**Time/Venue **Friday, February 21, 11 am in 325 LeConte**Host **Mike Zaletel

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Condensed Matter Theory Center

**Time/Venue **Friday, February 21, 11 am in 325 LeConte**Host **Mike Zaletel

Title

Abstract

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Comments Off on Special QM Seminar Speaker Bart Andrews (University of Zurich) Friday, February 21, 11 am in 325 LeConte

**Time/Venue** Wednesday, January 22, 2 pm in LeConte 402**Host** Feng Wang / Joel Moore**Title** Into the dark world of excitons in atomically thin semiconductors**Abstract** About a decade ago, the discovery of monolayers of transition metal dichalcogenides opened a new frontier in the study of optically excited states in semiconductors, and related opto-electronic technologies. These materials exhibit a plethora of robust excitonic states, such as bright excitons at the K & K’ valleys, momentum- and spin-forbidden dark excitons, and hot excitons. Optics-based experiments have revealed much about the bright excitonic states, but they remain largely unable to access their valley character, their scattering channels into other valleys within the Brilloin Zone, and the nature of the dark states in these valleys.

Angle-Resolved Photoemission Spectroscopy (ARPES) based techniques would be ideal to access the valley character, and momentum-resolved scattering channels of photoexcited states in 2D semiconductors. But these are very challenging experiments to perform on the typically-available, micron-scale, 2D semiconductors. In today’s talk, I will discuss the challenges involved, and progress made in my lab to date towards this aim. And – time permitting – we will end with an entertaining peek into the ‘quantum psychology of dark excitons’!

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**Time/Venue **Thursday, January 16, 2 pm in 325 LeConte hall**Host **Ehud Altman

Title

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Comments Off on Special QM Seminar Speaker Luca Delacretaz (University of Chicago) Thursday, January 16, 2 pm in 325 LeConte hall

**Time/Venue** Monday, January 13 at 2 pm in Old LeConte 325**Host** Ehud Altman**Title **The Measurement-Induced Transition in Open Quantum Systems

**Abstract** Quantum technologies fundamentally rely on quantum control, measurement, and feedback. Measurement-induced transitions are a recently uncovered class of critical phenomena that occur when many-body unitary dynamics are interspersed with measurements at a tunable rate [1,2]. We uncover precise connections between this phase transition and quantum error correction thresholds in the quantum channel capacity of open system dynamics [3,4]. We then show how to define a local order parameter for the transition that measures the ability of the system to store one bit of quantum information for exponentially long times [5]. Using this order parameter, we identify scalable probes of the transition that are immediately applicable to advanced quantum computing platforms such as trapped ions or superconducting qubits. Studying this class of measurement-driven many-body dynamics may potentially lead to more efficient realizations of scalable, fault-tolerant quantum computing, as well as deepen our understanding of the transition from quantum to classical physics in many-body systems.

[1] Y. Li, X. Chen, and M. P. A. Fisher, Phys. Rev B 98, 205136 (2018).

[2] B. Skinner, J. Ruhman, and A. Nahum, Phys. Rev. X 9, 031009 (2019).

[3] S. Choi, Y. Bao, X.-L. Qi, and E. Altman, arXiv:1903.05124

[4] M. J. Gullans and D. A. Huse, arXiv:1905.05195

[5] M. J. Gullans and D. A. Huse, arXiv:1910.00020

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Comments Off on Special QM Seminar Speaker Michael Gullans, (Princeton), Monday, January 13 at 2 pm in Old LeConte 325

**Time/Venue** Wednesday, January 8 at 2 pm in Old LeConte 402**Host **Joel Moore**Title/Abstract** Quantum Chaos; From classical diffusion to Anderson localization and beyond

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Comments Off on QM Seminar Speaker Professor Giulio Casati, (University of Insubria; Lake Como School of Advanced Studies), Wednesday, January 8 at 2 pm in Old LeConte 402

**Time/Venue **Thursday, December 19 at 2 pm, in LeConte 325**Host** Mike Zaletel

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Comments Off on Special QM Seminar Speaker Yijian Zou (Perimeter Institute) Thursday, December 19 at 2 pm, in LeConte 325

