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Applied Physics Letters

Free-space quantum key distribution with Rb vapor filters

Xin Jian Shan, Xianping Sun, Jun Luo, Zheng Tan, Mingsheng Zhan

Published Nov 6, 2006• 51 citations
Publisher

Abstract

By using Rb vapor filters, free-space quantum key distribution with the B92 protocol was realized experimentally; the filters based on the Faraday anomalous dispersion effect were used to suppress strong background light. With these atomic filters as spectral filtering, average transmission rates of sifted keys and corrected keys have reached as high as 3.14 and 1.56kbits∕s, respectively, while the error rate was maintained at a reasonable low level of 5.10%. The experimental results show that our Rb vapor filters with transmission efficiency of around 60% operate well under the condition of the mean photon number n¯⩽0.1.

DOI
10.1063/1.2387867

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By using Rb vapor filters, free-space quantum key distribution with the B92 protocol was realized experimentally; the filters based on the Faraday anomalous dispersion effect were used to suppress strong background light. With these atomic filters as spectral filtering, average transmission rates of sifted keys and corrected keys have reached as high as 3.14 and 1.56kbits∕s, respectively, while the error rate was maintained at a reasonable low level of 5.10%. The experimental results show that our Rb vapor filters with transmission efficiency of around 60% operate well under the condition of the mean photon number n¯⩽0.1.
One of the challenges of implementing free-space quantum key distribution (QKD) systems working in daylight is to remove unwanted background noise photons from sunlight. Elaborate elimination of background photons in the spectral, temporal, and spatial domains is an indispensable requirement to decrease the quantum bit error rate (QBER), which guarantees the security of the systems. However, quantitative effects of different filtering techniques and performance optimization in terms of the secure key rate have not been investigated. In this study, we quantitatively analyze how the performance of the QBER and the key rates changes for different combinations of filtering techniques in a free-space BB84 QKD system in daylight. Moreover, we optimize the conditions of filtering techniques in order to obtain the maximum secure key rate.
We present a theoretical and experimental study of the ultra-narrow bandwidth Faraday anomalous dispersion optical filter (FADOF) operating at the rubidium D1 line (795 nm). This atomic line gives better performance than other lines for the main FADOF figures of merit, e.g. simultaneously 71 % transmission, 445MHz bandwidth and 1.2GHz equivalent noise bandwidth. c © 2013 Optical Society of America OCIS codes: 020.1335, 010.3640 Ultra-narrow bandwidth optical filters are key el-ements in laser remote sensing (LIDAR), observa-tional astronomy, free-space communications and quan-tum optics. Relative to conventional interference fil-ters, FADOFs offer high background-rejection, mechani-cal robustness, imaging capability and high transmission. FADOFs have been developed for several alkali atom res-onances – Cs D2 [1] and 6S1/2 → 7P3/2 [2] lines, Rb D2 line [3, 4], K (three lines) [5], Na D lines [6], and for Ca [7]. We demonstrate a FADOF on the D1 line of Rb (wave-length 795 nm). This line, efficiently detected with Si detectors, accessible with a variety of laser technolo-gies, and with large hyperfine splittings, is a favorite for coherent and quantum optics with warm atomic va-pors. Applications include electromagnetically-induced transparency [8], stopped light [9], optical magnetom-etry [10, 11], laser oscillators [12], polarization squeez-ing [13, 14], quantum memory [15] and high-coherence heralded single photons [16,17]. Here we show that the Rb D1 line provides supe-rior FADOF performance. We demonstrate a FADOF surpassing other atoms and other Rb transitions for key figures of merit, including peak transmission Tmax, transmission bandwidth and equivalent noise bandwidth ENBW = T−1max T (ν)dν, where T (ν) is the filter trans-mission versus frequency ν [4].
To obtain the weak signal light detection from the high background noise,a study on ultra-narrow bandwidth Faraday anomalous dispersion optical filter(FADOF) at the 87Rb D1 line,5S1/2F=2→5P1/2 F′=1transmission(795 nm) is demonstrated experimentally.A sample cell filled with Rubidium atoms in which the proportion of 87Rb is greater than that of the nature rubidium is placed in a homogeneous magnetic field between two crossed Glan-Thompson polarizers.The incident probe light goes through the sample cell,and interacts with the atoms.Because of the Faraday rotation,the function of the filter can be realized.When the experiment condition changed,the result of transmission spectrum changed accordingly.With the temperature rising from 340 K to 360 K,the variation of the transmission spectrum is recorded particularly.And the reason for the variation is analyzed carefully.At a proper temperature and magnetic field,a transmission spectrum narrowed to ~220 MHz is obtained,and the peak transmission is about 48%.The performance of the87Rb D1 line is better than that of the 85Rb.