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

Time/Venue Friday, February 21, 11 am in 325 LeConte
Host
Mike Zaletel
Title 
tbd
Abstract 
tbd

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

QM Seminar Speaker Prof. Keshav Dani (OIST-Okinawa Institute of Science and Technology) Wednesday, January 22, 2 pm in 402 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’!

Posted in QM Seminar | Comments Off on QM Seminar Speaker Prof. Keshav Dani (OIST-Okinawa Institute of Science and Technology) Wednesday, January 22, 2 pm in 402 LeConte

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

Time/Venue Thursday, January 16, 2 pm in 325 LeConte hall
Host
Ehud Altman
Title 
Absence of Diffusion on the Edge
Abstract 
The edge of a FQH droplet supports gapless excitations that are protected by a U(1) anomaly. At small but finite temperature, diffusive spreading is expected to occur around the chiral ballistic front. I will show that this chiral diffusive fixed point is never stable. Hydrodynamic long-time tails give large corrections to dissipative transport on the edge, leading to a breakdown of diffusion and driving the edge to a dissipative fixed point in the KPZ universality class. Translation invariance is not assumed.
This rather simple setup presents several striking features: (i) a hydrodynamic theory describes a condensed matter system, but the long lived collective excitation is not momentum, and (ii) a quantum anomaly has dramatic consequences, leading to (iii) large hydrodynamic long-time tails.

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

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

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

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

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 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

Posted in QM Seminar | 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

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


Time/Venue Thursday, December 19 at 2 pm, in LeConte 325
Host
Mike Zaletel
Title Interacting cMERA: variational wavefunctional for strongly correlated QFTs
Abstract The continuous multiscale entanglement renormalization ansatz (cMERA) was proposed in 2011 by Haegeman et al. as a variational wavefunctional for quantum fields. However, so far cMERA is only well understood for non-interacting QFTs or within perturbation theory. In this talk we explain how to numerically generate and manipulate strongly correlated cMERA wavefunctionals for interacting QFTs in the non-perturbative regime. We efficiently produce scale-invariant wavefunctionals in one spatial dimension that are (i) highly non-gaussian and display (ii) power-law decay of correlations and (iii) logarithmic scaling of entanglement entropy. We discuss the potential application of this ansatz to solving scale-invariant quantum field theories variationally.

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

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 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.

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

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

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. 

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

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

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.

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

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

Time/Venue Monday, December 2 at 2:30 pm in 3 LeConte
Host Joel Moore
Title Quantized optical responses in chiral insulators and metals
Abstract
In this talk I will discuss how optical experiments can measure quantized observables, determined solely by fundamental constants.
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.

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