Gerald Bonner

Quantum Key Distribution (QKD) over a free-space channel is challenging due to inefficient coupling of received signals to the detection system. A narrow detector cross-section, such as with fibre coupling to a telescope, reduces the field of view (FOV) and thus increases the loss due to atmospheric turbulence. A larger detector cross-section increases the FOV; however, the corresponding increase in background noise also increases the QBER associated with a Discrete Variable (DV) QKD system. On the contrary, Continuous Variable (CV) QKD systems are highly tolerant to background noise, but their performance is still limited by the channel loss. The FOV of the receiver is defined as the area expressed in solid angle from which the detector can accepts signal. A simple geometrical analysis reveals FOV as $\Theta = 2\tan^{-1}(d/2F)$, where $d$ is the detector diameter and $F$ is the effective focal length of the receiver telescope. With a large-area detector, one can design the signal collection optics, such that the FOV of the telescope with a given aperture can be larger. In this work, we present a larger FOV receiver system for CVQKD which reduces the channel loss due to beam wandering and atmospheric turbulence. We will describe the performance of the receiver system in terms of shot-noise sensitivity, loss reduction and enhancement in secure key rate, compared to a typical fibre-coupled receiver system.