**Quantum Gravity** LABORATORY

# recent developments

*Rotational superradiant scattering in a vortex flow*

* 25 April 2017*

We report on the world's first detection of superradiance due to rotation, and our results are published in Nature Physics: When an incident wave scatters off of an obstacle, it is partially reflected and partially transmitted. In theory, if the obstacle is rotating, waves can be amplified in the process, extracting energy from the scatterer. Here we describe in detail the first laboratory detection of this phenomenon, known as superradiance. We observed that waves propagating on the surface of water can be amplified after being scattered by a draining vortex. The maximum amplification measured was 14% ± 8%, obtained for 3.70 Hz waves, in a 6.25-cm-deep fluid, consistent with the superradiant scattering caused by rapid rotation. We expect our experimental findings to be relevant to black-hole physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole, as well as to hydrodynamics, due to the close relation to over-reflection instabilities.

* Fluids and gravity: superradiance and analogue black holes *

*Monday 3rd and Tuesday 4th of April 2017*

Fluids and gravity: interdisciplinary workshop on fluid dynamics and rotating black hole physics at the University of Nottingham. A joint event between the School of Mathematical Sciences and the Centre for the Mathematics and Theoretical Physics of Quantum Non-Equilibrium Systems (CQNE).

*Research Associate/Fellow (2 posts)*

*21 Feb 2017*

Applications are invited for the above two posts to work with Dr Silke Weinfurtner, members of the Quantum Gravity Laboratory group and external collaborators (e.g. Vitor Cardoso, Stefano Liberati, Joerg Schmiedmayer, Mauricio Richartz and Bill Unruh).

The Quantum Gravity Laboratory group is part of the Quantum Gravity group headed by John Barrett, and includes Kirill Krasnov, Jorma Louko, Thomas Sotiriou, Alexander Schenkel and Silke Weinfurtner http://www.nottingham.ac.uk/mathematics/research/mathematical-physics/quantum-gravity.aspx.

The successful applicants will have strong knowledge of Classical and Quantum Field Theory in curved spacetimes and/or Analogue Gravity Systems. However, due to the interdisciplinary nature of the project objectives, a background in Ultra-cold atoms, Fluid Dynamics, Quantum Information, and AdS/CFT correspondence applied to condensed matter field theories will be considered.

Candidates must hold or be near completion of a PhD, or equivalent, in a relevant branch of theoretical/experimental physics. Candidates should be confident, organised, have good communication skills and enjoy working as part of a team as well as independently. The successful candidates will be expected to contribute to the publication of scientific papers, and to their dissemination at relevant workshops and conferences. They should have evidence that the quality and quantity of their previous research outputs is commensurate with their level of experience.

In recognition of its commitment to promoting women in science, The University of Nottingham is one of a small number of universities to hold a Silver Athena SWAN Award.

These two full-time positions are available from 1 April 2017 or as soon as possible thereafter.

For information about the School of Mathematical Sciences, which has strong and active research groups see: http://www.nottingham.ac.uk/mathematics/index.aspx.

Informal enquiries may be addressed to Dr Silke Weinfurtner, tel: +44(0)115 9567903 email: silke.weinfurtner@nottingham.ac.uk. Please note that applications sent directly to this email address will not be accepted.

The University of Nottingham is an equal opportunities employer and welcomes applications from all sections of the community.

Location: Nottingham

Salary: £26,052 to £32,004 per annum, depending on skills and experience (minimum £29,301 with relevant PhD)

Hours: Full Time

Contract Type: Permanent

Placed on: 14th February 2017

Closes: 15th March 2017

Job Ref: SCI022917

*PhD position in Analogue Gravity*

*3 Feb 2017*

We are currently carrying out an experiment to study the effects occurring around effective horizons in an analogue gravity system. In particular, the scientific goals are to explore superradiant scattering and the black hole evaporation process. To address this issue experimentally, we utilize the analogy between waves on the surface of a stationary draining fluid/superfluid flows and the behaviour of classical and quantum field excitations in the vicinity of rotating black. We are looking for a PhD student to get involved in the theoretical and/or experimental aspects of this projects. More information about the project can be found here: http://www.gravitylaboratory.com/

The students will be based at the University of Nottingham at the School of Mathematical Sciences. The external collaborators are Prof. Josef Niemela (ICTP, Italy), Prof. Vitor Cardoso (Instituto Superior Técnico, Portugal), and Prof. Stefano Liberati (SISSA, Italy). The external consultant for the experiment is Prof. Bill Unruh (University of British Columbia, Canada), who will be a regular visitor.

