Student Poster Abstracts
Please note that while there is not one required or mandatory size, all posters should fit in a space no longer than 36” (91 cm) wide and preferably be no more than about 48” (122 cm) in height. Arrangements can be made for other sizes as necessary.
If you are not able to get your poster printed before you leave home (or are unable to easily transport), you may get posters printed at FedEx Office (http://local.fedex.com/co/boulder/office0402/).
Silvia Cardenas Lopez Columbia University 
Manybody superradiance and dynamical symmetry breaking in waveguide QEDThe manybody decay of extended collections of twolevel systems remains an open problem. Here, we investigate whether an array of emitters coupled to a onedimensional bath undergoes Dicke superradiance, a process whereby a completely inverted system synchronizes as it decays, generating correlations between emitters via dissipation. This leads to the release of all the energy in the form of a rapid photon burst. We derive the minimal conditions for the burst to happen as a function of the number of emitters, the chirality of the waveguide, and the singleemitter optical depth, both for ordered and disordered ensembles. Manybody superradiance occurs because the initial fluctuation that triggers the emission is amplified throughout the decay process. We show that this avalanchelike behavior leads to a dynamical spontaneous symmetry breaking, where most photons are emitted into either the left or the rightpropagating optical modes, giving rise to an emergent chirality. Superradiant bursts may be a smoking gun for the generation of correlated photon states of exotic quantum statistics. This physics can be explored in diverse setups, ranging from atoms close to nanofibers to superconducting qubits coupled to transmission lines. 

Jeremy Côté Université de Sherbrooke 
Entanglement phase transition with spin glass criticalityWe define an ensemble of random Clifford quantum circuits whose output state undergoes an entanglement phase transition between two volumelaw phases as a function of measurement rate. Our setup maps exactly the output state to the ground space of a spin glass model. We identify the entanglement phases using an order parameter that is accessible on a quantum chip. We locate the transition point and evaluate a critical exponent, revealing spin glass criticality. Our work establishes an exact statistical mechanics theory of an entanglement phase transition. 

Haoyang Gao Harvard University 
Title to comeAbstract to come 

Hyunsoo Ha Princeton University 
Title to comeAbstract to come 

Jacob Hauser University of California, Santa Barbara 
Continuous symmetry breaking in adaptive quantum dynamicsAdaptive quantum circuits, in which unitary operations, measurements, and feedback are used to steer quantum manybody systems, provide an exciting opportunity to generate new dynamical steady states. To begin understanding what properties such target states can have, we introduce a circuit model with continuous symmetry where unitary operations, measurements, and local unitary feedback are used to drive ordering. In this setting, we find a pure steady state hosting symmetrybreaking order, which is the ground state of a gapless, local Hamiltonian. We explore the dynamical properties of the approach to this steady state. We find that this steadystate order is fragile to perturbations, even those that respect the continuous symmetry. 

Anish Kulkarni Princeton University 
Lindbladian dynamics of the SachdevYeKitaev modelWe study the open quantum dynamics of the SachdevYeKitaev (SYK) model. The SYK system is coupled to a Markovian bath. The dynamics is thus described by the Lindblad master equation. We compute various physical properties in the limit of large number of Majorana fermions. As a probe of the open quantum dynamics, we compute the averaged Loschmidt echo, namely the average overlap between the initial and timeevolved density matrices. The Loschmidt echo reveals first and secondorder dynamical phase transitions in the SYK Lindbladian models. We also compute the steadystate Green’s functions and associated decay rates and find an underdamped to overdamped transition. 

Abhishek Kumar University of Massachusetts, Amherst 
Title to comeAbstract to come 

Ethan Lake MIT 
Title to comeAbstract to come 

Cathy Li University of Pennsylvania 
Statistical Mechanics of Monitored Dissipative Random CircuitsDissipation is inevitable in realistic quantum circuits. We examine the effects of dissipation on a class of monitored random circuits that exhibit a measurementinduced entanglement phase transition. This transition has previously been understood as an ordertodisorder transition of an effective classical spin model. We extend this mapping to include onsite dissipation described by the dephasing and spontaneous emission channel and study the corresponding 2D Ising model with generalized interactions and develop diagrammatic methods for the exact Boltzmann weights of the bonds in terms of probability of measurement p, the dissipation rate Γ and the onsite Hilbert space dimension q. The dissipation plays the role of Z2symmetrybreaking interactions, while small measurement rates reduces the ratio of the symmetrybreaking interactions to the pairwise interactions, conducive to longrange order. We analyze the dynamical regimes of the R ́enyi mutual information and find that the joint action of monitored measurements and dissipation yields short time, intermediate time and steady state behavior that can be understood in terms of crossovers between different classical domain wall configurations. The presented analysis applies to monitored open or Lindbladian quantum systems and provides a tool to understand entanglement dynamics in realistic dissipative settings and achievable system sizes. 

