Talks

In recent years, ‘measurement-induced phase transitions’ (MIPT), have led to a new paradigm for dynamical phase transitions in quantum many-body systems. I will discuss a model of continuously monitored or weakly measured arrays of Josephson junctions (JJAs) with feedback. Using a variational self-consistent harmonic approximation, as well as analysis in the semiclassical limit, strong feedback and measurement limit, and weak coupling perturbative renormalization group, I will show that the model undergoes re-entrant superconductor-insulator MIPTs in its long-time non-equilibrium steady state as a function of measurement and feedback strength. I will contrast the phase diagram of monitored JJA with the well-studied case of dissipative JJA.
 

We discuss the entanglement between two critical spin chains induced by the Bell-state measurements, when each chain was independently in the ground state before the measurement. This corresponds to a many-body version of “entanglement swapping”. We employ a boundary conformal field theory (CFT) approach and describe the measurements as conformal boundary conditions in the replicated field theory. We show that the swapped entanglement exhibits a logarithmic scaling, whose coefficient takes a universal value determined by the scaling dimension of the boundary condition changing operator. We apply our framework to the critical spin-1/2 XXZ chain and determine the universal coefficient by the boundary CFT analysis, which is verified by a numerical calculation.

This talk is based on M. Hoshino, M. O., and Y. Ashida, arXiv:2406.12377
 

Measurement induced phase transitions (MIPT) occur when the natural entangling dynamics in many-body systems is overcome by persistent but random single particle measurements. The entanglement originates from two-qubit gates, and we consider circuits when this is fixed and the one qubit operations are random unitaries. This talk discusses how the entangling power and other local unitary invariants of special two-qubit gates modify the phase transition parameters with much more robust circuits possible than with typical gates. Apart from the usual bipartite entanglement, the possible relevance of some other characterizations of local entanglement structure in MIPT are also discussed. Entangling power, gate typicality and Measurement-induced Phase Transitions,

Based on: Sourav Manna, Vaibhav Madhok, Arul Lakshminarayan, arXiv:2407.17776
 

Climate Scientists are increasingly turning to Machine Learning to answer various questions in their domain. In this talk, we will discuss a few typical problems related to Indian Monsoon, and discuss ML-based approaches for them. Specifically, we will focus on multi-scale forecasting, downscaling, and attribution to large-scale drivers.

Many-body localization is one of several conceptual example of how an interacting system of many particles can fail to reach thermal equilibrium. This talk discusses the emerging understanding of systems that fail to thermalize, with a particular focus on quantum information quantities such as entanglement. The importance of entanglement as a constraint on classical computation is complemented by new approaches to measure entanglement in solid-state systems using old techniques such as neutron scattering. New experimental systems in quantum matter such as nitrogen vacancy centers in diamond are, at least on accessible time scales, neither localized nor conventionally thermalizing, and while simple phenomenological models seem to capture the physics in some cases, the reasons why such models work are so far not well understood.

Celestial Holography encompasses a decade-long endeavor to understand a flat space realization of the holographic principle starting from symmetries in the infrared.  But where does it fit within other attempts at constructing a flat hologram? This colloquium delves into some fun tensions in the literature and hopes for resolving them.

Abstract: Randomness is a powerful resource for information-processing applications. For example, classical randomness is essential for modern information security and underpins many cryptographic schemes. Similarly, quantum randomness can protect quantum information against noise or eavesdroppers who wish to access or manipulate that information. These observations raise a set of related questions: How quickly and efficiently can we generate quantum randomness? How much quantum randomness is necessary for a given task? What can we use quantum randomness for? In this talk, I address these questions using all-to-all Brownian circuits, a family of random quantum circuits for which exact results can often be obtained via mean-field theory. I will first demonstrate that all-to-all Brownian circuits form k-designs in a time that scales linearly with k. I will then discuss how these circuits can be applied to study Heisenberg-limited metrology and quantum advantage. In particular, I will discuss a time-reversal protocol that can achieve Heisenberg-limited precision in cavity QED and trapped ion setups; I will also discuss the application of these circuits to studying classical spoofing algorithms for the linear cross-entropy benchmark, a popular measure of quantum advantage.