Unless otherwise specified, all talks are held starting at 1.15pm in the Theory Library on the 4th floor of Physics East.

Everyone is encouraged to ask questions during the talk. You are welcome to leave when you need to, otherwise the talk will usually wrap up by 2.30pm, at which point there will be biscuits.

Thursday 13th November 2025: Lucas Sá (Cambridge)

Exactly solvable dynamics and spontaneous symmetry breaking in dissipative spin systems

I’ll discuss the dissipative dynamics of a class of interacting “gamma-matrix” spin models coupled to a Markovian environment. For spins on an arbitrary graph, we construct a Lindbladian that maps to a non-Hermitian model of free Majorana fermions hopping on the graph with a background classical Z2 gauge field. We show that its exponentially many steady states provide a concrete example of mixed-state topological order, in the sense of strong-to-weak spontaneous symmetry breaking of a one-form symmetry. While encoding only classical information, the steady states still exhibit long-range quantum correlations. We also examine the relaxation processes toward the steady state by computing decay rates, which, despite the model’s emerging integrability, exemplify several features typical of generic open system. We find the decay rates to be generically finite, even in the dissipationless limit (known as anomalous relaxation). We however identify symmetry sectors where fermion-parity conservation is enhanced to fermion-number conservation, where we can analytically bound the decay rates and prove that they vanish in the limits of both infinitely weak and infinitely strong dissipation. Our work establishes an analytically tractable framework to explore nonequilibrium quantum phases of matter and the relaxation mechanisms toward them.

Thursday 27th November 2025: Zlatko Papić (Leeds)

ScarFinder: Detecting Islands of Coherence in the Sea of Quantum Chaos

Understanding when and how isolated quantum systems fail to thermalise is a central challenge in nonequilibrium physics. Experiments with programmable quantum simulators, such as Rydberg atom arrays, have revealed striking examples of weak ergodicity breaking—long-lived coherent oscillations that defy the eigenstate-thermalisation hypothesis. Yet identifying such special dynamical trajectories in generic chaotic systems remains difficult. I will introduce ScarFinder, a variational framework that systematically searches for low-entanglement, non-thermal trajectories in arbitrary many-body models without prior knowledge of their structure. By iteratively evolving and projecting states within a variational manifold, ScarFinder filters out thermal components and converges to stable periodic or weakly relaxing orbits — islands of coherence within the sea of chaos. I will show that this versatile approach captures a wide range of ergodicity-breaking phenomena, including quantum many-body scars, anomalous thermalisation in mixed-field Ising chains, periodic trajectories in quantum models inspired by cellular automata, and quantum Mpemba effects where hotter states relax faster. These results establish ScarFinder as a general-purpose microscope for revealing the fine structure of quantum dynamics.

Thursday 4th December 2025: Stephen Powell (Nottingham)

TBA

TBA

Thursday 2nd October 2025: Jonathan Keeling (St Andrews)

Modelling realistic open quantum systems with process tensors

When an open quantum system is strongly coupled to a structured environment, describing the dynamics of that system becomes a challenging problem. Moreover, traditional approaches, based on time evolution of the reduced density matrix are generally harder to use when calculating higher-order or multi-time correlations.

I will review recent progress that addresses both these issues, by showing how the time evolution of the system can be efficiently simulated using tensor network methods [1]. Such a tensor network naturally leads one to consider the process tensor (PT), an object which encodes all multi-time correlations of the reservoir [2,3]. A key insight is that one can construct efficient MPO representations of the PT [4]. This idea makes possible many otherwise challenging tasks, including optimisation of non-Markovian systems [5,6], and modelling the non-Markovian dynamics of many-body open quantum systems [7], and calculation of two dimensional spectroscopy[8].

The algorithm underpinning this work is publicly available [9], and we are keen to help support other researchers in using this approach.

[1] A. Strathearn, P. Kirton, D. Kilda, J. Keeling, B. W. Lovett., Efficient non-Markovian quantum dynamics using time-evolving matrix product operators, Nature Commun. 9, 3322 (2018).

[2] M. R. Jørgensen and F. A. Pollock, Exploiting the causal tensor network structure of quantum processes to efficiently simulate non-Markovian path integrals, Phys. Rev. Lett. 123, 240602 (2019).

[3] M. Cygorek, M. Cosacchi, A. Vagov, V. M. Axt, B. W. Lovett, J. Keeling, E. M. Gauger, Simulation of open quantum systems by automated compression of arbitrary environments, Nat. Phys. 18, 662 (2022).

[4] J. Keeling, E. M. Stoudenmire, M.-C. Bañuls, D. R Reichman, Process Tensor Approaches to Non-Markovian Quantum Dynamics, arXiv:2509.07661 (2025).

[5] G. E. Fux, E. P. Butler, P. R. Eastham, B. W. Lovett, J. Keeling, Efficient exploration of Hamiltonian parameter space for optimal control of non-Markovian open quantum systems, Phys. Rev. Lett. 126, 200401 (2021).

