QUANT26: Quantum Dynamics - Fundamentals and Realizations

The rapid development of commercially available quantum devices is providing the research community with powerful new tools for exploring a broad range of topics in quantum physics. In this talk, I will outline our recent results on open-system atomic physics and coherent many-body dynamics using QuEra’s analog quantum simulator based on neutral-atom Rydberg arrays. First, I will discuss collective loss phenomena in Rydberg atoms. Our observations show that Rydberg atom loss depends strongly on the size and geometry of the array, suggesting that interactions and correlated processes play an important role beyond independent single-atom decay. Measurements of loss rates and spatial loss correlations place strong constraints on the mechanisms underlying the observed many-body loss dynamics. Next, I will switch gears and discuss how the same platform can be used to investigate many-body behavior in Ising models. By varying the detuning and amplitude of the Rydberg drive, we map out the phase diagram of the mixed-field Ising system and examine universal critical behavior near a continuous phase transition. Such universality connects systems with vastly different microscopic details.

Since this talk is being given as part of a workshop for young people embarking on their research careers, I have decided to describe one aspect of research, namely that sometimes it is worth trying to ‘go the distance’ on potentially important problems. I will illustrate this by describing long-standing research on the cuprate high temperature superconductor Tl2Ba2CuO6, trying to make contact with young scientists by concentrating on experiments done by two such people. This will make the talk somewhat unconventional compared to more standard colloquia, so I ask that the older generation bears with me if they choose to attend!

In this blackboard lecture, I will introduce the field of optical lattices, which is currently witnessing a paradigm shift from an analog quantum simulation platform to a combined analog-digital qubit architecture. I will cover the basics of tunnelling physics, interactions, as well as very recent developments in shuttling and gating mechanisms. Compared to other platforms, optical lattices stand out for their scalability, parallelism, low disorder, and high degree of coherence.

abstract

We all know from our day-to-day experience that a classical system becomes more chaotic if we shake it -- think of a drop of ink on a pot of water. If you shake it mixes more rapidly, approaching the "ergodic" state of a uniformly colored mixture. If one keeps on shaking without letting any energy escape, the system will keep on heating forever. However, this might not happen for a quantum system when the drive is strong enough. Some properties might freeze around their initial values, indicating that it does not keep on heating; rather, it stays stable, challenging the prediction of the famous ergodicity hypothesis extended to this scenario. I will explain how quantum interference can be responsible for such stability with an example of a single two-level system driven periodically by a sine wave drive. Recent experiments show how this Dynamical Freezing can stabilize quantum devices from losing their quantumness for a very long time in an open environment, and helps make them more accurate.

Programmable quantum simulators are rapidly evolving from proof-of-principle devices into experimental platforms for exploring and controlling many-body physics. Their openness to the environment, finite coherence times, and device imperfections are usually treated as obstacles to be overcome. We will see how some of these apparent bugs can instead become features: noise and metastability can be exploited, while initial-state engineering can be used to access regimes and dynamical phenomena that would otherwise remain out of reach. I will show how present-day quantum platforms can be “hacked” beyond their original scope, turning imperfect devices into versatile laboratories for probing relaxation, criticality, and the limits of control.

Artificial Intelligence (AI) is everywhere, but getting a model from notebook to production is another story — most projects never make it. I will outline the main obstacles data science teams face in industry, walk through concrete use cases from AI products we currently ship, and show where physics training — rigor, modeling intuition, comfort with uncertainty — gives you a real edge in tackling them.

Quantum observables in the form of few-point correlators are the key to characterizing the dynamics of quantum many-body systems. In dynamics with fast entanglement generation, quantum observables generally become insensitive to the details of the underlying dynamics at long times due to the effects of scrambling. In experimental systems, repeated time-reversal protocols have been successfully implemented to restore sensitivities of quantum observables. Using a 103-qubit superconducting quantum processor, we characterize ergodic dynamics using the second-order out-of-time-order correlators, OTOC(2). In contrast to dynamics without time reversal, OTOC(2) are observed to remain sensitive to the underlying dynamics at long time scales. Furthermore, by inserting Pauli operators during quantum evolution and randomizing the phases of Pauli strings in the Heisenberg picture, we observe substantial changes in OTOC(2) values. This indicates that OTOC(2) is dominated by constructive interference between Pauli strings that form large loops in configuration space. The observed interference mechanism endows OTOC(2) with a high degree of classical simulation complexity, which culminates in a set of large-scale OTOC(2) measurements exceeding the simulation capacity of known classical algorithms. Further supported by an example of Hamiltonian learning through OTOC(2), our results indicate a viable path to practical quantum advantage.

