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Conference on Lasers and Electro-Optics

A Daytime Free-Space Quantum-Optical Link using Atomic-Vapor spectral Filters

Christopher C. Evans, David N. Woolf, Justin M. Brown, Joel M. Hensley

Published May 5, 2019• 0 citations
Publisher

Abstract

We establish a free-space quantum-optical link that leverages narrow (~1 GHz) linewidth rubidium-based bandpass filters to drastically reduce solar background. To show its utility, we demonstrate the BB84 quantum key distribution protocol under daytime conditions. © 2019 The Author(s)
DOI
10.23919/cleo.2019.8749536

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Spectral filtering is essential in daytime quantum key distribution (QKD), which can suppress the strong background noise caused by scattered solar irradiation. An integrated Fabry-Perot filter is implemented based on a scheme that combines a Fabry-Perot etalon and a dense-wavelength-division-multiplex filter for narrow linewidth filtering and broad-spectrum noise suppression, respectively. This filter is integrated into a butterfly package with single-mode fibers for optical input and output, thereby enhancing high robustness and ease of use. The measurement results show that the filter has a linewidth of 25.6 pm, a noise suppression of over 44.7 dB ranging between 1380-1760 nm, an optical efficiency of 74.5% with variation less than 0.9% in 120 min, and a polarization fidelity after compensation exceeding 99.9%. The ability of fine-tuning the central wavelength with 9.5 pm/°C makes it very suitable for satellite-based applications under the Doppler effect. Further analysis is also given to demonstrate the prospects of applying this filter in future satellite-based daytime QKD applications.
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.
Abstract Entanglement-based quantum key distribution can enable secure communication in trusted node-free networks and over long distances. Although implementations exist both in fiber and in free space, the latter approach is often considered challenging due to environmental factors. Here, we implement a quantum communication protocol during daytime for the first time using a quantum dot source. This technology presents advantages in terms of narrower spectral bandwidth—beneficial for filtering out sunlight—and negligible multiphoton emission at peak brightness. We demonstrate continuous operation over the course of three days, across an urban 270 m-long free-space optical link, under different light and weather conditions.