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Adaptive-optics-enabled daytime field experiment for satellite-Earth quantum networking

Mark T. Gruneisen, Mark Eickhoff, Scott Newey, Kurt E. Stoltenberg, Jeffery F. Morris, Michael Bareian, Mark Anglin Harris, Denis W. Oesch, Michael D. Oliker, Michael Flanagan, Brian T. Kay, Jonathan D. Schiller, R. Nicholas Lanning

Published Jun 13, 2020• 1 citation
OpenAlex

Abstract

Daytime quantum communication was demonstrated under field-site conditions representative of space-to-Earth optical links. Higher-order adaptive optics increased quantum channel efficiencies significantly beyond those that were possible with tracking alone while tight spatial filtering rejected daytime background light without the need for an ultra-narrow spectral filter. High signal-to-noise probabilities and low quantum-bit-error rates were demonstrated over a wide range of relevant channel radiances and turbulence conditions. The impact to satellite-based quantum key distribution is discussed.

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Robust quantum channels between satellites and Earth are essential for the anticipated quantum Internet, but remain challenging in daylight, when background photons vastly outnumber qubit photons. Insufficient understanding of atmospheric propagation and turbulence compensation has led to approaches based on prohibitively narrow spectral filtering and unnecessarily low channel efficiencies. This field experiment uses adaptive optics to maximize efficiency while spatially filtering sky noise at the theoretical limit, enabling quantum communication over the daytime sky hemisphere. Requirements for spectral filtering are relaxed enough to accommodate present-day sources of entangled photons.

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arXiv:1906.04193•Advanced Optical Technologies•Sep 16, 2019•35 citations
Daytime quantum communication was demonstrated under field-site conditions representative of space-to-Earth optical links. Higher-order adaptive optics increased quantum channel efficiencies significantly beyond those that were possible with tracking alone while tight spatial filtering rejected daytime background light without the need for an ultra-narrow spectral filter. High signal-to-noise probabilities and low quantum-bit-error rates were demonstrated over a wide range of relevant channel radiances and turbulence conditions. The impact to satellite-based quantum key distribution is discussed.
Robust quantum channels between satellites and Earth are essential for the anticipated quantum Internet, but remain challenging in daylight, when background photons vastly outnumber qubit photons. Insufficient understanding of atmospheric propagation and turbulence compensation has led to approaches based on prohibitively narrow spectral filtering and unnecessarily low channel efficiencies. This field experiment uses adaptive optics to maximize efficiency while spatially filtering sky noise at the theoretical limit, enabling quantum communication over the daytime sky hemisphere. Requirements for spectral filtering are relaxed enough to accommodate present-day sources of entangled photons.
The rate of secure key generation (SKG) in quantum key distribution (QKD) is adversely affected by optical noise and loss in the quantum channel. In a free-space atmospheric channel, the scattering of sunlight into the channel can lead to quantum bit error ratios (QBERs) sufficiently large to preclude SKG. Furthermore, atmospheric turbulence limits the degree to which spatial filtering can reduce sky noise without introducing signal losses. A system simulation quantifies the potential benefit of tracking and higher-order adaptive optics (AO) technologies to SKG rates in a daytime satellite engagement scenario. The simulations are performed assuming propagation from a low-Earth orbit (LEO) satellite to a terrestrial receiver that includes an AO system comprised of a Shack-Hartmann wave-front sensor (SHWFS) and a continuous-face-sheet deformable mirror (DM). The effects of atmospheric turbulence, tracking, and higher-order AO on the photon capture efficiency are simulated using statistical representations of turbulence and a time-domain waveoptics hardware emulator. Secure key generation rates are then calculated for the decoy state QKD protocol as a function of the receiver field of view (FOV) for various pointing angles. The results show that at FOVs smaller than previously considered, AO technologies can enhance SKG rates in daylight and even enable SKG where it would otherwise be prohibited as a consequence of either background optical noise or signal loss due to turbulence effects.
Future quantum networks will be composed of both terrestrial links for metropolitan and continent-scale connections and space-based links for global coverage and infrastructure resilience. However, the propagation of quantum signals through the atmosphere is severely impacted by the effects of turbulence. This is even more the case for entanglement-based quantum communication protocols requiring two free-space channels to be considered simultaneously. In this work, we assess the advantage of turbulence mitigation by adaptive optics, in particular during daytime link operation, so as to increase the coupling of the received signal into an optical fiber. We show in particular that this improves the performance of entanglement-based quantum key distribution by up to a few hundred bits per second when compared with the uncorrected scenario
Spatial filtering is an important technique for reducing sky background noise in a satellite quantum key distribution downlink receiver. Atmospheric turbulence limits the extent to which spatial filtering can reduce sky noise without introducing signal losses. Using atmospheric propagation and compensation simulations, the potential benefit of adaptive optics (AO) to secure key generation (SKG) is quantified. Simulations are performed assuming optical propagation from a low-Earth-orbit satellite to a terrestrial receiver that includes AO. Higher-order AO correction is modeled assuming a Shack–Hartmann wavefront sensor and a continuous-face-sheet deformable mirror. The effects of atmospheric turbulence, tracking, and higher-order AO on the photon capture efficiency are simulated using statistical representations of turbulence and a time-domain wave-optics hardware emulator. SKG rates are calculated for a decoy-state protocol as a function of the receiver field of view for various strengths of turbulence, sky radiances, and pointing angles. The results show that at fields of view smaller than those discussed by others, AO technologies can enhance SKG rates in daylight and enable SKG where it would otherwise be prohibited as a consequence of background optical noise and signal loss due to propagation and turbulence effects.
Quantum key distribution (QKD) enables remote users to share cryptographic keys with information theoretic security. Satelliteto-ground QKD offers a route to extend the distances between users that is otherwise limited by losses through optical fibre. However, satellite QKD is typically limited to operation at night due to the background photons from sunlight. A proposed solution to reduce the background counts is tight spatial filtering via single-mode fibre coupling, but this comes at the cost of losses because of imperfect coupling efficiency due to wavefront distortions from atmospheric turbulence. To maximise the number of photons entering the fibre, adaptive optics (AO) can be used to correct for the atmospheric effects. We present results of end-to-end link simulations which show the trade-off between AO complexity and fibre coupling efficiency. These results show that for representative turbulence characteristics, a relatively small deformable mirror is required to obtain the required coupling efficiency for a reasonable key rate. This paves the way for affordable quantum communication in daylight conditions.
Abstract For quantum communications, the use of Earth-orbiting satellites to extend distances has gained significant attention in recent years, exemplified in particular by the launch of the Micius satellite in 2016. The performance of applied protocols such as quantum key distribution (QKD) depends significantly on the transmission efficiency through the turbulent atmosphere, which is especially challenging for ground-to-satellite uplink scenarios. Adaptive optics (AO) techniques have been used in astronomical, communication, and other applications to reduce the detrimental effects of turbulence for many years, but their applicability to quantum protocols, and their requirements specifically in the uplink scenario, is not well established. Here, we model the effect of the atmosphere on link efficiency between an Earth station and a satellite using an optical uplink and how AO can help recover from loss due to turbulence. Examining both low Earth orbit and geostationary uplink scenarios, we find that a modest link transmissivity improvement of about 3 dB can be obtained in the case of a coaligned downward beacon, while the link can be dramatically improved, up to 7 dB, using an offset beacon, such as a laser guide star. AO coupled with a laser guide star would thus deliver a significant increase in the secret key generation rate of the QKD ground-to-space uplink system, especially as reductions of channel loss have a favourably nonlinear key-rate response within this high-loss regime.