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To be announced

Past seminars

33 days ago
Chris Westbrook

Generation of entangled phonon modes in a modulated Bose-Einstein condensate

Abstract:Recent years have seen many attempts to realize various gravitational or cosmological analogs in Bose-Einstein condensates. These condensates are difficult to work with, but they have the important feature of being so cold that vacuum fluctuations can be comparable to or even stronger than thermal fluctuations so that the excitations of the fluid must be described in the context of quantum field theory. Thus, we can hope to observe quantum features when realizing such analogs. I will discuss recent experiments in which we have observed quantum features in an analog of the dynamical Casimir effect. I will also discuss the connections between our analog system and the preheating and reheating scenarios that have been posited for the primordial universe. Short bio: After receiving a PhD from the University of Michigan, Chris Westbrook spent 5 years working on laser cooling at the National Institute of Standards and Technology in Gaithersburg, Maryland. He then took a position as an employee of the French Centre National de la Recherche Scientifique working at the Laboratoire Charles Fabry de l'Institut d'Optique. His research interests are centered on quantum gases and atom optics.
47 days ago
Sam Dolan

The Maxwell fisheye lens, perfect focussing, and collapsing spheres in General Relativity

Abstract: Maxwell's fisheye is a gradient-index lens that perfectly focusses the rays emanating from a point source on its rim at the opposite point on its rim. In a simple geometric reinterpretation, lensed rays map to null geodesics on the curved geometry of a hypersphere. Hyperspherical geometries emerge naturally in General Relativity, up to an overall conformal factor, in the study of spheres of uniform density: well-known examples include the Friedmann spacetime, the Schwarzschild interior solution, and Oppenheimer-Snyder collapse. In this talk I will describe a general class of spheres of uniform density and isotropic pressure, embedded in the Schwarzschild spacetime, that are characterised by the radial motion of their surface (a free function). I show that these spacetimes are conformal to the hypersphere, and, by matching spacetime geometries at the surface, that the extent $\chi_0$ of the three-sphere geometry encompassed by the interior is an elementary function of the energy and proper acceleration of the surface. If the proper acceleration remains constant, then the extent $\chi_0$ also remains constant, and thus the geometry is conformal to part of a static Maxwell fisheye lens. I will consider the special case of the Schwarzschild interior solution that collapses to form a black hole under constant proper acceleration. I will also discuss some implications for analogue gravity and neutron stars. Bio: Dr Sam Dolan is a Senior Lecturer of Applied Mathematics at the University of Sheffield. His research interests include black hole perturbation theory, gravitational wave astronomy, modelling Extreme Mass-Ratio Inspirals with gravitational self-force, superradiant instabilities of black holes, and analogue gravity.
69 days ago
Petros Wallden

Quantum algorithms for causal sets

Abstract: Causal sets is an approach to quantum gravity that spacetime is fundamentally a “discrete” partially ordered set. The Lorentz invariance in such discrete structures results in “random” partial orders with high valence, making certain analytical calculations harder and the need for computational methods more important. Computational methods, on the other hand, are also challenging since the computational complexity of most interesting questions puts strict bounds on what can really be explored. Quantum computers promise significant, in certain cases exponential, speed-ups. Our focus is to analyse instances where quantum algorithms could provide useful speed-ups for causal sets questions. In this talk I will first briefly introduce causal sets and then some basic elements of quantum computing and quantum algorithms. Then I will give a brief exposition on certain causal sets questions and initial results and future thoughts on tackling them using quantum algorithms. Questions considered include: the calculation of Benincasa-Dowker action (causal sets analogue of Einstein-Hilbert action), the dimensionally restricted quantum gravity partition function and kinematic questions such as the emergence of continuous geometry. Bio: Petros Wallden is Reader (Associate Professor) in Quantum Informatics at the School of Informatics at the University of Edinburgh, is Deputy Director of the Quantum Software Lab and leads the Quantum Software activities of the Quantum Computing and Simulation Hub. His current research focuses on quantum algorithms and quantum machine learning, quantum cryptography and verification/benchmarking of quantum computing while in the past he has worked on quantum foundations and quantum gravity having obtained his PhD in Theoretical Physics from Imperial College. He is editor for the journal Quantum and the journal Cryptography and was two times general chair of the IACR international conference on Public Key Cryptography.
83 days ago
Cameron Bunney

Ambient temperature in the circular motion Unruh effect

Abstract: The Unruh effect lies at the interface of quantum field theory and general relativity, predicting that detectors in accelerated and inertial motion experience the quantum vacuum differently - the former measures a response when the latter does not. The excitation and de-excitation rates of this response have the characteristics of a thermal spectrum at a temperature directly proportional to the proper acceleration of the accelerated detector. Measuring this effect, however, has proven remarkably difficult: a 1K increase in temperature would require an acceleration of the order 10^20m/s^2. Since the discovery by Unruh in 1981 of the analogy between linearised surface perturbations in an inviscid fluid and a massless scalar field on a curved background, analogue gravity as a field has opened up previously inaccessible regimes to experimental probing in hydrodynamical, condensed matter, and optical systems. These systems offer analogue spacetimes as playgrounds for testing theoretical phenomena. However, the experiment must be contained within a finite size and such systems exhibit non-zero temperatures. As such, by expanding the theoretical framework to include a finite temperature with a detector in circular motion (constant acceleration), this work aims to bring experiment and theory closer together. In this talk, I will review the Unruh-DeWitt detector model applied to a field at a nonzero ambient, explore the detector's response function, and apply this formalism to a superfluid helium setup. Rather than being a hinderance, the ambient temperature can be found to enhance acceleration-dependent response. This talk is based on the works 2303.12690 and 2302.12023 Biosketch: Cameron Bunney is a final-year PhD student at University of Nottingham under Prof. Jorma Louko and Prof. Silke Weinfurtner. Before starting his PhD in 2020, he completed an MMath at University of Nottingham. His work has focused on experimental modelling for the circular motion Unruh effect.
96 days ago
David Kaiser

