Stefano Pirandola

The security of prepare-and-measure satellite-based quantum key distribution (QKD), under restricted eavesdropping scenarios, is addressed. We particularly consider cases where the eavesdropper, Eve, has limited access to the transmitted signal by Alice, and/or Bob’s receiver station. For instance, Eve can only receive an attenuated version of the transmitted signals. This results in settings where an uncharacterized bypass channel, inaccessible to Eve, can also carry signals to Bob. We obtain generic bounds on the key rate in the presence of bypass channels and apply them to continuous-variable QKD protocols with Gaussian encoding as well as to the family of BB84 protocols. We find regimes of operation in which the above restrictions on Eve can considerably improve system performance. Our work opens up new security frameworks for spaceborne quantum communications systems.

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.

Abstract Quantum communication is an important branch of quantum information science, promising unconditional security to classical communication and providing the building block of a future large-scale quantum network. Noise in realistic quantum communication channels imposes fundamental limits on the communication rates of various quantum communication tasks. It is therefore crucial to identify or bound the quantum capacities of a quantum channel. Here, we consider Gaussian channels that model energy loss and thermal noise errors in realistic optical and microwave communication channels and study their various quantum capacities in the energy-constrained scenario. We provide improved lower bounds to various energy-constrained quantum capacities of these fundamental channels and show that higher communication rates can be attained than previously believed. Specifically, we show that one can boost the transmission rates of quantum information and private classical information by using a correlated multi-mode thermal state instead of the single-mode thermal state of the same energy.