Please join us for this week’s AMO/QI seminar featuring Dr. Jon SImon from Stanford University. He will give a talk titled: Racing to the Bottom: Low Finesse, Small Waist Cavity QED

Abstract:
In this seminar I’ll tell the story of cavity quantum electrodynamics (cQED) from first principles, building towards a new type of sub-micron waist resonator recently developed in the Simon/Schuster collaboration at Stanford. We will start by developing physical intuition for cooperativity, the figure of merit that controls performance of light/matter coupled systems including photon collection efficiency, cavity-mediated information exchange fidelity, and even coherence of interactions of photonic quasi-particle. Maximizing cooperativity will push us in either of two directions; (1) high finesses or (2) small mode waists. The high-finesse route is well explored by many leaders in the field of cQED, so I will emphasize the quest to small mode waist resonators, motivating near-concentric resonators, bow-tie resonators, and finally, our lens cavities. These lens cavities sport mode waists below a micron, entering the strong coupling regime at finesses well below 100. I’ll share preliminary data demonstrating single-atom coupling to such a cavity, and a test-bench demonstration of an array of small waist cavities that we intend to integrate with an atom array.

Please join us for this week’s AMO/QI seminar featuring Dr. Jeremy Axelrod. We are celebrating the completion of his dissertation with the Mueller group. He will give a talk titled: A Laser Phase Plate for Transmission Electron Microscopy

Abstract:
Low image contrast is a major limitation in transmission electron microscopy, since samples with low atomic number only weakly phase-modulate the illuminating electron beam, and beam-induced sample damage limits the usable electron dose. The contrast can be increased by converting the electron beam's phase modulation into amplitude modulation using a phase plate, a device that applies a pi/2 radian phase shift to part of the electron beam after it has passed through the sample. Previous phase plate designs rely on material placed in or near the electron beam to provide this phase shift. This results in image aberrations, an inconsistent time-varying phase shift, and resolution loss when the electron beam charges, damages, or is scattered from the material.

In this seminar, I will present the theory, design, and implementation of the laser phase plate, which instead uses a focused continuous-wave laser beam to phase shift the electron beam. A near-concentric Fabry-Perot optical cavity focuses and resonantly enhances the power of the laser beam in order to achieve the high intensity required to provide the phase shift. We demonstrate that the cavity can surpass this requirement and generate a record-high continuous-wave laser intensity of 590 GW/cm^2. By integrating the cavity into a transmission electron microscope, we show that the ponderomotive potential of the laser beam applies a spatially selective phase shift to the electron beam. This enables us to make the first experimental observation of the relativistic reversal of the ponderomotive potential.

We then theoretically analyze the properties of the contrast transfer function generated by the laser phase plate. We experimentally determine that resolution loss caused by thermal magnetic field noise emanating from electrically conductive materials in the cavity can be eliminated by designing the cavity with a sufficiently large electron beam aperture. Finally, we show that the laser phase plate provides a stable pi/2 phase shift and concomitant contrast enhancement when imaging frozen hydrated biological macromolecules. We use these images to successfully determine the structure of the molecules. This demonstrates the laser phase plate as the first stable and lossless phase plate for transmission electron microscopy.

Please join us for this week’s AMO/QI seminar featuring Dr. Josh Combes from the University of Colorado, Boulder. He will be giving a talk titled: Nonclassical light interacting with matter and nonclassical measurements 

Abstract: In this talk, I'll begin by summarizing a decade of research into the effects on matter interacting with non-classical light pulses, such as Schrödinger cat states or photon number states. These interactions generate notable correlations between the incoming and radiated light, inducing non-Markovian behavior in the matter. The latter part of the talk will focus on utilizing non-classical light for advancing precision metrology techniques. In particular, I will consider using a Schrödinger cat state as a local oscillator to develop novel measurements in optical and atomic interferometry.

