Photon-mediated long range coupling of two Andreev level qubits

TL;DR

This paper demonstrates a novel long-range coupling method for Andreev level qubits using a microwave photon in a superconducting cavity, enabling scalable quantum architectures with entangled qubit states over six millimeters.

Contribution

It introduces a microwave photon-mediated coupling scheme for two Andreev level qubits, enabling long-distance entanglement and scalable quantum computing architectures.

Findings

Successful demonstration of remote ALQ coupling via microwave photon.

Observation of avoided-crossings in excitation spectra consistent with Tavis-Cummings model.

Identification of highly entangled two-qubit states over six millimeters.

Abstract

In a superconducting weak link, the supercurrent is carried by Andreev bound states (ABSs) formed by the phase-coherent reflection of electrons and their time-reversed partners. A single, highly transmissive ABS can serve as an ideal, compact two-level system, due to a potentially large energy difference to the next ABS. While the coherent manipulation of such Andreev levels qubits (ALQs) has been demonstrated, a long-range coupling between two ALQs, necessary for advanced qubit architectures, has not been achieved, yet. Here, we demonstrate a coherent remote coupling between two ALQs, mediated by a microwave photon in a novel superconducting microwave cavity coupler. The latter hosts two modes with different coupling rates to an external port. This allows us to perform fast readout of each qubit using the strongly coupled mode, while the weakly coupled mode is utilized to mediate the coupling between the qubits. When both qubits are tuned into resonance with the latter mode, we find excitation spectra with avoided-crossings, in very good agreement with the Tavis-Cummings model. Based on this model, we identify highly entangled two-qubit states for which the entanglement is mediated over a distance of six millimeters. This work establishes ALQs as compact and scalable solid-state qubits.