Student Poster Abstracts

Realizing and probing programmable 2D optical lattices with flexible geometries and connectivity

Over the past decade, ultracold atoms in optical lattices have become a vital platform for experimental quantum simulation, enabling precise studies of a variety of quantum many-body problems. For most experiments, the layout of the confining lattice beams restricts the accessible lattice configurations and thus the underlying physics. Here, we present a novel tunable lattice, which provides programmable unit cell connectivity and in principle allows for changing the geometry mid-sequence. Our approach builds on the generation of phase-stable realisation of a square or triangular base lattice combined with microscopically projected repulsive local potential patterns. With this technique, we realise Lieb and Kagome lattices, and benchmark the various configurations by exploring single particle quantum walks. We explore many body physics in these lattices by observing parity fluctuations associated with the superfluid-to-Mott insulator transition. As an outlook, we will explore how the presented lattices can be applied for spin-selective imaging as well as doublon detection.

Quantum computation from dynamic automorphism codes

We propose a new model of quantum computation comprised of low-weight measurement sequences that simultaneously encode logical information, enable error correction, and apply logical gates. These measurement sequences constitute a new class of quantum error- correcting codes generalizing Floquet codes, which we call dynamic automorphism (DA) codes. We construct an explicit example, the DA color code, which is assembled from short measurement sequences that can realize all 72 automorphisms of the 2D color code. On a stack of N triangular patches, the DA color code encodes N logical qubits and can implement the full logical Clifford group by a sequence of two- and, more rarely, three-qubit Pauli measurements. We also make the first step towards universal quantum computation with DA codes by introducing a 3D DA color code and showing that a non-Clifford logical gate can be realized by adaptive two-qubit measurements.

Many-Body Superradiance and Dynamical Mirror Symmetry Breaking in Waveguide QED

The many-body decay of extended collections of two-level systems remains an open problem. Here, we investigate whether an array of emitters coupled to a one-dimensional bath undergoes Dicke superradiance. This is a process whereby a completely inverted system becomes correlated via dissipation, leading 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 single-emitter optical depth, both for ordered and disordered ensembles. Many-body superradiance occurs because the initial fluctuation that triggers the emission is amplified throughout the decay process. In one-dimensional baths, this avalanchelike behavior leads to a spontaneous mirror symmetry breaking, with large shot-to-shot fluctuations in the number of photons emitted to the left and right. Superradiant bursts may thus be a smoking gun for the generation of correlated photon states of exotic quantum statistics.

Floquet codes with a twist

Floquet codes are a novel class of dynamical quantum error-correcting codes, whose ability to encode quantum information emerges from a periodic schedule of non-commuting measurements. Early examples of Floquet codes offer a promising alternative to the well-established surface code, since they exhibit relatively high error thresholds and can be efficiently implemented using only two-body measurements. In this work, we describe a new means for storing and processing quantum information in Floquet codes by introducing point-like twist defects. We focus on an example in which the dynamics possess an anomalous 1-form symmetry, which allows us to create twist defects by gauging the 1-form symmetry along a spatial path. Importantly, our construction maintains the connectivity of the lattice, preserves the period of the measurement schedule, and requires only two-body measurements. We also discuss how the notion of higher gauging can also be used to construct Floquet codes characterized by the topological order of certain Abelian twisted quantum doubles.