Nonperturbative Dynamics of Post-Inflation Reheating

Bio and Abstract Bio: David Kaiser is Germeshausen Professor of the History of Science and Professor of Physics at the Massachusetts Institute of Technology. He is the author of several award-winning books about modern physics. His latest book, Quantum Legacies: Dispatches from an Uncertain World (2020), was named a Choice Outstanding Academic Title and also honored as among the best books of the year by Physics Today and Physics World magazines. A Fellow of the American Physical Society, Kaiser has received MIT's highest awards for excellence in teaching. His work has been featured in Science, Nature, the New York Times, and the New Yorker magazine. His group's recent efforts to conduct a "Cosmic Bell" test of quantum entanglement, in collaboration with Nobel laureate Anton Zeilinger, were featured in the documentary film Einstein's Quantum Riddle. Abstract:Our understanding of the state of the universe between the end of early-universe inflation and big bang nucleosynthesis (BBN) is incomplete. To preserve the successes of the standard big bang model, the energy density that had driven inflation must be dispersed rapidly into a hot, thermal bath of Standard Model particles with a radiation-like equation of state. Although the early resonance phase of "preheating" can be accurately studied within a linearized approximation, the processes most critical for observational consistency are inherently nonlinear, and can unfold over distinct time-scales. In this talk I will review our current understanding of the nonperturbative dynamics at the end of inflation; some salient features of reheating in models that incorporate realistic features from high-energy particle physics; possible impacts of gravitational metric perturbations during reheating; and how the universe reaches a thermal state before BBN.
125 days ago
Smitha Vishveshwara

From nano-constrictions to event horizons: facets of the inverted harmonic oscillator

Bio & Abstract: Bio: Smitha Vishveshwara is a Professor of Physics at the University of Illinois at Urbana-Champaign. Her condensed-matter based theoretical research includes studies of fractionalization and anyons, topological superconductors, and non-equilibrium critical dynamics. Her explorations of ultracold atomic systems include collaborations with experimentalists on realizing quantum condensate bubbles in microgravity conditions aboard the International Space Station. Her interdisciplinary research identifies parallels between phenomena found in these realms and in gravitational physics and applies condensed matter concepts to biophysics. Vishveshwara also collaboratively explore the confluence of physics and the arts to create new work such as the theater piece Quantum Voyages and the circus performance Cosmic Tumbles, Quantum Leaps. Abstract: Time and again, one encounters unifying physics that links phenomena from the most miniscule to astronomical scales. Universal phase transitions in magnets, superconductors, and the cosmos; manifestations of the physics of fundamental particles in condensed matter; defect and structure formation in the early Universe and in liquid Helium---to name just a few. Here, I discuss how the inverted harmonic oscillator, the lesser-known sister of the simple harmonic oscillator, offers fertile ground for the same beautiful dynamics across scales. In its presence, deep parallels become manifest in quantum Hall systems, quantum optics, and curved spacetime. The non-commuting nature of the lowest Landau level sets the stage for the parallel between quantum Hall physics and quantum optics, including the construction of coherent states. The presence of a saddle potential in the lowest Landau level further links phenomena as disparate as tunneling across point contacts, squeezing, Hawking-Unruh radiation, and black hole ringdown. The inverted harmonic oscillator underlies these commonalities, providing fresh cross-disciplinary perspectives and new predictions.
138 days ago
Oliver Gould

Bubble nucleation for cosmological phase transitions

Abstract: First-order phase transitions proceed via bubble nucleation. This is true regardless of whether the transition is happening in your kettle or on a cosmological scale in the very early universe. For the latter, bubble collisions would produce a stochastic background of gravitational waves which may be observed today. This could yield quantitative information on physics beyond the Standard Model which is complementary to collider searches. Yet to realise this potential requires improving our understanding of bubble nucleation, as presently there are still huge theoretical uncertainties in the amplitude of the gravitational wave signal. I will give an overview of our present understanding of this signal, and of the tools that have been developed to compute it. Effective field theory has been at the forefront of this development, and I will outline how it can be used to solve a number of long-standing theoretical problems, with a focus on the bubble nucleation rate. Bio: Oliver Gould is a proleptic lecturer and Royal Society Dorothy Hodgkin Fellow at the University of Nottingham. Before joining the University of Nottingham, he worked as a postdoctoral researcher at Helsinki University, and he completed his PhD at Imperial College London. In the past few years his work has focused on the theory of cosmological phase transitions, with the aim to make reliable predictions of their gravitational wave signals.
166 days ago
Laura Batini

Real-time dynamics of false vacuum decay

ABSTRACT: False vacuum decay is a well-defined initial value problem in (real) time, in which the system starts from a metastable state that eventually decays and thermalizes due to fluctuations. It can be formulated in non-equilibrium quantum field theory on a closed time path and studied using correlation functions. We simulate the dynamics of a relativistic scalar field by classical-statistical field theory on a lattice in the high-temperature regime. In general, we find that the decay rates depend on time. Furthermore, we show that the decay rates in real time are comparable to those obtained by the conventional Euclidean (bounce) approach in the presence of a time-dependent effective potential. Finally, we show how quantum corrections to the equations of motion of the one- and two-point correlation functions can lead to transitions that are not captured by the statistical-classical approximations. BIOSKETCH: Laura Batini is a PhD student at the Heidelberg University. She studies under Prof. Dr. Jürgen Berges and she is presenting this paper on his behalf. Prior to starting her PhD in 2021, she completed a B.Sc. in Physics at the University of Milan in 2018, as well as a M.Sc. at the Institute of Theoretical Physics at Heidelberg University in 2021. Laura's research interests are Dynamical instabilities, dynamical critical phenomena, functional renormalisation group, ultracold atoms.
230 days ago
Dalila Pirvu