Topological condensed matter exhibits properties which can be unusually robust to local perturbations. Indeed, this property may make local information very hard to access, a phenomenon that may be called topological censorship. Here, we present two instances where this tension between local and topological physics brings up interesting physical phenomena. One is drawn from quantum Hall physics, where we show that the local distribution of the topologically quantised current flow is continuously tunable between qualitatively different regimes [1]. The other is a topological magnet, in which measurements of the global magnetisation dynamics reveal the appearance of a dynamical fractal and subdiffusion on the lattice scale in a stochiometric material [2]. [1] B. Doucot, D. Kovrizhin, R. M., PNAS 121 (39) e2410703121 (2024) [2] J. N. Hallen, S. A. Grigera, D. A. Tennant, C. Castelnovo, R.M. Science 378, 1218 (2022)

The Streda formula provides a remarkable connection between a simple thermodynamic response and the topological properties of quantum matter, relating the quantized Hall conductivity of an insulator to its density response under an external magnetic field. This lecture uses the Streda formula as a guiding thread to connect the quantum Hall effect, Chern insulators, and Floquet topological phases, showing how a simple density response provides physical insight into increasingly rich forms of topological quantum matter. Starting from the integer quantum Hall effect, we will develop an intuitive understanding of the Streda relation through Landau levels, spectral flow, and general thermodynamic arguments, before extending it beyond Landau quantization to lattice realizations of quantum Hall physics, such as Chern insulators. In this broader setting, the magnetic-field-induced density response acquires the interpretation of a local topological marker, providing experimentally accessible signatures of topology even in disordered and spatially inhomogeneous systems. The second part introduces Floquet engineering, where periodic driving is used to realize effective Hamiltonians with properties inaccessible in equilibrium. This naturally leads to Floquet topological phases, including anomalous edge states that challenge the conventional classification of static systems. Building on the Streda framework, we will show how the winding invariants characterizing Floquet phases can be understood from simple physical response principles, providing a physical perspective on the origin of their anomalous edge states.

The steady interest in understanding the thermodynamics of quantum systems has led to several approaches to defining work and heat in a quantum mechanically consistent way. Quantum thermodynamics as a subfield has grown steadily in the last 15 years, revealing the impact that coherence can have on the energetics of quantum systems. I will initially aim to provide an overview of some of the major lines of inquiry in the field, briefly discussing commonly employed definitions of quantum work, its experimental measurement, and some proof-of-principle demonstrations of nano-scale quantum heat engines. We will then zero in on some exciting new directions that the community has begun to explore, as recently collated in Ref [1]. We will aim to touch on the full breadth of applications, ranging from the foundational aspects, e.g. information propagation and scrambling which lie at the interface of quantum information and high energy physics, to the practical ramifications of the theory, which is driving the design of energetically efficient quantum technologies. [1] Roadmap on Quantum Thermodynamics, S. Campbell et al, Quantum Sci. Technol. 11, 012501 (2026).

In this lecture, I shall present some important ideas from the field of quantum magnetism. I shall begin with basic energetics, and then proceed to the notion of geometrical frustration and its consequences, before connecting these to more modern concepts such as deconfined quantum criticality. The emphasis will be on back-of-the-envelope and pictorial arguments that get across the essence of the physics under discussion.

Motivated by recent discovery of three sets of topologically non-trivial phases of matter in lattice models beyond established classification schemes, we generalise the framework of the quantum Hall effect (QHE) to that of the quantum skyrmion Hall effect (QSkHE). The essential generalisation of the QSkHE is that a single quantum spin of S small (e.g., 1/2, 1, 3/2, etc.), and multiplicity 2S+1, is more accurately treated in some cases as realizing an almost point-like---yet intrinsically 2+1-dimensional (2+1 D)---quantum Hall droplet formulated in terms of a generalisation of matrix gauge theory of Susskind and Polychronakos. The consequences are that a single spin can exhibit fractional statistics, quasi-hole/particle charged excitations (which we collectively refer to as quantum skyrmions), and incompressibility with measurable consequences for individual spins as well as extended systems of multiple spins. The quantum skyrmions can play the role, in the QSkHE, that charged particles play in the QHE.

Quantum circuits originated as a model of quantum computation consisting of sequentially applied operations acting on subsets of qubits. In recent years quantum circuits have also been investigated as a new paradigm for quantum many body dynamics discrete in both space and time, while retaining only the notion of locality. As well as being ripe for investigation on current quantum hardware, quantum circuits suggest new approximations and special cases in which the otherwise intractable problem of quantum dynamics can be simplified and characterised more completely, if not exactly. In this lecture I'll introduce the general idea of quantum circuits as many body systems and give some examples of this kind of simplification.