Waveguide Quantum Electrodynamics with Superconducting Slow-Light Waveguide Circuits

Citation

Ferreira, Vinicius Thaddeu dos Santos (2022) Waveguide Quantum Electrodynamics with Superconducting Slow-Light Waveguide Circuits. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/y4vk-a827. https://resolver.caltech.edu/CaltechTHESIS:05312022-105606161

Abstract

Waveguide quantum electrodynamics (QED) refers to the study of quantum emitters (qubits) coupled to a single mode waveguide - a 1D electromagnetic reservoir with a continuum of states. This paradigmatic quantum-optical system can serve as a test-bed for experimental investigations in many-body physics, quantum non-linear optics, reservoir engineering, non-Markovian physics, quantum networks, and quantum computing. While such a system can be realized in a variety of physical platforms, superconducting quantum circuits are well suited to the study of waveguide QED due their readily available strong light-matter interaction strengths.

Of particular interest is the ability to tailor the dispersion relation and modal properties of the waveguide beyond that of a conventional waveguide with linear dispersion. For example, through periodic modulation of the geometry of a waveguide, or through the fabrication of an array of coupled resonant elements, novel electromagnetic responses can be engineered. These include spectral constriction of the 1D continuum to a transmission band of finite bandwidth, enhanced or suppressed emission rates of quantum emitters into the waveguide that are dependent on their frequencies, and extreme slowing of the velocity of light. Such attributes of dispersive waveguides can be leveraged to substantially enrich the physics and applications of qubit-waveguide systems.

In this thesis, we demonstrate the design, fabrication, and characterization of a slow-light waveguide (SLWG) comprised of an array of coupled lumped-element superconducting microwave resonators, and present on various experiments involving superconducting transmon qubits coupled to the SLWG. We investigate the physics of a qubit strongly coupled to the SLWG reservoir by tuning its frequency across the passband of this waveguide, where we find substantial changes to the qubit emission rate, along with oscillatory energy relaxation of the qubit resulting from the beating of bound and radiative dressed qubit-photon states. Further, upon addition of a reflective boundary to one end of the waveguide, we observe revivals in the qubit population on a timescale 30 times longer than the inverse of the qubit's emission rate, corresponding to the round-trip travel time of an emitted photon.

In addition, we show how we leveraged the ability to induce this non-Markovian time-delayed feedback via the SLWG's long delay to generate multidimensional cluster states of itinerant microwave photonic qubits. By utilizing the SLWG as a delay line with 240 ns round-trip delay, a single flux-tunable transmon qubit as a quantum emitter, and a second auxiliary transmon as a switchable mirror, we achieve rapid, shaped emission of entangled photon wavepackets, and effect time-delayed feedback within the waveguide between previously emitted photons and the emitter qubit. We leverage these capabilities to generate a 2D cluster state of four photons with 70% fidelity, as verified by tomographic reconstruction of the quantum state. We conclude by discussing directly realizable novel follow-up experiments that involve a continuously driven qubit in the presence of time-delayed feedback, and discuss how our cluster-state generation scheme could be straightforwardly extended to generation of even larger multidimensional cluster states, thereby enabling utilization of such states for quantum information processing techniques in the microwave domain.

Thesis Files