Igor Konieczniak

Abstract A Two-Mode Squeezed Vacuum (TMSV) is a quantum resource proven useful in several aplications in Quantum Technology, one of them being Quantum Key Distribution (QKD). Here we report the building of a TMSV source for use in QKD. Our system will comprise of two OPO, with its squeezed vacuum outputs combined in a balanced beam splitters. Active controls are employed for cavities stabilization, squeezing phase lock and relative phase lock between squeezed fields. The new cavity for the first OPO was designed and is in operation. Our target is to obtain 13 dB of corrected squeezing for the amplitude quadrature and a combined Duan inequality violation of up to 10 dB. We will show the status and our more recent results towards those goals.

Continuous variable quantum key distribution (CV-QKD) uses shot-noise limited detection for measuring the quadratures of the signal sent by Alice over the quanutm channel. Typically, the signal transmission rate is limited by the bandwidth of the detection. One of the drawbacks of CV-QKD systems is low secure key generation rate at longer transmission distances. High bandwidth detectors—which allow higher signal transmission rates—cannot improve the key generation rate as these increase the electronic noise variance. Electronic noise variance in CV-QKD systems is considered as a trusted noise source and so theoretically, it does not have a great impact on the final key rate. However, from the practical point of view, there is impact on the performance of error correction codes—such as low density parity codes (LDPC). Increased electronic noise decreases the signalto-noise ratio (SNR). Constructing LDPC at lower SNR is a bottleneck for achieving long distance CV-QKD. Multi-mode signals can improve the SNR of CV-QKD system to a significant extent. In this work, we consider a multi-mode signal based CV-QKD system where signal modes are jointly measured in order to reduce the impact of electronic noise on the SNR.

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.