Charge qubits in the USC regime for quantum state transfer

TL;DR

This paper explores achieving ultrastrong and deep-strong light-matter coupling with superconducting charge qubits, demonstrating enhanced quantum state transfer capabilities in noisy environments, and advancing the understanding of extreme coupling regimes.

Contribution

It introduces a method to reach USC and DSC regimes exceeding classical bounds using a shunted charge qubit, and proposes a mediator system for efficient quantum state transfer in noisy conditions.

Findings

Maximal USC and DSC regimes surpass classical bounds.

Efficient quantum state transfer with mediator in USC regime.

Comparable transfer times without requiring state pre-selection.

Abstract

We study the feasibility of reaching the ultrastrong (USC) and deep-strong coupling (DSC) regimes of light-matter interaction, in particular at resonance condition, with a superconducting charge qubit, also known as Cooper-Pair box (CPB). We numerically show that by shunting the charge qubit with a high-impedance LC-circuit, one can maximally reach both USC and DSC regimes exceeding the classical upper bound $|g|\leq \sqrt{\omega_q\omega_r}/2$ between two harmonic systems with frequencies $\omega_q$ and $\omega_r$. As an application, we propose a hybrid system consisting of two CPBs ultrastrongly coupled to an LC-oscillator as a mediator device that catalyzes a quantum state transfer protocol between a pair of transmon qubits, with all the parties subjected to local thermal noise. We demonstrate that the QST protocol maximizes its efficiency when the mediator operates in the USC regime, exhibiting comparable times with proposals relying on highly coherent and controllable mediators requiring state-preparation or post-selection. This work opens the door for studying light-matter interactions beyond the quantum Rabi model at extreme coupling strengths, providing a new building block for applications within quantum computation and quantum information processing.