Special QM Seminar Speaker Matteo Ippoliti (Stanford & UT Austin) Thursday, June 16 at 1:00 pm Pacific Time

Time/Venue Thursday, June 16 at 1:00 pm Pacific Time in Physics North 375 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title Universal randomness beyond thermalization in quantum dynamics
Abstract Isolated quantum systems can achieve thermal equilibrium by generating entanglement between their degrees of freedom. As a result, local subsystems can be described by universal (equilibrium) ensembles of states that correctly reproduce thermal expectation values. Present-day quantum simulators, however, give us access to a much richer picture of quantum dynamics, comprising not just expectation values but also individual microscopic snapshots of the many-body state. This motivates the introduction of finer notions of quantum thermalization, based not only on expectation values but also on higher statistical moments that may be extracted from such snapshots. Recently, this idea was made concrete via the “projected ensemble”—the set of states on a subsystem obtained by projectively measuring its complement. Under chaotic dynamics at infinite temperature, this ensemble matches increasingly higher moments of the uniform distribution on the Hilbert space, i.e. it forms an approximate “quantum state k-design”. These criteria, one for each statistical moment k, define a hierarchy of time scales (the “design times”) beyond the thermalization time, recovered for k=1. Not much is known about the scaling of higher design times in general. I will review previous results and present recent work [1] in which we make progress on this question for (1+1)-dimensional quantum circuits by establishing a connection to measurement-induced entanglement phases.
[1] MI and Wen Wei Ho, arXiv:2204.13657

Posted in QM Seminar | Comments Off on Special QM Seminar Speaker Matteo Ippoliti (Stanford & UT Austin) Thursday, June 16 at 1:00 pm Pacific Time

QM Seminar Speaker Jamir Marino (University of Mainz) Wednesday, June 15 at 2:00 pm Pacific Time

Time/Venue Wednesday, June 15 at 2:00 pm Pacific Time in Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title  Kinetically Constrained Quantum Dynamics in Superconducting Circuits
Abstract We study the dynamical properties of the bosonic quantum East model at low temperature. We show that a naive generalization of the corresponding spin-1/2 quantum East model does not possess analogous slow dynamical properties. In particular, conversely to the spin case, the bosonic ground state turns out to be not localized. We restore localization by introducing a repulsive interaction term. The bosonic nature of the model allows us to construct rich families of many-body localized states, including coherent, squeezed, and cat states. We formalize this finding by introducing a set of superbosonic creation-annihilation operators that satisfy the bosonic commutation relations and, when acting on the vacuum, create excitations that are exponentially localized around a certain site of the lattice. Given the constrained nature of the model, these states retain memory of their initial conditions for long times. Even in the presence of dissipation, we show that quantum information remains localized within decoherence times that are tunable with the parameters of the system. We propose an implementation of the bosonic quantum East model based on state-of-the-art superconducting circuits, which could be used in the near future to explore the dynamical properties of kinetically constrained models in modern platforms.

Posted in QM Seminar | Comments Off on QM Seminar Speaker Jamir Marino (University of Mainz) Wednesday, June 15 at 2:00 pm Pacific Time

QM Seminar Speaker Shane Kelly (Johannes Gutenberg-Universität Mainz) Wednesday, June 8 at 2:00 pm


Time/Venue Wednesday, June 8 at 2:00 pm Pacific Time in Physics South 402 via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title: The dynamics of quantum coherence in measurement induced phase transitions
Abstract: The coherent superposition of quantum states is a key feature of quantum mechanics which distinguishes quantum dynamics and information from their classical counterparts.
It can be quantified by the relative entropy of coherence, and in this talk we investigate its dynamics in random hybrid circuits and its role in the protection of quantum information. We first discuss a coherence free circuit which provides a classical analog of the measurement induced phase transition, and which shows a transition in the classical channel capacity. We then investigate a circuit in which measurements dynamically change the coherence, and show how coherence dynamics can be used to control entanglement phase transitions.  Using the intuition developed by studying these hybrid circuits we conclude by proving that the coherence in any local basis gives an upper bound for the quantum code distance of any stabilizer quantum error correction code. Such a bound provides a rigorous quantification of the coherence resource requirements for quantum error correction.