Anyon response in field-driven quantum spin liquids

Whether the fractionalized excitations, such as those that emerge from Kitaev’s honeycomb model, have sharp signatures that can be detected in experiments? We show that while the response to single spin excitations is broad, the spectral function corresponding to two-spin excitations across a bond in the Abelian spin liquid phase has sharp signatures that can be attributed to specific anyons even at the linear response regime. Driven by a magnetic field, the composite gauge fermion therein disperses along a specific one-dimensional direction that provides a fingerprint of fractionalization. At lower anistropy in the center of the Abelian phase, the field generates a hybridization between the gauge fermion and the majorana fermion, resulting in a different one-dimensional dispersion pattern. These highly constrained fracton-like dispersions can be observable by inelastic light and neutron scattering, thereby providing “smoking gun” signatures of fractionalization in the Abelian QSL phase. We also discuss the deconfined spinon in the intermediate-field-driven phase of the Kitaev model, which harbors a neutral gapless spinon Fermi surface. By using a data-centric quantum-classical hybrid machine learning approach that characterizes complex quantum many-body states using interpretable classical machine learning, we demonstrate spinon Friedel oscillation at momenta 2kF potentially measurable in magnetic scattering or STM experiments.

Extensive long-range entanglement in a nonequilibrium steady state

Entanglement measures constitute powerful tools in the quantitative description of quantum many-body systems out of equilibrium. We analytically study entanglement in the steady state of noninteracting fermions, occupying a one-dimensional lattice that hosts a generic scatterer at its center and is coupled at its ends to two biased reservoirs. We show that disjoint intervals located on opposite sides of the scatterer, and within similar distances from it, maintain volume-law entanglement regardless of their separation, as measured by their fermionic negativity. The mutual information of the intervals, which quantifies the total correlations between them, follows a similar scaling. We find that this behavior arises whenever the occupation functions of the two reservoirs differ, thus capturing both the case of a chemical-potential bias and the case of a temperature bias (as well as any combination of the two). By deriving exact expressions for the extensive terms of the negativity and mutual information, we prove their simple and universal functional dependence on the scattering probabilities associated with the scatterer. This simple dependence allows to demonstrate that the strong long-range entanglement is generated by the coherence between the transmitted and reflected parts of propagating particles within the energy window of the bias.

This contribution is based on arXiv:2205.12991, as well as on newer results.

Many-Body Resonances in the Avalanche Instability of Many-Body Localization

Many-body localized (MBL) systems fail to reach thermal equilibrium under their own dynamics, even though they are interacting, nonintegrable, and in an extensively excited state. One instability toward thermalization of MBL systems is the so-called “avalanche,” where a locally thermalizing rare region is able to spread thermalization through the full system. The spreading of the avalanche may be modeled and numerically studied in finite one-dimensional MBL systems by weakly coupling an infinite-temperature bath to one end of the system. We find that the avalanche spreads primarily via strong many-body resonances between rare near-resonant eigenstates of the closed system. Thus we find and explore a detailed connection between many-body resonances and avalanches in MBL systems.

Measurement-induced phase transitions in the Sachdev-Ye-Kitaev model

Continuous monitoring of an otherwise closed quantum system has been found to lead to a measurement-induced phase transition (MIPT) characterized by abrupt changes in the entanglement or purity of the many-body quantum state. For an entanglement MIPT, entangling dynamics compete with measurement dynamics, pushing the system either to a phase with extensive entanglement or to a phase with low-level entanglement. For purification MIPTs, projective measurements effectively cool and localize the system, inducing a transition from a mixed state to an uncorrelated pure state. In this work, we numerically simulate monitored dynamics in the all-to-all Sachdev-Ye-Kitaev (SYK) model for finite N. We witness both entanglement and purification MIPTs in the steady-state. It is often said that there is an equivalence between entanglement and purification MIPTs, however we provide numerical evidence to the contrary, implying that entanglement and purification MIPTs are indeed two distinct phenomena. The reason for such a distinction is quite simple: entanglement can revive after a completely projective measurement – if measurements do not occur too often in time – but impurity cannot.