Bubble velocities and oscillon precursors in first order phase transitions

Abstract: Metastable `false' vacuum states are an important feature of the Standard Model of particles physics and many theories beyond it. In this talk I will introduce new observables describing the dynamics of a phase transition out of a false vacuum via the nucleation of bubbles. We study vacuum decay at finite temperature by numerically evolving ensembles of field theories in 1+1 dimensions from a metastable state. We demonstrate that for an initial Bose-Einstein distribution of fluctuations bubbles form with a Boltzmann-distributed spread of centre-of-mass velocities and that bubble nucleation events are preceded by an oscillon - a long-lived, time-dependent, pseudo-stable configuration of the field. We describe why these features are theoretically expected, and find quantitative agreement between simulations and theoretical predictions. We directly measure the critical bubble configuration from simulations by stacking nucleation events in their rest frame, finding agreement with the theoretical prediction for the bubble solution. We compute the total energy of the stacked configuration and the prediction for the energy of the critical bubble given by the measured velocity distribution, and find good agreement in both cases with the expected energy of the theoretical critical bubble solution. Bio: Dalila Pirvu is a graduate student in Cosmology at Perimeter Institute and University of Waterloo, Canada. She finished her undergraduate at Imperial College London. She works on lattice simulations of first order phase transitions in quantum field theories. She is also interested in dark matter and its interactions with large scale structure.
244 days ago
Laura Sberna

Nonlinear effects in the black hole ringdown

Abstract: When a binary merges to form a single black hole, the merger product emits a final burst of gravitational waves known as the “ringdown”. Ringdowns are currently being observed with LIGO and Virgo, and will be even stronger in future detectors like the Einstein Telescope or LISA. Observations of black hole ringdowns can be used to characterize binary merger remnants, and are particularly suited to test general relativity. So far, analyses of the ringdown have assumed linear black hole perturbation theory. In this talk, I will explore nonlinear effects during the ringdown, which could potentially spoil (or enhance) ringdown analysis. I will use numerical and analytic techniques to explore nonlinear effects both in a toy model (in anti-de Sitter), and then in asymptotically-flat black holes. I will conclude with some open problems in understanding the ringdown at nonlinear level, which could be an interesting avenue for analog experiments. Bio: Laura Sberna is currently a postdoc at the Max Planck Institute for Gravitational Physics, Potsdam, Germany. Besides the black hole ringdown, she is interested in the interplay between astrophysical environments and gravitational wave sources. Laura obtained her PhD at Perimeter Institute, Canada, under the supervision of Neil Turok.
257 days ago
David Horváth

Inhomogeneous quantum quenches in the sine-Gordon theory

ABSTRACT: In this talk inhomogeneous quantum quenches are discussed in the attractive regime of the sine-Gordon model. In the quench protocol under investigation, the system is prepared in an inhomogeneous initial state in finite volume by coupling the topological charge density operator to a Gaussian external field. After switching off the external field, the subsequent time evolution is governed by the homogeneous sine-Gordon Hamiltonian. Varying either the interaction strength of the sine-Gordon model or the amplitude of the external source field, an interesting transition is observed in the expectation value of the soliton density. This affects both the initial profile of the density and its time evolution and can be summarised as a steep transition between behaviours reminiscent of the Klein-Gordon, and the free massive Dirac fermion theory with initial external fields of high enough magnitude. The transition in the initial state is also displayed by the classical sine-Gordon theory and hence can be understood by semi-classical considerations in terms of the presence of small amplitude field configurations and the appearance of soliton excitations, which are naturally associated with bosonic and fermionic excitations on the quantum level, respectively. Features of the quantum dynamics are also consistent with this correspondence and comparing them to the classical evolution of the density profile reveals that quantum effects become markedly pronounced during the time evolution. These results suggest a crossover between the dominance of bosonic and fermionic degrees of freedom whose precise identification in terms of the fundamental particle excitations can be rather non-trivial. Nevertheless, their interplay is expected to influence the sine-Gordon dynamics in arbitrary inhomogeneous settings. BIO: Dávid Horváth was an undergraduate student at the Budapest University of Technology and Economics from 2009 to 2014 where he obtained a bachelor’s and a master’s degree in Physics. He worked together with Prof. János Kertész and studied spreading phenomena on complex networks for his master thesis. Under the supervision of Prof. Gábor Takács he then started to investigate the out-of-equilibrium dynamics of low dimensional quantum systems and obtained his PhD in 2019 at the same university. He was awarded the ‘UNKP’ fellowship of the ‘New National Excellence Program’ by the Hungarian state. In 2020 Dávid Horváth obtained his first postdoctoral fellowship in the group of Prof. Pasquale Calabrese (SISSA) with whom he developed a novel method to study non-trivial entanglement properties of integrable quantum field theories. In the meanwhile, he kept his interest also in non-equilibrium physics, which mostly manifested in studies of the sine-Gordon theory. Additionally he took part in substantial methodological development of the truncated space approach, which is suitable for non-equilibrium setups as well. He joined King’s College as a Postdoctoral Research Associate in October 2023 under the guidance of Prof. Benjamin Doyon. Dávid Horváth’s research activities focus on the out-of-equilibrium dynamics of low dimensional, strongly correlated quantum systems and on the entanglement properties thereof. He is particularly interested in phenomena related to anomalous equilibration, the impact of spatial inhomogeneities and the emergence of effective, large-scale descriptions in isolated systems, and in describing real-world experimental setups. He uses both analytical (TBA, GHD, QGHD, FF bootstrap and related methods) and numerical techniques (TCSA).
307 days ago
Seth Lovett, University of Sheffield