This studentship is linked to the EPSRC project " Black Hole Superradiance in Rotating Fluids (SURF)".

*Dissertation projects at qgLab*

*3 Feb 2017*

We offer two dissertation projects this year:

1) Slow cooperative relaxation and non-equilibrium at macroscopic

scales, a joint project with Juan Garrahan.

2) Mimicking astrophysical superradiance in fluid laboratories

*First experimental results from the new setup*

*20 December 2016*

In an amazing team effort qgLab (Theo Torres, Sam Patrick, Antonin Coutant and Silke Weinfurtner) and two external collaborators (Mauricio Richartz and Edmund W. Tedford) have completed the first experimental study entitled 'Observation of superradiance in a vortex flow' as part of the EPSRC program 'Black Hole Superradiance in Rotating Fluids' before the Christmas break. Well done everyone!

*Proposing new ways of detecting superradiance in fluid laboratories*

*November 2016*

Rotational superradiance has been predicted theoretically decades ago, and is the chief responsible for a number of important effects and phenomenology in black hole physics. However, rotational superradiance has never been observed experimentally. Here, with the aim of probing superradiance in the lab, we investigate the behaviour of sound and surface waves in fluids resting in a circular basin at the center of which a rotating cylinder is placed. We show that with a suitable choice for the material of the cylinder, surface and sound waves are amplified. Two types of instabilities are studied: one sets in whenever superradiant modes are confined near the rotating cylinder; the other, which does not rely on confinement, corresponds to a local excitation of the cylinder. Our findings are experimentally testable in existing fluid laboratories and hence offer experimental exploration and comparison of dynamical instabilities arising from rapidly rotating boundary layers in astrophysical as well as in fluid dynamical systems.

Phys. Rev. Lett. 117, 271101 2016

*Workshop in Paris: Gravity and Experiment *

*5-9 December 2016*

ABOUT the WORKSHOP

Gravity appears to be one of the ingredients of open problems that span an impressive range of length scales and several research fields - theoretical and mathematical physics, cosmology, astrophysics. Characteristic examples are the quantum nature of spacetime, vacuum energy and the cosmological constant, dark matter and dark energy, black holes physics. One of the key problems in gravity is the limited amount of experimental guidance. The purpose of this workshop is to bring together leading experimentalists and a handful of theorists and discuss how ongoing and future experiments can lead to new insights on the gravitational interaction. We will cover experiments at all scales, from tabletop to space missions, and across all fields, from gravitational waves and relativistic astrophysics to cosmology and analogue gravity.

THE GOAL

The hope is to build bridges between different fields and encourage interdisciplinary interactions. To this end, the proposed format is the following: invited overview talks on the latest experiments will be given by experimentalists and they will be followed by topical discussions led by theorists. The program will allow ample time for discussions and round tables.

*EPSRC funded!*

*1 November 2016*

The EPSRC funded our project 'Black Hole Superradiance in Rotating Fluids' to continue our experimental studies to explore black hole physics processes in fluid laboratories.

*Zack Fifer top of the class and will join qgLab in January 2017*

*17 October 2015*

Congratulations to MSc student Zack Fifer, who graduated first in his course at The University of Nottingham. Zack spent his dissertation project in the Black Hole Laboratory, and won a Vice-Chancellor's Scholarship for Research Excellence to continue his studies as PhD student in the qgLab group.

*The black hole machine*

*16 August 2016*

Our 'Black hole laboratory' is now part of the videos by Brady Haran for the University of Nottingham:

The Black Hole Laboratory at the University of Nottingham uses a special water tank and cameras to simulate and measure some conditions near a black hole. Interview with Dr Silke Weinfurtner.