Unitary k-designs from random number-conserving quantum circuits

Local random circuits scramble efficiently and accordingly have a range of applications in quantum information and quantum dynamics. With a global U(1) charge however, the scrambling ability is reduced; for example, such random circuits do not generate the entire group of number-conserving unitaries. We show that finite moments cannot distinguish the ensemble that local random circuits generate from the Haar ensemble on the entire group of number-conserving unitaries. Specifically, the circuits form a k_c-design with k_c = O(L^d) for a system in d spatial dimensions with linear dimension L. For k < k_c, the depth τ to converge to a k-design scales as τ \gtrsim k L^d+2.  In contrast, without number conservation τ \gtrsim k L^d. The convergence of the circuit ensemble is controlled by the low-energy properties of a frustration-free quantum statistical model which spontaneously breaks k U(1) symmetries. The associated Goldstone modes are gapless and lead to the predicted scaling of τ.  Our variational bounds hold for arbitrary spatial and qudit dimensions; we conjecture they are tight.

Page curves and typical entanglement in linear optics

Bosonic Gaussian states are a special class of quantum states in an infinite dimensional Hilbert space that are relevant to universal continuous-variable quantum computation as well as to near-term quantum sampling tasks such as Gaussian Boson Sampling. In this work, we study entanglement within a set of squeezed modes that have been evolved by a random linear optical unitary. We first derive formulas that are asymptotically exact in the number of modes for the Rényi-2 Page curve (the average Rényi-2 entropy of a subsystem of a pure bosonic Gaussian state) and the corresponding Page correction (the average information of the subsystem) in certain squeezing regimes. We then prove various results on the typicality of entanglement as measured by the Rényi-2 entropy by studying its variance. Using the aforementioned results for the Rényi-2 entropy, we upper and lower bound the von Neumann entropy Page curve and prove certain regimes of entanglement typicality as measured by the von Neumann entropy. Our main proofs make use of a symmetry property obeyed by the average and the variance of the entropy that dramatically simplifies the averaging over unitaries. In this light, we propose future research directions where this symmetry might also be exploited. We conclude by discussing potential applications of our results and their generalizations to Gaussian Boson Sampling and to illuminating the relationship between entanglement and computational complexity.

Lindbladian dynamics of the Sachdev-Ye-Kitaev model

We study the open quantum dynamics of the Sachdev-Ye-Kitaev (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 time-evolved density matrices. The Loschmidt echo reveals first and second-order dynamical phase transitions in the SYK Lindbladian models. We also compute the steady-state Green’s functions and associated decay rates and find an underdamped to overdamped transition.

Multipole Symmetries in Many-Body Physics

I will discuss recent work on the many body physics of systems with multipolar conservation laws, in which a total charge and various multipole moments thereof are conserved. Particular focus will be placed on Hubbard models with dipole moment conservation, which enjoy a natural experimental realization in strongly tilted optical lattices. The bosonic versions of these models possess unusual insulating Bose-Einstein condensates and unconventional patterns of spontaneous symmetry breaking, while the fermionic versions host exotic non-Fermi liquids. I will also discuss the ramifications that multipole conservation has for diffusive dynamics, where it leads to unusually large dynamical exponents and exponentially localized steady states.

Dynamical phase transitions in noisy random circuits

Understanding the dynamics of noisy random circuits holds significant theoretical importance and practical relevance. By examining the dynamics of quantum and classical correlations under various error rates, we observe and analyze sharp transitions in both time and error rate. Specifically, the entanglement dynamics exhibit distinct behaviors dependent on the scaling of the error rate in terms of system size N. (1) When the error rate decays faster than 1/N, entanglement rapidly saturates and decreases at a constant rate. (2) When the error rate decays at a rate of 1/N, entanglement gradually increases and then decreases at a constant rate. (3) When the error rate decays slower than 1/N, entanglement accumulates and abruptly disappears thereafter. Remarkably, for all cases, the entanglement vanishes non-analytically at the transition time, marking a sharp boundary between these different dynamical regimes. To investigate these dynamical phase transitions, we introduce the bipartite semiglobal model—a simplified circuit that captures the essential interplay between scrambling and decoherence. Our analysis is further supported by numerical simulations, validating our conclusions.

