Nitin Jain

Quantum key distribution (QKD) is a well-known application of quantum information theory that guarantees information-theoretically secure key exchange. While QKD systems are becoming commercially available, large-scale deployment of next-generation QKD systems requires photonic and electronic devices that are low-cost, small, and easily integrated with existing network infrastructure. Continuous variable (CV) QKD is a promising option for large-scale deployment due to its compatibility with standard telecom technology. Despite this, the secret key rates of CV-QKD systems have been limited to a few megabits per second due to the bandwidth bottleneck of the receiver and the limited symbol rate of the transmitter. Here, we present the first discrete-modulated coherent state CV-QKD system operating at a classical telecom symbol rate of 10 GBaud. This system generates keys at rates exceeding 0.7 Gb/s over a distance of 5 km and 0.3 Gb/s over a distance of 10 km while being secure against collective attacks in both the asymptotic and finite-size regimes. This is made possible by using a high-speed, co-integrated phase-diverse receiver consisting of a silicon photonics optical front-end and a custom-designed integrated transimpedance amplifier. Additionally, well-engineered digital signal processing is used for quantum state preparation and measurement. Our experiment sets a new record for secure quantum communication and paves the way for the next generation of CV-QKD systems.

Squeezed states of light promise significant advantages for enhancing the performance of continuous-variable quantum key distribution (CV-QKD) systems. These advantages include the ability to reach longer distances, tolerate higher levels of excess noise, and operate at lower information reconciliation efficiency. So far those advantages were only predicted in theory. In this work, we experimentally demonstrate a CV-QKD system over 40 km fibre using squeezed light achieving a secret key rate of 0.0318 bits per channel use, surpassing the equivalent coherent state system. Similar to state-of-the-art coherent state QKD systems our system employs digital signal processing for impairment compensation eliminating the need for complex locking mechanisms and enhancing its suitability for practical implementations.

Quantum key distribution (QKD) enables two remote parties to share encryption keys with security based on physical laws. Continuous variable (CV) QKD based on coherent states and coherent detection is a suitable scheme for integration into existing telecom networks. However, thus far, long-distance CV-QKD has only been demonstrated using a highly complex transmitted local oscillator scheme, opening security loopholes for eavesdroppers and limiting its potential applications. Here, we report a long-distance CV-QKD experiment with a locally generated local oscillator over a 100 km fiber channel. This record-breaking distance is enabled by controlling the phase-noise component of excess noise, using a machine-learning framework for carrier recovery and optimizing the modulation variance. We consider the full CV-QKD protocol implementation and demonstrate the generation of keys secure against collective attacks in asymptotic and finite-size regimes. Our results set an essential milestone for CV quantum access networks realization, where a high loss budget is required, and pave the way for large-scale deployment of secure QK.

Advances in the security analysis of continuous-variable quantum key distribution (CVQKD) protocols with true discrete modulation aim to unlock the same performance as that obtained from `traditional' protocols based on Gaussian modulation. We report a CVQKD experiment using 4 states that utilizes a composable security proof to generate a secret key fraction of $5.6 \times 10^{-3}$ bits/symbol over 10 km channel, while providing security against collective attacks.