Canonical quantisation of telegrapher's equations coupled by ideal nonreciprocal elements

TL;DR

This paper presents a systematic method for quantising Hamiltonians of transmission line networks with nonreciprocal elements, enabling better design of complex quantum circuits with broken time-reversal symmetry.

Contribution

It introduces a doubled flux-charge description for quantising nonreciprocal transmission line networks, resolving ambiguities and divergences in the Hamiltonian derivation.

Findings

Provides a unified quantisation framework for nonreciprocal networks.

Demonstrates the approach with a circulator-Josephson junction example.

Enhances the design capabilities for quantum circuits with nonreciprocal components.

Abstract

We develop a systematic procedure to quantise canonically Hamiltonians of light-matter models of transmission lines coupled through lumped linear lossless ideal nonreciprocal elements, that break time-reversal symmetry, in a circuit QED set-up. This is achieved through a description of the distributed subsystems in terms of both flux and charge fields. We prove that this apparent redundancy is required for the general derivation of the Hamiltonian for a wider class of networks. By making use of the electromagnetic duality symmetry in transmission lines (waveguides), we provide unambiguous identification of the physical degrees of freedom, separating out the nondynamical parts. This doubled description can also treat the case of other extended lumped interactions in a regular manner that presents no spurious divergences, as we show explicitly in the example of a circulator connected to a Josephson junction through a transmission line. This theory enhances the quantum engineering toolbox to design complex networks with nonreciprocal elements.