The Infosys Prize 2024 in Physical Sciences is awarded to Prof. Vedika Khemani, who has made wide ranging and groundbreaking contributions to theoretical and experimental non-equilibrium quantum matter, most notably the discovery of time-crystals.
Scope and Impact of Work
Prof. Vedika Khemani works in the area of non-equilibrium many-body physics. Recent years have witnessed breakthrough experimental developments in building well-isolated and highly controllable quantum devices from complex systems of many quantum entities (atoms, ions, photons, superconducting qubits etc.) While these developments are broadly motivated by the quest to build quantum computers, they are also exceptional new classes of physics experiments which give us controlled access to the dynamics of many-particle quantum systems. This is in sharp contrast to the “traditional” setting in many-body condensed matter physics, which studies phases of matter in nature in equilibrium.
These developments have opened a new subfield of physics, and have motivated a number of fundamental questions which are at the center of Prof. Khemani’s research: What types of novel phases are possible in non-equilibrium quantum systems? How do isolated quantum systems reach thermal equilibrium? How does hydrodynamics emerge in such quantum systems? Can quantum systems evade ergodicity and thermal equilibrium? How do systems transition from ergodic to localized behavior?
Prof. Khemani’s work has contributed to our theoretical understanding of these foundational questions. She has also collaborated extensively with experimental groups, including Prof. Immanuel Bloch’s group and the Google Quantum AI team, to study novel non-equilibrium phenomena in quantum experiments - including many-body localization, pre-thermalization, anomalous hydrodynamics, measurement-driven entanglement phase transitions and time-crystals.
One of Khemani’s notable results is on the topic of Hilbert Space Fragmentation. Conventional conservation laws divide phase space into “sectors” with different values of the conservation law. Likewise, a large part of the physics of many-body localization comes from the emergence of unusual local conservation laws that continue to exist under small perturbations. In several papers, Prof. Khemani has studied novel mechanisms for non-ergodicity by which the dynamics of a system breaks up into a large number of sectors for reasons other than conservation laws.
In the interest of brevity only the time crystal will be discussed in any detail because it is the most fascinating and easiest to describe. Can a quantum system break time-translation symmetry and show time-dependent oscillations forever, the same way that ordinary crystals break spatial translation symmetry by forming a lattice starting with the continuum? A pendulum clock seems to be an answer, because it breaks the time continuum into periods of length T. The problem is that a pendulum bob - when viewed as a many-particle system (comprised of all the atoms in the bob) will eventually reach equilibrium and come to rest by equipartition of energy. Indeed, theorems have established that the laws of thermodynamics rule out time-crystals as an equilibrium phase. On the other hand, Prof. Khemani’s work showed how this phase can be realized in the intrinsically non-equilibrium setting of a periodically driven many-body localized system. The time crystal oscillates at a period equal to twice that of the driving field, never heats up, and being a non-equilibrium state to begin with, can go on forever. The key to this is many-body localization. The absence of heating and robust spatiotemporal order, not just at one point in parameter space but over a range of parameters and from all initial states is the signature of this phenomenon.
The theoretical formulation of this work was laid out in a PRL paper led by Khemani. In a subsequent Nature paper in collaboration with Google’s AI group, Vedika Khemani and collaborators demonstrated an experimental realization of the time-crystal: a system of spins flips back and forth between two many-body localized states, no matter which state it starts from.
Bio
Vedika Khemani is a theoretical physicist and Associate Professor at Stanford University. Her research lies at the intersection of condensed matter physics and quantum information theory. Prof. Khemani obtained her Ph.D. at Princeton University in 2016, advised by Prof. Shivaji Sondhi. Following that, she was a Junior Fellow at the Harvard Society of Fellows and then joined the faculty at Stanford University, earning tenure in four short years.
Khemani’s work has been recognized by several fellowships and awards: a Sloan Fellowship (2020), the Willam L. McMillan Award (2020), a Packard Fellowship (2021), and the APS George E. Valley Jr. Prize (2021). For her work on time crystals, she won the Breakthrough New Horizons Prize in 2022.
Today Prof. Khemani mentors her own group of talented students and postdocs. She is known for her collaborative research style, with broad interests spanning several subfields, and for her ability to engage deeply with experimental groups in pursuit of new phenomena.
Prof. Khemani has spoken at over 100 international conferences including at the American Physical Society, German Physical Society, Aspen Center for Physics, International Centre for Theoretical Physics (ICTP), Max Planck Institute, Kavli Institute for Theoretical Physics (KITP), International Centre for Theoretical Sciences (ICTS), and the Solvay Conference (2022). She has organized numerous workshops, and has lectured at the Cargese, Les Houches, Boulder, Princeton IAS, and ICTP summer schools.
Timeline
Jury Citation
Prof. Vedika Khemani has made extensive contributions to the broad area of non-equilibrium many-body systems. With her unique command of theory and track-record of meaningful collaborations with experimentalists, she has managed to spearhead their symbiosis, attacking and solving a wide array of problems from both ends.
Prof. Khemani has investigated fundamental theoretical issues such as thermalization and the hydrodynamic description of quantum many-body systems. She and her collaborators showed how to unify traditional hydrodynamics and a recently elegant explanation of how a closed quantum system manages to dissipate. Another landmark contribution of Khemani is the discovery of a new mechanism for ergodicity breaking, now called Hilbert space fragmentation.
Vedika Khemani’s work has led to a deeper understanding of the many-body-localized (MBL) phase and most importantly in realizing that periodically driven MBL systems provide a paradigm for realizing time crystals. She theoretically formulated this phase, and then collaborated with the scientists at Google to use their quantum processor to provide a working model of a time-crystal.
Whereas in the earlier part of her career she was collaborating with senior physicists on equal terms, Prof. Khemani has now turned to guiding the next generation of physicists who flock to her as students and postdocs.
Vedika Khemani is a powerful force for diversity by wordlessly setting a shining example. In addition to being an outstanding physicist, she has remarkable pedagogical skills. She is a widely sought-after speaker at summer schools, international conferences, and as the organizer of workshops around the world. In recognition of her far-reaching contributions in the field of driven quantum system from theory to experiment, and the dissemination of knowledge to a huge variety of audiences, the Infosys panel of jurors is pleased to recognize this once-in-a-generation physicist with the Infosys Prize 2024 in Physical Sciences.
Vedika Khemani
Congratulations Prof. Vedika Khemani! Your work addressed a fundamental topic in physics: crystals. Ordinary crystals are periodic in space and last forever. For a long time, physicists have wondered about the temporal equivalent, time crystals. With your deep theoretical knowledge and a keen awareness of practicality you teamed up with experimentalists and demonstrated time crystals—systems which are periodic in time and lasting forever. You have been a real transfer agent between theory and experimental work. The jury panel noted that not only your work has been fundamental contribution to physics but also your unique approach to fundamental quantum mechanics. On behalf of the jury panel I congratulate you on being the winner of the Infosys Prize 2024 in Physical Sciences.