Gunhee Park Caltech 
Connections between tensor network influence functional and realtime density matrix embedding theoryWe develop connections between tensor networkbased influence functional method and realtime density matrix embedding theory (DMET) for computing nonequilibrium electron dynamics in strongly correlated systems, specifically in nonequilibrium quantum impurity problems. In realtime DMET theory, the equation of motion is derived using the timedependent variational principle. Here, we formulate tensor network method that can project bath degrees of freedom into finite degrees of discretized bath. We derive the equation of motion for the coupled impurity and bath wavefunction within the discretized bath using tensor network truncations. The numerical performance is compared in the quench dynamics of single impurity Anderson model. 

Klée Pollock Iowa State University 
Variational Microcanonical EstimatorWe propose a variational quantum algorithm for estimating microcanonical expectation values in models obeying the eigenstate thermalization hypothesis. Using a relaxed criterion for convergence of the variational optimization loop, the algorithm generates weakly entangled superpositions of eigenstates at a given target energy density. An ensemble of these variational states is then used to estimate microcanonical averages of local operators, with an error whose dominant contribution decreases as a power law in the size of the ensemble. We apply the algorithm to the onedimensional mixedfield Ising model, where it converges for ansatz circuits of depth roughly linear in system size. The most accurate thermal estimates are produced for intermediate energy densities and for local operators that appear in the Hamiltonian. In our error analysis, we find connections with recent works investigating the underpinnings of the eigenstate thermalization hypothesis. In particular, the failure of energybasis matrix elements of local operators to behave as independent random variables is a potential source of error that the algorithm can overcome by averaging over an ensemble of variational states. 

Michael Rampp Max Planck Institute for the Physics of Complex Systems 
Title to comeAbstract to come 

Wesley Roberts Northeastern University 
Fidelity of the Kitaev honeycomb model under a quenchMotivated by rapid developments in the field of quantum computing and the increasingly diverse nature of qubits, we theoretically study the influence that quenched outside disturbances have in an intermediately long time limit. We consider localized imperfections, uniform fields, noise, and couplings to an environment which we study in a unified framework using a prototypical but idealized interacting quantum device  the Kitaev honeycomb model. Our study focuses on the quantum state robustness in response to an outside magnetic field, a magnetic bath, magnetic noise, magnetic impurities, and a noisy impurity. As indicators for quantum robustness, we use the Uhlmann fidelty of the ground state and excited spinon states after a quench. We find that the time dependence of the fidelity often depends crucially on whether the system is gapped. We find that in the gapped case the fidelity decays to a constant value under noiseless quenches, while in a gapless system it exhibits algebraic decay. In all other situations studied, such as coupling to a bath and noisy quenches, both gapped and gapless systems exhibit a universal form for the longtime fidelity, Ce−αtt−β, where the values of C, α, and β depend on physical parameters such as system size, disturbance strength, etc. Therefore, our work provides estimates for the intermediatelong time stability of a quantum device and it suggests under what conditions there appear the hallmarks of an orthogonality catastrophe in the timedependence of the fidelity. Our work provides engineering guidelines for quantum devices in quench design and system size. 

Sibaram Ruidas Indian Institute of Science 
Classical limit of measurementinduced transition in manybody chaos in integrable and nonintegrable oscillator chainsWe discuss the dynamics of integrable and nonintegrable chains of coupled oscillators under continuous weak position measurements in the semiclassical limit. We show that, in this limit, the dynamics is described by a standard stochastic Langevin equation, and a measurementinduced transition appears as a noise and dissipationinduced chaoticto nonchaotic transition akin to stochastic synchronization. In the nonintegrable chain of anharmonically coupled oscillators, we show that the temporal growth and the ballistic lightcone spread of a classical outoftime correlator characterized by the Lyapunov exponent and the butterfly velocity are halted above a noise or below an interaction strength. The Lyapunov exponent and the butterfly velocity both act like order parameters, vanishing in the nonchaotic phase. In addition, the butterfly velocity exhibits a critical finitesize scaling. For the integrable model, we consider the classical Toda chain and show that the Lyapunov exponent changes nonmonotonically with the noise strength, vanishing at the zero noise limit and above a critical noise, with a maximum at an intermediate noise strength. The butterfly velocity in the Toda chain shows a singular behavior approaching the integrable limit of zero noise strength. 

