Protocol for heralding microwave-optical resource states from superconducting quantum processors

Utilizing the strength of different quantum hardware components may enhance the capabilities of emerging quantum processors. We propose an architecture that leverages the non-local connectivity of optics and the exquisite control in superconducting quantum circuits to produce resource states for quantum computing, communication and sensing. Our proposal uses squeezing between microwaves and optics to produce heralded microwave-optical Bell pairs in a dual-rail encoding from a microwave quantum processor. The microwave quantum processor facilitates deterministic entanglement of microwave-optical Bell pairs into larger cluster states, from which entangled optical photons can be extracted through microwave measurements. By entangling only a few microwave-optical Bell pairs into a resource state, our architecture enables the construction of a modular, fusion-based quantum computer, in which each module is a small superconducting processor inside a typical dilution refrigerator. We analyze the performance of heralded Bell-pair generation using noisy transducers and identify regimes where the performance approaches the requirements of known fault-tolerant quantum instruments.