Abstract
Experimental quantum key distribution through free-space channels requires accurate pointing-and-tracking to co-align telescopes for efficient transmission. The hardware requirements for the sender and receiver could be drastically reduced by combining the detection of quantum bits and spatial tracking signal using two-dimensional single-photon detector arrays. Here, we apply a two-dimensional CMOS single-photon avalanche diode detector array to measure and monitor the single-photon level interference of a free-space time-bin receiver interferometer while simultaneously tracking the spatial position of the single-photon level signal. We verify an angular field-of-view of 1.28° and demonstrate a post-processing technique to reduce background noise. The experimental results show a promising future for two-dimensional single-photon detectors in low-light level free-space communications, such as quantum communications.
Highlights
At the receiver interferometer’s output, a 30 mm focal length convergent lens focused the beam onto the 2D single-photon avalanche diode (SPAD) array plane, which created a focal spot of ~60 μm in diameter for the single-mode, non-turbulent, channel
It is worth noting that other designs of 2D SPAD arrays have independent pixel post-processing capability [55], but this was not deemed necessary for the experiments described in this paper
The combined detection capability is beneficial for a free-space single-photon level communications, such as quantum key distribution (QKD), alleviating the requirement for high-power optical alignment beacons, as the fine pointing and tracking detection could be performed using the single-photon level signal and a 2D SPAD array
Summary
The exchange of quantum bits via optical fiber or free-space links offers the potential of unconditionally verifiably secure quantum key distribution (QKD) for sharing encryption keys between two [1,2] or more [3] parties, unforgeable digital signatures [4,5], secure bit commitment [6], digital fingerprinting [7], oblivious transfer [8], and more. Since quantum repeaters are still far from a mature technology, the fastest route to global quantum networks will be to connect separate metropolitan networks via long distance free-space channels and trusted quantum satellite nodes [18]. Due to the unguided and turbulent nature of free-space channels, optical beacons and spatial position sensors are required to actively co-align the transmitter and receiver telescopes during communications, in order to minimize losses in the optical link [35]. We demonstrate the feasibility of simultaneous detection of the temporal information encoded in single-photon per clock-cycle level optical pulses as well as their spatial position for telescope pointing-and-tracking. We implement a free-space time-bin receiver interferometer with an optical relay to address wave-front distortion [36] and employ a two-dimensional (2D) siliconcomplementary metal–oxide–semiconductor (CMOS) single-photon avalanche diode (SPAD). Satellites in particular have a severely restricted SWAP allowance [37] and could greatly benefit from a rollout of future-generation CMOS SPAD arrays
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