#### Center for Quantum Sensors

#### Axion Dark Matter Experiment (ADMX)

#### Quantum Circuits

The mixing chamber stage of our dilution refrigerator. |

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 are focused on understanding and controlling open quantum systems. These are quantum systems that necessarily interact with their environment, a process that can be deleterious to the quantum properties of the system, but can also be harnessed for control and to induce desired dynamics. In studying these systems, we hope to develop new ways of using precision quantum measurement to study novel phases of condensed matter, prepare quantum states, and probe chemical and biological systems.

Superconducting qubits are a promising system for the realization of quantum schemes for computation, simulation, and data encryption. While the fabrication of these systems allows for exquisite control over the properties of the quantum systems, their complex material nature results in coupling to uncontrolled degrees of freedom in the surrounding environment, eventually leading to decoherence of some states of these systems. Our research focuses on engineering the quantum system-environment interaction to preserve coherence, to prepare complex many body states, and to create interfaces with atomic systems such as cold neutral atoms, trapped ions, and solid state spins such as nitrogen vacancy centers in diamond.

##### Superconducting qubits

A SQUID (superconducting quantum interfering device) shunted with a capacitor forms transmon qubit. |

Superconducting qubits are artificial atoms that are formed from low-loss superconducting circuits that use Josephson junctions as a source of nonlinearity. These artificial atoms are powerful systems for studying fundamental questions in quantum mechanics because many properties of the system, including how it couples to its environment can be engineered precisely.

##### Quantum Thermodynamics

Thermodynamics is a field of physics which describes quantities such as heat and work and their relationship to entropy and temperature. Originally developed as a theory to optimize the efficiency of heat engines, two extensions of thermodynamics in the last century advanced the theory to the point at which quantum mechanics should be incorporated. First, the identification of the role of information in thermodynamics makes strong connections between heat, entropy and information. Second, extensions of thermodynamics to the realm of microscopic systems in which fluctuations are significant allow the application of thermodynamics at the level of single trajectories of classical particles. Quantum mechanics requires both of these features as information and fluctuations are central to the behavior of quantum systems. The experimental control over single quantum systems that has been achieved in this century places us in a unique position to extend thermodynamics into the quantum regime. Understanding quantum thermodynamics will be increasingly important as quantum machines become more complicated and as classical machines are further miniaturized. Our experimental work utilizes microscopic superconducting circuits as artificial atoms and the interaction of these atoms with microwave light to control the atom's environment.

##### Time symmetry in quantum mechanics

We can
track the evolution of the quantum state by calculating the trajectory for a density matrix. For separate iterations of the experiment, they trajectories behave differently. We can also consider the evolution of backwards propagated trajectories, given by the matrix E. Are these trajectories the time reverse of the forward propagated trajectories? |

The laws of quantum mechanics that govern the atomic and molecular building blocks of our physical world are fundamentally time symmetric. What then allows us to conclude that time moves forward rather than backwards? Clues to this puzzle have long been thought to reside in the process of wave function collapse: a quantum system can be initialized in a superposition of states but will collapse randomly into a classically allowed state if it is measured. Our recent experimental innovations allow us to examine quantum measurement as a continuous process, revealing the evolution induced by measurement. An open question in continuously measured systems is the presence of time reversal symmetry in this process. The persistence of forward/backward symmetry in quantum measurement allows for new tests of causality which can be constructed with time reversed quantum feedback loops. The purpose of this research is to investigate the origins and degree of time symmetry in continuously measured quantum systems, and to examine the extent to which quantum systems and measurements validate or question the notion of determinism.

##### Synthesizing artificial quantum materials

Researchers create novel forms of matter using exceptionally pure ingredients. Examples range from materials formed from cold atoms trapped in uniform optical potentials to controlled deposition of ultra-pure single-crystal materials. While this research enables studies of novel quantum phenomena, it also allows the creation of macroscopic quantum systems that themselves can serve as building blocks for even more exotic materials. Breakthroughs in superconducting circuit based "artificial atoms" have recently enabled studies that address fundamental questions in atomic and optical physics that have eluded experiments with natural atoms for decades. Here, we explore a new class of quantum materials that are constructed out of superconducting artificial atoms with engineered interactions and quantum environments.