Please join us for this week’s AMO/QI seminar featuring Dr. Nelson Oppong, JILA. He will be presenting a talk titled: Making, probing, and using 'Schrödinger cat' states in an optical clock

While optical atomic clocks continue to reach unprecedented levels of precision and accuracy, an important question concerns how quantum entanglement can be harnessed to improve their performance below the standard quantum limit. Our experiment combines techniques from atom arrays with the optical clock qubit of strontium to address this question. In this talk, I will discuss how we use laser-controlled Rydberg interactions to implement a multi-qubit gate for preparing 'Schrödinger cat' states on the clock qubit. We directly probe the performance of these Greenberger–Horne–Zeilinger type cat states in an atom-laser comparison and demonstrate performance below the standard quantum limit for short dark times. Using the programmability of our platform for the preparation of multiple atomic ensembles with variable sizes, we also explore a potential pathway toward Heisenberg-limited scaling of atomic clock precision.

Please join us for this week’s AMO/QI seminar with Dr. Sara Murciano from CalTech. She will present a talk titled: Measurements and symmetries on the fate of entanglement

Abstract:
 Entanglement plays a key role in different fields of physics. This talk focuses on two aspects where understanding its behaviour yields intriguing results: measurements and symmetries. The first topic explores how weak measurements alter the properties of critical models: We identify different protocols wherein measurements (i)  weakly modify the universal long-range entanglement and (ii) they completely obliterate it. As a potential practical application of this setup, I will show how it can be used to enable the teleportation of quantum states between distant parties and to what extent the entanglement of a many-body wavefunction transfers under imperfect teleportation protocols. The second subject concerns the study of the symmetry breaking in a subsystem. This investigation leads to the definition of the entanglement asymmetry, which neatly detects novel physical out-of-equilibrium features, in particular an unexpected quantum version of Mpemba effect.

Please join us for this week’s AMO/QI seminar, Qual Club.

The AMO Qual Club is meant for our early-career graduate students who are learning broadly about AMO physics in preparation for their qualifying examination.  This event is for graduate students, and in an effort to keep the gatherings more informal and less stressful, we ask professors, postdocs, undergraduate students, visitors and other researchers not to join us for this meeting.

Please join us for a talk by Dr. Nathanan Tantivasadakarn from CalTech.

Please join us for a talk by Dr. Loïc Anderegg from Harvard University titled:  Laser Cooled Molecules for Quantum Science and Fundamental Physics

Abstract:
 Ultracold molecules offer a diverse array of potential applications ranging from fundamental physics to quantum simulation and computation. Motivated by potential discoveries in these areas, significant advances in controlling molecules at the single-quantum-state level have occurred over the past decade. Recently, molecules have been loaded into optical tweezer arrays allowing both high-fidelity readout and quantum control of individual molecules. In this talk, we will discuss creating and employing optical tweezer arrays of diatomic (CaF) and polyatomic (CaOH) molecules to unlock new quantum science applications. We demonstrate second scale coherence times for molecular qubits in optical tweezer traps, parametrizing the potential performance of polar-molecule-based quantum simulators or computers. Additionally, we show progress towards realizing the goal of high-fidelity molecular qubits by demonstrating dipolar interactions and entanglement between CaF molecules. The full quantum state control afforded by this platform allows us to study quantum state specific collisions and control the dynamics of the molecular collisions. Finally, extending the tools of quantum control to polyatomic molecules leads to powerful new scientific avenues, including significant improvements to searches for physics beyond the Standard Model. We demonstrate the experimental protocols needed for such a search and leverage the structural characteristics of polyatomic molecules to extend coherence times, paving the way towards orders-of-magnitude improved experimental sensitivity to time-reversal-violating physics.

Please join us for a talk by Dr. Harry Levine, from Amazon Web Services, who will be presenting a talk titled: Novel strategies for hardware-efficient quantum processors

Abstract:
Quantum error correction is an exciting scientific frontier at the interface of many fields including quantum information science, many-body physics, and computer science. The field has developed rapidly in the last several years, with major milestones marking the first glimpses into a future of error-corrected quantum computers. At the same time, these advances have also illuminated the major science and engineering challenges that remain on the road to useful fault-tolerant quantum computers due to large resource overheads and demanding performance and control requirements. In this talk, I will discuss recent progress in strategies to ease the demands of error correction with a focus on two leading quantum information platforms: superconducting circuits and cold atoms. First, I will discuss the paradigm of “erasure qubits” which are qubits for which errors can be flagged in real-time and are consequently easier to correct. In this context, I will discuss recent experiments showing how erasure qubits can be realized using a “dual-rail” encoding in superconducting transmons, offering a way to package standard qubit components into better error correction building blocks. Second, I will discuss the recent, rapid progress in neutral atom quantum computers and highlight how the unique capabilities for efficient and flexible control can ease the path towards scalable operation of error-corrected quantum processors.