Gapped interfaces in fracton models and foliated fields

This work investigates the gapped interfaces of foliated field theories that describe fracton phases of matter, focusing on the X-cube model as an example. We analyze the gapped interfaces between X-cube and the vacuum (gapped boundary), 3+1D toric code, and another X-cube model. The interfaces can be either undecorated or decorated, with the latter involving adding $K$-matrix Chern-Simons type terms to specific undecorated interfaces. We systematically identify the undecorated interfaces for the aforementioned cases and explore general decorations, supported by many examples. We further demonstrate the formulation of lattice models for the general gapped interfaces found in foliated field theories and present several illustrative instances. While previous literature has discussed the gapped boundaries of the X-cube model, our formalism facilitates the straightforward establishment of new decorated boundaries. This work is a collaboration with Po-Shen Hsin and Ananth Malladi.

Programmable adiabatic demagnetization for systems with trivial and topological excitations

Preparing the ground state of a many-body Hamiltonian on a quantum device is of central importance, both for quantum simulations of molecules and materials, and for a variety of quantum information task. Different approaches to ground state preparation have been proposed, including variational quantum simulation, adiabatic evolution and, more recently, also simulated cooling.

We propose a simple, robust protocol to prepare a low-energy state of an arbitrary Hamiltonian on a quantum computer. The protocol is inspired by the “adiabatic demagnetization” technique, used to cool solid state systems to extremely low temperatures. The adiabatic cooling protocol is demonstrated via an application to the transverse field Ising model. We use half of the qubits to model the system and the other half as a bath. Each bath spin is coupled to a system spin. In a strong magnetic field, the bath spins are prepared in the polarized ground state. By an adiabatic downward sweep of the magnetic field, we change the energy of the bath spins and allow for resonant processes that transfer entropy from the system to the bath qubits. After each cycle, the bath is reset to the ground state.

We find that the performance of the algorithm in the presence of a finite error rate depends on the nature of the excitations of the system; systems with non-local (topological) excitations are more difficult to cool. Finally, we explore ways to partially mitigate this problem.

arXiv:2210.17256

Effect of (non)-magnetic disorder in quasi-1D singlet superconductors

We investigate the effects of the combination of interactions and disorder in a quasi 1D system. In this case, the critical temperature of superconductivity is an interesting observable for this purpose. Anderson’s theorem indeed states that BCS-type superconductivity is resistant to non-magnetic disorder because time- reversal invariance is still preserved. In quasi 1D systems, since there the effect of disorder and interactions is more important than in higher dimensional systems, the Anderson theorem is generally not respected.

We here study the competition between disorder and interactions in such systems by considering forward scattering disorder, both for magnetic and non-magnetic impurities. Using a field theory representation and renormalization, we show that non magnetic disorder preserves Tc in agreement with Anderson theorem. However, for the magnetic disorder, we find a reduction of the spin-gap and compute the reduction of Tc. We investigate the consequences for systems made of fermionic tubes with at- tractive interactions.

Measurement-induced phase transition in Toric code

We show how distinct phases of matter can be generated by performing random single-qubit measurements on a subsystem of toric code. Using a parton construction, such measurements map to random Gaussian tensor networks, and in particular, random Pauli measurements map to a classical loop model in which watermelon correlators precisely determine measurement-induced entanglement.  Measuring all but a 1d boundary of qubits realizes hybrid circuits involving unitary gates and projective measurements in 1+1 dimensions.  We find that varying the probabilities of different Pauli measurements can drive transitions in the un-measured boundary between phases with different orders and entanglement scaling, corresponding to short and long loop phases in the classical model.  Furthermore, by utilizing single-site boundary unitaries conditioned on the bulk measurement outcomes, we generate mixed state ordered phases and transitions that can be experimentally diagnosed via linear observables. This demonstrates how parton constructions provide a natural framework for measurement-based quantum computing setups to produce and manipulate phases of matter.