Observation of Zitterbewegung in photonic microcavities

ABSTRACT The study of analogues to effects appearing in the domain of high energy physics is among the trends of modern condensed matter physics. By using analogue quantum systems in the lab, we can simulate effects that are otherwise out of reach using current technology. A prime example of such an effect is the ‘Zitterbewegung’ effect, something that has remained somewhat elusive since it was first predicted by Schrodinger for relativistic free electrons. These predictions concerned the counterintuitive trembling motion of propagating electrons perpendicular to the direction of travel and were later extended for all particles governed by the Dirac equation. It was recently proposed that by using optical microcavities this effect should be observable in the propagation path of highly photonic polaritons with a well-defined wavevector,1,2. These observations are possible due to the fact that Fabry–Perot optical microcavities allow direct imaging of the internal spinor wavefunction via photon tunnelling through the mirrors, along with supporting polarisation wave-vector coupling via TE-TM splitting (spin-orbit coupling analogue) 3 and in some cases birefringent coupling. Such systems have already allowed observation of a range of important physical effects such as optical spin-Hall effect4, the emergence of monopoles5 and the onset of the non-Abelian gauge fields6,7. This seminar aims to discuss the history of the Zitterbewegung effect, discuss its origins in a photonic microcavity system and present our findings in both non-periodic and periodic systems both of which contain direct observations of the effect. References: 1. Sedov, E. S., Rubo, Y. G. & Kavokin, A. V. Zitterbewegung of exciton-polaritons. Phys. Rev. B 97, 245312 (2018). 2. Whittaker, C. E. et al. Optical analogue of Dresselhaus spin–orbit interaction in photonic graphene. Nat. Photonics 15, 193–196 (2021). 3. Shelykh, I. A. et al. Polariton polarization-sensitive phenomena in planar semiconductor microcavities. Semicond. Sci. Technol. 25, 013001 (2010). 4. Leyder, C. et al. Observation of the optical spin Hall effect. Nat. Phys. 3, 628–631 (2007). 5. Hivet, R. et al. Half-solitons in a polariton quantum fluid behave like magnetic monopoles. Nat. Phys. 8, 724–728 (2012). 6. Polimeno, L. et al. Experimental investigation of a non-Abelian gauge field in 2D perovskite photonic platform. Optica 8, 1442–1447 (2021). 7. Biegańska, D. et al. Collective excitations of exciton-polariton condensates in a synthetic gauge field. Phys. Rev. Lett. 127, 185301 (2021). BIOSKETCH Seth Lovett is a postdoctoral research associate at the University of Sheffield currently working in the Low-dimensional structures and devices group (LDSD). He completed his PhD in 2023 (with minor corrections to his thesis ongoing) regarding the study of light-matter interactions in low dimensional structures with a focus on exciton-polaritons in inorganic semiconductor devices. His work so far has covered the demonstration of tuneable band gaps via a magnetic response for polaritons in 2D slab waveguides, the first direct observation of the Zitterbewegung effect for highly photonic polaritons in microcavity structures and more recently the observation of linear and non-linear compact localised states in micropillar lattice structures. He is also currently involved in projects centred around analogue gravity in polaritonic systems with a long-term goal of using said systems to study Penrose-like effects around an analogue polariton black-hole.
398 days ago
Jose Luis Jaramillo

Pseudospectrum and black hole quasinormal mode instability: an ultraviolet universality conjecture

Abstract: Can we measure the ‘effective regularity’ of spacetime from the perturbation of quasi-normal mode (QNM) overtones? Black hole (BH) QNMs encode the resonant response of black holes under linear perturbations, their associated complex frequencies providing an invariant probe into the background spacetime geometry. In the late nineties, Nollert and Price found evidence of a BH QNM instability phenomenon, according to which perturbed QNMs of Schwarzschild spacetime migrate to new perturbed branches of different qualitative behaviour and asymptotics. Here we revisit this BH QNM instability issue by adopting a pseudospectrum approach. Specifically, we cast the QNM problem as an eigenvalue problem for a non-selfadjoint operator by adopting a hyperboloidal formulation of spacetime. Non-selfadjoint (more generally non-normal) operators suffer potentially of spectral instabilities, the notion of pseudospectrum providing a tool suitable for their study. We find evidence that perturbed Nollert & Price BH QNMs track the pseudospectrum contour lines, therefore probing the analytic structure of the resolvent, showing the following (in)stability behaviour: i) the slowest decaying (fundamental) mode is stable, whereas ii) (all) QNM overtones are ultraviolet unstable (for sufficiently high frequency). Building on recent work characterizing Burnett’s conjecture as a low-regularity problem in general relativity, we conjecture that (in the infinite-frequency limit) generic ultraviolet spacetime perturbations make BH QNMs migrate to ‘Regge QNM branches’ with a precise universal logarithmic pattern. This is a classical general relativity (effective) low-regularity phenomenon, agnostic to possible detailed (quantum) descriptions of gravity at higher-energies and potentially observationally accessible. Bio: Jose Luis Jaramillo works at the Institut de Mathématiques de Bourgogne (IMB) in Dijon, in the Mathematical Physics group. He did his Ph.D at the Instituto de Astrofísica de Andalucía (IAA-CSIC), followed by postdoctoral stays at the Observatoire de Paris-Meudon, the Albert Einstein Institut (Max-Planck Institut for Gravitational Physics) in Golm and the Laboratoire de Physique Océanographique (LPO) in Brest (France). Since 2015 he is professor in the Mathematics Department at the Université de Bourgogne in Dijon.
460 days ago
Jonathan Braden