Please join us for a talk by Dr. Kevin Singh from the University of Chicago. He will be giving a talk titled: A dual-apecies Rydberg array
Abstract:
Rydberg atom arrays are a leading platform for quantum information science. Such arrays comprise hundreds of long-lived qubits that are used for highly coherent analog quantum simulation and digital quantum computation. Advanced quantum protocols such as quantum error correction, however, require midcircuit qubit operations, including readout, reset, and replenishment of a subset of qubits. A compelling strategy to achieve these capabilities is a dual-species architecture in which a second atomic species can be controlled without crosstalk and entangled with the first via Rydberg interactions. In this talk, I will present our realization of a dual-species Rydberg array consisting of rubidium (Rb) and cesium (Cs) atoms. I will discuss the richness of interaction regimes that can be accessed in the system and how we achieve enhanced interspecies interactions by electrically tuning the Rydberg states close to a Forster resonance. In this regime, we demonstrate interspecies Rydberg blockade and use this blockade to generate Bell states between Rb and Cs hyperfine qubits. I will discuss how we combine this interspecies entanglement with native midcircuit readout to achieve quantum non-demolition measurement of a Rb qubit using an auxiliary Cs qubit. Finally, I will discuss how these techniques enable scalable measurement-based protocols and real-time feedback control in large-scale quantum systems.

Please join us on Monday, Feb 12 for a talk by Dr. Sylvia Biscoveanu from Northwestern University titled:
Compact-object astrophysics with gravitational waves and their electromagnetic counterparts

The growing catalog of over one hundred detected gravitational-wave signals from compact-object mergers is enabling studies of the properties of black hole and neutron star binaries with increasing precision. However, the processes governing the formation and evolution of these systems and their electromagnetic counterparts remain largely unconstrained. The current observing run of the LIGO-Virgo-Kagra gravitational-wave detector network has already doubled the number of observed sources, and the next generation of detectors will be sensitive to nearly all compact-object mergers across cosmic time. In this talk, I will highlight how measurements of the population properties of black hole and neutron star binaries can be used to constrain the astrophysical processes driving their formation and evolution, focusing on binary black hole spins and neutron star-black hole mergers. I will conclude by contextualizing the prospects for probing compact-object astrophysics with gravitational waves within the observational landscape of the next decade.

Please join us for this week’s AMO/QI seminar featuring Dr. Sophia Economou from Virginia Tech. She will be presenting a talk titled: Spin-photon interfaces: control and distribution of entanglement

Abstract:
Spin-photon interfaces are ubiquitous in quantum information processing and also feature intriguing physics culminating from the interplay of spin-spin, spin-photon, and spin-field interactions. In these systems, of particular interest are entangled states of nuclear spins, which can be used as quantum memories, and of photonic qubits, which can be used to transmit information robustly. I will discuss the dynamics of these systems and applications to quantum networks and photonic quantum computing.

Bio:
Sophia Economou is a Professor and the T. Marshall Hahn Chair in Physics at Virginia Tech. She is also the director of the Virginia Tech Center for Quantum Information Science and Engineering. She focuses on theoretical research in quantum information science, including quantum computing, quantum communications, and quantum simulation algorithms.

Please join us for a talk by Dr. Tibor Rakovszky from Stanford University.