Efficient embedding framework for tensor network influence functional of Anderson impurity model

We develop a quantum embedding theory for constructing a tensor network-based influence functional method in quantum impurity models, such as the Anderson impurity model. Here, we formulate a tensor network method that can project bath degrees of freedom into the embedding bath orbital space. The original discrete-time formalism is extended to the continuous-time formulation, which is expressed by the differential equation of motion in the embedding space. This framework leads to the algorithm that requires only linear computational costs to the number of timesteps with smaller timestep errors. The numerical performance is compared in the quench dynamics of the single impurity Anderson model.

Non-Equilibrium Steady States of Driven-Dissipative Lattice Models: Exact Solutions, Topological Light, and Interaction Induced Stability

The intersection of many-body quantum systems with open dynamics can lead to a number of diverse phenomena that do not appear in their closed counterpart. Specifically in this poster, we begin by looking at a few examples of reservoir engineering – the process by which we can couple a quantum system to a structured (Markovian) bath that deterministically relaxes the quantum system into a nontrivial steady state. We study a non-integrable open spin chain whose steady state is a so-called rainbow state, as well as a way to use correlated dissipation to selectively excite topological edge modes in a photonic lattice. In addition to this, we study ways one can systematically add Hamiltonian interactions to a dynamically unstable photonic lattice to cut off an instability, leading to surprising phenomena like topological lasing as well as the dissipative stabilization of highly non-classical states of light.

Hayden-Preskill recovery in chaotic and integrable unitary-circuit dynamics

Quantum many-body systems distribute localized information over many non-local degrees of freedom in the course of their time evolution, a process called information scrambling. Information scrambling is related to fundamental questions in quantum dynamics such as thermalization, quantum chaos, and entanglement spreading, and to the control of information dynamics in quantum computing platforms. The Hayden-Preskill protocol probes the capability of information recovery from local subsystems after unitary transformations. It has mostly been discussed for global unitary transformations in the thermodynamic limit without any internal time or length scales. Here, we investigate the use of Hayden-Preskill recovery as a dynamical probe of scrambling in local quantum many-body systems. We present exact results for certain classes of unitary circuit models, both structured (dual-unitary) and Haar-random circuits, and discuss different dynamical signatures corresponding to information transport or scrambling, respectively. Surprisingly, certain chaotic circuits can transport information with perfect fidelity. In integrable dual-unitary circuits, we relate the information transmission to the transport and scattering of quasiparticles. Using numerical and analytical insights, we argue that the qualitative features of information recovery extend away from these solvable points. Our results suggest that information recovery protocols can serve to distinguish chaotic and integrable behavior, and that they can be used to identify characteristic dynamical features, such as quasiparticles or underlying dual-unitarity.

Chaos in perturbed dual unitary models

Dual unitary models have been proven to follow quantum chaotic dynamics. In this work, we investigate if the chaotic properties of a wide class of dual unitary models are stable under perturbations away from the dual unitary point.

Classical limit of measurement-induced transition in many-body chaos in integrable and non-integrable oscillator chains

We discuss the dynamics of integrable and non-integrable 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 measurement-induced transition appears as a noise- and dissipation-induced chaotic-to- non-chaotic transition akin to stochastic synchronization. In the non-integrable chain of anharmonically coupled oscillators, we show that the temporal growth and the ballistic light-cone spread of a classical out-of-time 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 non-chaotic phase. In addition, the butterfly velocity exhibits a critical finite-size scaling. For the integrable model, we consider the classical Toda chain and show that the Lyapunov exponent changes non-monotonically 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.

