# Transmission lines and resonators based on quantum Hall plasmonics: electromagnetic field, attenuation and coupling to qubits

## TL;DR

This paper analyzes quantum Hall edge states as low-loss, high-impedance microwave transmission lines and resonators, demonstrating their potential for strong coupling with solid-state qubits through a microscopic and numerical approach.

## Contribution

It provides a detailed microscopic and numerical analysis of quantum Hall-based transmission lines and resonators, highlighting their suitability for qubit coupling.

## Key findings

- High impedance of quantum Hall resonators enhances qubit coupling.
- Numerical estimates suggest achievable strong photon-qubit coupling.
- A simple formula for the Q factor of quantum Hall resonators is proposed.

## Abstract

Quantum Hall edge states have some characteristic features that can prove useful to measure and control solid state qubits. For example, their high voltage to current ratio and their dissipationless nature can be exploited to manufacture low-loss microwave transmission lines and resonators with a characteristic impedance of the order of the quantum of resistance $h/e^2\sim 25\mathrm{k\Omega}$. The high value of the impedance guarantees that the voltage per photon is high and for this reason high impedance resonators can be exploited to obtain larger values of coupling to systems with a small charge dipole, e.g. spin qubits. In this paper, we provide a microscopic analysis of the physics of quantum Hall effect devices capacitively coupled to external electrodes. The electrical current in these devices is carried by edge magnetoplasmonic excitations and by using a semiclassical model, valid for a wide range of quantum Hall materials, we discuss the spatial profile of the electromagnetic field in a variety of situations of interest. Also, we perform a numerical analysis to estimate the lifetime of these excitations and, from the numerics, we extrapolate a simple fitting formula which quantifies the $Q$ factor in quantum Hall resonators. We then explore the possibility of reaching the strong photon-qubit coupling regime, where the strength of the interaction is higher than the losses in the system. We compute the Coulomb coupling strength between the edge magnetoplasmons and singlet-triplet qubits, and we obtain values of the coupling parameter of the order $100\mathrm{MHz}$; comparing these values to the estimated attenuation in the resonator, we find that for realistic qubit designs the coupling can indeed be strong.