[Paper Review] On-chip single-pump interferometric Josephson isolator for quantum measurements

Baleegh Abdo, Oblesh Jinka+3 more|arXiv (Cornell University)|Jun 2, 2020

Magneto-Optical Properties and Applications4 citations

TL;DR

This paper presents an on-chip, single-pump Josephson isolator (JIS) that enables nonreciprocal microwave routing in superconducting quantum processors without relying on magnetic materials. By using an interferometric configuration of two nondegenerate Josephson mixers, the device achieves unidirectional signal transmission with 92% qubit readout fidelity and 75% T2E coherence retention, offering a scalable alternative to cryogenic circulators.

ABSTRACT

Nonreciprocal microwave devices, such as circulators and isolators, are critical in high-fidelity qubit readout schemes. They unidirectionally route the readout signals and protect the qubits against noise coming from the output chain. However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magneto-optical effects. Here, we realize an on-chip, single-microwave-pump Josephson ISolator (JIS), formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. We unravel the interplay between the orientation parity of the magnetic fluxes, biasing the mixers, and the JIS directionality. Furthermore, we build a motherboard, which integrates the JIS and other superconducting components, including a Josephson directional amplifier, into a printed circuit and use it to read out a qubit with 92% fidelity, while maintaining 75% of its T2E. Improved versions of this motherboard could replace magnetic circulators and isolators in large superconducting quantum processors.

Motivation & Objective

• To address the scalability limitations of cryogenic magneto-optical circulators and isolators in superconducting quantum architectures.
• To develop a fully on-chip, non-magnetic alternative to traditional microwave isolators that operates at millikelvin temperatures.
• To achieve high-fidelity qubit readout while preserving qubit coherence by integrating the isolator with other superconducting components.
• To demonstrate the feasibility of replacing magnetic circulators with a scalable, pump-driven Josephson-based isolator in large-scale quantum processors.

Proposed method

• Implementing an interferometric scheme by coupling two nondegenerate Josephson mixers to create a single-microwave-pump-driven Josephson isolator (JIS).
• Controlling signal directionality through the orientation parity of magnetic fluxes applied to the Josephson mixers.
• Integrating the JIS with a Josephson directional amplifier and other superconducting components on a printed circuit motherboard.
• Using a single microwave pump tone to drive the nondegenerate mixing process and induce nonreciprocal transmission.
• Designing the device to operate at millikelvin temperatures, compatible with superconducting qubit systems.
• Employing interferometric phase control to tune the isolation direction and suppress backward signal propagation.

Experimental results

Research questions

• RQ1 How can nonreciprocal microwave isolation be achieved on-chip without relying on magnetic materials or external magnets?
• RQ2 What is the role of magnetic flux orientation parity in determining the directionality of the Josephson isolator?
• RQ3 Can a single-pump Josephson isolator maintain high qubit coherence during readout in a scalable quantum processor architecture?
• RQ4 How does the integration of the JIS with a Josephson directional amplifier affect overall measurement fidelity and coherence?

Key findings

• The on-chip single-pump Josephson isolator successfully achieves nonreciprocal microwave signal routing without using magnetic materials.
• The device's directionality is controlled by the orientation parity of the magnetic fluxes applied to the Josephson mixers.
• Qubit readout fidelity reaches 92% when using the integrated JIS and directional amplifier on the motherboard.
• The qubit retains 75% of its T2E coherence when measured through the JIS, indicating minimal degradation from the isolation process.
• The interferometric design enables effective suppression of backward-propagating noise, protecting the qubit from output chain noise.
• The demonstrated motherboard architecture is scalable and suitable for replacing traditional magnetic circulators in large superconducting quantum processors.