Probing the Tavis-Cummings level splitting with intermediate-scale superconducting circuits

TL;DR

This paper demonstrates local control and measurement of up to eight superconducting qubits interacting with a microwave cavity, confirming the square root scaling of collective coupling strength as predicted by the Tavis-Cummings model.

Contribution

It introduces a method for controlling and reading out multiple superconducting qubits simultaneously, with calibration techniques to mitigate interference effects, advancing intermediate-scale quantum simulation.

Findings

Observed collective coupling strength scaling with square root of qubit number.

Achieved control of up to eight qubits with individual frequency tuning.

Identified limitations due to reduced signal visibility and off-resonance shifts.

Abstract

We demonstrate the local control of up to eight two-level systems interacting strongly with a microwave cavity. Following calibration, the frequency of each individual two-level system (qubit) is tunable without influencing the others. Bringing the qubits one by one on resonance with the cavity, we observe the collective coupling strength of the qubit ensemble. The splitting scales up with the square root of the number of the qubits, which is the hallmark of the Tavis-Cummings model. The local control circuitry causes a bypass shunting the resonator, and a Fano interference in the microwave readout, whose contribution can be calibrated away to recover the pure cavity spectrum. The simulator's attainable size of dressed states with up to five qubits is limited by reduced signal visibility, and -- if uncalibrated -- by off-resonance shifts of sub-components. Our work demonstrates control and readout of quantum coherent mesoscopic multi-qubit system of intermediate scale under conditions of noise.