The Big Bang in the Lab : Analog Preheating with Bose-Einstein Condensates

Abstract: In inflationary cosmology, the Universe must transition from an exponentially expanding state dominated by the nearly homogeneous inflaton condensate, into a state dominated by a hot bath of Standard Model particles. This process is known as reheating. Detailed modelling of this transition generically reveals exponentially growing instabilities, whose dynamics is typically referred to as preheating. A canonical example arises when the inflaton oscillates around a potential minimum shortly after the inflationary phase ends. Linear fluctuations of fields coupled to the inflaton (or the inflaton itself) then obey a wave equation with an oscillating mass, leading to exponential growth of certain bands of wavenumbers. Eventually these growing modes become large enough to undergo strong mode-mode coupling, resulting in a fracturing of the inflaton. This nonlinear stage can only be studied using semiclassical lattice simulations. While these lattice simulations are believed to provide a good approximation to the full quantum dynamics, they have never been tested with experiments leaving the possibility for novel quantum phenomena. I will show how we can emulate end-of-inflation dynamics in the lab using two coupled dilute gas Bose-Einstein condensates (BECs), providing an opportunity to experimentally study preheating. By appropriately tuning parameters, the evolution of the relative phase between the two BECs is well described by the relativistic sine-Gordon model. To study preheating, we begin with two nearly homogeneous BECs and imprint an initial relative phase between them. The relative phase then undergoes oscillations analogous to a rigid pendulum. Of course, quantum mechanics ensures that the condensates cannot be perfectly homogeneous, and small quantum fluctuations are initially present. In the sine-Gordon theory, there is a single band of linearly unstable modes which grow exponentially in the presence of a homogeneous oscillating background. As a first step, I will demonstrate that the cold atom system replicates the linear instability of the sine-Gordon model. I will also quantify the deviations from the pure sine-Gordon limit, showing they are small. I then use lattice simulations of the BECs to study the full nonlinear evolution of the condensates. I will show that once the fluctuations enter the nonlinear regime, the relative phase fractures into a collection of localized oscillating field configurations, known as oscillons. This behaviour matches that seen in simulations of the pure sine-Gordon model. Bio: Jonathan Braden is a Senior Research Associate at the Canadian Institute for Theoretical Astrophysics. His primary research interest is early Universe cosmology, with a particular focus on strong nonlinearity in the early Universe. Some specific topics include the dynamics of phase transitions, (p)reheating, and particle production during inflation; and the nonGaussian imprints of these phenomena in observational data. He is also interested in using analog early Universe experiments to build a "Universe on a table-top". He also has extensive experience in high performance computing, specifically the application of spectral and symplectic integration schemes to cosmology. He was previously a postdoctoral fellow at University College London. He received his PhD in Physics from the University of Toronto.
502 days ago
Ian Spielman

Accurate Determination of Hubble Attenuation and Amplification in Expanding and Contracting Cold-Atom Universes

Abstract: In the expanding universe, relativistic scalar fields are thought to be attenuated by “Hubble friction,” which results from the dilation of the underlying spacetime metric. By contrast, in a contracting universe this pseudofriction would lead to amplification. Here, we experimentally measure, with fivefold better accuracy, both Hubble attenuation and amplification in expanding and contracting toroidally shaped Bose-Einstein condensates, in which phonons are analogous to cosmological scalar fields. We find that the observed attenuation or amplification depends on the temporal phase of the phonon field, which is only possible for nonadiabatic dynamics. References: [1] Accurate Determination of Hubble Attenuation and Amplification in Expanding and Contracting Cold-Atom Universes; S. Banik, M. G. Galan, H. Sosa-Martinez, M. Anderson, S. Eckel, I. B. Spielman, and G. K. Campbell; Phys. Rev. Lett. 128 090401 (2022). doi:10.1103/PhysRevLett.128.090401 [2] A Rapidly Expanding Bose-Einstein Condensate: An Expanding Universe in the Lab; S. Eckel, A. Kumar, T. Jacobson, I. B. Spielman, and G. K. Campbell; Phys. Rev. X 8 21021 (2018). doi:10.1103/PhysRevX.8.021021 Ian Spielman is a fellow at the Joint Quantum Institute, National Institute of Standards and Technology and the University of Maryland.
517 days ago
Stephen Green

Machine Learning for Gravitational Wave Astronomy

Abstract: Since 2015, the LIGO-Virgo-KAGRA Collaboration has detected 90 signals from merging compact objects such as black holes and neutron stars. Each of these is analyzed using Bayesian inference, employing a stochastic algorithm such as Markov Chain Monte Carlo to compare data against models—thereby characterizing the source. However, this is becoming extremely costly as event rates grow with improved detector sensitivity. In this talk I will describe a powerful alternative using probabilistic deep learning to analyze each event in orders of magnitude less time while maintaining strict accuracy requirements. This uses simulated data to train a normalizing flow to model the posterior distribution over source parameters given the data—amortizing training costs over all future detections. I will also describe the use of importance sampling to establish complete confidence in these deep learning results. Finally I will describe prospects going forward for simulation-based inference to enable improved accuracy in the face of non-stationary or non-Gaussian noise. Bio: I am a Nottingham Research Fellow in gravitational waves at the School of Mathematical Sciences. My current interests are in black hole perturbation theory and machine-learning methods for data analysis. Prior to Nottingham, I spent five years at the Albert Einstein Institute in Potsdam, where I became a member of LIGO. I also did postdocs at the University of Guelph and Perimeter Institute, studying turbulence in gravitational waves and black hole instabilities. I obtained my PhD in physics at the University of Chicago under Robert Wald. My dissertation was on general relativistic backreaction effects in cosmology.
530 days ago
Gabriele Ferrari