*Bill Unruh visiting qgLab for 3 months*

*Sep-Dec 2016*

Bill Unruh is spending part of his sebatiacl at The University of Nottingham to work with qgLab.

*Sam Patrick joins qgLab*

*July 2016*

Sam Patrick joins qgLab to work on 'Hydrodynamic Simulation of Black Holes'.

*A solution to a long-standing problem in analogue gravity: Hawking radiation without horizons?*

*9 Mar 2016*

We study the propagation of low frequency shallow water waves on a one dimensional flow of varying depth. When taking into account dispersive effects, the linear propagation of long wavelength modes on uneven bottoms excites new solutions of the dispersion relation which possess a much shorter wavelength. The peculiarity is that one of these new solutions has a negative energy. When the flow becomes supercritical, this mode has been shown to be responsible for the (classical) analog of the Hawking effect. For subcritical flows, the production of this mode has been observed numerically and experimentally, but the precise physics governing the scattering remained unclear. In this work, we provide an analytic treatment of this effect in subcritical flows. We analyze the scattering of low frequency waves using a new perturbative series, derived from a generalization of the Bremmer series. We show that the production of short wavelength modes is governed by a complex value of the position: a complex turning point. Using this method, we investigate various flow profiles, and derive the main characteristics of the induced spectrum.

*Theo Torres joins qgLab*

*March 2016*

Theo Torres joined qgLab to work on Hydrodynamic Simulations of Rotating Black Holes. Theo is partially funded by Royal Society.

*Biology application for Ripple Catcher*

*2 October 2015*

Hartman Thomas (Biology, Nottingham University) and Tommy Napier (Physics, Nottingham University) asked us to use the Ripple Catcher to scan a giraffe skull.

*PhD position openings*

*We are looking for two new students*

We are currently carrying out various experiments to study the effects occurring around effective horizons in an analogue gravity system. In particular, the scientific goals are to explore superradiant scattering, the black hole evaporation process and dynamical instabilities related to rotating blakc holes and stars. To address this issue experimentally, we utilize the analogy between waves on the surface of a stationary draining fluid/superfluid flows and the behavior of classical and quantum field excitations in the vicinity of rotating black holes and stars.

This project will be based at the University of Nottingham at the School of Mathematical Sciences. The two external collaborators are Prof. Josef Niemela (ICTP, Italy), Prof. Stefano Liberati (SISSA, Italy) and Prof. Vitor Cardoso (Instituto Superior Técnico, Portugal).

The PhD student(s) will be involved in all aspects of the experiments theoretical as well experimental. We require an enthusiastic graduate with a 1st class degree in Mathematics/Physics/Engineering (in exceptional circumstances a 2(i) class degree can be considered), preferably of the MMath/MSc level. Candidates would need to be keen to work in an interdisciplinary environment and interested in learning about

quantum field theory in curved spacetimes, fluid dynamics, analogue gravity, and experimental techniques such as flow visualisation (i.g. Particle Imaging or Laser Doppler Velocimetry) and surface measurements (i.g. profilometry methods).

The studentship period will depend on the training needs of the successful applicant.

Early application is strongly encouraged.

Funding Notes:

The studentship is available for immediate start and provides an annual stipend at the

standard rate (currently £13,863 per annum) and full payment of Home/EU Tuition

Fees.

*The Ripple Catcher up and running*

*22 September 2015*

The Ripple Catcher is up and running and mounted into possition. The first results look fantastic! Over the next couple of days we have to optimize the dye concentration and add some more optical components to the setup to reduce errrors due to light scattering. We have already posted some first results on site.

*The Ripple Catcher*

*21 September 2015*

Our high speed 3D air-fluid interface sensor arrived in Nottingham.

Our 3D sensor technology is based on a stereo-photogrammetry approach. It includes a pattern projector, two cameras, two cameras are directed onto the water surface. Because of fluorescent dye of average concentration, the projector light is converted at the surface of the water. The two cameras take pictures of the water surface with differing statistical patterns from the projector illuminating it.

