In fall 2021 talks are held primarily in person every Wednesday at 10:30AM in Lyman 425. In spring 2022 talks are held both in person and over Zoom. The directions to the physics department can be found here. Whenever available, slides of the talks will be provided. The details such as title and abstract can be found below. For spring semester, most of the talks are recorded. You can request them by contacting me at tikhanovskaya"at"harvard.edu. See more details below.
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Organizers (Fall 2021  Spring 2022): Paul Hanakata, Grace Zhang, Maria Tikhanovskaya
Talks Spring 2022
May 3, 2022  12:10PM  Google calendar 
Speaker: Juven Wang (Harvard)Title: Ultra Quantum Matter and Beyond the Standard Model PhysicsAbstract: Ongoing ideas developed from the quantum matter and quantum field theory frontier may guide us to explore new physics beyond the 4d Standard Model. I will mention a few such ideas. First, new physics for neutrinos: righthanded neutrinos can carry a Z_16 class mixed gaugegravitational global anomaly index, which could be replaced by 4d or 5d topological quantum field theory, or 4d interacting conformal field theory. These theories provide possible new neutrino mass mechanisms [arXiv:2012.15860]. Proton stability may also be protected by a discrete symmetry [2204.08393]. Second, deconfined quantum criticality between Grand Unified Theories: dictated by a Z_2 class global anomaly, a gapless quantum critical region can happen between GeorgiGlashow and PatiSalam models as deformation of Standard Model, where Beyond Standard Model physics like deconfined dark photons occur as neighbor phases [arXiv:2106.16248, arXiv:2112.14765, arXiv:2202.13498]. Third, the familiar C, P, and T symmetries together with fermion parity (1)^F could "fractionalize" to form a nonabelian finite group structure. Examples include the familiar 4d spin1/2 Dirac fermion [arXiv:2109.15320], and 1d Majorana zero modes [arXiv:2011.12320, arXiv:2011.13921], etc.Additional notes: This is a Zoom talk. Sign up for the mailing list to get the zoom link. Recording is available per request. 

April 26, 2022  12:10PM  
Speaker: Shreya Vardhan (MIT)Title: Bound entanglement and information recovery in thermalized states.Abstract: For mixed states, a quantity called logarithmic negativity provides a better measure of entanglement than mutual information. I will discuss the behavior of this quantity in various universality classes of mixed states that are in a macroscopic thermal equilibrium. At finite temperature, this quantity reveals a rich entanglement phase diagram with some surprising features. In particular, there are regimes where the logarithmic negativity is much larger than the mutual information, implying that there is a large amount of "bound entanglement," which cannot be distilled into EPR pairs. I will also comment on applications to evaporating black holes and related results for the HaydenPreskill experiment at finite temperature.Additional notes: This is a hybrid talk. Recording is available per request. 

April 19, 2022  12:10PM  
Speaker: Ruben Verresen (Harvard)Title: Topological order and longrange entanglement from measuring SPT phases.Abstract: A fundamental distinction between manybody quantum states are those with short and longrange entanglement (SRE and LRE). The latter, such as cat states, topological order, or critical states cannot be created by finitedepth circuits. Remarkably, examples are known where LRE is obtained by performing singlesite measurements on SRE, such as the toric code from measuring a sublattice of a 2D cluster state. In this talk, I will discuss a vast generalization, where LRE appears quite generally by performing singlesite measurements on symmetryprotected topological (SPT) phases. As a special case of such measurementbased conversion of SRE into LRE, we can implement the KramersWannier transformation in finite time, which amounts to gauging a symmetry. As an application, I will outline how this is a scalable method to prepare LRE states using existing platforms such as Rydberg atom arrays, where toric code, nonAbelian topological order, and even exotic fracton phases of matter can be prepared with high fidelity. Time permitting, I will also discuss various open questions raised by this new connection between SPT and LRE.