Ferromagnetism in an extended coherently-coupled atomic superfluid

ABSTRACT: Extended atomic superfluids in the presence of a coherent coupling between two internal states represent a flexible platform to address open problems in condensed matter and fundamental physics. I will discuss their application to the experimental observation of a quantum phase transition from a para- to a ferromagnetic state. The nature of the transition is assessed by looking at the phase diagram as a function of the control parameters, at hysteresis phenomena, and at the magnetic susceptibility and the magnetization fluctuations around the critical point. Domain walls separating regions of opposite magnetization in the ferromagnetic state are created deterministically. I will also discuss the local decay of the metastable state into the real ground state leading to the probabilistic nucleation of bubbles. REFERENCE: Revealing the ferromagnetic phase transition in an extended two-component atomic superfluid; Riccardo Cominotti, Anna Berti, Clement Dulin, Chiara Rogora, Giacomo Lamporesi, Iacopo Carusotto, Alessio Recati, Alessandro Zenesini, Gabriele Ferrari; arXiv:2209.13235. SHORT BIO: Gabriele Ferrari is associate professor at the Physics Department of the University of Trento, Italy.
545 days ago
Dmitry Solnyshkov

Analogue black holes in quantum fluids of light

ABSTRACT Analogue physics is an interesting direction in modern Physics, based on the similarities of the mathematical models describing different systems. Such similarities were known for a very long time, the most famous example being the ubiquitous harmonic oscillator. However, the idea to use these similarities to study inaccessible systems and regimes in the lab has appeared relatively recently. The directions of research in analogue physics and associated effects include analogue gravity (Hawking emission1), early Universe (Kibble-Zurek mechanism2), high-energy physics (Klein tunnelling3, Zitterbewegung4), quantum simulations (Heisenberg model5), and others. In this talk, I will discuss a particularly interesting platform: a quantum fluid of strongly-coupled exciton-polaritons, often called a quantum fluid of light. Starting with an overall description of the state of the art on analogue black holes6, I will then turn to recent advances in polariton black hole simulations7. In particular, I will discuss the topological defects called quantum vortices, which can also be considered as charged relativistic particles8 obeying to the equations of analogue electrodynamics and gravity and allowing to simulate a Kerr black hole9 and to reproduce the Penrose effect. 1 W. G. Unruh, Phys. Rev. Lett. 46, 1351 (1981). 2 W. Zurek, Nature 317, 505 (1985). 3 M. I. Katsnelson et al, Nature Physics 2, 620 (2006). 4 J. Schliemann et al, Phys. Rev. Lett. 94, 206801 (2005). 5 C. Gross, I. Bloch, Science 357, 995 (2017). 6 C. Barcelo, Nat. Physics 15, 210 (2019). 7 M. J. Jacquet et al, Ph. Trans. Roy. Soc. A 378, 20190225 (2020). 8 D. Solnyshkov et al, Phys. Rev. B 85, 073105 (2012). 9 D. Solnyshkov et al, Phys. Rev. B 99, 214511 (2019) BIOSKETCH Dmitry Solnyshkov is a Professor of Physics (Solid State Physics) in University Clermont Auvergne (Clermont-Ferrand, France). Honorary member of the Institut Universitaire de France. Master of Physics obtained in St. Petersburg State Polytechnical University (Russia). PhD and second thesis (HDR) in University Blaise Pascal (France).Theoretician, author of more than 160 publications cited more than 6000 times, h=41 (WoS). Contributed to 4 books. Co-supervised 9 PhD students. Research topics in fundamental and applied physics: Bose-Einstein condensation, strong light-matter coupling, quantum fluids, topological physics, lasers and optical devices, 2D materials, analogue physics.
586 days ago
Sebastian Erne

Extracting and designing effective field theory descriptions of complex many-body systems

Abstract: "In this seminar I will discuss methods for extracting, testing, and designing effective field theory descriptions for continuous (quantum) field theory simulators. After introducing general concepts for the preparation, design, and detection of effective field theories in cold atom systems, I will present our latest results on the floquet engineered sine-Gordon model, emerging as the effective quantum field theory description of two tunnel-coupled quantum wires, and on parametrically driven classical two-fluid interfaces as analogue simulators for the onset of nonlinearities in preheating dynamics. Special focus will be on factorization properties of higher-order correlations as sensitive probes for nonlinear dynamics in strongly correlated field theories which opens a window in the experimental extraction of emergent field theory descriptions for complex many-body systems." Biosketch: Sebastian Erne is a Senior Research Associate at the Atominstitut of the Vienna Technical University. His primary research is focused on non-equilibrium quantum many-body systems in the interface between experiment and theory. His main research interests are analogue cold-atom simulators for precision experiments of quantum field theory with applications to early Universe cosmology, quantum field theory in curved spacetime, the Unruh effect, equal time formulations of QFT, universal physics close to and far from equilibrium, and high performance computing for the modeling of complex quantum systems.
601 days ago
Markus Oberthaler