**Time/Venue** Thursday, December 5 at 4 pm in Old LeConte 402**Host** Ehud Altman**Title** A glimpse at chaos in the quantum realm**Abstract** From three-body problem to butterfly effect, chaos is one of the most mathematically fascinating yet tangible phenomena in the nature. Chaos in the classical world has been (more or less) well understood formulated in the language of dynamical system, but its quantum counterpart remains elusive — even its definition is not clear despite decades of efforts. We will discuss a new way to look at the old problem: instead of focusing on quantum states, we aim to uncover the chaotic behavior of a quantum system by studying *the entanglement of operators.* We will test this idea in the context of conformal field theories (CFTs), a special type of quantum field theory where the existence of abundant symmetries allow concrete analytic calculations to be done. Among other results, this ”theoretical experiment” reveals a striking property of a peculiar type of CFT, dubbed holographic CFT, that it is perhaps the most chaotic quantum field theory to date, mirroring its dual relation with black holes via the AdS/CFT correspondence.

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Comments Off on Special QM Seminar Speaker Lamei Nie (University of Chicago) Thursday, December 5 at 4 pm in Old LeConte 402

**Time/Venue** Thursday, December 5 at 2 pm LeConte 325 **Host** Norman Yao**Title** NV centers in diamond as quantum sensors for high-pressure physics**Abstract** The nitrogen-vacancy (NV) color center is a point defect of diamond. Behaving as an artificial atom it can be used as a magnetic field, pressure, and temperature solid-state quantum sensor down to the atomic scale. I will describe how this sensitivity can be applied inside a diamond anvil cell in order to investigate the magnetic and superconducting properties of high-pressure materials. This NV-based high-pressure sensing technique is also compatible with a synchrotron-based characterization of the crystalline structure. The implementation of these complementary techniques in a single set-up will open a broad range of applications, for instance for the discovery of novel superconductors.

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Comments Off on Special QM/AMO seminar speaker Jean-François Roch (ENS Paris-Saclay), Thursday, December 5 at 2 pm inLeConte 325

**Time/Venue** Wednesday, December 4 at 2 pm in Old LeConte 402**Host** Ehud Altman**Title **Multiscale entanglement clusters at the many-body localization phase transition**Abstract** In the presence of strong disorder, interacting systems can localize and avoid thermalization due to the emergence of an extensive set of local integrals of motions. These many-body localized (MBL) systems break ergodicity, and present interesting entanglement properties. In this talk, I will focus on the entanglement structure of the wave-functions at the phase transition between ergodic and MBL phases. After an overview of the properties of the many-body localization and a brief discussion of the different phenomenological renormalization group descriptions of the phase transition, I will discuss how we can access the entanglement structure of the wave-function. Critical states close to the transition have a structure compatible with fractal or multiscale-entangled states, characterized by entanglement at multiple levels: small strongly entangled clusters are weakly entangled together to form larger clusters. The critical point therefore features subthermal entanglement and a power-law distributed cluster size, while the localized phase presents an exponentially decreasing cluster distribution. These results are consistent with some of the recently proposed phenomenological renormalization-group schemes characterizing the many-body localized critical point, and may serve to constrain other such schemes.

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Comments Off on QM Seminar Speaker Loïc Herviou (KTH Royal Institute of Technology) Wednesday, December 4 at 2 pm in Old LeConte 402

**Time/Venue** Monday, December 2 at 2:30 pm in 3 LeConte **Host** Joel Moore ** Title **Quantized optical responses in chiral insulators and metals

I will discuss two effects triggered by circular polarized light: circular dichroism, and the circular photogalvanic effect. The former is a linear response that is quantized in higher order topological insulators with chiral edge modes. The latter is a non-linear response that is quantized, in units of e^3/h^2, in Weyl semimetals where all mirror symmetries are broken. I will discuss how these probes can be used to distinguish these phases from trivial states, and some of the subtleties to interpret experiments capable of measuring them.

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Comments Off on Special QM Seminar Speaker Adolfo Grushin (Institut Néel), Monday, December 2 at 2:30 pm in 3 LeConte