Talks

CondMat for Dummies: Many-body quantum chaos

In classical mechanics, the initial state of a system uniquely determines its state at a later time. The system is defined as chaotic if it shows an exponentially large sensitivity to initial conditions, and the phenomenon is termed (deterministic) chaos. Chaos is ubiquitous in nonlinear dynamical systems and thus central to the foundations of statistical mechanics. A quantum system's chaotic nature is diagnosed by the statistical properties of its energy spectrum. These statistical properties are universal for chaotic systems and are captured in a mathematical formalism called random matrix theory. It is unclear why a physical chaotic system behaves like a random matrix with no microscopic structure. Further, physical systems are divided into three classes based on the presence or absence of time reversal symmetry. Random matrix theory reveals three different forms of spectral correlations for chaotic systems. We present a unified description explaining the emergence of random matrix behavior in chaotic systems from each of the three classes. We consider a generic form of Hamiltonian describing chaotic periodically kicked many-body systems. We analytically compute a measure of spectral correlation known as spectral form factor in the presence or absence of time reversal symmetry and show that it matches the results from random matrix theory in the ergodic phase.

Specificity and tolerance of the immune T cell repertoire

The adaptive immune system protects the body from an ever-changing landscape of foreign pathogens. The two arms of the adaptive immune system, T cells and B cells, mount specific responses to pathogens by utilizing the diversity of their receptors, generated through hypermutation. T cells recognize and clear infected hosts when their highly variable receptors bind sufficiently strongly to antigen-derived peptides displayed on a cell surface. To avoid auto-immune responses, randomly generated receptors that bind strongly to self-peptides are eliminated in the “central" process of thymic selection, ensuring a mostly self-tolerant repertoire of mature T cells. “Peripheral” tolerance, including a quorum mechanism further protects against self-targeting T cells that escape thymic selection. We discuss how these mechanisms can still fail during persistent infections.

Topological mechanics

In this talk, I will discuss a few incarnations and implications of topology in low-dimensional (classical) physical and biological systems - from artificial muscle fibres to topological robotics, from fast(!) anacondas to stiff bird nests, from reconstructive surgery of skin to kirigami and the singing saw.

Multiscale methods and microswimmer models

Swimming on the microscale has long been the subject of intense research efforts, from experimental studies of bacteria, sperm, and algae through to varied theoretical questions of low-Reynolds-number fluid mechanics. The biological and biophysical settings that drive this ongoing research are often confoundingly complex, a fact that has driven the development and use of simple models of microswimmers. In this talk, we will showcase how we can often exploit separated scales present in these problems and models to reveal surprisingly simple emergent dynamics, including predictions of globally attracting, long-term behaviours. In doing so, we'll also uncover a surprising cautionary tale that calls into question much of the intuition gained from commonplace models of microswimming. In particular, we'll see that a wave-of-the-hands, which I have been guilty of before, can drastically and qualitatively change the dynamics that simple models predict, and we'll see how such missteps can be addressed through systematic multiscale methods.

Subexponential decay of local correlations from diffusion-limited dephasing

Chaotic quantum systems at finite energy density are expected to act as their own heat baths, rapidly dephasing local quantum superpositions. We argue that in fact this dephasing is subexponential for chaotic dynamics with conservation laws in one spatial dimension: all local correlation functions decay as stretched exponentials or slower. The stretched exponential bound is saturated for operators that are orthogonal to all hydrodynamic modes. This anomalous decay is a quantum coherent effect, which lies beyond standard fluctuating hydrodynamics; it vanishes in the presence of extrinsic dephasing. Our arguments are general, subject principally to the assumption that there exist zero-entropy charge sectors (such as the particle vacuum) with no nontrivial dynamics: slow relaxation is due to the persistence of regions resembling these inert vacua, which we term "voids". In systems with energy conservation, this assumption is automatically satisfied because of the third law of thermodynamics.

Quantum Dynamics Seminar: tba

Bright squeezed vacuum for strong-field physics

Strongly pumped parametric down-conversion produces a state of light that, on the one hand, is as intense as laser light, and on the other hand, has pronounced quantum features. This state, known as bright squeezed vacuum, has zero mean electric field and consists only of quantum fluctuations, enhanced or suppressed on a subcycle scale. Recently, we have used this state to drastically modify the dynamics of strong-field effects, such as non-perturbative harmonics generation and electron ejection from needle tips.

Exotic metals, fractionalization, and quantum criticality

The quest for novel states of matter is important both on fundamental grounds and in view of possible applications, with superconductivity and the various quantum Hall effects being outstanding examples. This talk will summarize recent developments in the field, with an emphasis on the effects on frustration and intrinsic topological order. I will highlight frustration-based routes to novel forms of order and disorder, non-Fermi liquid metals and exotic superconductivity, and I will discuss aspects on quantum phase transitions between the various phases. Connections to experiments on kagome and pyrochlore metals as well as cuprate high-temperature superconductors will be made.

Colloquium

Colloquium