The AMO/QI seminar resumes for the Spring Semester with our first talk by Dr. Kenneth Brown from the Duke Quantum Center. His talk is titled Simulating a conical intersection with a trapped ion quantum computer 

Abstract:
Conical intersections often control the reaction products of photochemical processes and occur when two electronic potential energy surfaces intersect. Theory predicts that the conical intersection will result in a geometric phase for a wavepacket on the ground potential energy surface, and although conical intersections have been observed experimentally, the geometric phase has not been directly observed in a molecular system. Here we use a trapped atomic ion system to perform a quantum simulation of a conical intersection. The ion’s internal state serves as the electronic state, and the motion of the atomic nuclei is encoded into the motion of the ions. The simulated electronic potential is constructed by applying state-dependent optical forces to the ion. We experimentally observe a clear manifestation of the geometric phase using adiabatic state preparation followed by motional state measurement. Our experiment shows the advantage of combining spin and motion degrees for quantum simulation of chemical reactions. We conclude with a discussion of future simulation directions. 

The AMO Qual Club is meant for our early-career graduate students who are learning broadly about AMO physics in preparation for their qualifying examination.  This event is for graduate students, and in an effort to keep the gatherings more informal and less stressful, we ask professors, postdocs, undergraduate students, and other researchers not to join us for this meeting.

Lunch will follow the meeting to encourage further conversation.

This will be our final AMO/QI seminar for the semester. We look forward to seeing you again in Spring.

Please join us for a special CIQC Seminar featuring Dr. Aziza Suleymanzade (Harvard). She will give a talk titled: Quantum networking with solid-state defects in diamond nanophotonic cavities.

Abstract: Silicon-vacancy (SiV) defect centers in diamond coupled to nanophotonic crystal cavities offer a promising platform for quantum network applications. Our system utilizes long qubit coherence times, high optical cooperativity, and on-chip scalability, providing a unique path to the practical implementation of long-distance quantum networking. In this talk, I will present our recent results on generating long-distance distributed entanglement across a two-node network, demonstrating entanglement across a deployed 35-km telecom fiber network in the Boston/Cambridge area. I will also go over our ongoing projects, including the realization of blind delegated computing and applications for long-baseline entangled telescopes using our platform.

Please join us for this week’s AMO/QI seminar, on a special day, MONDAY Nov. 20 due to the Thanksgiving holiday.
Our speaker will be Dr. Lincoln Carr from the Colorado School of Mines. His talk is titled, Entangled quantum cellular automata, physical complexity, and Goldilocks rules

Abstract:
Cellular automata are interacting classical bits that display diverse emergent behaviors, from fractals to random-number generators to Turing-complete computation. We discover that quantum cellular automata (QCA) can exhibit complexity in the sense of the complexity science that describes biology, sociology, and economics. QCA exhibit complexity when evolving under 'Goldilocks rules' that we define by balancing activity and stasis. Our Goldilocks rules generate robust dynamical features (entangled breathers), network structure and dynamics consistent with complexity, and persistent entropy fluctuations. Present-day experimental platforms—Rydberg arrays, trapped ions, and superconducting qubits—can implement our Goldilocks protocols, making testable the link between complexity science and quantum computation exposed by our QCA.
The inability of classical computers to simulate large quantum systems is a hindrance to understanding the physics of QCA, but quantum computers offer an ideal simulation platform. I will discuss our recent experimental realization of QCA on a digital quantum processor, simulating a one-dimensional Goldilocks QCA rule on chains of up to 23 superconducting qubits. Employing low-overhead calibration and error mitigation techniques, we calculate population dynamics and complex network measures indicating the formation of small-world mutual information networks. Unlike random states, these networks decohere at fixed circuit depth independent of system size, the largest of which corresponds to 1,056 two-qubit gates.  This quantum circuit depth result presents a strong contrast to the quantum volume concept used to characterize many current quantum computers in industry. Such computations may open the door to the employment of QCA in applications like the simulation of strongly-correlated matter or beyond-classical computational demonstrations.

