Quantum dial

TL;DR

The paper introduces a quantum dial device that dynamically controls the coupling of a qubit to its environment, enabling fast switching between isolation for coherence and strong coupling for control, thereby improving quantum processor performance.

Contribution

It presents the first implementation of a quantum dial using a band-stop filter to tune qubit-environment coupling on nanosecond timescales without significant Stark shifts.

Findings

Achieved >150 μs T1 in reset mode, reduced to ~200 ns

Demonstrated high-fidelity qubit control with 99.99% idle and 99.9% gate fidelities

Enabled sensitive thermometry of the qubit environment

Abstract

Accurate control of quantum degrees of freedom is promising for sensing, communication, and computing, but building a useful quantum computer faces a central isolation-and-control challenge: qubits must remain well isolated from their environment to preserve coherence, yet also be coupled strongly enough for control, readout, and reset. Existing approaches address this challenge only partially, using separate reset elements, drive schemes, and Purcell filters, often with added complexity and tradeoffs such as heating and crosstalk. Here we introduce and demonstrate a first-generation quantum dial: a device that on demand mediates the coupling of a qubit to selected auxiliary degrees of freedom. Our implementation uses a band-stop filter between a high-coherence transmon qubit and a broadband transmission line, enabling the coupling strength to be tuned by several orders of magnitude on nanosecond timescales without significant Stark shift. In the reset configuration, we reduce the qubit energy relaxation time T1 from >150 $\mu$s to about 200 ns and demonstrate reset independent of the initial state. In the control configuration, we obtain 99.99% idle fidelity and 99.9% gate fidelities for 40 ns pulses at about -110 dBm. The same device also enables thermometry of the qubit environment, reaching a noise-equivalent temperature of 0.6 mK/$\sqrt{Hz}$ at 60 mK and approaching the Cram\'er-Rao bound at higher temperatures. The quantum dial thus offers fast, on-demand switching between isolation and strong coupling, with potential to reduce noise and errors in future quantum processors.