After capturing a sequence of images with both cameras, correspondence assignment is applied to the images to achieve many, accurately located homologous image points. Afterwards, the corresponding image points are triangulated with a preliminary calibration of the camera parameters. The obtained 3D points can now be taken for manifold investigations, dispersion relation, amplitude magnification and so on.

The University of Nottingham and EnShape have submitted a joint-pattent application to protect our invention. If you are interested in purchasing our sensor, please contact EnShape. The first Hight speed 3D air-fluid interface sensor has now arrived in Nottingham. On site we will document the arrival, setup and first results obtained with the newest member of QG-Lab.

*3D printer up and running*

*15 July 2015*

Congratulations to Zack Fifer! Over the last month Zack has built and callibrated a 3D printer from scratch. We will use the 3D printer for various applicaitons, e.g. to print flow straightners and callibration surfaces for our 3D surface wave detection sensors.

*Undergraduate Research Bursary application successful*

*11 May 2015*

Andrew Scoins (University of Nottingham) wins an Undergraduate Research Bursary Fellowship from the London Mathematical Society to join QG-LAB.

*Marie Curie Fellowship*

*4 February 2014*

Antonin Coutant has been awarded a Marie Curie Fellowship to join QG-LAB. The title of his proposal was 'From fluid dynamics to quantum gravity'.

Antonin is an expert in theoretical analogue gravity studies in fluids and superfluids, and has already made imporant contributions in the field. Antonin will be a valuable addition to QG-LAB.

*Spektrum der Wissenschaft*

*09 January 2015*

Article about analouge gravity experiments by Franziska Konitzer (in German), including qg-LAB experiments.

Analoge Gravitation

Das Schwarze Loch in der Badewanne

Sind Schwarze Löcher wirklich schwarz? Wie bildeten sich Galaxien? Erste Antworten auf solche Fragen liefern jetzt raffinierte Nachbauten: Mini-Universen auf dem Labortisch.

*Spektrum der Wissenschaft*

*14 November 2014*

Article about analogue table-top experiments by Sophie Hebden (in German), including qg-LAB experiments.

Experimente

Große Physik ganz klein

Manche Experimente sind schlicht undurchführbar: Für die einen müsste der weltgrößte Teilchenbeschleuniger noch 100-mal größer sein, für andere bräuchte man gar ein Schwarzes Loch. Doch es gibt analog funktionierende Systeme, die auf einen Labortisch passen und trotzdem Erkenntnisse versprechen.

*BECosmology @ UoN*

*23/24 June 2014*

Workshop on 'Non-equilibrium Bose-Einstein condensates and its application to Cosmology'

List of participants:

John Barrett • Tom Barrett • Clare Burrage • Ed Copeland • Antonin Coutant • Daniele Faccio • Jorge Ferreras • Andreas Finke • Juan Garrahan • Ed Hinds • Ted Jacobson • Piyush Jain • Dieter Jaksch • Claus Kiefer • Peter Kruger • Tim Langen • Emanuele Levi • Jorma Louko • Fedja Orucevic • Parentani Renaud • Wolfgang Rohringer • Joerg Schmiedmayer • Thomas Sotiriou • Andrea Trombettoni • Bill Unruh • Silke Weinfurtner • Chris Westbrook

*Workshop for Science Writers: Quantum Theory*

*27-29 August 2014*

Quantum Gravity LABORATORY will participate at the workshop for science writers organized by Prof. Sabine Hossenfelder and George Musser to give an overview on analogue gravity.

"Quantum physics is a notoriously challenging subject even for the experts. The goal of this workshop is to give science writers the opportunity to take a step back and gain a broader perspective on this field. At the same time, we want to give researchers in the field the possibility to interact with science writers and share experiences about the pitfalls of science communication.

Some of the topics that will be covered at this workshop are: Quantum computing, quantum optics and novel tests of the foundations of quantum mechanics, topologial insulators, tests of emergent quantum mechanics, analog gravity, the gauge-gravity duality and its applications in condensed-matter physics, and searching for new physics in atomic, molecular and optical physics."

