Saturable Purcell filter for circuit quantum electrodynamics

TL;DR

This paper proposes a saturable artificial atom-based Purcell filter in circuit QED that suppresses qubit decay during measurement, enabling high-fidelity control and measurement with a single transmission line, thus improving scalability.

Contribution

Introduction of a saturable artificial atom filter that suppresses Purcell decay and enables combined control and measurement in circuit QED systems.

Findings

Purcell decay is effectively suppressed by the filter.

High-fidelity Pauli σ_x gates are achievable with simple pulses.

The filter allows multiple qubits to be controlled and measured via a single line.

Abstract

We consider a typical circuit QED setup where an artificial atom encodes a qubit and is dispersively coupled to a measurement resonator that in turn is coupled to a transmission line. We show theoretically that by placing another artificial atom in this transmission line to act as a filter, the Purcell decay of the qubit into the transmission line is suppressed. When strong control fields are applied in the transmission line, the filter is saturated and effectively switched off. Such a Purcell filtering capability permits both the control and measurement of the qubit using the single transmission line, while maintaining the long coherence time of the qubit in the absence of the control pulses. We show that high fidelity Pauli $\sigma_x$ gates on the qubit can be realized using simple pulse shapes. For devices that already use one transmission line both for control and measurement of the qubit, our work provides a way to completely filter out the qubit frequency without removing the possibility of controlling the system. Further, combining the proposed filter with frequency multiplexing potentially enables both control and measurement of several qubits using a single Purcell-filtered transmission line. This will enhance the scalability of superconducting quantum processors by decreasing the number of the required transmission lines.