Talks

In the first part of my talk, I will discuss anomalous subdiffusive phases that appear in clean long-range fermionic lattice systems and their origin. I will then talk about how such a long-range lattice, when subjected to dephasing noise that acts at all lattice sites, shows an interesting crossover from super-diffusive to diffusive transport regime as one tunes the long-range hopping exponent.

In this plenary lecture, I will discuss various aspects of seasonal and decadal monsoon forecasting for the Indian monsoon. In the initial years, the seasonal forecasts for the Indian summer monsoon rainfall (ISMR) are based on statistical methods using predictors representing the monsoon teleconnections. In 2017, the India Meteorological Department (IMD) introduced the statistical ensemble forecasting system, which was used by IMD for operational forecasts. With the rapid advancements in coupled modelling strategies and tools, the seasonal forecasts for monsoon rainfall are now produced using coupled ocean-atmosphere models. In 2021, IMD introduced multi-model ensemble forecasts based on different dynamical models. These models have shown more skill compared to the current statistical models. However, the current seasonal forecast skill is still far below the potential predictability. The ways in which the seasonal forecasts can be further improved will be discussed. Since the monsoon precipitation exhibits significant multi-decadal variability, attempts are being made to predict the decadal variability of the monsoon precipitation. However, recent results suggest that the decadal prediction for south Asian monsoon region is currently not very skilful.

Spontaneous symmetry breaking underpins some of the most important phenomena in condensed matter and statistical physics. A description of direct transitions between symmetry breaking phases in terms of local order parameters is formulated when the symmetry breaking patterns were Landau-compatible i.e. when the unbroken symmetries of one phase is a subset of the other. About twenty years ago, Senthil et al [1] demonstrated that a direct transition between Landau-incompatible symmetry breaking phases was also possible in two-dimensional quantum magnets. Such 'deconfined quantum critical' (DQC) transitions are believed to be exotic and found in interacting quantum systems, often with anomalous symmetries (e.g.: constrained by Lieb-Schultz-Mattis theorems).

In this talk, based on recent work with N. Jones [2], I will demonstrate that such special conditions are unnecessary and Landau-incompatible transitions can be found in a well-known family of classical statistical mechanical models introduced by Jose, Kadanoff, Kirkpatrick and Nelson [3]. All smoking-gun DQC features such as charged defect melting and enhanced symmetries are present and readily understood. I will also show that a closely related family of models also exhibits another unusual critical phenomenon found in quantum systems- 'unnecessary criticality' where a stable critical surface exists within a single phase of matter analogous to the first-order line separating liquid and gases.

[1] SCIENCE, Vol 303, Issue 5663
[2] arXiv: 2404.19009
[3] Phys. Rev. B 16, 1217 (1977)
 

One of the first nontrivial examples of quantum matter to be understood at equilibrium was the behavior of a chain of two-state spins, or qubits, entangled by nearest-neighbor interactions. Hans Bethe’s solution of the ground state in 1931 eventually led to the concept of Yang-Baxter integrability, and the thermodynamics were fully understood in the 1970s. However, the dynamical properties of this spin chain at any nonzero temperature remained perplexing until some unexpected theoretical and experimental progress beginning around 2019. Atomic emulators and quantum computers are beginning to complement solid-state quantum magnetism experiments, and computer scientists, physicists, and mathematicians all have their own reasons to care about the dynamics of simple arrangements of quantum spins. The last part of the talk covers how dynamics of more complicated spin models in higher dimensions are being used to search for emergent gauge fields and other phenomena.

The long-range force between neutrinos is poorly constrained. In the late-time universe, a long-range force that is a few orders of magnitude stronger than gravity can induce Jeans perturbation instability in the non-relativistic cosmic neutrino background, drastically changing its large-scale behavior. In this talk, I will describe how the cosmic neutrino background evolves and forms nonlinear bound states in the presence of a long-range force. I will then discuss the impact of these neutrino bound states on the matter structures in the universe, and the constraints due to the absence of these signals.

Deviations from a parity-symmetric Universe would strongly signal the presence of new physics beyond the Standard Model. The lowest-order scalar observable sensitive to parity in a homogeneous and isotropic Universe is the connected four-point function of a given cosmological field. Geometrically, this function is described by a suitable average of this field over many tetrahedral configurations. As the CMB represents a spherical projection of three-dimensional curvature fluctuations, its four-point function serves as a unique, but potentially limited, probe of this fundamental symmetry. In this talk, I will discuss progress in rigorously understanding the CMB’s sensitivity to parity-violating physics from both early- and late-time sources.

Compositions are fundamental to how we understand the world, but in the quantum realm, they reveal a deeper and more profound complexity. In composite quantum systems, intriguing phenomena such as Bell nonlocality, quantum entanglement, and quantum discord emerge—features entirely absent in classical systems. These nonclassical correlations are crucial for developing advanced information and communication protocols. In this talk, drawing from our recent works, I will explore foundational aspects of composition as they apply to quantum systems. I will also discuss new insights into the nonclassical correlations arising in these systems, and introduce a novel form of composition in the temporal domain, proposing it as a new primitive for information processing.
 

This talk will summarize our current thinking on the role of ocean-atmosphere coupling to intraseasonal oscillations including the Madden-Julian oscillation and its boreal summer counterpart. The importance of surface flux and SST variability for maintenance and propagation of intraseasonal oscillations will be examined, including insights derived from models. The issue of how coupling affects models simulations of intraseasonal variability will be examined, including whether it is mean state changes caused by the act of coupling versus the active role of coupling for improving the simulation of intraseasonal oscillations. Cross scale interactions between intraseasonal oscillations and other modes of coupled tropical variability will also be examined, including the Indian Ocean Dipole and El Nino-Southern Oscillation. An upcoming field program in the tropical Pacific to better understand some of these cross-scale interactions will also be highlighted.

Quantum resources have entered the many body stage over the last two decades. It is by now widely appreciated that entanglement plays a key role in characterizing physical phenomena, as diverse as topological order and critical behaviour. However, entanglement alone is not informative about state complexity, and in fact, it is only one side of the medal. In this talk, I will flip the coin and tackle quantum state complexity of many-body systems under the lense of non-stabilizerness - also known as magic. Magic quantifies the difficulty of realizing states in most error corrected codes, and is thus of fundamental practical importance. However, very little is known about its significance to many-body phenomena.

I will present method(s) to measure magic in tensor network simulations, and illustrate a series of applications to many body systems, including: (a) how state magic and long-range magic behave in conformal field theories - illustrating the limit of the former, and the capabilities of the latter; (b) how magic characterizes phases of lattice gauge theories, both in the context of spin liquids/error correction (toric code), and in the context of theories describing coupling between matter and light (Schwinger model); and (c) how our computational tools are presently more advanced than the largest scale experimental demonstration of magic in Rydberg atom quantum simulators.

Finally, I will discuss the broader impact of these findings on state complexity - indicating that realizing generic state quantum dynamics may require a very large amount of resources in error correcting quantum computers, but at the same time, providing interesting perspectives on new classes of variational states more powerful than tensor networks.