The workshop will take place at NORDITA in Stockholm.

*New Scientist feature article on analogue gravity experiments*

*10 March 2014*

The New Scientist reports on 'A black hole in a bath: Big physics on a bench-top': "Over the event horizon Where analogues really come into their own is with objects in the universe that we know exist, but that are impossible to investigate directly. Black holes are a good example. These cosmic monsters are predicted by Einstein's theory of gravity, the general theory of relativity. They form when large stars collapse and die, and supermassive versions are thought to skulk at the heart of most large galaxies. They are also portals to the ultimate prize of physics – a theory that explains what happens when the quantum particles of matter meet the extremes of gravity, the only force not covered by the quantum rules of the standard model. But given that it emits no light, it is not easy to discern exactly what a black hole is doing. Silke Weinfurtner at the University of Nottingham, UK, aims to lift the veil in the lab, using just water and laser light to simulate a black hole's emission of Hawking radiation. This process, proposed by the physicist Stephen Hawking in the 1970s, is thought to occur when a fluctuation in the quantum vacuum near a black hole's event horizon – its point of no return – causes a quantum-entangled pair of matter and antimatter particles to form. If one of the pair falls into the black hole while the other is just far enough away to escape it, the two particles can separate, with one trapped inside the black hole forever and one radiated away. Weinfurtner's analogue actually simulates a "white hole" that, instead of sucking everything in to it, deflects everything away. Reverse the direction of time in the underlying equations, however, and conclusions drawn for white holes are just as valid for black ones. The analogue consists simply of water flowing along a channel containing a smooth obstacle. The team induced ripples on the surface of the water travelling in the opposite direction, and used a 2D sheet of laser light to analyse the properties of the surface waves as they hit the obstacle region and are reflected off. They found that the amplitude and spread of wave frequencies corresponded to those expected of Hawking radiation at a black hole's event horizon (arxiv.org/abs/1302.0375). "It was a very clear, conclusive detection of the effect," says Weinfurtner. "It was a big surprise to us how robust these experiments are." The work has already triggered theoretical studies into how an entirely classical-physics experiment can even crudely reproduce aspects of Hawking radiation, which is a fundamentally quantum effect. A full-blown lab demonstration – one that also shows that the particles remain entangled as predicted by the theory – would require a more sophisticated, quantum analogue. Together with her Nottingham colleague Peter Kruger, Weinfurtner is working out how to detect the effect using supercooled atoms."

*Cover page*

*7 March 2014*

Our article Reducing spacetime to binary information published in the Journal of Physics A: Mathematical and Theoretical was selected for the cover page.

*Perimeter Welcomes Five New Emmy Noether Fellows*

*7 March 2014*

The Perimeter Institute announced the appointment of five young scientists to its Emmy Noether Fellowships program: Alejandra Castro, Belén Paredes, Catherine Pépin, Silke Weinfurtner (QG-LAB), and Kathryn Zurek.

*3 year project funded by Royal Society Research Grant Scheme*

*10 February 2014*

The RS has funded a thre year project to outsource some of the ativities taking place to Nottingham. In particular, we will further develop the 2D surface weve detection mechanism in Nottigham. Besides its purpuse for the analogue gravity experiment, this project has various applications for industry, such as 3D printing.

*The Quantum Gravity LABORATORY group moves to Nottingham (UK)*

*1 October 2013*

From October 2013 onwards the Quantum Gravity LABORATORY will be part of the Quantum Gravity Group at the School of Mathematics at the University of Nottingham.

*Audience favorite*

*6 July 2013*

Matt Penrice presented a poster on his PhD project 'Experimental Hydrodynamic Simulations of Rotating Black Holes ' and wins the 'Audience favorite Certificat'.