April 12, 2022  12:10PM  
Speaker: Michael Wang (UMass Amherst)Title: Selflimiting assembly of frustrated puzzlemers: 2D and beyond!Abstract: Geometrically frustrated selfassembly, a process by which the local preferred arrangement of selfassembling particles cannot be realized perfectly on scales much larger than the particles themselves, has recently emerged as a paradigm for potentially controlling the growth and ultimately, the size of selfassembled structures (selflimiting assembling). In this talk, I will describe and analyze a model system of trapezoidalshaped particles (puzzlemers) to understand how particle shape and interactions can generate frustration and affect the morphology and size of the resulting assemblies. By examining the energy landscapes for 2D sheets of puzzlemers, I will show that the assemblies exhibit polymorphism with puzzlemers assembling into either tall, straight or curved, annuluslike structures with finite dimensions, each with unique energetics. In addition, I will identify some key ingredients necessary for assembling these selflimiting structures with wellcontrolled sizes much larger than the individual particles themselves, but still finite. Finally, I will briefly discuss recent attempts to understand how these 2D sheets of puzzlemers can escape or relieve some frustration by buckling into 3D to form tubes or wrinkled sheets.Additional notes: This is a hybrid talk. Recording is available per request (contact Grace). 

April 5, 2022  12:10PM  
Speaker: Nishad Maskara (Harvard)Title: Amplifying topological order parameters with local quantum error correction.Abstract: In this talk, I will discuss ongoing work to develop a technique for amplifying Wilson loops in a Z2 topologically ordered phase by applying local quantum error correction (QEC). Extending the QCNN framework for phase classification of Cong et al (2018), we interpret the application of local QEC as generating a real space RG flow towards the topological fixed point. In the Heisenberg picture, local QEC coarsegrains the bare Wilson loops, with the width of the new Wilson loops dependent on the range of the local QEC procedure. We present theoretical arguments that in the thermodynamic limit, if the coarsegrained Wilson loops to go one then this provides a sufficient condition for certification of topological order. We demonstrate the technique with numerical simulations of the canonical toric code model as well as the recently proposed and observed dimer model on a ruby lattice [Verresen et al. (2020) and Semeghini et al. (2021)]. Further, local error correction can also catch and correct local incoherent errors, suggesting the technique also can be applied to mixed states. We show this using a classical bitflip error model as well. Finally, we present some preliminary results on experimental data from Semeghini et al.Additional notes: This is a hybrid talk. Recording is available per request. 

March 29, 2022  12:00PM  
Speaker: Wen Wei Ho (Stanford)Title: Emergent quantum state designs from quantum chaotic dynamics: a new random matrix universality and applications for quantum information science.Abstract: In this talk I will introduce the idea of a novel kind of emergent random matrix universality that quantum manybody systems can exhibit. Specifically, I will consider quench dynamics of systems which are believed to be quantum chaotic, occuring at infinite temperatures. I will show that the distribution of pure states supported on a small subsystem, generated from projective measurements of the remainder of the system in a local basis, generically approaches a universal form: it becomes uniformly distributed in Hilbert space. This goes beyond the standard paradigm of quantum thermalization, which only dictates that the subsystem at late times relaxes to an ensemble of states that reproduces the expectation values of local observables in a thermal state. Instead, our results imply more generally that the distribution of quantum states themselves is indistinguishable from random states, i.e. the ensemble forms a "quantum state design" in the parlance of quantum information theory. I will show how this phenomenology can be demonstrated in an exactlysolvable toy model, the kicked Ising model, leveraging the dual unitary nature of its dynamics. Lastly, I will briefly sketch how such universal randomness can be harnessed to design algorithms for quantum state learning, i.e. the learning of important physical features of a system, which are implementable in existing quantum simulator platforms despite their limited controls.Additional notes: This is a hybrid talk. Recording is NOT available for this talk. 

March 22, 2022  12:10PM  
Speaker: Ruihua Fan (Harvard)Title: Floquet conformal field theory with spatially deformed Hamiltonians.Abstract: In this talk, I will introduce a class of exactly solvable Floquet manybody systems: 1+1D conformal field theories driven by spatially deformed Hamiltonians. It supports both heating and nonheating phases distinguished by the pattern of energy and entanglement growth. I will introduce a quasiparticle picture to explain the dynamics and why the heating phase can be used for cooling. If time permits, I will also explain what happens if the driving is quasiperiodic or random.Additional notes: This is an online talk. Recording is available per request. 

