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

Tailoring the decay rate of structured quantum emitters into their environment opens possibilities for quantum-state stabilization, amplification, and nonlinear quantum optics. 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) parity are predominantly coupled to one (the other) waveguide. In a first-generation device, we achieve a selectivity of 2 orders of magnitude for the coupling rates. 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 parity in the single-excitation manifold of the molecule. Using that process, we implement frequency conversion across the waveguides, mediated by the molecule, with efficiency in excess of 90%. 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.