[6] E. P. Butler, G. E. Fux, C. Ortega-Taberner, B. W. Lovett, J. Keeling, and P. R. Eastham, Optimizing performance of quantum operations with non-Markovian decoherence: The tortoise or the hare?, Phys. Rev. Lett. 132 060401 (2024).

[7] P. Fowler-Wright, B. W. Lovett, J. Keeling, Efficient many-body non-Markovian dynamics of organic polaritons, Phys. Rev. Lett. 129 173001 (2022).

[8] R. de Wit, J. Keeling, B. W. Lovett, A. W. Chin, Process tensor approaches to modeling two-dimensional spectroscopy, Phys. Rev. Research 7, 013209 (2025).

[9] The OQuPy package, https://github.com/tempoCollaboration/OQuPy

Thursday 9th October 2025: Nigel Cooper (Cambridge)

Ideal Optical Flux Lattices

We present a new approach for engineering fractional quantum Hall (FQH) states in cold atoms, overcoming the limitations of current methods. By introducing a simple scalar potential to a two-state optical flux lattice, we show how one can create Chern bands that are both exceptionally flat and “ideal” for hosting strongly correlated phases. Drawing inspiration from “magic-angle” physics in moiré materials, our method allows for precise tuning of the band geometry, and stabilizing robust abelian and non-abelian FQH states. The scheme is compatible with existing experimental techniques, providing a practical path toward exploring topological quantum matter with ultracold atoms.

Thursday 23rd October 2025: Thomas Siday (Birmingham)

Atomic nonlinearities bring ultrafast optical spectroscopy to the shortest lengthscales

Sampling the interaction of light and matter over the smallest possible length- and timescales has been a long-sought goal in both photonics and condensed matter physics. By exploiting linear evanescent fields confined to miniscule objects, near-field microscopy can access ultrafast light-matter interaction on nanometer length scales [1]. In this talk, I will first discuss several recent applications of ultrafast near-field microscopy. Then, I will demonstrate a fundamentally new paradigm for ultrafast microscopy which exploits strong atomic nonlinearities within optical near-fields. In doing so, simultaneous atomic-scale spatial resolution and subcycle time resolution become possible [2]. This emergent nonlinear response originates from electromagnetic radiation emitted by tunnelling currents flowing in response to the THz electric field [3,4]. This fundamentally new imaging mechanism – near-field optical tunnelling emission (NOTE) – provides the first subcycle videography of atomic-scale quantum dynamics.

[1] M. Plankl et al., “Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures”, Nat. Photonics, 15, 594 (2021).
[2] T. Siday et al., “All-optical subcycle microscopy on atomic length scales”, Nature, 629, 329 (2024).
[3] T. L. Cocker et al., “An ultrafast terahertz scanning tunnelling microscope”, Nat. Photonics, 7, 620 (2013).
[4] T. L. Cocker et al., “Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging”, Nature, 539, 263 (2016).

Thursday 6th November 2025: Alessandro Romito (Lancaster)

Measurement-Induced Phase Transitions: From Quantum Trajectories to Most Likely Dynamics

Quantum systems exhibit complex dynamics marked by information scrambling, leading to phenomena such as long-range entanglement and thermalization. Local measurements can significantly alter these dynamics by freezing local degrees of freedom, giving rise to new non-equilibrium phases and associated Measurement-induced Phase Transitions (MiPTs). While measurement-induced dynamics is inherently stochastic, post-selecting specific detector readouts to single “quantum trajectories” yields deterministic dynamics governed by a non-Hermitian Hamiltonian, with post-selected MiPTs exhibiting distinct universal characteristics.

In this presentation, I will revisit the foundational principles of quantum measurements and contrast the quantum dynamics of individual post-selected trajectories with their collective statistical behaviour. I will introduce a novel partially post-selected stochastic Schrödinger equation that captures controllable subsets of quantum trajectories and apply it to introduce the concept of the most likely trajectory. I will then demonstrate these frameworks on two paradigmatic systems: Gaussian Majorana fermions, where two-replica and renormalization group techniques reveal the robustness of non-Hermitian MiPT universality under limited stochasticity; and interacting bosons in the Sine-Gordon model, where the most likely trajectory method captures dynamics through self-consistent harmonic approximation and reveals an entanglement phase transition from area-law to logarithmic-law scaling.

These complementary approaches provide powerful tools for studying MiPTs in monitored quantum systems, opening new avenues for understanding measurement-induced phenomena in both fermionic and bosonic platforms.

Based on:

[1] Chun Y. Leung, Dganit Meidan, and Alessandro Romito, Phys. Rev. X 15, 021020 (2025)

[2] Anna Delmonte, Zejian Li, Rosario Fazio, and Alessandro Romito, ArXiv:2509.24520 (2025)