Posted in QM Seminar | Comments Off on QM Seminar Speaker Shane Kelly (Johannes Gutenberg-Universität Mainz) Wednesday, June 8 at 2:00 pm

Special Emergent Phenomena Seminar 290S/QM Seminar Speaker Jonah Herzog-Arbeitman (Princeton) Monday, May 16 at 3:00 pm

Time/Venue Monday, May 16 27 at 3:00 pm, 325 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 , Meeting ID: 995 2349 9113 Passcode: 600704
Host Mike Zaletel
Title Exact Results for the Cooper Pair Spectrum of Flat Band Hubbard Models
Abstract In a flat band superconductor, bosonic excitations disperse while the unpaired electrons are completely immobile. To study this strongly interacting system, Tovmasyan et al. identified a class of Hubbard models possessing an eta-pairing symmetry and analytical ground states when a requirement called the uniform pairing condition is satisfied. In fact, we show that perfectly flat bands with uniform pairing can be guaranteed systematically from the orbitals alone in all space groups, thereby showing this family of Hamiltonians is very large. We study their many-body excitations and find that the Cooper pair (and Goldstone) bound states obey an effective single-particle Hamiltonian. It possesses a unique gapless band, akin to the Feynman mode, whose quadratic dispersion is determined by the minimal quantum metric, thereby connecting the bulk topology of the flat bands to the many-body spectrum. Perturbing away from the flat band limit, we show that the strongest superconductivity occurs at half filling, not at the highest density of states. This is similar to the superconductivity observed in twisted bilayer graphene.

Posted in QM Seminar | Comments Off on Special Emergent Phenomena Seminar 290S/QM Seminar Speaker Jonah Herzog-Arbeitman (Princeton) Monday, May 16 at 3:00 pm

Special Emergent Phenomena Seminar 290S/QM Seminar Speaker Hart Goldman (MIT) Thursday, April 28 at 11:00 am

Time/Venue Thursday, April 28 at 11:00 am 375 Physics North and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 , Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title Exact results for metallic quantum critical points
Abstract I discuss how exact, non-perturbative results can be obtained for both optical transport and static susceptibilities in “Hertz-Millis” theories of Fermi surfaces coupled to critical bosons. Such models possess a large emergent symmetry and anomaly structure, which we leverage to fix these quantities. In particular, I will show that in the infrared limit, the boson self energy at zero wave vector is a constant independent of frequency, and the real part of the optical conductivity is purely a delta function Drude peak with no other corrections. I will also obtain exact relations between Fermi liquid parameters as the critical point is approached from the disordered phase.

Posted in QM Seminar | Comments Off on Special Emergent Phenomena Seminar 290S/QM Seminar Speaker Hart Goldman (MIT) Thursday, April 28 at 11:00 am

Emergent Phenomena 290S/QM Speaker Lingyuan Kong (CalTech) Wednesday, April 27 at 2:00 pm

Time/Venue Wednesday, April 27 at 2:00 pm Pacific Time in Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore / Michal Papaj
Title Iron-based superconducting vortex: a nature-gifting platform for Majoranas
Abstract Iron-based superconducting vortex is emerging as a promising platform for Majorana quasiparticle. After four-year intensive studies, substantial achievements have been made on several compounds, including Fe(Te,Se), (Li,Fe)OHFeSe, CaKFe4As4and LiFeAs. For example, the discovery of integer series of quantized bound states manifests nontrivial topology of vortex zero mode, promises a hope to the field of Majorana research. In this talk, I will depict the main profile of this emerging Majorana platform, on the aspects of materials, bound states, experimental configurations, and etc. Its advantages on physics study and the major controversies owing to the practical diversity will be discussed in short. The systematic investigations accomplished on this platform is promoting the entrance to a second-phase research for manipulating Majorana zero modes in iron-based superconducting vortices.

