Cosmo Lupo

Quantum key distribution (QKD) promises everlasting security based on the laws of physics. Most common protocols are grouped into two distinct categories based on the degrees of freedom used to carry information, which can be either discrete or continuous, each presenting unique advantages in either performance, feasibility for near-term implementation, and compatibility with existing telecommunications architectures. Recently, hybrid QKD protocols have been introduced to leverage advantages from both categories. In this work we provide a rigorous security proof for a protocol introduced by Qi in 2021, where information is encoded in discrete variables as in the widespread Bennett Brassard 1984 (BB84) protocol but decoded continuously via heterodyne detection. Security proofs for hybrid protocols inherit the same challenges associated with continuous-variable protocols due to unbounded dimensions. Here we successfully address these challenges by exploiting symmetry. Our approach enables truncation of the Hilbert space with precise control of the approximation errors and lead to a tight, semi-analytical expression for the asymptotic key rate under collective attacks. As concrete examples, we apply our theory to compute the key rates under passive attacks, linear loss, and Gaussian noise.

Emerging communication and cryptography applications call for reliable, fast, unpredictable random number generators. Quantum random number generation allows for the creation of truly unpredictable numbers thanks to the inherent randomness available in quantum mechanics. A popular approach is using the quantum vacuum state to generate random numbers. While convenient, this approach was generally limited in speed compared to other schemes. Here, through custom co-design of opto-electronic integrated circuits and side-information reduction by digital filtering, we experimentally demonstrated an ultrafast generation rate of 100 Gbit/s, setting a new record for vacuum-based quantum random number generation by one order of magnitude. Furthermore, our experimental demonstrations are well supported by an upgraded device-dependent framework that is secure against both classical and quantum side-information and that also properly considers the non-linearity in the digitization process. This ultrafast secure random number generator in the chip-scale platform holds promise for next generation communication and cryptography applications.

Continuous-variable quantum key distribution offers a practical way for doing secure key exchange by means of broadband modulators and coherent detectors operating in the telecom band. Recent advances in theory and practice have improved the security and eased the system implementation. These include composable security with a finite number of distributed Gaussian-modulated coherent states and the use of pilot/reference signals and a real local oscillator for sharing the phase reference across the communicating parties. Here we report the first prepare-and-measure continuous-variable quantum key distribution experiment that can produce composable keys in the finite-size regime with security against collective attacks. Through novel improvements in the existing security proofs and a fast, yet low-noise and highly stable system operation, we obtain a secret key rate $>$5 Mbps over a 20 km long fiber channel. Our demonstration verifies the security of practical continuous-variable quantum key distribution when used for encryption or other cryptographic tasks.

Quantum enhanced receivers are endowed with resources to achieve higher sensitivities than conventional technologies. For application in optical communications, they provide improved discriminatory capabilities for multiple non-orthogonal quantum states. In this work, we propose and experimentally demonstrate a new decoding scheme for quadrature phase-shift encoded signals. Our receiver surpasses the standard quantum limit and outperforms all previously known non-adaptive detectors at low input powers. Unlike existing approaches, the receiver only exploits linear optical elements and on-off photo-detection. This circumvents the requirement for challenging feed-forward operations that limit communication transmission rates and can be readily implemented with current technology.

Symmetry plays a fundamental role in the security analysis of quantum key distribution (QKD). Here we review how symmetry is exploited in continuous-variable (CV) QKD to prove the optimality of Gaussian attacks in the finite-size regime. We then apply these results to improve the efficiency, and thus the key rate, of these protocols. First we show how to improve the efficiency and practicality of the energy test, which is one important routine aimed at establishing an upper bound on the effective dimensions of the otherwise infinite-dimensional Hilbert space of CV systems. Second, we show how the routine of parameter estimation can be made resource efficient in measurement-device independent (MDI) QKD. These results show that all the raw data can be used both for key extraction and for the routines of energy test and parameter estimation. Furthermore, the improved energy test does not require active symmetrization of the measured data, which can be computationally demanding.