Quantum Dynamics

Thermalization and its Alternatives

Advances in trapped neutral atoms, ions, & polar molecules, superconducting- and quantum-dot qubits, and ultra-fast optical techniques have given physicists a remarkable toolkit to start exploring the quantum coherent dynamical properties of interacting quantum systems driven far from thermal equilibrium. Our groups research aims to understand the universal "laws" governing far-from-equilibrium quantum dynamics. These questions are particularly important in isolated quantum systems, such as qubit arrays, where there is no coupling to the external environment and the familiar rules of thermodynamics and statistical physics need not apply!

A common fate of isolated quantum systems with generic interactions and many-particles is that they have enough internal sources of dissipation to act as their own heat bath, even when isolated from their environment. Understanding the dynamics of this process, known as "thermalization" has revealed new insights into the quantum mechanical underpinnings of thermodynamics, established deeper understanding of ergodicity and quantum chaos, and exposed intriguing connections to the quantum mechanics of black-holes and holography.

While common, thermalization is not inevitable. A dramatic example is many-body localization (MBL), in which random disorder potentials pin the excitations responsible for carrying heat and causing dissipation. MBL systems never reach thermal equilibrium, and exhibit quantum coherent dynamics on very long time scales.

Our group aims to understand how localization occurs, by studying the phase transition between MBL and thermalizing systems in a variety of contexts. We also examine how non-thermalizing systems can exhibit dynamical quantum phases and critical points even at infinite (microcanonical) temperature! To this end, we develop both new types of analytic and numerical methods to model non-equilbirium dynamics, and try to propose and design new experiments to test these predictions in experimental systems.

A big question going forward is: are there other types of universal fates besides thermalization and localization?

Driven quantum phases

Driving a quantum system with time-dependent forces can lead to new types of quantum dynamical phenomena that are impossible in static, equilibrium settings. The simplest types of drive are are periodically repeating, or "Floquet" drives. By developing new analytic tools for understanding the dynamics of quantum correlations and entanglement in Floquet systems, theorists, including our group, have recently uncovered a number of intriguing dynamical quantum phenomena, from "Floquet time-crystal" phases that spontaneously break the time-translation symmetry of their underlying drive, to Floquet topological phases that enable new robust ways of dynamically protecting and manipulating quantum information.

Our group aims to develop new theoretical tools to characterize exotic new phases and behaviors in driven quantum systems, designs experiments to realize and test their properties, and investigates ways to harness their unique capabilities for quantum information processing and quantum computing applications. We also explore other types of time dependent drives, from quenches, to quasi-periodic driving, and explore what types of new dynamical phenomena can arise outside the Floquet setting.