Universal stabilization of a parametrically coupled qubit

TL;DR

This paper presents a method for stabilizing any qubit state using parametric modulation of a tunable coupling between a superconducting qubit and resonator, achieving high fidelity and fast state exchange.

Contribution

It introduces a tunable coupler design enabling both static and dynamic control of qubit-resonator coupling for universal qubit state stabilization.

Findings

Achieved qubit coherence times over 20 μs across the tuning range.

Demonstrated stabilization of arbitrary qubit states with fidelity exceeding 80%.

Enabled single photon exchange in 6 ns through dynamic coupling modulation.

Abstract

We autonomously stabilize arbitrary states of a qubit through parametric modulation of the coupling between a fixed frequency qubit and resonator. The coupling modulation is achieved with a tunable coupler design, in which the qubit and the resonator are connected in parallel to a superconducting quantum interference device. This allows for quasi-static tuning of the qubit-cavity coupling strength from 12 MHz to more than 300 MHz. Additionally, the coupling can be dynamically modulated, allowing for single photon exchange in 6 ns. Qubit coherence times exceeding 20 $\mu$s are maintained over the majority of the range of tuning, limited primarily by the Purcell effect. The parametric stabilization technique realized using the tunable coupler involves engineering the qubit bath through a combination of photon non-conserving sideband interactions realized by flux modulation, and direct qubit Rabi driving. We demonstrate that the qubit can be stabilized to arbitrary states on the Bloch sphere with a worst-case fidelity exceeding 80 %.