March 8, 2022  12:00PM  
Speaker: Asja Radja (Harvard)Title: Deciphering the physical rules of biological patterns: how marine glass skeletons form from biological foams.Abstract: Silica (glass) skeletons of radiolarians and phaeodarians, singlecelled protists found in all oceans on earth, produce striking geometric structures that have inspired scientists, artists, and architects for decades. While several attempts have been made to rationalize the formation of these intricate structures, little clarity has emerged as to whether it is more appropriate to consider the patterned skeletons as developing from directly encoded, active cellular processes, or as an energetically passive result of material equilibration. Here I provide the first mathematical model of skeletal patterning based on liquidgas phase separation, a physicochemical process dictated purely by organic constituents of these cells. I show how silica deposition in some radiolarians and phaeodarians across the evolutionary tree may be templated by biological foams and quantify how the dynamics of sphericallybound foams resemble many final forms of radiolarian and phaeodarian skeletons seen in nature.Additional notes: This is a Zoom talk. Recording is available per request (contact Grace). 

March 1, 2022  12:00PM  
Speaker: Xueyang Song (MIT)Title: Doping a chiral spin liquid towards topological superconductivity and duality of critical theories.Abstract: This talk will discuss a novel route to obtain a topological superconducting state with dwave pairing symmetry. This is achieved by doping an exotic quantum spin state  chiral spin liquid(CSL) which breaks time reversal. I'll first introduce the parton formalism used in the study of strongly correlated systems, where the electrons are decomposed into two slave particles. Applied in the CSL, one species of slave particles, called spinons, form a chern insulator. If upon doping the CSL, another slave particle species carrying charge, enters the bosonic integer quantum hall state, then the physical state has vanishing resistivity, i.e. becomes superconducting. In this way a wavefunction is projected from a product of two quantum hall states and describes a d+id superconductor. The gauge flux nucleates electron cooper pairs.
Additional notes: This is a hybrid talk. Recording is available per request. 

February 22, 2022  12:00PM  
Speaker: Gregory KahanamokuMeyer (Berkeley)Title: Classical verification of quantum advantage.Abstract: An important milestone on the path to applicationready quantum computing is the demonstration of quantum computational advantage: solving some problem faster on a quantum computer than would be possible on any classical computer. Excitingly, several experiments have already performed sampling problems which are believed to be intractable for even the world's top supercomputers. But a challenge arises in the verification of these experiments: checking the quantum computer's output requires exponential classical resources, so correctness cannot be explicitly verified at classically intractable system sizes. Here we present protocols for efficientlyverifiable quantum advantage, through cryptographic "proofs of quantumness." These protocols have the combined advantages of polynomialtime classical verification, and security against even adversarial classical impostors via wellstudied cryptographic hardness assumptions. After discussion of the protocols we present progress toward their implementation on nearterm quantum devices.Additional notes: This is a hybrid talk. Recording is available per request. 

February 15, 2022  12:00PM  
Speaker: Dries Sels (NYU  Flatiron institute)Title: Taking arms against a sea of troubles, a critical discussion on manybody localization.Abstract: Understanding the properties of far from equilibrium quantum manybody systems has been at the forefront of quantum condensed matter research for the past decade. In particular, understanding when and how systems can avoid thermalizing under their own dynamics has captivated many of us. Current wisdom dictates that 1D systems with a local Hamiltonian can evade thermalization when subject to sufficiently strong disorder, a phenomenon called manybody localization (MBL).
Additional notes: This is a Zoom talk. Recording is available per request. 