Posted in QM Seminar | Comments Off on Emergent Phenomena 290S/QM Speaker Lingyuan Kong (CalTech) Wednesday, April 27 at 2:00 pm

Emergent Phenomena 290S/QM Speaker Chandra Varma (Visiting Scholar/Chancellor’s Professor, UC Riverside) Wednesday, April 20 at 2:00 pm

Time/Venue Wednesday, April 20 at 2:00 pm Pacific Time in Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Linear in T and in H resistivity at Quantum criticality with emphasis on Moire’ Bilayer Graphene (abstract will not be posted)

Posted in QM Seminar | Comments Off on Emergent Phenomena 290S/QM Speaker Chandra Varma (Visiting Scholar/Chancellor’s Professor, UC Riverside) Wednesday, April 20 at 2:00 pm

Special Joint 290S/QM and High Energy Seminar Speaker Paolo Glorioso Thursday, April 14 at 1:00 pm

Time/Venue Thursday, April 14 at 1:00 pm Pacific Time in Physics North 375 
Host Ehud Altman / Raphael Bousso
Title Breakdown of hydrodynamics in fracton fluids below 4+1 dimensions
Abstract Fracton phases are characterized by that their elementary excitations have restricted mobility. In the context of hydrodynamics it has been shown theoretically, and confirmed experimentally, that such restricted mobility leads to novel emergent scaling laws. In this talk, I will introduce a framework to describe the hydrodynamics of fractons and predict such scaling laws, with particular focus on systems with conserved dipole and momentum. This hydrodynamics turns out to have rather exotic properties, owing to the fact that translation invariance leads to a non-trivial extension of spacetime symmetries. After developing an effective theory approach that allows accounting for fluctuations, I will show that such theory contains relevant nonlinearities that lead to the emergence of a stochastic non-Gaussian universality class, thus constituting a breakdown of its local hydrodynamic description.

Posted in QM Seminar | Comments Off on Special Joint 290S/QM and High Energy Seminar Speaker Paolo Glorioso Thursday, April 14 at 1:00 pm

Emergent Phenomena 290S/QM Speaker Daniel Parker (Harvard) Wednesday, April 13 at 2:00 pm

Time/Venue Wednesday, April 13 at 2:00 pm Pacific Time in Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore / Ehud Altman
Title Quantum Geometry and Its Application to Fractional Chern Insulators in Twisted Bilayer Graphene
Abstract In contrast to the fractional quantum Hall (FQH) effect, where electron density fixes the applied magnetic field, fractional Chern insulators (FCIs) can realize FQH states in comparatively weak or even zero magnetic fields. Previous theoretical work highlighted magic angle graphene (TBG) as a promising FCI platform, satisfying the twin requirements of flat bands and lowest-Landau-level-like quantum geometry. Indeed, recent experiments have demonstrated FCIs in magic angle graphene with weak magnetic fields.
In the first part of this talk I will discuss how the humble position operator naturally leads to ideal (holomorphic) quantum geometry and present the generalization to many bands and higher Chern numbers.
In the second part of the talk, I will apply combine ideal geometry with numerics and the Hofstadter butterfly to answer three practical questions about the FCIs observed in TBG:1. Why do FCIs appear in TBG?2. Why have they only appeared at finite magnetic field so far?3. What are the experimental parameters required for FCIs at zero magnetic field?
This talk is based on arXiv: 2107.10854, arXiv: 2112.13837, and forthcoming work.

Posted in QM Seminar | Comments Off on Emergent Phenomena 290S/QM Speaker Daniel Parker (Harvard) Wednesday, April 13 at 2:00 pm

Emergent Phenomena 290S/QM Speaker Ruihua Fan (Harvard) Wednesday, April 6 at 2:00 pm


Time/Venue Wednesday, April 6 at 2:00 pm Pacific Time in Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host
Ehud Altman
Title
Fun with geometric flow in conformal field theories: from Floquet dynamics to t’ Hooft anomalies
Abstract Real-time evolution in many-body systems is usually hard to describe. It gets simplified in one-dimensional conformal field theory, where certain evolution can be reduced to geometric flow. We will start with reviewing the quasi-particle picture that was introduced in the study of entanglement in CFT, the intuition built on which will suggest some dynamical problems to study. One is a Floquet problem, where we can design the Floquet driving to have both non-heating and various heating phases. The heating phase is not thermalizing but, paradoxically, can be used for cooling. The other problem is to extract some ’t Hooft anomalies, or equivalently, some edge properties from a wavefunction.

Posted in QM Seminar | Comments Off on Emergent Phenomena 290S/QM Speaker Ruihua Fan (Harvard) Wednesday, April 6 at 2:00 pm