10 October 2023

Title: Security of Discrete-Modulated Continuous-Variable Quantum Key Distribution

Speaker: Dr. Stefan Baeuml, Institute of Photonic Sciences

Abstract of Talk: Continuous variable quantum key distribution with discrete modulation has the potential to provide quantum physical security using widely available optical elements and existing telecom infrastructure.  While their implementation is significantly simpler than that for protocols based on Gaussian modulation, proving their finite-size security against coherent attacks poses a challenge.  In this work we apply the entropy accumulation theorem, a tool that has previously been used in the setting of discrete variables, to prove finite-size security against coherent attacks for a discrete-modulated quantum key distribution protocol involving four coherent states and heterodyne detection.  To do so, and contrary to previous approaches, we consider a protocol in which all the information is discretized.  We first bound its asymptotic rate under a realistic photon number cutoff assumption.  This bound is then upgraded into a finite-size security proof using entropy accumulation.  [The talk is based on the recent work https://arxiv.org/abs/2303.09255.]

Profile of Speaker: Stefan Baeuml obtained his Ph.D. at the University of Bristol in the United Kingdom, under the supervision of Andreas Winter.  His Ph.D. thesis is focused on quantum repeaters and bound entanglement.  A main result has been an upper bound on the achievable key rate in a quantum repeater that can become arbitrarily small if bound entanglement is used as initial resource for the repeater.  He then moved to the group of Bill Munro at NTT Basic Research Labs in Japan as a postdoc, focusing on upper bounds on key rates achievable in general quantum network scenarios, as well as capacities for quantum channels with multiple inputs and outputs.  After continuing his work on quantum networks during another postdoc position in the group of Stephanie Wehner and David Elkouss at Delft University of Technology in the Netherlands, he moved to the group of Antonio Acin at the Institute of Photonic Sciences (ICFO) in Barcelona, Spain.  His current research at ICFO is focused on security proofs for discrete modulated continuous variable quantum key distribution.