February 8, 2022  12:00PM  
Speaker: Alexander Mietke (MIT)Title: Defect braiding and spontaneous chiral symmetry breaking in dihedral liquid crystals.Abstract: Dihedral (“katic”) liquid crystals (DLCs) are assemblies of microscopic constituent particles that exhibit kfold discrete rotational and reflection symmetries. Generalizing the halfinteger defects in nematic liquid crystals, twodimensional katic DLCs can host point defects of fractional topological charge ±m/k. Starting from a generic microscopic model, we derive a unified hydrodynamic description of DLCs with aligning or antialigning shortrange interactions in terms of GinzburgLandau and LandauBrazovskiiSwiftHohenberg theories for a universal complex orderparameter field. Building on this framework, we demonstrate in particle simulations how adiabatic braiding protocols, implemented through suitable boundary conditions, enable defectpairs to carve out subdomains with an emerging anyonic exchange symmetry: Every braidingexchange of the defectpair shifts the subdomain orientation by a fractional angle 2π/k and k consecutive braids restore the initial configuration of the DLC. Analytic solutions and simulations of the meanfield theory further predict a novel spontaneous chiral symmetrybreaking transition in antialigning DLCs, in quantitative agreement with the patterns observed in particle simulations.Additional notes: This is a Zoom talk. Recording is available per request. 

February 1, 2022  12:00PM  
Speaker: Iris Cong (Harvard)Title: Recognizing Topological Phases of Matter with Quantum Convolutional Neural Networks.Abstract: In this talk, I introduce our recently developed quantum machine learning model called the quantum convolutional neural network (QCNN), which is inspired by the widely successful convolutional neural network (CNN) used for image recognition. Our quantum convolutional neural network (QCNN) makes use of only O(log(N)) variational parameters for input sizes of N qubits, allowing for its efficient training and implementation on realistic, nearterm quantum devices. To explicitly illustrate its capabilities, I show that QCNNs can accurately recognize quantum states associated with a onedimensional symmetryprotected topological phase, with performance surpassing existing approaches. Finally, motivated by recent experimental breakthroughs in the realization of twodimensional topological phases, I then present our latest work on constructing a generic QCNN framework for recognizing twodimensional phases of matter and demonstrating its potential through two examples involving the toric code topological order.Additional notes: This is a Zoom talk. Recording is available per request. 
Talks Fall 2021
December 8, 2021  10:30AM  Google calendar 
Speaker: Botond Tyukodi (Brandeis)Title: Sizecontrol and escape mechanisms in selflimiting assemblies with open boundaries.Abstract: Selflimiting assembly refers to a selfassembly process which autonomously terminates at a large, but welldefined finitesized structures. Such selflimited assembly underlies crucial functions in many biological systems; examples include viral protein capsids, bacterial microcompartments, and other proteinshelled organelles. Achieving similar capabilities in synthetic systems is of great interest for nanotechnology, and recently humanengineered programmable subunits have been developed that form similar selflimited capsid structures. In these biological and synthetic examples, selflimitation is driven by a preferred curvature of the subunits, which causes the structure to close upon itself and thus eliminate boundaries at which additional subunits could assemble. However, mechanisms to achieve selflimited structures with open boundaries have yet to be realized.


December 1, 2021  10:30AM  
Speaker: Ethan Lake (MIT)Title: Dipole conservation and the BoseHubbard model.Abstract: I will discuss a simple model of interacting bosons whose dynamics conserves both boson charge and boson dipole moment. This model, the ``dipolar BoseHubbard model'', provides a simple framework in which the consequences of dipolar conservation laws can be explored. I will discuss the phase diagram of this model in various dimensions, and show how it realizes several rather unusual phases of matter (including an ``insulating superfluid''). This talk is based on joint work with M. Hermele and T. Senthil.Additional notes: This is a hybrid talk. Sign up for the mailing list to get the zoom link. 

November 10, 2021  10:30AM  
Speaker: Luyi Qiu (Harvard University)Title: Bacterial shape: instabilities of rodshaped cells and formation of helices.Abstract: Bacteria are diverse in shape, and their shape is dictated by a rigid, meshlike cell wall. I will first discuss how rodshaped bacteria, modeled as cylindrical shells, can experience an instability upon bending. The study of such instabilities was pioneered by Brazier nearly a century ago, for open tubes (e.g., a common drinking straw). For the case of highly pressurized closed shells  relevant for bacteria such as E. coli  we find that this instability is significantly postponed by its internal pressure, while the cell wall bending stiffness has little influence. This suggests a novel experimental method to infer internal pressure in rodshaped bacteria. Next, I will discuss how rodshaped bacteria may transform into helices due to the combination of inner pressure and heterogeneity in the cell wall elastic modulus, motivated by recent experiments on the helical shaped bacterium H. pylori. We hope these results will shed light on the biological significance of bacteria mechanical structure as well as novel design of transformable materials inspired by biology.Additional notes: This is a hybrid talk. Sign up for the mailing list to get the zoom link. 

