17 November 2021

Iacopo Carusotto (Universitá di Trento)

Quantum Fluids of Light

In this colloquium, we will give an overview of the emerging field of quantum fluids of light: in the presence of a finite effective mass due to light confinement and binary interactions mediated by the optical nonlinearity of the material medium, an assembly of photons behaves as a fluid of particles and displays intriguing hydrodynamic phenomena.

After reviewing early work on Bose-Einstein condensation and superfluidity in such systems, I will sketch some among the most exciting recent developments at the crossroad of many-body physics, non-equilibrium statistical mechanics, and quantum optics.

The new features of topological lasing, aka non-equilibrium condensation into a topological edge mode, are capturing the interest of a wide community spanning across fundamental and applied research, from a remarkable topological protection of long-distance phase-locking to fabrication disorder to Kardar-Parisi-Zhang features in the emission coherence.

Strong efforts are presently devoted to the realization of strongly quantum correlated phases of photonic matter in a novel non-equilibrium context. After presenting recent realizations of Mott insulator and baby fractional quantum Hall fluids, I will sketch the new perspectives that these advances are opening for many-body physics and quantum technologies in general.

1 December 2021

Christopher Monahan (William & Mary)

Peering inside the proton

Calculating the internal three-dimensional structure of protons and neutrons has been a long-standing goal for nuclear physics. The strong nuclear force binds together quarks and gluons into protons, neutrons and other hadrons, but the strongly coupled nature of the strong force ensures that calculations of hadron structure are very challenging. Recent theoretical developments mean these calculations, using lattice quantum chromodynamics (QCD), have now become possible. Last year, the first proof-of-principle calculations of generalised parton distributions, which capture the correlations between the longitudinal momentum structure of nucleons and their transverse structure, marked the advent of a new era in lattice QCD calculations of hadron structure. I introduce the ideas that underpin these developments, and summarise some of the most exciting recent results.

Zoom link:


15 December 2021

Janet Anders (Exeter)

Quantum Brownian Motion for Magnets

In this talk I will discuss how a system+bath Hamiltonian, similar to the Caldeira-Leggett and spin-boson models, can be used to derive a general spin dynamics equation [1]. I will show how in the Ohmic (Markovian) limit the new equation reduces to the Landau Lifshitz Gilbert equation, a phenomenological equation widely used in magnetism. I will demonstrate how resonant Lorentzian couplings can be used as a general tool for the systematic comparison of spin dynamics of Markovian and non-Markovian regimes [1], and present numerical results of a classical spin's dynamics under classical and quantum noise.
In the second part of the talk we will explore long time steady states. The dynamical convergence to the Gibbs state is a standard assumption across much of classical and quantum thermodynamics. However, for nanoscale and quantum systems the interaction with their environment becomes non-negligible.  Is the system steady state then still the Gibbs state? And if not, how exactly does it depend on the interaction details? I will briefly outline several aspects of this timely topic [2, 3, 4].
[1] Quantum Brownian Motion for Magnets, 
J Anders, C Sait, S Horsley, arxiv 2009.00600 (2020).
[2] Weak and ultrastrong coupling limits of the quantum mean force Gibbs state, 
arXiv:2104.12606 to be published in PRL, JD Cresser, J Anders.
[3] Open quantum system dynamics and the mean force Gibbs state, 
arXiv:2110.01671, M Merkli, AS Trushechkin, JD Cresser, J Anders. 
[4] Quantum-classical transition of steady states and mean force Gibbs states in the spin boson model, 
in preparation, F Cerisola, M Berritta, S Scali, JD Cresser, SAR Horsley, J Anders.


Zoom link:


26 January 2022

Hannah Price (Birmingham)

Quantum Simulation with Synthetic Dimensions

In the field of quantum simulation, a quantum system, like a cold atomic gas, is artificially engineered so as to emulate phenomena such as topological phases of matter, which are now of great interest across many areas of physics. In this context, the concept of synthetic dimensions has emerged over the last decade as a powerful and general experimental approach. The main idea of a synthetic dimension is to couple together suitable degrees of freedom in order to mimic the motion of a particle along an extra spatial dimension. This approach provides a way to engineer controllable lattice models and to explore condensed-matter phenomena with different spatial dimensionalities. In this talk, I will give a brief overview of this topic, before presenting a very recent experiment to explore Bloch oscillations along a synthetic dimension of atomic trap states. I will then review how synthetic dimensions are used to implement topological models, before finally discussing perspectives for exploring physics with more than the usual three spatial dimensions. 

Zoom link:


9 February 2022

Tom Hayward (Sheffield)

From Stochasticity to Functionality: Harnessing Magnetic Domain Walls for Probabilistic and Neuromorphic Computing

Domain walls (DWs) in soft, ferromagnetic nanowires have been a topic of intense research interest due to proposals to use them as data carriers in energy efficient, non-volatile logic and memory devices. However, despite their apparent technological potential, these devices have been challenging to realise, because DWs pinning and propagation is highly stochastic, making digital devices unreliable [1]. While materials engineering approaches can be used to supress stochasticity [2], it is also interesting to consider whether alternative computer paradigms could prove more resistant to, or even benefit from, the DWs’ stochastic behaviours, thus converting it from a technologically inhibitive phenomenon into a functional property. 

 In this talk I will introduce the basic physical behaviours of domain walls in magnetic nanowires and show how these lead to complex and stochastic interactions between DWs and defect sites. I will then present a combination of experimental measurements and simulations that illustrate how these behaviours can be used to realise two different neuromorphic (brain-like) computing paradigms in hardware: feed-forward neural networks and reservoir computing [3]. The work presented will include experimental demonstrations of how these devices can be used to performed benchmark machine learning tasks such as spoken/written digit recognition.

 [1] T.J. Hayward and K.A. Omari, Beyond the quasi-particle: stochastic domain wall dynamics in soft ferromagnetic nanowires, Journal of Physics D: Applied Physics 50, 8, 084006 (2017).

[2] T.J. Broomhall, A.W. Rushforth, M.C. Rosamond, E.H. Linfield, and T.J. Hayward, Physical Review Applied 13, 024039 (2020).

[3] R. W. Dawidek, T. J. Hayward, I. T. Vidamour, T. J. Broomhall, M. Al Mamoori, A. Mullen, S. J. Kyle, P. W. Fry, N. J. Steinke, J. F. K. Cooper, F. Maccherozzi, S. S.Dhesi, L. Aballe, M. Foerster, J. Prat, E. Vasilaki, M. O. A. Ellis, D. A. Allwood, submitted to Advanced Functional Materials (2020).

Zoom link: