Abstract
In the diffraction-limited near-field propagation regime, free-space optical quantum key distribution (QKD) systems can employ multiple spatial modes to improve their key rate. This improvement can be effected by means of high-dimensional QKD or by spatial-mode multiplexing of independent QKD channels, with the latter, in general, offering higher key rates. Here, we theoretically analyze spatial-mode-multiplexed, decoy-state BB84 whose transmitter mode set is either a collection of phase-tilted, flat-top focused beams (FBs) or the Laguerre-Gaussian (LG) modes. Although for vacuum propagation the FBs suffer a QKD rate penalty relative to the LG modes, their potential ease of implementation make them an attractive alternative. Moreover, in the presence of turbulence, the FB modes may outperform the LG modes.
Highlights
Quantum key distribution (QKD) secures fiber and free-space optical (FSO) channels against the most powerful adversary allowed by physics [1,2,3]
After describing our system setup, we show that the orthogonality of the LG modes allows them to outperform flat-top focused beams in vacuum, the latter does capture a significant portion of the possible multiplexing gain
We show that even though turbulence increases the cross-talk between the focused beams and, degrades their QKD rate, they outperform the LG modes
Summary
Quantum key distribution (QKD) secures fiber and free-space optical (FSO) channels against the most powerful adversary allowed by physics [1,2,3]. In the far-field regime ( f ≪ 1) only one transmitter-pupil spatial mode couples significant power into the receiver pupil (with transmissivity 1 ≈ f) [4], precluding appreciable improvement in the achievable QKD rate from multiple orthogonal spatial modes. Each beam acts as its own channel These modes are not orthogonal even when propagated in vacuum, and the cross-talk between the overlapping beams limits the achievable QKD rate. We constrain their transmitter to a square pupil, their receiver to a 100%-fill-factor tiling of equal-area square pixels, and the protocol to decoy-state (DS) discrete-variable (DV) laser-light BB84 [19] This allows computation of a QKD rate-distance envelope for our flat-top FB array. We evaluate and compare the QKD rate attainable with these mode sets over vacuumpropagation and turbulent channels
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