Photon Shot Noise Dephasing in the Strong-Dispersive Limit of Circuit QED

TL;DR

This paper investigates photon shot noise dephasing in superconducting transmon qubits within the strong-dispersive regime, confirming theoretical models and showing how cavity parameters influence qubit coherence.

Contribution

It provides experimental verification of pure dephasing mechanisms due to photon shot noise and demonstrates control over dephasing rates by tuning cavity properties.

Findings

Dephasing rate matches theoretical predictions.

Increasing cavity decoupling enhances coherence times.

Intrinsic coherence exceeds several hundred microseconds with Hahn echo.

Abstract

We study the photon shot noise dephasing of a superconducting transmon qubit in the strong-dispersive limit, due to the coupling of the qubit to its readout cavity. As each random arrival or departure of a photon is expected to completely dephase the qubit, we can control the rate at which the qubit experiences dephasing events by varying \textit{in situ} the cavity mode population and decay rate. This allows us to verify a pure dephasing mechanism that matches theoretical predictions, and in fact explains the increased dephasing seen in recent transmon experiments as a function of cryostat temperature. We investigate photon dynamics in this limit and observe large increases in coherence times as the cavity is decoupled from the environment. Our experiments suggest that the intrinsic coherence of small Josephson junctions, when corrected with a single Hahn echo, is greater than several hundred microseconds.