References:
1.     LE Hillberry, MT Jones, DL Vargas, P Rall, N Yunger Halpern, N Bao, S Notarnicola, S Montangero, LD Carr, “Entangled quantum cellular automata, physical complexity, and Goldilocks rules,” Quantum Science and Technology, v. 6, p. 045017 (2021)
2.     EB Jones, LE Hillberry, MT Jones, M Fasihi, P Roushan, Z Jiang, A Ho, C Neill, E Ostby, P Graf, E Kapit, and LD Carr, “Small-world complex network generation on a digital quantum processor,” Nature Communications v. 13, p. 4483 (2022)
3.     LE Hillberry, M Fasihi, L Piroli, N Yunger Halpern, T Prosen, and LD Carr, “Thermodynamics and integrability in quantum cellular automata,” Phys. Rev. Lett, to be submitted shortly (2023)

This week’s AMO/QI seminar features Dr. Benjamin Bloom, Founder and CTO of Atom Computing. His talk is titled: The Alkaline-Earth Playground: Gates, Mid-Circuit Measurement, and Scaling up to 1000+ qubits

Abstract:
Atom Computing, with locations in Berkeley and Boulder, has been laser focused on how we build large scale neutral atom systems compatible with error correction. An astonishing number of challenges stand in the way of building a useful system. Here I will give an overview of our Strontium and Ytterbium systems and how they are attacking many of these issues.

Please join us for this week’s AMO/QI seminar with Dr. Andrew Ludlow from NIST Boulder. He will be presenting a talk titled: Towards the next-generation of optical lattice clocks

Abstract:
Over the last five years, optical clocks based on ultracold atoms confined in an optical lattice have realized a staggering 18 digits of precision and accuracy.  This has facilitated their application to searches for dark matter, tests of fundamental physics, and metrology supporting the re-definition of the SI second.  Further advances promise to open the door to gravitational studies, including beyond-state-of-the-art Earth-based geodesy, space-based tests of general relativity, and even clock-based observations of gravitational waves.  Here, I highlight a toolbox of quantum control techniques aimed at realizing the next-generation of optical lattice clocks with orders of magnitude higher performance.  This begins with novel laser cooling techniques using the ultra-narrow clock transition, capable of cooling alkaline-earth species deep into the nK regime and useful for trapping in very shallow optical lattices. Next, we demonstrate controlled tunneling in excited bands of a Wannier-Stark lattice in order to suppress collisional effects via coherent delocalization. A complementary technique, which we refer to as ‘ratchet loading’, allows us to programmably fill optical lattices up to cm-scale spatial distributions.  Finally, we demonstrate a moving cryogenic shield for strongly suppressing the blackbody radiation that shifts the clock transition frequency of lattice confined atoms.  To conclude, I also briefly summarize our efforts to deliver the precision of optical lattice clock systems in a transportable apparatus beyond the lab.

The AMO Qual Club is meant for our early-career graduate students who are learning broadly about AMO physics in preparation for their qualifying examination.  This event is for graduate students, and in an effort to keep the gatherings more informal and less stressful, we ask professors, postdocs, undergraduate students, and other researchers not to join us for this meeting.

Lunch will follow the meeting to encourage further conversation.

Please join us for this week’s AMO/QI seminar featuring Dr. Eleanor Rieffel from NASA Ames. She will be giving a talk titled: A NASA Perspective on Quantum Computing, with Emphasis on Two Applications of Quantum Networks

Abstract:
This talk will begin with a brief overview of the NASA QuAIL team’s ongoing quantum computing investigations. It will then transition to cover two specific technical topics, one in distributed quantum computing and one in fundamental quantum physics.

I’ll introduce both the classical and quantum CONGEST-CLIQUE Models of distributed computation, and then outline two quantum algorithms that succeed with high probability; one yields an approximately optimal Steiner Tree, and the other an exact directed minimum spanning tree, each using asymptotically fewer rounds of communication than any known algorithms in the classical CONGEST-CLIQUE model. We achieve these results by combining classical algorithms with fast quantum subroutines. Additionally, we characterize the constants and logarithmic factors involved in our algorithms, as well as related prior classical and quantum algorithms, revealing the importance of “small” factors and highlighting that advances are needed to render both the classical and quantum algorithms practical.

Within the last few years, researchers have proposed extended Wigner friend scenarios that lead to new theorems, along the lines of Bell’s theorems but with weaker assumptions. This work is tied to deep questions related to reversibility, observation, and measurement. We map out an ultimate experiment that would include a human-level artificial intelligence (HLAI) QUALL-E running on a quantum computer playing the role of a friend. On the one hand, a proof-of-principle experiment in which a photon played the role of a “friend” has already been carried out. On the other hand, we were able to provide rough upper bounds for the resources required to implement the full experiment. An interesting open question is to design attractive intermediate experiments. The intent of this work is to give a clear goal for future experimentalists, and a clear motivation for trying to achieve that goal.