Boundary conditions dependence of the phase transition in the quantum Newman-Moore model

We study the triangular plaquette model (TPM, also known as the Newman-Moore model) in the presence of a transverse magnetic field on a lattice with periodic boundaries in both spatial dimensions. We consider specifically the approach to the ground state phase transition of this quantum TPM (QTPM, or quantum Newman-Moore model) as a function of the system size and type of boundary conditions. Using cellular automata methods, we obtain a full characterization of the minimum energy configurations of the TPM for arbitrary tori sizes. For the QTPM, we use these cycle patterns to obtain the symmetries of the model, which we argue determine its quantum phase transition: we find it to be a first-order phase transition, with the addition of spontaneous symmetry breaking for system sizes which have degenerate classical ground states. For sizes accessible to numerics, we also find that this classification is consistent with exact diagonalization, Matrix Product States and Quantum Monte Carlo simulations

Tunable superdiffusion in integrable spin chain using correlated initial states

Although integrable spin chains only host ballistically propagating particles they can still feature diffusive spin transport. This diffusive spin transport originates from quasiparticle charge fluctua- tions inherited from the initial state’s magnetization Gaussian fluctuations. We show that ensembles of initial states with quasi-long range correlations lead to superdiffusive spin transport with a tun- able dynamical exponent. We substantiate our prediction with numerical simulations and explain how deviations arise from finite time and finite size effects.

Robust Ergodicity Breaking in Quantum Loop Models

I will present a quantum loop model which provably exhibits topologically stable ergodicity breaking. The results follow from a one-form symmetry, which can be exact or emergent. In the latter case, the ergodicity breaking becomes an exponentially long-lived prethermal phenomenon. The model has nonlocal conserved quantities that correspond to a pattern of system-spanning domain walls, and which are robust to the addition of arbitrary k-local perturbations.

Linear and Non-Linear Optics of Lindbladian Systems

Non-equilibrium quantum systems can exhibit fundamentally different qualitative behavior than their equilibrium counterparts. Here we leverage Keldysh Green’s functions to develop a linear and non-linear response formalism to probe the properties of open quantum systems subject to dissipative evolution by Lindbladian superoperators. We present results for fermionic systems and place a specific emphasis on optical conductivity and the example of Bernal bilayer graphene. Contrary to Hermitian systems: (1) the diamagnetic response may be real and exceptional points manifest themselves in the k-resolved diamagnetic response, (2) the paramagnetic response has a similar magnitude to the equilibrium system but does not manifest universal Dirac behavior, and (3) asymmetric dissipation can enable a second order response in a centrosymmetric system.

Preparing ground states of nearly-frustration-free models

Solving for quantum ground states is important for understanding the properties of quantum many-body systems, and quantum computers are potentially well-suited for solving for quantum ground states. The complexity of this problem has been thoroughly investigated, and the worst-case runtime scaling is known: in particular, it depends inversely on the spectral gap of the Hamiltonian of interest. On the other hand, for the special case of frustration-free Hamiltonians, the scaling relaxes to an inverse square-root dependence. In this work we determine the exact details of how that scaling interpolates between the two cases, by considering the ground state preparation problem restricted to a special subset of Hamiltonians which we term “nearly-frustration-free.” For this subclass, we describe an algorithm whose dependence on the gap is asymptotically better than the worst-case upper bound, and prove that this new dependence is the best achievable up to logarithmic factors. In addition, we give examples of physically motivated Hamiltonians which live in this subclass. Finally, we describe an extension of this method which allows the preparation of excited states both for generic Hamiltonians as well as, at a similar speedup as the ground state case, for those which are nearly frustration-free.

Iterative construction of most necessary conserved quantities

General Gibbs ensembles (GGEs) have been proposed as a local steady state description of integrable many-body systems after a quantum quench. Similarly, GGEs also approximately describe the non-equilibrium steady states of integrable systems weakly coupled to non-equilibrium 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 integrability-breaking perturbations, the space of conserved quantities is spanned by the action of the baths’ super-operators.

