Talks

Tunable Matter -- Many More is More Different

In 1972 Phil Andersen articulated the motto of condensed matter physics as “More is different.” However, for most many-body systems the behavior of a trillion bodies is nearly the same as that of a thousand. Here I argue for a class of condensed matter, “tunable matter," in which many more is different. The ultimate example of tunable matter is the brain, whose cognitive capabilities increase as size increases from 302 neurons (C. Elegans) to a million neurons (honeybees) to 100 billion neurons (humans). I propose that tunable matter provides a unifying conceptual framework for understanding not only a wide range of systems that perform biological functions, but also physical systems capable of being trained to develop special collective behaviors without using a processor.

Effect of interaction softness on the collective properties of active Brownian particles

Motility-induced phase separation (MIPS), exhibited by a system of active Brownian particles with repulsive interaction has been extensively studied before. In this talk, we will discuss the effect of a crucial parameter, namely the softness of the inter-particle repulsion, on MIPS. We find that increasing the particle softness leads to a delayed transition to the MIPS state. Furthermore it can destabilize the MIPS phase, leading to the formation of a porous cluster. We extend our work to study a binary mixture of hard and soft particles -- we find that apart from the transitions seen for the homogenous system, there is a complex structure formation within the MIPS state that depends on the relative softness of the binary system. With these additional phases, the phase-diagram appears to be more complex than previously perceived.

Quantum algorithms and quantum control

While quantum processors have progressed immensely over the last decade, they still face significant hurdles such as short coherence times and high error rates. As a result, they are not able to compete with classical information processing in solving problems of practical interest unless big advances take place both at the bottom of the stack (hardware, control) and at the top (algorithms). I will discuss our contributions across the quantum information processing stack, from the control of qubits to quantum algorithm development and back.

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.

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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.

Colloquium SPEQED25

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