This week our AMO/QI seminar is a celebration of Dr. Julian Wolf’s completed dissertation. Please join us at 11am in 375 Physics North for his talk titled: Coherent and Incoherent Quantum Feedback in an Atom--Cavity System.

Abstract:
Feedback control allows a wide range of systems to be stabilized to out-of-equilibrium states. An ultracold atomic gas coupled to a high-finesse optical cavity offers a convenient testbed for feedback in the quantum regime. In this talk, I will discuss two instances of this: a coherent quantum feedback system, in which the energy of a collective atomic spin is autonomously stabilized to a set point conditioned on the detuning of the pump light from cavity resonance; and an incoherent feedback system, in which light escaping from the cavity offers a real-time measurement of the number of atoms present in the cavity during evaporative cooling, which is then used to stabilize to a desired atom number. Along the way, I will discuss other interesting findings that have been made along the way, as well as some techniques that we've found particularly useful.

The AMO Qual Club is meant for our early-career graduate students who are learning broadly about AMO physics in preparation for their qualifying examination.  This event is for graduate students, and in an effort to keep the gatherings more informal and less stressful, we ask professors, postdocs, undergraduate students, and other researchers not to join us for this meeting.

Lunch will follow the meeting to encourage further discussion

Please join us for a presentation by Dr. David Schuster of Stanford University.

His talk is titled: Assembling and probing highly entangled quantum matter with superconducting circuits

Abstract: Manipulating quantum systems composed of interacting particles represents a central challenge of modern quantum science, with applications from quantum computation to many-body physics. I will present our recent work in constructing low-entropy quantum fluids of light by employing particle-resolved assembly combined with robust adiabatic preparation. This experiment is performed in a 1D Bose-Hubbard circuit implemented with an array of capacitively coupled transmon qubits. We leverage strong lattice disorder to inject individual photons into known localized eigenstates, then adiabatically remove this disorder to melt the photons into a fluid via tunneling-induced quantum fluctuations. It is becoming increasingly clear that in the quantum regime, state preparation and characterization should not be treated separately -- entangling the two processes provides a quantum advantage in information extraction.  I will present a new approach which we term ``manybody Ramsey interferometry'' that combines adiabatic state preparation and Ramsey spectroscopy: leveraging our recently-developed one-to-one mapping between computational-basis states and manybody eigenstates, we prepare a superposition of manybody eigenstates controlled by the state of an ancilla qubit, allow the superposition to evolve relative phase, and then reverse the preparation protocol to disentangle the ancilla while localizing phase information back into it.  At the end, we can discuss ways for quantum computers to efficiently probe quantum matter.

Please join us for this week’s AMO/QI seminar featuring Alisher Duspayev from the University of Michigan.

He will be giving a talk titled: Rydberg atoms for molecular physics and field sensing
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Abstract: Neutral atoms in highly-excited Rydberg states are actively utilized in a variety of research directions such as ultracold chemistry and many-body physics, precision measurements and emerging quantum technologies. This talk is focused on using Rydberg atoms for creating long-range molecular states and for sensing AC/DC electric fields. First, I will present a novel type of Rydberg dimer formed through long-range electric-multipole interactions between a Rydberg atom and an ion. Its vibrational spectra and stability against nonadiabatic effects will be discussed. In the second part, I will present our recent experimental investigations on whether Rydberg-atom-based sensing would be practical for electric microfields that occur in ion clouds and plasmas. The talk will be concluded with a discussion of the prospects of using high-angular-momentum Rydberg states for DC-electric-field sensing in room-temperature vapor cells with and without buffer gas.

The AMO/QI seminar returns for the Fall 2023 semester!
Please join us for our first seminar of the semester, featuring Dr. Chris Anderson from University of Illinois, Urbana-Champaign.

