ICTS

Quantum steering is a protocol made up of successive measurements of the system, employing the back-action generated to push the system towards a desired target state. The latter may be the ground state of the system’s Hamiltonian, thus cooling the system to “zero temperature”. I will address here two questions: (1) Can we achieve such cooling by acting only on a small section of the system (a protocol we denote “dilute cooling”)? (2) When such cooling to zero
temperature is not feasible - how low in temperature can we go?
 

Over the past two decades our understanding of the charge and heat transport properties of graphene has evolved progressively as the quality of the graphene devices improved. A key crossover occurs when the scattering between electrons themselves become more frequent than the scattering between electrons and disorder. In this regime, the electrons gas behaves as a hydrodynamic fluid, whose properties exhibit emergent universalities close to the charge neutrality point. In this talk, I shall present some new experimental result on the electrical and thermal transport measurements in very high-quality graphene devices where electron-electron scattering dominates over the momentum relaxation rate. I shall show that the transport in such graphene devices is unique in multiple ways, ranging from unconventional functional dependence of the dc conductivity on carrier density to giant violation of the Wiedemann-Franz law, where effective Lorenz number varies over six orders of magnitude with carrier density. We find that the transport properties of ultra-clean graphene close to charge neutrality are quantitatively consistent with that of a hydrodynamic Dirac fluid where both charge and heart flow are determined by a single universal transport constant.
 

We consider a gas of non-interacting fermions that is released from a box into the vacuum. This provides a simple analytically tractable model that reproduces many features of the Page curve characterizing the evolution of entanglement entropy during evaporation of a black hole. Apart from the entropy we consider several other physical observables and show that generalized hydrodynamics provides a rather surprisingly accurate description of the quantum dynamics.
 

Quantum resources have entered the many body stage over the last two decades. It is by now widely appreciated that entanglement plays a key role in characterizing physical phenomena, as diverse as topological order and critical behaviour. However, entanglement alone is not informative about state complexity, and in fact, it is only one side of the medal. In this talk, I will flip the coin and tackle quantum state complexity of many-body systems under the lense of non-stabilizerness - also known as magic. Magic quantifies the difficulty of realizing states in most error corrected codes, and is thus of fundamental practical importance. However, very little is known about its significance to many-body phenomena.

I will present method(s) to measure magic in tensor network simulations, and illustrate a series of applications to many body systems, including: (a) how state magic and long-range magic behave in conformal field theories - illustrating the limit of the former, and the capabilities of the latter; (b) how magic characterizes phases of lattice gauge theories, both in the context of spin liquids/error correction (toric code), and in the context of theories describing coupling between matter and light (Schwinger model); and (c) how our computational tools are presently more advanced than the largest scale experimental demonstration of magic in Rydberg atom quantum simulators.

Finally, I will discuss the broader impact of these findings on state complexity - indicating that realizing generic state quantum dynamics may require a very large amount of resources in error correcting quantum computers, but at the same time, providing interesting perspectives on new classes of variational states more powerful than tensor networks.
 

Atomic and solid-state spin ensembles are promising platforms for implementing quantum technologies, but the unavoidable presence of noise imposes the needs for error correction. Typical quantum error correction (QEC) requires addressing specific qubits, but this is practically challenging in most realistic architectures. In this work, we propose QEC schemes for unresolvable spin ensembles. By using degenerate superpositions of excited states, which are fundamentally mixed, we find codes that can protect against both individual and collective errors, including dephasing, decay, and pumping. We show how information recovery can be achieved with only collective measurement and control, and illustrate its applications in extending memory lifetime. 

Ref: arXiv:2408.11628v1
 

Certain natural geometric properties of electron wavefunctions in a crystal turn out to explain a vast range of experimentally relevant properties. The original example was the explanation of the integer quantum Hall effect by Thouless and co-workers in terms of the “Berry curvature” derived from Bloch states. We now understand that a kind of gauge field in the Brillouin zone is the key to many equilibrium and linear-response properties, and current work is seeking to generalize these results to nonlinear and non-equilibrium properties as well. This talk reviews the basic concepts of wavefunction geometry starting from basic notions of undergraduate quantum mechanics, then covers more recent applications to new topological states, with a particular focus on effects beyond the standard adiabatic limit.

Projected ensemble, formed by the collection of pure states obtained by partial measurement of a quantum many body system and labeled by the measurement outcome provides a means to define a distribution of states in the Hilbert space, thereby going beyond the limitations of density matrices that can describe the unlabeled states. We discuss this and some results in the context of a many body system where conserved charges have local support.
 

One of the first nontrivial examples of quantum matter to be understood at equilibrium was the behavior of a chain of two-state spins, or qubits, entangled by nearest-neighbor interactions. Hans Bethe’s solution of the ground state in 1931 eventually led to the concept of Yang-Baxter integrability, and the thermodynamics were fully understood in the 1970s. However, the dynamical properties of this spin chain at any nonzero temperature remained perplexing until some unexpected theoretical and experimental progress beginning around 2019. Atomic emulators and quantum computers are beginning to complement solid-state quantum magnetism experiments, and computer scientists, physicists, and mathematicians all have their own reasons to care about the dynamics of simple arrangements of quantum spins. The last part of the talk covers how dynamics of more complicated spin models in higher dimensions are being used to search for emergent gauge fields and other phenomena.

In the first part of my talk, I will discuss anomalous subdiffusive phases that appear in clean long-range fermionic lattice systems and their origin. I will then talk about how such a long-range lattice, when subjected to dephasing noise that acts at all lattice sites, shows an interesting crossover from super-diffusive to diffusive transport regime as one tunes the long-range hopping exponent.

We discuss the entanglement between two critical spin chains induced by the Bell-state measurements, when each chain was independently in the ground state before the measurement. This corresponds to a many-body version of “entanglement swapping”. We employ a boundary conformal field theory (CFT) approach and describe the measurements as conformal boundary conditions in the replicated field theory. We show that the swapped entanglement exhibits a logarithmic scaling, whose coefficient takes a universal value determined by the scaling dimension of the boundary condition changing operator. We apply our framework to the critical spin-1/2 XXZ chain and determine the universal coefficient by the boundary CFT analysis, which is verified by a numerical calculation.

This talk is based on M. Hoshino, M. O., and Y. Ashida, arXiv:2406.12377
 

Many-body localization is one of several conceptual example of how an interacting system of many particles can fail to reach thermal equilibrium. This talk discusses the emerging understanding of systems that fail to thermalize, with a particular focus on quantum information quantities such as entanglement. The importance of entanglement as a constraint on classical computation is complemented by new approaches to measure entanglement in solid-state systems using old techniques such as neutron scattering. New experimental systems in quantum matter such as nitrogen vacancy centers in diamond are, at least on accessible time scales, neither localized nor conventionally thermalizing, and while simple phenomenological models seem to capture the physics in some cases, the reasons why such models work are so far not well understood.

The effects of disorder and chaos on quantum many-body systems can be superficially similar, yet their interplay has not been sufficiently explored. We study this using an all to all interacting spin chain with disordered interacting term in presence of periodic kicks. The disorder free version of this model shows regular and chaotic dynamics within permutation symmetric subspace as the interaction strength is increased. When the disorder is increased, we find a transition from a dynamics within permutation symmetric subspace to full Hilbert space where the expectation values of various operators are given by random matrix theory in full Hilbert space. Interestingly, finite size scaling predicts a continuous phase transition at a critical disorder strength.