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Optics Letters

Ultranarrow bandwidth spectral filtering for long-range free-space quantum key distribution at daytime

David Höckel, Lars Koch, Eugen Martin, Oliver J. Benson

Published Oct 8, 2009• 11 citations
Publisher

Abstract

We describe a Fabry-Perot-based spectral filter for free-space quantum key distribution (QKD). A multipass etalon filter was built, and its performance was studied. The whole filter setup was carefully optimized to add less than 2 dB attenuation to a signal beam but block stray light by 21 dB. Simulations show that such a filter might be sufficient to allow QKD satellite downlinks during daytime with the current technology.

DOI
10.1364/ol.34.003169

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We describe a Fabry-Perot-based spectral filter for free-space quantum key distribution (QKD). A multipass etalon filter was built, and its performance was studied. The whole filter setup was carefully optimized to add less than 2 dB attenuation to a signal beam but block stray light by 21 dB. Simulations show that such a filter might be sufficient to allow QKD satellite downlinks during daytime with the current technology.
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.
Free-space optical communication links promise better security and increase bandwidths but can suffer from noise in daylight. This is particularly detrimental in quantum communications where current mitigation techniques, such as spectral, temporal, and spatial filtering, are not yet sufficient to make daylight tolerable for satellite quantum key distribution (SatQKD). As all current SatQKD systems are polarization-encoded, polarization filtering has not been investigated. However, by using time- and phase-encoded SatQKD, it is possible to filter in polarization in addition to existing domains. Scattered daylight can be more than 90% polarized in the visible band, yielding a reduction in detected daylight between 3 dB and 13 dB, such that polarization filtering can reduce the brightness of 780 nm daylight to below the unfiltered equivalent at 1550 nm. Simulations indicate that polarization filtering increases the secure key rate and allows for SatQKD to be performed at dawn and dusk. This could open the way for daylight SatQKD utilizing shorter near-infrared wavelengths and retaining their benefits.
Current distance limitations of quantum key distribution (QKD) over fibre optic networks suggest that satellite (free-space optical) QKD networks will be required to enable global quantum communications. However, the operational availability of these systems is limited by background noise and strong attenuation caused by turbulence and adverse weather conditions. Using the decoy-state BB84 QKD protocol, we evaluate the secret key rate for a range of wavelengths, receiver sizes and initial beam waists through a variety of atmospheric conditions. We combine filtering techniques, adaptive optics, and wavelength selection to optimize the performance of satellite QKD. This study is simulation-based.
We describe a combined ultranarrow bandpass filtering setup for single-photon experiments in quantum optics. The filter is particularly suitable for single-photon electromagnetically induced transparency (EIT) experiments, but can also be used in several similar applications. A multipass planar Fabry-Perot etalon together with polarization filters and spatial filtering allows 114 dB pump beam suppression, while the signal beam is attenuated by just 4 dB, although both wavelengths are only separated by 0.025 nm (9.2 GHz). The multipass etalon alone accounts for 46 dB suppression while it has a peak transmission of 65%. We demonstrate EIT experiments in Cs vapor at room temperature with probe power in the femtowatt regime using this filter.