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Leeds-Loughborough-Nottingham Seminars Archive 2023/24

Following the tradition from the previous years, the Universities of Leeds (Prof Zlatko Papic), Loughborough (Dr Achilleas Lazarides) and Nottingham (Dr Adam Smith) ran a joint hybrid seminar series on non-equilibrium physics during 2023-24.

The seminar series for 2024-25 is ongoing. For previous years, see the archives on the left.

Seminar schedule:

  • 12/07/2024, 11am: Bhaskar Mukherjee (UCL)

    Please note the unusual time!
    Location: GR18, Bragg Building, Leeds
    Title: Emergent strong zero mode through local Floquet engineering

    Abstract: Decoherence is the main obstacle for quantum information processing and fault-tolerant quantum computation. That's why it is extremely important to explore ways to suppress the decoherence of a quantum system. The constituents of a generic interacting many-body system usually decohere under their own dynamics but some sophisticated (integrable) interaction can lead to long-lived emergent quasiparticles, for example, edge Majorana zero mode of a Kitaev chain. The notion of strong zero mode (SZM) was extended to systems with genuine (yet integrable) interaction but their existence in non-integrable systems has always been an open question. In this talk, I will discuss how local Floquet engineering can be used to help emerge a SZM in a non-integrable system. Individual spins of a non-integrable system (spin-1/2 staggered Heisenberg model with a global field) can not be dynamically frozen even by the most optimal choice of drive parameters. Interestingly, a special sequence of optimally driven sites can stabilize a SZM, enhancing the decoherence time of the edge spin exponentially in system size.

  • 03/07/2024, 3pm: Dmitry Kovrizhin (University Cergy-Pontoise)

    Location: Nottingham and Zoom
    Title: Meandering "edge states" in Chern Insulators

    Abstract: Our work addresses the question of where the famously quantised current flows in Chern insulators. This question has received considerable attention in the context of the quantum Hall effect, but progress has been hampered by the lack of local probes, and no consensus has emerged. The fundamental problem is: topological protection hides most local information (such as the spatial distribution of the current), a phenomenon we call topological censorship. Two recent remarkable experiments, which use local probes to determine the current distribution, have remedied the dearth of experimental data for the anomalous quantum Hall effect in the Chern insulating compound (Bi,Sb)2Te3. They have reached new, and very different, conclusions. Here, we provide a theory explaining one of these experiments.

  • 19/06/2024, 3pm: Andrea Legramandi (Trento)

    Location: Zoom
    Title: New perspectives into the Black Hole Information Paradox

    Abstract: The black-hole information paradox has puzzled physicists for decades. In this talk, I will introduce it by modeling black holes as generic non-integrable many-body systems. Their time evolution can be effectively described using Haar random unitaries, while they radiate degrees of freedom in the form of Hawking radiation.

    I will bridge this toy model with the more realistic gravitational scenario and demonstrate how the information paradox can be addressed using the concept of entanglement islands.

    While a significant portion of this talk will be dedicated to reviewing the topic, I will also present original findings, including a simple formula for calculating the entropy of arbitrary subsets of Hawking radiation, from which the quantum-information properties of the radiation can be investigated. Additionally, I will also show how to connect this formula with a generalization of Page's theorem that involves a nested temporal sequence of random unitaries.

  • 05/06/2024, 3pm: Sreejith GJ (IISER-Pune)

    Location: Nottingham (room TBC)
    Title: A quantum Monte Carlo exploration of the finite temperature phase diagram of the 2D quantum dimer model

    Abstract: The quantum dimer model is a paradigmatic example of a strongly constrained quantum many body system. We present a quantum Monte Carlo algorithm that can sample the finite temperature configuration space of the 2D quantum dimer model. Due to the constraints in the local degrees of freedom, conventional stochastic series expansion techniques fail. An improvement was made in Yan et al Phys. Rev. B 99, 165135 (2019) where they introduce the sweeping cluster updates, however this is not ergodic. We improve on the sweeping cluster updates to produce an algorithm that appears to be ergodic and can measure the thermal expectation values of interest. We use this to map out the finite-temperature phase diagram of the quantum dimer model on the square lattice. We find a high-temperature critical phase with power-law correlations that extends down to the Rokhsar-Kivelson point, in the vicinity of which a reentrance effect in the lines of constant exponent is found. For small values of the kinetic energy strength, we find finite-temperature transitions to ordered states (columnar and staggered) which match those of interacting classical dimer models. I will discuss the details of the algorithm and the parameter regimes where the algorithm fails.

