SmallTalks: Computing with microwaves: building a bosonic quantum processor

Abstract 

Demonstrating quantum speed-up, meaning outpacing any classical machine at a well-defined task, is the outstanding goal of the quantum computing community. One route to it is boson sampling, which relies on the interference of many indistinguishable photons through a large network of modes, a process believed inefficient to simulate classically and hard even to verify.

Rather than using optical light, we build our system from superconducting circuits confining microwave photons, a type of boson, in high-quality cavities, forming so-called bosonic modes. These bosonic modes offer large Hilbert spaces and a hardware-efficient path to scalable processing, but only if the operations mixing the bosonic modes stay well controlled and linear even when the cavities are populated.

Retaining linearity in the operations is our central challenge and result. Using an asymmetrically threaded SQUID (ATS) biased where the dominant nonlinearities cancel, we realize linear beamsplitting between cavities and benchmark its fidelity at large photon numbers. We show how we build this multi-mode processor on a single chip, the beamsplitting we have demonstrated, and the path toward the multi-mode interference needed for boson sampling.