantage of well compatibility with classical coherent optical communications. However, there exists a performance trade-off between CVQKD and classical communication on single-mode fiber (SMF) because of the spontaneous Raman scattering. Space-division multiplexing (SDM) technique may provide a feasible way to mitigate this performance trade-off in short-distance communication while CVQKD coexisting with classical signals on few-mode fiber (FMF). Here, we examine the feasibility of CVQKD coexisting with classical signals on FMF and analyze the noise impact in weak coupling regime. We find that the inter-mode crosstalk generated from the mode coupling and re-coupling between modes and the group delay spread originated from the differential group delay (DGD) contribute the main noise sources. DGD may become one of the main limits for FMF-based CVQKD towards high-speed system. In addition, a well channel wavelength management is needed to suppress the inter-mode four-wave-mixing for achieving the positive secret key rates. The numerical simulations identify the key parameters for CVQKD system, enabling a helpful insight for realizing security analysis of the Gaussian modulated coherent state protocol. It shows that CVQKD coexisting with high power classical signals on FMF is feasible to implement with standard telecommunication components and able to operate at higher secret key rates. The results may provide a potential guideline for the practical high-rate CVQKD integrating with the FMF-based configuration.
We report a fiber-based four-state discrete modulation continuous variable quantum key distribution system based on homodyne detection. A secret key rate of 1kbit/s is achieved at a transmission distance of 30.2 km. Two factors that result in the excess noises of the quantum key distribution system are analyzed. The first is the relative phase dithering between the signal and local fields, and the second is the local field leakage into the signal field due to the scattering process that depolarizes the local field. It is found that the latter has a significant impact on the excess noise, which is the main limiting factor to the long-distance secure quantum transmission. Some protocols are also given to decrease the excess noise effectively.
In silica glass fibers, thermally excited acoustic phonons scatter light into the beam propagating in the forward direction. At acoustic frequencies up to several hundreds of megahertz, the wave vectors of the phonons interacting with the light propagate essentially transversally to the fiber axis. This effect is known as Guided Acoustic Wave Brillouin Scattering (GAWBS) and leads to phase and polarization noise in the guided light. For fiber-based quantum optics experiments, this excess noise is a major limitation. In Photonic Crystal Fibers (PCFs), light is guided by a microstructure simultaneously acting as a 2D transversal phononic crystal which modifies the acoustic noise spectrum. We demonstrate a GAWBS-noise reduction in commercially available PCFs. This gives rise to the prospect of fiber-based quantum optic devices exhibiting less excess noise, thus resulting in higher quantum state purity. Further improvement can be achieved by tailoring the photonic microstructure such that a reduction of phonon noise by design is achieved.