Quantum field simulator for dynamics in curved spacetime

"In this seminar I will introduce the capabilities of ultracold gases to address fundamental questions in quantum field theory. I will introduce the experimental capabilities in preparation and detection of quantum fields in atomic gases. A special focus will be on our latest result on the implementation of curved spacetime for a scalar massless quantum field. We have shown that positive as well as negative curvature can be experimentally realized and can be dialed in as needed. We also used the experiment/simulator to reveal particle production in an accelerating, decelerating and constantly expanding spacetime. Employing Sakharov oscillations for detecting the production of excitations opens a window to add to the power spectrum also phase information about the excitations." Markus Oberthaler is a professor, chair of experimental physics at Kirchhoff-Institute for Physics,Heidelberg University. His main research fields are Quantum Entanglement in many particle systems and quantum simulation, precision experiment testing quantum field theory, quantum metrology, immersed quantum systems, universal physics far from equilibrium and connection to high energy physics, environmental physics and dating of water and ice with Argon Trap Trace Analysis (ATTA).
649 days ago
Jonathan Braden

The Role of Short-Wavelength Modes in Vacuum Decay

ABSTRACT False vacuum decay plays an important role in many cosmological scenarios, while also acting as an important keystone model for nonequilibrium quantum field theory. It was recently realized that dilute gas Bose-Einstein condensates (BECs) can be used to emulate the dynamics of relativistic vacuum decay, providing a unique experimental window into the early Universe. Making optimal use of these experiments requires a detailed understanding of the theoretical predictions for vacuum decay. I will discuss some recent work on the role of short-wavelength fluctuations in real-time simulations of false vacuum decay, focussing on the case of pure scalar field theory. This includes renormalization effects, which capture averaged effects of fluctuations on the long-wavelength dynamics, as well as stochastic contributions where the long-wavelength modes are sensitive to the particular realization of the short-wavelengths. I will comment on the implications of these results for BEC experiments. Finally, time (and audience interest) permitting, I will discuss how the evolution of a dilute gas BEC system can be mapped onto another important nonlinear epoch in the early Universe --- the end-of-inflation. BIO Jonathan Braden is a Senior Research Associate at the Canadian Institute for Theoretical Astrophysics. His primary research interest is early Universe cosmology, with a particular focus on strong nonlinearity in the early Universe. Some specific topics include the dynamics of phase transitions, (p)reheating, and particle production during inflation; and the nonGaussian imprints of these phenomena in observational data. He is also interested in using analog early Universe experiments to build a "Universe on a table-top". He also has extensive experience in high performance computing, specifically the application of spectral and symplectic integration schemes to cosmology. He was previously a postdoctoral fellow at University College London. He received his PhD in Physics from the University of Toronto.
761 days ago
Ulrich Schneider

Realizing a first-order quantum phase transitions in a driven optical lattice

ABSTRACT: Phase transitions and critical phenomena have been at the heart of many-body physics, and quantum simulations with cold atoms from the beginning. While almost all phase transitions in cold atoms systems are continuous, there is a renewed interest also in discontinuous (first-order) phase transitions and the quantum metastability, whose relativistic analogues are believed to play an important role in early-universe cosmology (false vacuum decay). We experimentally demonstrate a novel level of control over a quantum phase transition by combining an optical lattice with an uncommon type of Floquet engineering based on a resonant drive. Contrary to most applications of periodic driving, where the drive frequency is selected to avoid all resonances, we resonantly couple the lowest two bands of a lattice. With this drive, we can not only induce the superfluid to Mott insulator transition but are furthermore able to control its character and turn the Mott transition from a continuous into a discontinuous transition, thereby opening the door to quantum simulations of the early universe and interacting topological transitions in condensed matter systems. BIO: Prof. Ulrich Schneider is a Professor of Many-Body Physics at the University of Cambridge. His work is centred on employing ultracold atoms in optical lattices as a testbed to study Quantum Many-Body Dynamics. His scientific interests range from quantum thermodynamics, low-dimensional systems, and strongly correlated systems to topological effects and many-body localization. Prof. Schneider studied physics in Kaiserslautern and Sheffield, received his PhD from the Johannes-Gutenberg University Mainz, and worked as a senior scientist at the Ludwig-Maximilians-University (LMU) and the Max-Planck-Institute for Quantum Optics (MPQ) in Munich. He is a fellow of Jesus College, received the 2015 Rudolf-Kaiser Prize, and the recipient of ERC Starting (2016, Quasicrystal) and Consolidator (2021, Kagome) grants. In 2020 he also joined the AION collaboration building an Atom Interferometer and Observatory Network. The seminar will last 1 hour including Q&A and be held at 3.00pm UK Time/10.00am Toronto Time. This seminar will be held on Zoom.
789 days ago
Ian Moss

False vacuum decay in an ultracold spin-1 Bose gas

Abstract: After some background on first order transitions in the early universe I’ll describe a new physical system that might be used as a laboratory analogue. The system is based on a spin 1 Bose gas with Raman and RF induced interactions. It does not require the Feshbach resonance and modulated interactions of previous proposals. Based on work by Ian Moss, Tom Billam and Kate Brown. Biosketch: Ian Moss is Professor of Theoretical Cosmology at Newcastle University. He has been been researching early universe phase transitions for what seems like forever and he is half of the Hawking-Moss instanton. The seminar will last 1 hour including Q&A and be held at 3.00pm UK Time/10.00am Toronto Time. This seminar will be held on Zoom.
804 days ago
Maxime Jacquet

