Ulrik L. Andersen

Quantum key distribution (QKD) stands as the most successful application of quantum information science, providing information-theoretic security for key exchange. While it has evolved from proof-of-concept experiments to commercial products, widespread adoption requires chip-based integration to reduce costs, enable mass production, facilitate miniaturization, and enhance system performance. Here, we demonstrate the first fully photonic-integrated continuous-variable QKD (CVQKD) system operating at a classical telecom symbol rate of 16 GBaud. Our system integrates a silicon photonic transmitter circuit (excluding the laser source) and a 20 GHz photonic-electronic receiver, which features a phase-diverse silicon photonic integrated circuit and custom-designed GaAs pHEMT transimpedance amplifiers. Advanced digital signal processing allows our system to achieve the highest reported secure key rate to date, reaching 0.289 Gb/s and 0.246 Gb/s over a 20 km fiber link in the asymptotic and finite-size regimes, respectively. These results establish a record key rate and represent a critical step toward scalable, cost-effective, and mass-deployable quantum-secure communication using photonic-integrated CVQKD systems.

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.

Integrating quantum key distribution (QKD) with classical data transmission over the same fiber is crucial for scalable quantum-secured communication. However, noise from classical channels limits QKD distance. We demonstrate the longest-distance continuous-variable QKD (CVQKD) over 120 km (20 dB loss) coexisting with a fully populated coarse wavelength division multiplexing system. Natural mode filtering of the local oscillator and phase noise mitigation enabled this without additional filtering or wavelength reallocation. Benchmarking against a commercial discrete-variable QKD system and considering finite-size effects confirms the feasibility of CVQKD as a plug-and-play solution for typical 80–100 km long-haul optical networks. Our results set a record distance for CVQKD, showing its potential for cost-effective, large-scale deployment in existing network infrastructure.

We present the results of our experimental polarisation diverse continuous variable quantum key distribution system operating over 20km SMF at 100 Mbaud, implemented using a 10 kHz laser as a free running local oscillator. A composable finite size key is achieved with 200 million states.

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.

We propose a long-distance device-independent conference key agreement (DI-CKA) protocol. We use an efficient GHZ state distribution protocol based on entanglement swapping. We calculate a key rate of our protocol from violation of a multipartite Bell inequality and show that our protocol can distribute a secret key over longer distance than a direct transmission DI-CKA protocol. We also consider practical displacement-based measurement and show experimental feasibility of our protocol.

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.

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.

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.

This work experimentally investigates the impact of residual phase noise on CVQKD systems using phase profiles obtained through simulated Wiener phase processes and experimental measurements, and compares the experimental measurements to the theoretical calculation.

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.