Nanako Shitara University of Colorado, Boulder 
Noise Spectroscopy Without Dynamical Decoupling PulsesSpectral characterization of noise environments that lead to the decoherence of qubits is critical to developing robust quantum technologies. While dynamical decoupling offers one of the most successful approaches to characterize noise spectra, it necessitates applying large sequences of π pulses that increase the complexity and cost of the method. In this talk, I will introduce a noise spectroscopy method that utilizes only the Fourier transform of free induction decay measurements, thus removing the need for the application any π pulses. This method faithfully recovers the correct noise spectra and outperforms previous dynamical decoupling schemes while significantly reducing its experimental overhead. I will also discuss the experimental feasibility of our proposal and demonstrate its robustness in the presence of statistical measurement noise. Our method is applicable to a wide range of quantum platforms and provides a simpler path toward a more accurate spectral characterization of quantum devices, thus offering possibilities for tailored decoherence mitigation. 

Hansveer Singh University of Massachusetts, Amherst 
Title to comeAbstract to come 

Spenser Talkington University of Pennsylvania 
Response of Flat Bands and Exceptional Points in Fermionic Lindbladian SystemsExceptional points in parameter space where eigenvalues and eigenvectors coalesce are a key feature of nonHermitian systems. NonHermiticity can originate with dissipation processes that lead to the gain or loss of energy and/or particles. In open quantum systems, these processes can be represented via Lindbladian superoperators that determine the time evolution of the density matrix of a system subjected to continuous measurements by a memoryless bath. For quadratic Hamiltonians the Lindbladian can be reformulated in terms of right and left superfermions/superbosons with a lowdimension matrix representation. We showed (in PRB 106, 161109) conditions under which symmetries of the dissipation (jump operators) ensure the formation of flat bands that are protected from dissipation. This matrix representation can then be rotated to obtain the retarded, advanced, and Keldysh Green’s functions, which in turn can be used to calculate expectation values and response properties of materials. Here we review this work and in particular consider the optical conductivity of systems with flat bands and systems with exceptional points. 

Nathanan Tantivasadakarn Caltech 
title to comeAbstract to come 

Julian Thoenniss University of Geneva 
An efficient method for quantum impurity problems via matrixproduct states in the temporal domainWe introduce an efficient method to simulate dynamics of an interacting quantum impurity coupled to noninteracting fermionic reservoirs. Viewing the impurity as an open quantum system, we describe the reservoirs by their FeynmanVernon influence functionals (IF). The IF are represented as matrixproduct states in the temporal domain, which enables an efficient computation of dynamics for arbitrary interactions. We apply our method to study quantum quenches and transport in an Anderson impurity model, including highly nonequilibrium setups, and find favorable performance compared to stateoftheart methods. The computational resources required for an accurate computation of dynamics scale polynomially with evolution time, indicating that a broad class of outofequilibrium quantum impurity problems are efficiently solvable. 

Foster Thompson University of Minnesota 
Title to comeAbstract to come 

Iris Ulčakar Jožef Stefan Institute, Slovenia 
Iterative construction of most necessary conserved quantitiesGeneral Gibbs ensembles (GGEs) have been proposed as a local steady state description of integrable manybody systems after a quantum quench. Similarly, GGEs also approximately describe the nonequilibrium steady states of integrable systems weakly coupled to nonequilibrium Markovian baths. We present an iterative method for the latter case, constructing the most necessary conserved quantities for an efficient truncated GGE description. Guided by the integrabilitybreaking perturbations, the space of conserved quantities is spanned by the action of the baths’ superoperators. 

Carlo Vanoni SISSA  International School for Advanced Studies 
Interface dynamics in the 2D quantum Ising modelWe consider the nonequilibrium dynamics of the 2d quantum Ising model in the regime of strong ferromagnetic coupling. We study the dynamics of domain walls separating regions of reversed spin orientation. For many initial configurations, we are able to map the problem to a fermionic chain, and show that at leading order there is an emergent integrability. The particular case of a large corner is investigated in details and we then discuss how integrability is broken when the ferromagnetic coupling is large but finite. We show that a symmetrybreaking longitudinal field gives rise to a robust ergodicity breaking in two dimensions, a phenomenon underpinned by Stark manybody localization of the emergent fermionic excitations of the interface. 

Melissa Will Technical University of Munich 
Hilbert space fragmentation in a tilted, twodimensional BoseHubbard modelQuantum manybody systems out of equilibrium can exhibit very rich and exciting phenomena. A particularly relevant question is whether and how a quantum system thermalizes under its own unitary evolution. In this context, three classes of systems can be distinguished: ergodic, localized and intermediate regimes exhibiting quantum manybody scars. We consider a Bose Hubbard model on a square lattice in presence of a tilted field and demonstrate perturbatively the emergence of Hilbert space fragmentation (HSF) when fine tuning the onsite interactions. We investigate the quench dynamics of this system and find that the relaxation dynamics strongly depends on the chosen initial state—which we interpret as a signature of HSF. Furthermore, we identify point like excitations on top of certain frozen states, for which we analyze their dynamics using a cellular automaton. 