We are an experimental research group that conducts research at the interface of Atomic, Molecular, and Optical (AMO) and condensed matter physics. We use nano-fabrication techniques to construct superconducting quantum circuits that allow us to probe fundamental questions in quantum mechanics. Quantum optics focuses on the interaction of light with matter and condensed matter physics is concerned with macroscopic phenomena resulting from the interaction between microscopic components. The goal is to use light, and more broadly, the electromagnetic environment to mediate these interactions in a controllable way resulting in new materials, new phenomena and a deeper understanding of physics. The light-matter interaction forms the basis for measurement in quantum mechanics and we seek to examine how measurement can control quantum systems and elucidate the boundary between quantum and classical measurement.
We currently have four primary research directions in the lab, 1) exploration of open quantum system dynamics in the context of non-Hermitian Hamiltonians, 2) investigation of fundamental questions in quantum thermodynamics and quantum energetics, 3) work advancing quantum sensors and quantum sensing, and 4) development of a novel circuit QED quantum memory based on the spin states of single electrons trapped on solid neon.
Non-Hermitian Dynamics
The quintessential light matter interaction is the radiative decay of an atom by spontaneous emission. To mathematically describe the time evolution of such radiative decay one typically uses a master equation treatment for a density operator ρ. The density operator in this case reflects a lack of knowledge about the state of the system; the interaction entangles the atom with the modes of the environment and in the absence of direct monitoring of such modes, the state of the atom is mixed. A wavefunction description of the atomic evolution can be restored by introducing a trick known as quantum jumps—that the atom abruptly jumps from excited to ground state at a random time. Under this description, the lone effect of the dissipative coupling to the environment enters the dynamics as an effective non-Hermitian Hamiltonian. We have developed an experimental method to selectively isolate this non-Hermitian evolution, allowing us to study, for the first time, the evolution of a quantum eigenstates of this non-Hermitian Hamiltonian.
Non-Hermitian Hamiltonians exhibit degeneracies known as Exceptional Points (EPs), where both the eigenvalues and eigenvectors coalesce. The complex energies in the vicinity of such EPs are described by Riemann manifolds, and the topology of these manifolds enables new methods for control of quantum states. We are currently exploring how adiabatic operations on the parameters of these non-Hermitian Hamiltonians enables fundamentally new levels of control over quantum systems. Future work will characterize topologically protected geometric phases arising from this control. We will also extend these techniques to the synthesis of non-Hermitian quantum materials by creating arrays of superconducting artificial atoms.
Quantum thermodynamics
Thermodynamics plays a pivotal role in our understanding of physical systems, accurately describing the behavior of macroscopic systems consisting of an enor- mous number of constituents ranging from gases and liquids to superconductors and black holes. There is growing interest in extending the laws of thermodynamics to the quantum regime, where quantum coherent effects are present. In the past few years, my group has become one of the lead- ing experimental groups performing research in this new area. We performed the first experimental study of a fully quantum realization of Maxwell’s demon; a case where the demon uses the physics of weak measurement and quantum trajectories to make measurements of a particle without projecting it onto eigenstates, thereby elucidating the role of quantum coherence in thermodynamics. Our currently funded work will explore quantum heat engines, where quantum measurement and information is harnessed as a type of fuel. Just as the study of engines was instrumental in the formation of classical thermodynamics, the extension to quantum information engines will be key to understanding the role of quantum measurement, coherence, entanglement and backaction in quantum thermodynamics.
Quantum sensors
An ultimate goal of our research into quantum measurement is to apply our techniques and understanding to other areas of physics. The LIGO collaboration currently utilizes squeezed light as a routine technical tool to lower the noise floor of the detector. Our vision is to extend the use of quantum resources into many areas of science, bringing the scientific community into the second quantum revolution. To facilitate these advances, together with other members of the Washington University community, we founded the Center for Quantum Leaps, which is serving as a hub for collaboration and innovation.
We are also closely involved with an international collaboration searching for dark matter. The ADMX collaboration searches for an Axion—a dark matter candidate-using a microwave frequency cavity in a large magnetic field. We have been working on design and operation of superconducting quantum amplifiers, that both reduce the noise level in the detection chain, but can also be used to squeeze the quantum vacuum—similar to what is employed by the LIGO collaboration—to further reduce the noise floor. As we search through higher mass ranges, and correspondingly higher frequencies, the experiment will become more challenging because the quantum fluctuations scale with the frequency. In this case, photon detection becomes the preferred measurement approach. We are planning to apply the technologies developed for quantum computers to create error corrected microwave photon detectors with very low dark count rates and high quantum efficiency.
Electrons on neon
The two spin states of a single electron are the paradigmatic example of a qubit. We aim to create a long-lived quantum memory based on single electrons that are trapped in vacuum near a solid crystal of neon in a cryogenic environment. The purpose of the neon crystal is to form a one dimensionally confining potential along the direction perpendicular to the crystal surface. This is achieved through the combination of the polarizability of the crystal, which produces a positive image charge attracting the electron to the surface, and the positive electron affinity of neon, which prevents the electron from being accepted onto the crystal surface. The combination of these two effects provides tight axial confinement of an electron, allowing static electrodes to provide confinement in the other two directions. By tuning this radial confinement, the motional quanta of the resulting “charge qubit” can have energy scales in the GHz range, compatible with superconducting qubit quantum computing architectures. The system bears a similarity to spin qubits, where electrons are isolated from a two-dimensional electron gas, with the exception that the electron-on-neon architecture traps the electron in vacuum, resulting in high coherence. This qubit is also similar to the long-sought electrons on helium architecture, but with the advantage of a solid-rather than superfluid-trapping substrate.
First demonstrations of this electron-on-neon qubit in the circuit quantum electrodynamics architecture resulted in strong coupling of the qubit to a superconducting microwave resonator allowing initialization, control, and readout. Using qubit control methods from circuit quantum electrodynamics we observed an energy relaxation coherence time of 15 microseconds. We expect that further refinements will lead to dramatic improvements. The purpose of this research thrust is torealize a long-lived quantum memory based on the spin states of the electron (rather than the motional degrees of freedom), which are expected to have coherence times measured in seconds, an unfathomably dramatic advance for solid state quantum architectures. The resulting quantum memory would naturally be compatible with superconducting qubit processor technology since they both require similar superconducting structures and cryogenic environments.