Coherent control of microwave pulse storage in superconducting circuits

Engineering Symmetry-Selective Couplings of a Superconducting Artificial Molecule to Microwave Waveguides

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 waveguides in our device. In addition, we implement a 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.

Quantum Chaos Driven by Long-Range Waveguide-Mediated Interactions

We study theoretically quantum states of a pair of photons interacting with a finite periodic array of two-level atoms in a waveguide. Our calculation reveals two-polariton eigenstates that have a highly irregular wave function in real space. This indicates the Bethe ansatz breakdown and the onset of quantum chaos, in stark contrast to the conventional integrable problem of two interacting bosons in a box. We identify the long-range waveguide-mediated coupling between the atoms as the key ingredient of chaos and nonintegrability. Our results provide new insights in the interplay between order, chaos, and localization in many-body quantum systems and can be tested in state-of-the-art setups of waveguide quantum electrodynamics.

Phase control of reservoir engineering for quantum entanglement

It is shown that the reservoir engineering can be controlled by the collective phase Φ of three coherent fields interacting with a closed Δ-type atom. We find that the atomic system acts as a one-channel dissipation reservoir when Φ = 0(π), but it behaves as a two-channel dissipation reservoir for Φ ≠ 0(π). The phase-dependent reservoir engineering provides a convenient way to produce robust two-mode squeezing and entanglement, which may find potential applications in quantum information processing.