May 2021

Abstracts of the QSIT Lunch Seminar, Thursday, May 6, 2021

Scheduled Zoom meeting

Implementation and network integration of high repetition rate QKD systems

Fadri Grünenfelder – Quantum Technologies (Zbinden group), University of Geneva

With the advent of the quantum computer, currently used asymmetric cryptosystems can be broken. Therefore, an alternative solution for private communication is needed. Quantum key distribution (QKD) enables sharing of secret keys over a distance which can be used in the One-Time-Pad to allow private communication even if an adversary has access to a quantum computer. Although the first QKD protocol was proposed already more than 35 years ago and was proven to be information-theoretically secure, there are still open questions both on the theory and implementation of QKD systems. In this talk, we present one of our state-of-the-art QKD systems. First, we recapitulate the working principle of QKD. Then we discuss the integration of a QKD system in existing optical ber networks. While classical communication signals are launched with power levels of the order of milliwatts, the power of QKD signals are typically below tens of picowatts. Therefore, already weak scattering or cross-talk of the classical signal can degrade the quantum signal. Nevertheless, it is possible to use QKD in a network environment. We demonstrate the coexistence of a QKD system with time-bin encoding at 2.5 GHz repetition rate and classical telecom signals in the same fiber.

A Luttinger Liquid coupled to Ohmic-class environments

Andisheh Khedri – Electronic and photonic quantum engineered systems (Zilberberg group), ETH Zurich

We investigate the impact of an Ohmic-class environment on the conduction and correlation properties of one-dimensional interacting systems. Interestingly, we reveal that inter-particle interactions can be engineered by the environment's noise statistics. Introducing a backscattering impurity to the system, we address Kane-Fisher's metal-to-insulator quantum phase transition in this noisy and realistic setting. Within a perturbative renormalization group approach, we show that the Ohmic environments keep the phase transition intact, while sub- and super-Ohmic environments, modify it into a smooth crossover at a scale that depends on the interaction strength within the wire. The system still undergoes a metal-to-insulator-like transition when moving from sub-Ohmic to super-Ohmic environment noise. We cover a broad range of realistic experimental conditions, by exploring the impact of a finite wire length and temperature on transport through the system.