  • 29/05/2024, 2pm: Kiryl Pakrouski  (ETH)

    Please note the earlier start time!
    Location: GR18, Bragg Building, Leeds
    Title: Weak ergodicity breaking in multi-band fermionic models

    Abstract: Many-body scars are states that do not obey ETH and exhibit indefinite revivals if the system is initialised to a superposition of them. I will first briefly review a general approach to constructing group-invariant many-body scars and give some examples for single-band electrons in Hubbard-like models. I will provide an argument why the group-invariant scars always have lower entanglement entropy compared to generic thermal states. Then I will discuss how our formalism can be applied to the Hilbert space of multi-band/multi-flavour fermions. This leads to a natural generalization of the eta-pairing states known to be scars in single-band Hubbard-like models. For small number of bands we are able to calculate scars’ entanglement entropy analytically and show that it grows logarithmically with the system size. Several features are unique to the multi-band setup. First, the number of scars is much larger and forms a sizeable fraction in the DOS in small systems. Choosing the Hamiltonian parameters the spectrum within the scar subspace can be made either completely chaotic or integrable, leading to revivals. If the Hamiltonian is strictly short-range, there are certain “unbreakable” degeneracies in the scars’ spectrum which might enable new quantum computing applications.

    Based on:

  • 22/05/2024, 3pm: Bastien P. Lapierre (Princeton University)

    Location: Zoom
    Title: Floquet engineered inhomogeneous quantum chaos in critical systems

    Abstract: In this talk, I will present universal chaotic dynamics of a large class of periodically driven critical systems described by spatially inhomogeneous conformal field theories. By employing an effective curved spacetime approach, I will show that the onset of quantum chaotic correlations, captured by the Lyapunov exponent of out-of-time-order correlators (OTOCs), is set by the Hawking temperature of emergent Floquet horizons. Furthermore, scrambling of quantum information will be shown to be strongly inhomogeneous, leading to transitions from chaotic to non-chaotic regimes by tuning driving parameters. I will finally describe a concrete protocol to simulate and measure OTOCs in quantum simulators, by designing a simple stroboscopic backward time evolution.

  • 11/11/2023, 3pm: Dr. Ahsan Nazir (University of Manchester)

    Location: Room B17, University of Nottingham
    Title: Quantum work statistics at strong reservoir coupling.

    Abstract: Calculating the stochastic work done on a quantum system while strongly coupled to a reservoir is a formidable task, requiring the calculation of the full eigenspectrum of the combined system and reservoir. In this talk I will show that this issue can be circumvented by using a polaron transformation that maps the system into a new frame where weak-coupling theory can be applied. It is shown that the work probability distribution is invariant under this transformation, allowing one to compute the full counting statistics of work at strong reservoir coupling. Crucially this polaron approach reproduces the Jarzynski fluctuation theorem, thus ensuring consistency with the laws of stochastic thermodynamics. I will apply the formalism to a system driven across the Landau-Zener transition, where clear signatures in the work distribution arising from a non-negligible coupling to the environment are identified. These results provide a new method for studying the stochastic thermodynamics of driven quantum systems beyond Markovian, weak-coupling regimes.

    O. Diba, H. J. D. Miller, J. Iles-Smith, and A. Nazir, arXiv:2302.08395

  • 25/10/2023, 3pm: Dr. Mark Mitchison (Trinity College Dublin)

    Location: Sir William Henry Bragg Building 4.03, University of Leeds
    Title: Current fluctuations in continuously measured quantum systems: lessons from the critical Kerr model.

    Abstract: Continuously measured quantum systems are characterized by an output current, in the form of a stochastic and correlated time series which conveys crucial information about the underlying quantum system. In this talk, I will present a toolbox for describing current fluctuations, which unifies concepts from continuous measurement theory and full counting statistics [1]. As an application, I will discuss current fluctuations in the parametrically pumped Kerr (PPK) model. The PPK model describes a driven-dissipative nonlinear cavity, whose nonequilibrium phase diagram features both continuous and discontinuous quantum phase transitions. When continuously measuring light leaking from the cavity, the photocurrent shows interesting divergences near criticality, whose nature depends both on the order of the phase transition and the choice of measurement basis [2]. These findings highlight the rich features of current fluctuations near nonequilibrium phase transitions in quantum-optical systems. Time permitting, I will also provide an outlook on some interesting future directions related to strongly coupled transport problems [3,4] and the thermodynamics of clocks [5,6].[1] G. T. Landi, M. J. Kewming, M. T. Mitchison & P. P. Potts, arXiv:2303.04270[2] M. J. Kewming, M. T. Mitchison & G. T. Landi, Phys. Rev. A 106, 033707 (2022)[3] M. Brenes et al., Phys. Rev. X 10, 031040 (2020)[4] A. M. Lacerda et al., Phys. Rev. B 107, 195117 (2023)[5] P. Erker et al., Phys. Rev. X 7, 031022 (2017)[6] O. Culhane et al., arXiv:2307.09122