November 10, 2021  10:30AM  
Speaker: David Long (Boston University)Title: Boosting the Quantum State of a Cavity with Floquet Driving.Abstract: The striking nonlinear effects exhibited by cavity QED systems make them a powerful tool in modern condensed matter and atomic physics. A recently discovered example is the quantized pumping of energy into a cavity by a strongly coupled, periodically driven spin. I will uncover a remarkable feature of these energy pumps: they coherently translate, or boost, a quantum state of the cavity in the Fock basis. Furthermore, I will argue that the required ultrastrong coupling may be achieved in a rotating frame. Boosting thus enables the preparation of highly excited nonclassical cavity states in near term optical cavity and circuit QED experiments. One need only boost low occupation states.Additional notes: This is a hybrid talk. Sign up for the mailing list to get the zoom link. 

November 3, 2021  10:30AM  
Speaker: Brian Swingle (Brandeis)Title: Quantum Chaos in Adamantane.Abstract: In the context of chaotic quantum manybody systems, we show that operator growth, as diagnosed by outoftimeorder correlators of local operators, also leaves a sharp imprint in outoftimeorder correlators of global operators. In particular, the characteristic spacetime shape of growing local operators can be accessed using global measurements without any local control or readout. Building on an earlier conjectured phase diagram for operator growth in chaotic systems with powerlaw interactions, we show that existing nuclear spin data for outoftimeorder correlators of global operators in adamantane is consistent with our theory. We also predict superpolynomial operator growth in dipolar systems in 3d and discuss the potential observation of this physics in future experiments with nuclear spins and ultracold polar molecules. Forthcoming work with Tianci Zhou.Additional notes: This is a hybrid talk. Sign up for the mailing list to get the zoom link. 

October 27, 2021  10:30AM  
Speaker: Anna Golubeva (MIT)Title: The efficiency of machinelearning quantum states.Abstract: Machine learning has provided a variety of new computational tools for physics that have proven successful for many problems. An important application of ML in quantum manybody physics is the use of generative modeling for wavefunction reconstruction. Since the number of classical parameters needed to encode a quantum wavefunction scales rapidly with the number of qubits, the ability to learn efficient representations is of critical importance. In this talk, I will present the results from two empirical studies that systematically evaluate the scaling of computational resources for reconstructing positivereal wavefunctions with Restricted Boltzmann Machines (RBMs) and investigate pruning as a way to compress the RBM wavefunction representation.Additional notes: This is a hybrid talk. Sign up for the mailing list to get the zoom link. 

October 20, 2021  10:30AM  
Speaker: Anne Meeussen (Harvard University)Title: Shapemorphing matter.Abstract: Thin sheets show unusual behaviour, from crumpling to stiffening when curved. These phenomena arise from a geometrymediated competition between stretching and bending energies, which produces rich emergent effects. Here, we harness a thin sheet with a simple geometrya "groovy sheet" with parallel undulationsto create a multistable structure with minimal preprogramming. We show that our groovy sheets exhibit rapid, easytoactuate, and reversible shapeswitching between a large array of outofplane states via snapthrough instabilities. Our work opens up new vistas for the design of shapeshifting materials, leveraging thin sheets' potential for large elastic deflections.Additional notes: This is an online talk. We still encourage you to come to Lyman 425 and listen to it from there. Food will be provided. For a zoom link please sign up for the mailing list. 

October 13, 2021  10:30AM  
Speaker: Pavel (Pasha) Dolgirev (Harvard University)Title: Coherent terahertz emission in photoexcited striped superconductors.Abstract: In this talk, I will first present our experimental finding that impulsive photoexcitation of striped superconductor LBCO results in a coherent terahertz emission at the Josephson Plasma frequency. This phenomenon is unexpected because no external magnetic field or electric bias has been applied. This effect appears to be correlated with the strength and coherence length of the charge order. I will then argue that behind the observed terahertz emission is the nonlinear optical effect called shift current, which encodes the downconversion of the highfrequency electric field into lowfrequency charge current. In the rest of the talk, I aim to address two questions: 1) given the generation of the shift currents, how do we get sharp coherent emission? and 2) what is the origin of the shift current in the first place? The likely answer to the first question is that the shift currents drive the Surface Josephson Plasmons, which can radiate out because their dispersion is backfolded onto the light cone due to the stripes. For the second question, I will discuss lattice symmetries of various stripes patterns and their implications on the probable origin.Additional notes: This is a hybrid talk. Sign up for the mailing list to get the zoom link. 