*Royal Society*

*13 June 2013*

QG-LAB wins Royal Society University Research Fellowship (URF) with the University of Oxford. The Royal Society is the national Academy of science in the UK and its fundamental purpuse is to "to recognise, promote, and support excellence in science and to encourage the development and use of science for the benefit of humanity." The Royal Society offers various grant schemes for all career stages. More information can be found on their website.

*Andreas Finke joins QG-LAB*

*20 May 2013*

Andreas won a RESEARCH TRAINING FELLOWSHIPS FOR UNDERGRADUATE STUDENTS at SISSA for the period of 1 Oct 2013 till 31 Dec 2013 at SISSA to study analogue gravity systems.

In 2014 he will start his PhD on a joint-project linking the School of Mathematics (supervisor: Silke Weinfurtner) and the School of Physics & Astronomy (supervisor: Peter Krueger) at the University of Nottingham.

*Vidi*

*15 May 2013*

QG-LAB wins NWO VIDI grant with the Radboud University. NWO is the Netherlands Organisation for Scientific Research is the national research council in the Netherlands, with a yearly budget of 625 million euros, promoting quality and innovation in science.

*Lorenz G. Straub award*

*30 April 2013*

Ted Tedford, working on the joint SISSA-ICTP experimental activity in analog gravity models at the ICTP Applied Physics/Fluid Dynamics Laboratory at Elettra, was awarded the Lorenz G. Straub Prize Friday, 30 April 2013 at the University of Minnnesota. The award, established under the Lorenz G. Straub Memorial Fund, is given for the most outstanding thesis in hydraulic engineering, ecohydraulics, or related fields in a given year and is open internationally. Ted will be giving live-streamed lecture. More information can be found at the link below.

*New record on ICTP Facebook website*

*23 April 2013*

ICTP pulishes some pictures of our experimental setup, and we set a new record in the number of 'Likes'.

*qg-LAB in The Economist*

*16 March 2013*

qg-LAB was interviewed for The Economist. The overall article is about * How to build a multiverse*, and the section about is called

**A black hole in a bath…**

"Creating a star in a laboratory is small beer compared with creating a black hole. This is an object that is so massive and dense that not even light can flee its gravitational field. Looking inside one is therefore, by definition, impossible. All the more reason to try, says Silke Weinfurtner of the International School for Advanced Studies, in Trieste, Italy.

Dr Weinfurtner plans to make her black hole in the bath. The bath in question, properly called a flume, is a water-filled receptacle 3 metres by 1.5 metres and 50cm deep, across which carefully crafted trains of ripples can pass. In the middle of the tank is a plug hole. If the water going down the hole rotates faster than the ripples can propagate, the ripples which stray beyond the aqueous “event horizon” (a black hole’s point of no return) will not make it out. They are sucked down the drain.

Then the researchers will check whether the simulacrum affects water waves in a way analogous to that which general relativity predicts for light—itself a wave—approaching an astrophysical black hole. According to Albert Einstein’s theory, a region immediately outside the event horizon of a rotating black hole will be dragged round by the rotation. Any wave that enters this region but does not stray past the event horizon should be deflected and come out with more energy than it carried on the way in. To detect this super-radiant scattering, as the effect is called, Dr Weinfurtner will add fluorescent dye to the water and illuminate the surface waves with lasers. The waves, often no bigger than one millimetre, can then be detected using high-definition cameras.

Stefano Liberati, Dr Weinfurtner’s colleague in Trieste, reserves the greatest enthusiasm for another aspect of the experiment. It might, if the researchers are lucky enough, offer clues to the nature of space-time. Could the cosmic fabric be made up of discrete chunks, atoms of space if you like, rather than being continuous, as is assumed by relativity? This problem has perplexed physicists for decades. Many suspect black holes hold the answer, because they are sites where continuous relativity meets chunky quantum physics.

Waterborne holes serve as a proxy. Water is, after all, made up of just such discrete chunks: molecules of H₂O. As wavelengths fall—equivalent to rising energy—waves reach a point where the size of molecules may begin to influence how they behave. If Dr Weinfurtner and Dr Liberati observe some strange behaviour around their event horizons, theorists will be thrilled."