Fragmentation and Novel Prethermal Dynamical Phases in Disordered, Strongly-Interacting Floquet Systems

We explore how interactions can facilitate classical-like dynamics in Floquet models with sequentially activated hopping. Specifically, we add local and short range interaction terms to classes of Floquet Hamiltonians and ask for conditions ensuring the evolution acts as a permutation on initial local number Fock states. We show that at certain values of hopping and interactions, determined by a set of Diophantine equations, such evolution can be realized. When only a subset of the Diophantine equations is satisfied the Hilbert space can be fragmented into frozen states, states obeying cellular automata like evolution, and subspaces where evolution mixes Fock states and is associated with eigenstates exhibiting high entanglement entropy and level repulsion.  When disorder is added to these systems, the dynamics may be stabilized in regions of parameter space away from these special parameter values - via k-body (or many-body) localization - to create new families of interesting phases.  These families of phases also include phases already of current interest such as discrete time crystals and (correlation-induced) anomalous Floquet topological insulators.

Scrambling Transition in a Radiative Random Unitary Circuit

We study quantum information scrambling in a random unitary circuit that exchanges qubits with an environment at a rate p. As a result, initially localized quantum information not only spreads within the system, but also spills into the environment. Using the out-of-time-order correlator (OTOC) to characterize scrambling, we find a nonequilibrium phase transition in the directed percolation universality class at a critical swap rate pc: for p < p_c the ensemble-averaged OTOC exhibits ballistic growth with a tunable light cone velocity, while for p > p_c the OTOC fails to percolate within the system and vanishes uniformly within a finite timescale, indicating that all local operators are rapidly swapped into the environment. To elucidate its information-theoretic consequences, we demonstrate that the transition in operator spreading coincides with a transition in an observer’s ability to decode the system’s initial quantum information from the swapped-out, or “radiated,” qubits. We present a simple decoding scheme which recovers the system’s initial information with perfect fidelity in the nonpercolating phase, and with continuously decreasing fidelity with decreasing swap rate in the percolating phase. Depending on the initial state of the swapped-in qubits, we further observe a corresponding entanglement transition in the coherent information from the system into the radiated qubits.

Detecting emergent continuous internal symmetries at quantum criticality

New or enlarged symmetries can emerge at the low-energy spectrum of a Hamiltonian that does not possess the symmetries, if the symmetry breaking terms in the Hamiltonian are irrelevant under the renormalization group flow. In this letter, we propose a tensor network based algorithm to numerically extract lattice operator approximation of the emergent conserved currents from the ground state of any quantum spin chains, without the necessity to have prior knowledge about its low-energy effective field theory. Our results for the spin-1/2 J-Q Heisenberg chain and a one-dimensional version of the deconfined quantum critical points (DQCP) demonstrate the power of our method to obtain the emergent lattice Kac-Moody generators. It can also be viewed as a way to find the local integrals of motion of an integrable model and the local parent Hamiltonian of a critical gapless ground state.

Observation of hydrodynamization and local prethermalization in 1D Bose gases

Hydrodynamics accurately describe relativistic heavy-ion collision experiments well before local thermal equilibrium is established. This unexpectedly rapid onset of hydrodynamics—which takes place on the fastest available timescale—is called hydrodynamization. It occurs when an interacting quantum system is quenched with an energy density that is much greater than its ground-state energy density. During hydrodynamization, energy gets redistributed across very different energy scales. Hydrodynamization precedes local equilibration among momentum modes, which is local prethermalization to a generalized Gibbs ensemble in nearly integrable systems or local thermalization in non-integrable systems. Although many theories of quantum dynamics postulate local prethermalization, the associated timescale has not been studied experimentally. Here we use an array of one-dimensional Bose gases to directly observe both hydrodynamization and local prethermalization. After we apply a Bragg scattering pulse, hydrodynamization is evident in the fast redistribution of energy among distant momentum modes, which occurs on timescales associated with the Bragg peak energies. Local prethermalization can be seen in the slower redistribution of occupation among nearby momentum modes. We find that the timescale for local prethermalization in our system is inversely proportional to the momenta involved. During hydrodynamization and local prethermalization, existing theories cannot quantitatively model our experiment. Exact theoretical calculations in the Tonks–Girardeau limit show qualitatively similar features.