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arXiv:2006.07745 · Physical Review Applied

Adaptive-Optics-Enabled Quantum Communication: A Technique for Daytime Space-To-Earth Links

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

Published Jul 29, 2021• 73 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevapplied.16.014067

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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.
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.
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.
The efficient coupling of photons from a free-space quantum channel into a single-mode optical fiber (SMF) has important implications for quantum network concepts involving SMF interfaces to quantum detectors, atomic systems, integrated photonics, and direct coupling to a fiber network. Propagation through atmospheric turbulence, however, leads to wavefront errors that degrade mode matching with SMFs. In a free-space quantum channel, this leads to photon losses in proportion to the severity of the aberration. This is particularly problematic for satellite-Earth quantum channels, where atmospheric turbulence can lead to significant wavefront errors. This report considers propagation from low-Earth orbit to a terrestrial ground station and evaluates the efficiency with which photons couple either through a circular field stop or into an SMF situated in the focal plane of the optical receiver. The effects of atmospheric turbulence on the quantum channel are calculated numerically and quantified through the quantum bit error rate and secure key generation rates in a decoy-state BB84 protocol. Numerical simulations include the statistical nature of Kolmogorov turbulence, sky radiance, and an adaptive-optics system under closed-loop control.
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
The efficient coupling of photons from a free-space quantum channel into a single-mode optical fiber (SMF) has important implications to quantum network concepts involving SMF interfaces to quantum detectors, atomic systems, integrated photonics and direct coupling to a fiber network. Propagation through atmospheric turbulence however leads to wavefront errors that degrade mode-matching with SMFs. In a free-space quantum channel, this leads to photon losses in proportion to the severity of the aberration. This is particularly problematic for satellite-Earth quantum channels where atmospheric turbulence can lead to significant wavefront errors. This report considers propagation from a transmitter in low-Earth orbit to a terrestrial ground station and evaluates the efficiency with which photons couple either through a circular field stop or into a SMF situated in the focal plane of the optical receiver. The effects of atmospheric turbulence on the quantum channel are calculated numerically and quantified through the quantum bit error rate and secure key generation rates in a decoy-state BB84 protocol. Numerical simulations include the statistical nature of Kolmogorov turbulence, sky radiance, and an adaptive optics system under closed loop control.
Satellite based quantum communication has been proven as a feasible way to achieve global scale quantum communication network. Very recently, a low-Earth-orbit (LEO) satellite has been launched for this purpose. However, with a single satellite, it takes an inefficient 3-day period to provide the worldwide connectivity. On the other hand, similar to how the Iridium system functions in classic communication, satellite constellation (SC) composed of many quantum satellites, could provide global real-time quantum communication. In such a SC, most of the satellites will work in sunlight. Unfortunately, none of previous ground testing experiments could be implemented at daytime. During daytime, the bright sunlight background prohibits quantum communication in transmission over long distances. In this letter, by choosing a working wavelength of 1550 nm and developing free-space single-mode fibre coupling technology and ultralow noise up-conversion single photon detectors, we overcome the noise due to sunlight and demonstrate a 53-km free space quantum key distribution (QKD) in the daytime through a 48-dB loss channel. Our system not only shows the feasibility of satellite based quantum communication in daylight, but also has the ability to naturally adapt to ground fibre optics, representing an essential step towards a SC-based global quantum network.