October 6, 2021  10:30AM  
Speaker: Dominic Else (Harvard University)Title: Critical drag as a mechanism for resistivity, with application to strange metals.Abstract: In this talk, I will discuss very general ways to reason about electrical resistivity in metals, beyond the conventional Fermi liquid theory. This will lead to a proposal for a new mechanism for resistivity that we call "critical drag", where the resistivity originates from critical fluctuations. This is strikingly different to more conventional resistivity mechanisms that involve violation of conservation laws such as momentum conservation. Furthermore, I will argue on general grounds that critical drag is the only resistivity mechanism that is compatible with certain basic experimentally motivated assumptions about the socalled "strange metals" seen in cuprates and other materials.
Additional notes: This is an online talk. We still encourage you to come to Lyman 425 and listen to it from there. Food will be provided. For a zoom link please sign up for the mailing list. 

September 29, 2021  10:30AM  
Speaker: Soonwon Choi (MIT)Title: Emergence of universal randomness from chaotic quantum dynamics.Abstract: Universality  the emergence of features independent of precise microscopic details  allows us to simplify the analysis of complex systems and to establish important general principles. For example, quantum thermalization constitutes a universal behavior emerging from outofequilibrium dynamics, as it prescribes that the density matrix of a local subsystem is driven to a Gibbs ensemble under generic dynamics of isolated quantum manybody systems, independent of the details of initial states.
Additional notes: 

September 22, 2021  10:30AM  
Speaker: Jinghui Liu (MIT)Title: Topological defects and information flows on the membrane of a living cell.Abstract: Topological defects determine the structure and function of physical and biological matter over a wide range of scales, from the turbulent vortices in planetary atmospheres, oceans or quantum fluids to bioelectrical signaling in the heart and brain. While many advances have been made in understanding the defect dynamics in passive nonequilibrium fluids, it remains elusive whether physical laws that govern their statistics and dynamics extend to information flows in biologically active processes. Here, I will discuss several recent studies on a defectmediated turbulence that underlies the complex wave propagation patterns of RhoGTP signaling protein on the membrane of starfish egg cells, a process closely relevant to information processing in early developments. Combining direct experimental observations with quantitative analysis and mathematical modeling, we show that the phase velocity field extracted from the RhoGTP concentration waves exhibits vortical defect motions and annihilation dynamics reminiscent of those seen in quantum fluids and active nematics. Moreover, these spiral wave cores undergo spontaneous braiding dynamics and can be mapped quantitatively to predictions from a generic continuum theory. We hope these results will shed light on designing and controlling biological structures with logical capabilities which feature robust and efficient informationprocessing units.Additional notes: 

September 15, 2021  10:30AM  
Speaker: Daniel Parker (Harvard University)Title: Fractional Chern Insulators and Hofstadter Band Geometry in MagicAngle GrapheneAbstract: Fractional Chern Insulators (FCIs) generalize the celebrated fractional quantum hall effect to the lattice setting. A number of theoretical proposals have suggested (hBNaligned) magicangle graphene (hBNMATBG) is a prime candidate for realizing FCIs, as its bandstructure and quantum geometry are relatively close to that of the lowest Landau level. Indeed, this was borne out in a recent experiment from the Yacoby group [1], which observed 8 FCIs in hBNMATBG at magnetic fields as low as 5 Tesla. This talk will examine a constellation of questions surrounding this experiment. Can we understand the appearance of these FCIs? What quantum geometric conditions are necessary to favor FCIs in the minibands of the Hofstadter butterfly? Can MATBG support FCIs without an external field?[1] Xie et al., arXiv: 2107.10854 Additional notes: 