Adnan A.E. Hajomer

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.

We develop a novel multi-user protocol and report the first continuous-variable quantum passive optical network (CV-QPON), that supports secure key generation for eight users simultaneously. This is achieved considering practical PON topology with an 11 km span of access links. Depending on the trust assumptions about users we reach 1.5 Mbits/s and 2.1 Mbits/s of total network key generation. Novel CV-QPON protocol exploits the multi-user nature of the network allowing to extend the network size and enhance individual keys, thus offering a pathway toward establishing low-cost, high-rate, and scalable quantum access networks using standard telecom technologies that directly benefits from the existing access network infrastructure.

We report the experimental demonstration of multi-user continuous-variable quantum key distribution based on a passive optical network (QPON) that supports se- cure key generation for 5 users simultaneously. This is achieved considering practical PON topology with an 11 km span of access links.

Measurement-device-independent (MDI) QKD removes all side-channel attacks on detectors. Continuous variable (CV) MDI-QKD based on coherent states is a promising candidate for integration into existing telecom infrastructure. Despite previous demonstrations of the concept and the potential for secure communication offered by CV MDI-QKD, a practical implementation of the system for real-world data encryption has yet to be achieved. Here, we introduce a simple and practical CV MDI-QKD system that can coexist with classical telecommunications channels. This is achieved through the use of a new relay structure, a real-time phase locking system and a well-designed digital signal-processing pipeline. Our design demonstrates the first practical CV MDI-QKD system, operating at a symbol rate of 20 MBaud and generating keys that are secure against collective attacks in both the finite-size and asymptotic regimes. This sets an important milestone towards in-field implementation and integration of high-performance CV MDI-QKD into telecom networks.

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.