Talks

Beyond transport: a quantum information perspective for strong correlations in quantum impurity systems

The characterization of strongly correlated effects in quantum impurity systems (QIS) is particularly challenging due to the infinite size of the environment and the buildup of long-range entanglement in the system. A notable example of strong correlation is the Kondo effect, where the impurity's spin is screened by surrounding conduction electrons, forming a many-body singlet state. Moving beyond standard (perturbative) transport analysis, we introduce a quantum information-based approach to resolve Kondo and other strongly correlated effects in QIS. Specifically, we propose the elements of the impurity’s reduced density matrix as witnesses for the formation of both intra-impurity and impurity-environment strong correlations. Additionally, we develop a method to identify the environment’s role in the screening of Kondo impurities. We apply our scheme to several mesoscopic devices, including semiconductor quantum dots coupled to structured environments, as well as graphene islands. In my talk, I will focus on the example of a double-dot device, where the dots are coupled via off-resonant ballistic whispering gallery modes. Here, we find that (i) Kondo hybridization can dominate and form on each dot individually, (ii) the whispering gallery modes can dominate and mediate a singlet between the two dots, or (iii) the modes hybridize with the dots forming a macroscopic Kondo impurity. These results motivate the use of our quantum information-based approach for studying unconventional Kondo impurities and other strongly-correlated effects in emerging platforms beyond mesoscopic devices, such as quantum computers and cold atom systems.

Quantum Dynamics Seminar: Simulating spectroscopy in mesoscale molecular materials

Recent developments in spatially resolved non-linear spectroscopies have offered novel probes of excited-state dynamics on the 100 nm length scale in a variety of molecular materials. Mechanistic interpretation of these measurements, however, is often obscured by material heterogeneities, electronic delocalization, and spectral congestion. While simulation and modeling provide a powerful approach to uncovering the essential dynamics underlying spectroscopic observation, most computational methods remain intractable for mesoscale molecular materials where the number of molecules is massive and the material parameters tend to fall into the broad “intermediate regime” where perturbative techniques breakdown. I will briefly review recent developments in my group of a formally exact algorithm for simulating exciton dynamics in open quantum systems with O(1) scaling – i.e. for sufficiently large materials, the computational cost stops increasing with system size. I will then describe ongoing work developing a local representation of non-linear response functions that enables O(1) scaling simulations of spectroscopic measurements in molecular materials, starting with absorption and fluorescence. Looking forward, we expect the resulting framework to enable efficient simulations of spatially resolved non-linear spectroscopies.

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.

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.

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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.

Quantum Dynamics Seminar: tba

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