Quantum vacuum excitation of a quasi-normal mode in an analog model of black hole spacetime

Maxime Jacquet (Laboratoire Kastler Brossel,Quantum Optics Group, Paris, France), on "Quantum vacuum excitation of a quasi-normal mode in an analog model of black hole spacetime" Biosketch: Maxime is currently leading experimental and theoretical research in analogue gravity with quantum fluids of light in the Quantum Optics group at Laboratory Kastler Brossel, Sorbonne University and CNRS, France. Abstract: Analogue gravity enables the laboratory investigation of effects of quantum field theories on curved spacetimes. The archetypal example is the parametric amplification of vacuum quantum fluctuations of the field on the curved spacetime, as in the Hawking effect (the correlated emission of waves) at the event horizon. In this talk, I will review theoretical work on the Hawking effect in transsonic, quantum fluids of microcavity polaritons [1]. Because of the driven-dissipative dynamics of the fluid, the system is out of thermal equilibrium. I will explain how this impacts correlated emission by the Hawking effect [2], and also show that dissipation may be harnessed to observe novel effects like the quantum vacuum excitation of quasi-normal modes of the acoustic field [3]. This, I will argue, opens a range of new questions pertaining to all quantum fields on black hole spacetimes beyond analogue models. Refs: [1] arxiv:2002.00043, [2] arxiv:2201.02038, [3] arxiv:2110.14452. The seminar will last 1 hour including Q&A and be held at 3.00pm UK Time/10.00am Toronto Time. This seminar will be held on Zoom.
817 days ago
Dalila Pirvu

Bubble Clustering in Cosmological First Order Phase Transitions

Abstract: False vacuum decay in quantum mechanical first order phase transitions is a phenomenon with wide implications in cosmology and presents interesting theoretical challenges. In the standard approach, it is assumed that false vacuum decay proceeds through the formation of bubbles that nucleate at random positions in spacetime and subsequently expand. In our work, we investigated the presence of correlations between bubble nucleation sites using a recently proposed semiclassical stochastic description of vacuum decay. The procedure sampled vacuum fluctuations which were evolved using classical lattice simulations. We computed the two-point function for bubble nucleation sites from an ensemble of simulations, demonstrating that nucleation sites cluster in a way that is qualitatively similar to peaks in random Gaussian fields. I will comment qualitatively on the phenomenological implications of bubble clustering in early Universe phase transitions, which include features in the power spectrum of stochastic gravitational waves and an enhancement or suppression of the probability of observing bubble collisions in the eternal inflation scenario. I finish by explaining briefly how our results can be tested empirically using a table-top analogue of vacuum decay. Biosketch: Dalila Pirvu is a second year PhD student at the Perimeter Institute and University of Waterloo in Canada. She did her undergraduate at Imperial College London. Her work is focused on dynamical lattice simulations of phase transitions in quantum field theories. The seminar will last 1 hour including Q&A and be held at 3.00pm UK Time/10.00am Toronto Time. This seminar will be held on Zoom. Details to follow
832 days ago
Niayesh Afshordi

Stimulating Hawking Radiation of Gravitational Waves

Abstract: I will outline the diverse theoretical arguments for why quantum mechanics of merging black holes should lead to the stimulation of Hawking Radiation of gravitational waves, which may manifest as delayed echoes in observations. I will then summarize the status and outlook of the observational program to search for these echoes. Biosketch: Niayesh Afshordi is an Astrophysicist whose research spans from Early Universe Cosmology and Quantum Gravity, to Dark Matter, Dark Energy, Extragalactic Astronomy, Large Scale Structure of the Universe, and High Energy Astrophysics. He received his PhD at Princeton in 2004. He then did postdocs at Harvard College Observatory and Perimeter Institute for Theoretical Physics. He is currently a professor of Physics and Astronomy at the University of Waterloo. He is also an associate faculty at Perimeter Institute, and a founding faculty of the Waterloo Centre for Astrophysics. The seminar will last 1 hour including Q&A and be held at 3.00pm UK Time/10.00am Toronto Time. This seminar will be held on Zoom.
867 days ago
Maria Chiara Braidotti

Measurement of Penrose superradiance in a photon fluid

Superradiance is the amplification ofwaves scattered by a rapidly rotating object, first proposed by Roger Penroseas a way to extract energy from rotating black holes. Despite being afundamental process in wave physics, astrophysical superradiance has not beenobserved yet due to the large distances involved. However, proposal based onanalogue gravity studies have demonstrated their versatility providing the firstmeasurement of superradiance in a hydrodynamic experiment. Here we report the first measurementof Penrose superradiance in nonlinear optical systems, especially in a 2Dsuperfluid. A weak signal beam with orbital angular momentum is focused ontothe core of a pump vortex. In the scattering, a negative norm idler wave isgenerated and trapped inside the pump core, while the signal (positive normwave) gets amplified. Our results demonstrate the presence of Penrosesuperradiance in superfluids unveiling the key role of the negative norm modein the amplification process. Maria Chiara Braidotti is a research associateat the School of Physics & Astronomy of the University of Glasgow (UoG),Scotland. She joined works in the ExtremeLight group lead by Prof. Faccio, UoG( Her main expertise is theoretical andexperimental nonlinear optics, especially using it as a tool to testfundamental processes at the interface between gravitation and quantummechanics. In recent years, her research activity has focused on the test ofPenrose process and Zel’dovich effect both involving amplification ofelectromagnetic modes from the scattering with a rotating body. She is author of more than 15 publications ininternational journals (with 2 publications on Physics Review Letters journal)and over 30 invited presentations to national and international conferences.