Yoann Piétri

Quantum communication is advancing toward large-scale quantum networks, with quantum key distribution (QKD) serving as a key driving technology. However, seamless interoperability between fiber-based and free-space links remains a major challenge for heterogeneous quantum networks. Here we report, to the best of our knowledge, the first continuous-variable QKD (CV-QKD) system distributing secret keys using both coherent and squeezed states over a hybrid channel composed of a 620m free-space link followed by 2km of optical fiber, corresponding to a total loss of 20 dB. Daylight operation is enabled by intrinsic mode filtering provided by a locally generated local oscillator, eliminating the need for complex spectral or spatial filtering. In addition, we introduce an optimized binning strategy that mitigates free-space transmittance fluctuations, resulting in an average of 45% increase in the secure key rate. These results demonstrate the feasibility of CV-QKD across hybrid optical channels and highlight its potential as a plug-and-play solution for heterogeneous quantum networks integrating fiber and free-space infrastructure.

Continuous‑Variable Quantum Key Distribution (CV‑QKD) and Quantum Random Number Generation (CV‑QRNG) are crucial technologies relying on shot‑noise‑limited coherent detection to enable secure communication and high‑speed randomness generation. Integrated photonics plays a central role in advancing these technologies, offering compact, scalable, and efficient implementations. In this work, we introduce Femtosecond Laser Micromachining (FLM) on borosilicate glass as a novel platform for Photonic Integrated Circuits (PICs) tailored to coherent detection in quantum information processing. Using off‑chip detectors, we exploit the versatility of FLM to realize a PIC designed for CV‑QKD and CV‑QRNG. The device features fully tunable optical components, low insertion loss ($\leq$ 1.28 dB), polarization‑insensitive operation, and a Common‑Mode Rejection Ratio (CMRR) exceeding 73 dB. These capabilities enable the experimental demonstration of a Source‑device‑Independent CV‑QRNG with a secure rate of 42.74 Gbps and a QPSK‑based CV‑QKD system achieving a 3.2 Mbit/s secret key rate. Our results establish FLM as a promising integrated‑photonics platform for scalable, high‑performance quantum communication systems.

Key exchange over an untrusted communication channel is a crucial step in modern cryptographic protocols, yet current approaches rely on computational assumptions to ensure their security. However, these assumptions may be undermined by an adversary possessing quantum computational capabilities. Quantum Key Distribution (QKD), both in its Discrete Variable (DV) and Continuous Variable (CV) format, is a way to counter this threat. By exploiting the principles of quantum mechanics, QKD enables the development of unconditionally secure protocols, providing a lasting solution that can withstand adversaries regardless of their computational resources. Between DV and CV, the latter is particularly appealing if speed and compatibility with the current fiber telecom infrastructure are considered. In fact, CV-QKD systems can benefit from coherent receivers and widely accessible fiber-based commercial telecom components. Despite this, several practical tasks must be addressed to enable such systems targeting field-deployed real-world scenarios. Among these are the integration of a Quantum Random Number Generator, real-time estimation of key system parameters necessary to compute the Secret Key Rate and network reconfigurability. This study presents a CV-QKD architecture integrating a coherent receiver at the transmitter, enabling secure quantum randomness generation and in-line estimation of key parameters for secret-key-rate computation, advancing the deployment over existing telecom networks.

Quantum Random Number Generators (QRNGs) are essential components for modern cryptography, as they are a practical source of true randomness, without which the security of such protocols cannot be guaranteed. Among the proposed schemes, Semi-Device-Independent QRNGs (SDI-QRNGs) based on photon-number constraints offer an appealing balance of security, speed, and experimental simplicity, but demonstrations have so far focused mainly on binary encoding and conditional min‑entropy certification. In this work, we report the first experimental implementation of a Continuous‑Variable SDI‑QRNG in a prepare‑and‑measure configuration building on the SDP‑based (Semi-Definite Programming) framework of Ref.~\cite{carceller2025}. This approach allows us to directly lower‑bound the conditional Shannon entropy, employ more complex modulation formats, and apply entropy‑accumulation techniques beyond the i.i.d. assumption. The experiment combines a low-loss integrated photonic heterodyne receiver with a simple transmitter built from commercial components, generating coherent‑state Quadrature-Phase-Shift-Keying (QPSK) modulation, to preserve experimental practicality while enabling high‑speed operation.

We report, to the best of our knowledge, the first experimental demonstration of wavelength-division multiplexing between independent continuous-variable (CV) and discrete-variable (DV) quantum key distribution (QKD) systems operating simultaneously on the same optical link. We validate coexistence over a fiber and a daylight free-space channel, benchmarking the Secret Key Rate (SKR) versus channel attenuation while both systems operate simultaneously. We observe the expected CV-DV complementarity, with CV-QKD providing higher SKR at low loss and DV-QKD becoming advantageous in the high-loss regime. In free-space daylight, both systems sustain Mbit/s key rates under atmospheric fluctuations. In all scenarios analyzed, simultaneous operation introduces negligible multiplexing-induced penalty. These results provide a validation of hybrid CV-DV architectures for heterogeneous quantum communication infrastructures, where high-throughput metropolitan users and long-reach links can be simultaneously served on the same physical channel.

Quantum Key Distribution (QKD) is a field with a potentially major impact on cybersecurity and telecommunications. QKD protocols allow two distant parties to share a secret key regardless of the capacities of an eavesdropper. Thanks to quantum physics laws, an attempt to measure the signal on a quantum channel will necessarily introduce a disturbance, thus an eavesdropper cannot go unnoticed. The first protocols studied used Discrete Variables (DV), but their implementation requires specific technology such as single-photon detectors. Protocols using so-called continuous variables (CV) allow for the use of standard telecommunication components. Recent studies show that high key rates can be achieved using CV-QKD. Exail coordinates the QKISS project, as part of the development of the European Quantum Communication Infrastructure (EuroQCI), together with Thales SIX, LIP6 (CNRS/Sorbonne Université) and Institut d'Optique (CNRS), with the aim of industrializing CV-QKD systems.We have explored different hardware and software configurations to reduce the excess noise and increase the secret key rate of our prototype system. Based on these studies, we have built a demonstrator, used to realize field tests. We are also inlvolved in different European projects, to concretize the European Quantum Communication Infrastructure.

Quantum Key Distribution (QKD) is arguably the most mature application of principles of quantum mechanics to cryptography, and several lab and field demonstrations have been realized. However the realization of QKD in deployed networks, with high distances and/or complex network architecture is still a challenge. Trusted nodes is a known solution to these issues, but requires the delegation of trust to third parties. Here, we propose a trusted node protocol where the requirements of trust delegation are lowered, with no overhead in the consumption of the key exchanged with QKD, allowing to keep the same secret key rate. This protocol is then applied to 2 links in the Parisian Quantum Network, composed of dark dedicated fibers between 8 nodes in the Parisian region, for a total fiber distance of 57 km. Our results show the overall key exchange with no degradation of the key rate.