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.
A team of researchers at IBM Quantum, USA, has realized an on-chip, single-microwave-pump Josephson ISolator (JIS), formed by coupling two non-degenerate Josephson mixers in an interferometric scheme.
They unraveled the interplay between the orientation parity of the magnetic fluxes, biasing the mixers, and the JIS directionality. Furthermore, they built 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.
The paper can be found here.