Active protection of a superconducting qubit with an interferometric Josephson isolator

TL;DR

This paper introduces an active, non-magnetic Josephson isolator for superconducting qubits, enabling high-fidelity quantum measurements with enhanced noise protection, crucial for scalable quantum computing.

Contribution

The work presents a novel interferometric Josephson isolator that replaces magnetic-based isolators, improving scalability and integration in superconducting quantum processors.

Findings

Achieved over 20 dB noise protection in qubit readout.

Demonstrated fast, high-fidelity quantum nondemolition measurements.

Implemented a magnetic-free, active isolation scheme for superconducting circuits.

Abstract

Nonreciprocal microwave devices play several critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They separate input from output, impose unidirectional routing of readout signals, and protect the quantum systems from unwanted noise originated by the output chain. However, state-of-the-art, cryogenic circulators and isolators are disadvantageous in scalable superconducting quantum processors because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the Josephson mixers, which play the role of the magnetic field in a Faraday medium. To demonstrate the applicability of this Josephson-based isolator for quantum measurements, we incorporate it into the output line of a superconducting qubit, coupled to a fast resonator and a Purcell filter. We also utilize a wideband, superconducting directional coupler for coupling the readout signals into and out of the qubit-resonator system and a quantum-limited Josephson amplifier for boosting the readout fidelity. By using this novel quantum setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing more than 20 dB of protection against amplified noise reflected off the Josephson amplifier.