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.

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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.

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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. 

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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.

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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.

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Special QM Seminar Speaker Arpit Dua (Yale) Monday, November 25 at 11 am LeConte 325


Time/Venue Monday, November 25 at 11 am LeConte 325 (325 is reconfirmed-see you there!)
Host Norman Yao and Mike Zaletel
Title Structure of fracton stabilizer models
Abstract
In two dimensions, all translation-invariant topological Pauli stabilizer models are equivalent to copies of the 2D toric code under a local unitary. Much less is known about 3D stabilizer models due to the existence of fracton topological order. In this talk, I will discuss how tools such as compactification, commutation quantities and entanglement renormalization shed light into the properties and structure of fracton stabilizer models.

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Special QM/AMO Seminar Speaker Philip Crowley (Boston University), Thursday, November 14 at 2 pm in Old LeConte 402

Time/Venue Thursday, November 14 at 2 pm in Old LeConte 402
Host Norman Yao
Title Topological classes of quantum dynamics in quasi-periodically driven systems
Abstract Advances in the control and manipulation of experimental quantum systems has allowed us to realize new driven phases of quantum matter in the laboratory. In periodically driven systems new phases occur when the steady states, determined by Bloch-Floquet theorem, have novel spatio-temporal or topological order.
In this talk I show how the Bloch-Floquet theorem is generalized to cases when the drives are not periodic, but rather quasi-periodic.
I apply this framework to the simplest case of a few level system, and show that steady state dynamics admit a topological classification. When the classification is non-trivial the system exhibits a quantized pumping of energy, and a sensitivity to initial conditions, neither of which is present in the trivial case.
I further discuss the stability of this classification, the behavior near the critical point where the topological class changes, and ongoing work to observe this in experiments.




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QM Seminar Speaker Aavishkar Patel (UCB), Wednesday, November 13 at 2 pm in LeConte 402

Time/Venue Wednesday, November 13, 2:00 pm, LeConte 402
Host Ehud Altman
Title An exactly solvable model of a strongly correlated metal with “Planckian dissipation”
Abstract The apparently universal value, kBT/ℏ, of the transport scattering rate in a variety of non-Fermi liquid metals with T-linear resistivity has been a longstanding mystery in condensed matter physics. We present a lattice model of fermions with N flavors and random interactions that describes a non-Fermi metal at low temperatures T → 0, in the solvable limit of large N. We begin with quasiparticles around a Fermi surface with effective mass m∗, and then include random interactions that lead to fermion spectral functions with frequency scaling with kBT/ℏ. The resistivity ρ obeys the Drude formula ρ = m∗/(ne2τtr), where n is the density of fermions, and the transport scattering rate is 1/τtr = fkBT/ℏ; we find f of order unity and essentially independent of the strength and form of the interactions. The random interactions are a generalization of the Sachdev-Ye-Kitaev models; it is assumed that processes nonresonant in the bare quasiparticle energies only renormalize m∗, while resonant processes are shown to produce the Planckian behavior. We point out some predictions of this theory that are, in principle, testable in photoemission experiments.

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Special QM Seminar Speaker Leonid Glazman (Yale), Thursday, November 7 at 2 pm in LeConte 402

Time/Venue Thursday, November 7 at 2 pm in LeConte 402
Host Norman Yao
Title Microwave spectroscopy of a weakly-pinned charge density wave in a superinductor
Abstract A chain of small Josephson junctions (aka superinductor) emerged recently as a high-inductance, low-loss element of superconducting quantum devices. We noticed that the intrinsic parameters of a typical superinductor in fact place it into the Bose glass universality class for which the propagation of waves in a sufficiently long chain is hindered by pinning. Its weakness provides for a broad crossover from the spectrum of well-resolved plasmon standing waves at high frequencies to the low-frequency excitation spectrum of a pinned charge density wave. We relate the scattering amplitude of microwave photons reflected off a superinductor to the dynamics of a Bose glass. The dynamics on the scales long and short compared to the Larkin pinning length determines, respectively, the low- and high-frequency asymptotes of the reflection amplitude.

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QM Seminar Speaker Thomas Klein (UCB/Stockholm University), Wednesday, November 6 at 2:00 pm in LeConte 402

Time/VenueWednesday, November 6 at 2:00 pm in LeConte 402
Host
Ehud Altman
Title (Revised Nov. 6)
Time-evolving local density matrices
Abstract (Revised Nov. 6)
I will take the opportunity to discuss the unfinished project I am currently working on — a new algorithm to do quantum time-evolution starting from an initial product state. The idea is to write an approximate time-evolution for just the local density matrices, which at least in some cases can be shown to capture the time-evolution accurately.

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