*NARF*

*14 March 2013*

QG-LAB wins Nottingham Advanced Research Fellowship (NARF), a fellowship scheme to target outstanding early career researchers.

27 Jan 2017

*Presentation by Koen J.Groot*

BiGlobal Stability Theory – Applications to Measured Mean Flows. Abstract: Flow stability theory studies small perturbations to steady "base flows." The classical equation applied to boundary layer flows is the Orr-Sommerfeld equation, which returns the perturbations' shape and growth rate. The growth indicates where the perturbations become large and can breakdown non-linearly to yield a turbulent flow; the process referred to as laminar-turbulent transition. Where the Orr-Sommerfeld equation is restricted to predominantly one-dimensional flows, like boundary layers, the BiGlobal stability equations handle two-dimensional flows.

Laminar-turbulent transition depends sensitively on the specific disturbance environment. To take into account realistic disturbance environments, windtunnel experiments are performed. The general stability approach presumes the base flow to be a steady Navier-Stokes solution. In some cases, however, the wintunnel's disturbance environment strongly influences the base flow. Computing the corresponding flow can therefore be difficult and expensive, especially in cases where they are higher dimensional. In that case, it is sensible to perform the stability analysis on the measured time-averaged flow.

This approach is applied to two application cases. The first involves the secondary instability to swept-wing boundary layers and the second the instability of a micro-ramp wake. For consistency purposes, verifications are performed regarding statistical convergence and the impact of practical limitations associated to the experiments. The results are validated with the instantaneous measurement data. The stability results are converged with respect to the ensemble size of the measured mean flow and independent of the spatial limitations of the measurement. This shows one can bring the stability approach closer to the experiment, which can yield a better representation and physical understanding of the ultimate transition process.

Physics building, room B13 at 11am

9 Nov 2016

*Presentation by Julian Barbour*

A dynamical theory of time's arrows. Abstract: Because all known physical laws are time-reversal symmetric, it is almost

universally believed the various arrows of time can only be explained under

the assumption that the solution to the law of the universe in which we

find ourselves is very special. This idea goes back to Boltzmann and today

is often expressed in the assumption that the universe began at the big

bang with an exceptionally low entropy. In PRL *113*, 181101 (2014),

Koslowski, Mercati and I identified bidirectional gravitational arrows of

time that are present in all N-body solutions with non-negative energy. The

oppositely pointing arrows arise at a unique 'Janus point', which divides

any given solution into two qualitatively symmetrical halves, thus

reflecting the time-reversal symmetry of the dynamical law. Any observer

must be on one or the other side of the Janus point and will observe an

arrow time while simultaneously establishing that the underlying law is

time-reversal symmetric. As I will explain, this initial insight has led us

to think many accumulated ideas about the arrows of time could be wrong.

Pope C1 from 2pm till 4pm

10 November 2016

*Colloquium presentation by Bill Unruh*

Analogues for Horizons. Abstract: Hawking's discovery of black hole emissions via quantum effects was one of the most surprising results of the latter half of the 20th century. The experimental observation of this seems impossible (small black holes being pretty dangerous to have on earth, and astrophysical ones being far too cold for the radiation to be observable). Analogue systems which have low frequency horizons and suffer the same type of quantum instability promise experimental observations of the analogue effects, and a few such experiments have recently been carried out. This talk will report on the quantum instability, and describe some of the experiments.

C05 (Physics building)

16 November 2017

*Colloquium presentation by Joerg Schmiedmayer*

Does an isolated quantum system relax? Abstract: The evolution of an isolated quantum system is unitary. This is simple

to probe for small systems consisting of few non-interacting particles. But what happens if the system becomes large and its constituents interact? In general, one will not be able to follow the evolution of the complex many body eigenstates.

Ultra cold quantum gases are an ideal system to probe these aspects of many body quantum physics and the related quantum fields. Our pet systems are one-dimensional Bose-gases. Interfering two systems allows studying coherence between the two quantum fields and the full distribution functions and correlation functions give detailed insight into the many body states and their non-equilibrium evolution.