He will be presenting a talk titled: Building a quantum internet with photons and electron spins

Abstract:
How do we get quantum systems to ‘talk’ to each other? How can we distribute entanglement at global scales? I will describe our work tackling these challenges by using light as a robust mediator of quantum interactions between matter qubits. First, I overview the development of optically-active electron spins in silicon carbide as a platform to realize long-distance quantum links. These qubits uniquely combine world-record spin coherence, noiseless single photon emission, and nanophotonic device integration- all in a wafer-scale semiconductor. I will then present our recent discovery of a quantum material with an electro-optic tunability orders of magnitude greater than leading systems, enabled by harnessing quantum phase transitions. This large cryogenic optical nonlinearity paves the way for photonic quantum computing, scaling of superconducting processors, and the microwave-to-optical transduction needed to link leading quantum systems to optical networks. These results highlight the power of controlling electron spins and photons in condensed matter systems to enable a future quantum internet.

Special 290S/290K Quantum Materials Seminar speaker Federica Surace (Caltech), Thursday, May 25 at 2:00 pm in 375 Physics North

Time/Venue Thursday, May 25 at 2:00 pm in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host  Ehud Altman/Alessandra Lanzara
Title Weak perturbations of integrable models
Abstract A quantum integrable system slightly perturbed away from integrability is typically expected to thermalize on timescales of order τ∼λ^(-2), where λ is the perturbation strength. We here study classes of perturbations that violate this scaling, and exhibit much longer thermalization times τ∼λ^(-2k) where k>1 is an integer.  Systems with these "weak integrability breaking'' perturbations have an extensive number of quasi-conserved quantities that commute with the perturbed Hamiltonian up to corrections of order λ^k. We demonstrate a systematic construction to obtain families of such weak perturbations of a generic integrable model for arbitrary k. We then apply the construction to various models, including the Heisenberg, XXZ, and XYZ chains, the Hubbard model, models of spinless free fermions, and the quantum Ising chain. Our analytical framework explains the previously observed evidence of weak integrability breaking in the Heisenberg and XXZ chains under certain perturbations.

Special 290S/290K Quantum Materials Seminar speaker Sara Murciano (Caltech), Thursday, May 25 at 11:00 pm in 375 Physics North

Time/Venue Thursday, May 25 at 11:00 am in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host  Ehud Altman/Alessandra Lanzara
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Special 290S/290K Quantum Materials Seminar speaker Dan Mao (Cornell), Monday, May 22 at 3:00 pm in 375 Physics North

Time/Venue Monday, May 22 at 3:00 pm in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host  Ehud Altman/Alessandra Lanzara
Title Superconducting and excitonic phase-stiffness for interacting isolated narrow bands
Abstract Inspired by the discovery of superconductivity in moir\'e materials with isolated narrow bandwidth electronic bands, I will analyze the question of what is the maximum attainable $T_c$ in interacting flat-band systems. I will focus specifically on the low-energy effective theory, where the density-density interactions are projected to the set of partially-filled flat bands. The resulting problem is inherently non-perturbative, given that the interaction energy scale can be comparable or larger than the band width of the low energy narrow bands, where the standard mean-field approximation is not applicable. Recently, we develop a Schrieffer-Wolff transformation based approach to compute the effective electromagnetic response and the superconducting phase-stiffness in terms of ``projected'' gauge-transformations and extend the formalism to compute the stiffness for excitonic superfluids. Importantly, our method requires neither any ``wannierization'' for the narrow bands of interest, regardless of their (non-)topological character, nor any knowledge of an underlying pairing symmetry, and can be set up directly in momentum-space. We use this formalism to derive upper bounds on the phase-stiffness for sign-problem-free models, where their values are known independently from numerically exact quantum Monte-Carlo computations. We also illustrate the analytical structure of these bounds for the superconducting and excitonic phase-stiffness for perfectly flat-bands with Landau-level-like wavefunctions.

Special 290S/290K Quantum Materials Seminar speaker Carolyn Zhang (University of Chicago), Monday, May 22 at 2:00 pm in 375 Physics North

Time/Venue Monday, May 22 at 2:00 pm in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host  Ehud Altman/Alessandra Lanzara

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Abstract tba