Enhancing the Capabilities of Superconducting Quantum Processors with Microwave Quantum Networks*

Superconducting electronic circuits are ideally suited for studying quantum physics and its applications. Since complex circuits containing hundreds or thousands of elements can be designed, fabricated, and operated with relative ease, they are one of the prime contenders for realizing quantum computers. Large-scale universal fault-tolerant quantum computers operated using quantum-error-corrected logical qubits will likely require millions of physical qubits. Operating this many physical qubits will benefit from a modular approach to realizing quantum computers. In this talk I will present work in which we explore approaches to network modular superconducting quantum processors using microwave-frequency quantum channels. We previously used such channels to transfer quantum states and create entanglement between two physically separated modules in a single cryostat [1] and between modules housed in two cryostats over meter-scale distances [2]. In a recent experiment, we have operated a coherent microwave quantum channel to deterministically entangle a pair of superconducting qubits across 30 meters [3]. Performing fast, and high-fidelity qubit readout along randomly chosen bases we have demonstrated a loophole-free violation of Bell’s inequality with superconducting circuits. We have evaluated a CHSH-type Bell inequality for more than one million experimental trials and determined an average S-value of 2.0747 ± 0.0033, violating Bell’s inequality by more than 22 standard deviations [3]. Our work demonstrates that non-locality is a viable new resource in networked quantum information processors based on superconducting circuits. This work also points at the potential of creating general-purpose microwave-quantum networks for applications in quantum computing, communication, and sensing, and for fundamental physics.

[1] P. Kurpiers et al., Nature 558, 264–267 (2018)

[2] P. Magnard et al., Phys. Rev. Lett. 125, 260502 (2020)

[3] S. Storz et al., Nature 617, 265-270 (2023)