Engineering symmetry-selective couplings of a superconducting artificial molecule to microwave waveguides

TL;DR

This paper demonstrates a novel symmetry-selective coupling scheme between a superconducting artificial molecule and microwave waveguides, enabling efficient frequency conversion and entangled state generation for quantum information applications.

Contribution

The authors introduce a new coupling scheme that achieves high symmetry-based selectivity and enables coherent control and entanglement of microwave photons in superconducting circuits.

Findings

Coupling selectivity ratio exceeds 30 for both waveguides.

Achieved 95% efficiency in frequency conversion.

Generated spatially-separated Bell states.

Abstract

Tailoring the decay rate of structured quantum emitters into their environment opens new avenues for nonlinear quantum optics, collective phenomena, and quantum communications. Here we demonstrate a novel coupling scheme between an artificial molecule comprising two identical, strongly coupled transmon qubits, and two microwave waveguides. In our scheme, the coupling is engineered so that transitions between states of the same (opposite) symmetry, with respect to the permutation operator, are predominantly coupled to one (the other) waveguide. The symmetry-based coupling selectivity, as quantified by the ratio of the coupling strengths, exceeds a factor of 30 for both the waveguides in our device. In addition, we implement a two-photon Raman process activated by simultaneously driving both waveguides, and show that it can be used to coherently couple states of different symmetry in the single-excitation manifold of the molecule. Using that process, we implement frequency conversion across the waveguides, mediated by the molecule, with efficiency of about 95%. Finally, we show that this coupling arrangement makes it possible to straightforwardly generate spatially-separated Bell states propagating across the waveguides. We envisage further applications to quantum thermodynamics, microwave photodetection, and photon-photon gates.