In our experiments we study how the coherence created between the two isolated one-dimensional quantum gases by coherent splitting slowly degrades by coupling to the many internal degrees of freedom available [1]. We find that a one-dimensional quantum system relaxes to a pre-thermalisatized quasi steady state [2] which emerges through a light cone like spreading of ’de-coherence’ [3]. The pre-thermalized state is described by a generalized Gibbs ensemble [4]. Finally, we investigate the further evolution away from the pre-thermalized state. On one hand we show that by engineering the Quasiparticles we can create many body quantum revivals. On the other hand, we point to two distinct ways for further relaxation towards a final state that appears indistinguishable from a thermally relaxed state. The system looks like two classically separated objects. This illustrates how classical physics can emerge from unitary evolution of a complex enough quantum system.

We conjecture that our experiments points to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by scrambling of the eigenmodes of long lived excitations (quasi particles) [5].

Supported by the Wittgenstein Prize, the Austrian Science Foundation

(FWF) SFB FoQuS: F40-P10 and the EU through the ERC-AdG /QuantumRelax/

[1] S. Hofferberth et al. Nature, *449*, 324 (2007).

[2] M. Gring et al., Science, *337, *1318 (2012); D. Adu Smith et al.

NJP, *15,* 075011 (2013).

[3] T. Langen et al., Nature Physics, *9*, 640–643 (2013).

[4] T. Langen et al., Science *348* 207-211 (2015).

[5] T. Langen, T. Gasenzer, J. Schmiedmayer, J. Stat. Mech. 064009 (2016)

B13 (Physics Building)

2 June 2016

*Presentation by Renaud Parentani*

Observability of quantum entanglement due to pair creation effects. Abstract: In cosmology, the expansion of the universe induces the spontaneous amplification of density fluctuations. In the inflationary scenario, this explains the observed properties of the large scale structures. In homogeneous cond-mat systems, temporal changes induces pair creation of quasi-particles for the same reason. Depending of the initial state and the presence of dissipative effects, the final state can be quantum mechanically entangled. Interestingly, this property can be observationally tested by rather simple in situ measurements of the 2-point correlation function. Seminar based on arXiv:1305.6841, 1311.3507, and 1404.5754.

C04 (Physics Building)

1 June 2016

*Presentation by Florent Michel*

"No-hair" and uniqueness results for analogue black holes. Abstract: The seminal work of Unruh (1981) established a precise correspondence between linear perturbations in non-relativistic moving media and free relativistic fields in curved space-times. It lead to analogue models aimed at better understanding Hawking radiation through condensed matter and conversely. Surprisingly, the analogy partially extends to the non-linear level, where analogue black hole flows obey uniqueness and no-hair results reminiscent of those for black holes in general relativity. In this talk, after a brief introduction to analogue gravity I will show the uniqueness of black hole flows in Bose-Einstein condensates. I will then introduce the basics of Whitham's modulation theory and show how it allows for a precise characterization of the waves emitted by the formation of an analogue black hole.

C27 (Physics Building)

25 May 2016

*Presentation by Sebastian Erne*

Analyzing and simulating quantum many-body systems. Abstract: The relaxation of isolated quantum many-body systems is a fundamental unsolved problem connecting many different fields of physics. Examples range from the dynamics of the early universe and quark-gluon plasmas to coherence and transport in condensed matter physics. Recent progress in cold atom experiments, make them

ideal systems to study the fundamental aspects of quantum field theories in and out of equilibrium.

An extensive semi-classical field library was developed enabling numerical investigation for a wide range of experimentally accessible setups. Specifically we consider a system of linearly coupled quasi one-dimensional condensates, realizing the quantum Sine-Gordon or Lieb-Liniger theories. Studying the relaxation following a quantum quench we show that these systems can reach non-thermal quasi-steady states, determined by emergent universal properties from the unitary quantum evolution. Further we analyze higher order correlations and their factorization properties in and out of equilibrium. This enables a detailed analysis of the quantum-state and is a crucial ingredient towards the implementation and verification of (analogue) quantum simulators.

Physics Room C12