Abstracts
Wednesday 19 September
Wednesday 19 September
09:00-9:40 Alexei A Starobinsky (Landau Institute for Theoretical Physics, Russia), Looking for observable gravitational quantum effects in the Universe
The quantum effect of creation of particles-antiparticles and corresponding field fluctuations including space-time metric ones in strongly curved space-time is one of the cornerstones of present viable inflationary models, as well as of some alternative scenarios. It includes the purely quantum-gravitational effect of generation of gravitational waves during inflation. It leads to definite and falsifiable predictions for primordial metric perturbation spectra, and some of them have been confirmed by observations. However, this does not mean that we can exclude a possibility of some other purely deterministic process leading to the same spectra completely. I discuss perspectives of detection of the primordial GW background predicted by the simplest inflationary models and of gravitational quantum corrections to the background behaviour expected in these models, as well of other effects in the scalar spectrum which quantum origin would be more pronounced.09:45-10:25 Clifford P. Burgess (McMaster University, Canada), Gravitational Fields as Open Systems: Potential Surprises from QFT in Curved Space
Quantum fields in curved spaces with horizons have long been understood as open systems, and this talk explores how this observation changes some common assumptions about the domain of validity of EFT methods in curved space.11:00-11h40 Jérôme Martin (IAP Paris), Quantum inequalities and the CMB
According to the theory of inflation, the quantum state of cosmological perturbations is a strongly squeezed state. This state is close to an EPR state and thus possesses highly non classical properties. We study whether those non classical features can be revealed by designing CMB cosmic Bell inequalities which could allow us to highlight quantum correlations in the sky.11:45-12:25 Daniel Sudarsky (México University, Mexico), Reassessing the scalar tensor mode relationship in the context of modified treatments of the gravity / quantum interface
Standard treatments of the generation of primordial perturbations during inflation link the expectations for scalar and tensor modes in a relatively stringent manner. In this talk we will argue that, in searching for a satisfactory answer to the question “ how is the primordial homogeneity and isotropy of early inflationary stages broken?” we are naturally lead to modified treatments in which such link might be drastically changed, leading to dramatic departures from standard expectations.14:00-14:40 Sean Carroll (Caltech, USA), Quantum Circuit Cosmology
I will discuss how to take the quantum nature of our cosmological spacetime seriously, even without a complete theory of quantum gravity. In the conventional approach of quantum field theory in a classical spacetime, there are an infinite number of modes, each of which begins with sub-Planckian wavelength and emerges in its ground state. But the existence of a cosmological constant implies that our observable universe only has a finite number of degrees of freedom. Quantum circuits provide a potentially useful formalism for treating finite-dimensional quantum systems, and we can model the expansion of the universe as entangling new degrees of freedom as time passes.14:45-15:25 Tejinder Pal Singh (Tata Institute of Fundamental Research, Mumbai, India), Space and time as a consequence of Ghirardi-Rimini-Weber quantum jumps
The Ghirardi-Rimini-Weber (GRW) theory of spontaneous collapse offers a possible resolution of the quantum measurement problem. In this theory, the wave function of a particle spontaneously and repeatedly localises to one or the other random position in space, as a consequence of the hypothesised quantum jumps. In between jumps the wave function undergoes the usual Schrodinger evolution. In the present work we suggest that these jumps take place in Hilbert space, with no reference to physical space, and physical three-dimensional space arises as a consequence of localisation of macroscopic objects in the universe. That is, collapse of the wave-function is responsible for the origin of space. We then suggest that similar jumps take place for a hypothetical time operator in Hilbert space, and classical time as we know it emerges from localisation of this time operator for macroscopic objects. More generally, the jumps are suggested to take place in an operator space-time in Hilbert space, leading to an emergent classical space-time. Based on [https://arxiv.org/abs/1806.01297].15:30-16:10 Nelson Pinto Neto (CBPF, Rio de Janeiro, Brazil), The de Broglie-Bohm approach to quantum cosmology
Quantum gravity aims to describe gravity in quantum mechanical terms. How exactly this needs to be done is an open question. Various proposals have been put on the table, such as canonical quantum gravity, loop quantum gravity, string theory, etc. These proposals encounter technical and conceptual problems. In this contribution we focus on canonical quantum cosmology, and we show that many conceptual problems, such as the measurement problem, the problem of time and the problem of space-time singularities, can be overcome by using the de Broglie-Bohm quantum theory. In addition, the Bohmian approach yields a conceptually clear framework to study quantum cosmological perturbations, their quantum-to-classical transition, and the comparison with observations. In particular, we exhibit a non-singular cosmological model with a quantum bounce and a dark energy expanding phase, where the amplitudes and spectra of scalar and tensor perturbations are consistent with observations.16:45-17:25 Andrew Jaffe (Imperial College London, UK), Intersection of QBism and Cosmology
QBism is a way of looking at quantum mechanics that takes its probabilistic predictions seriously and, hence, to be fully Bayesian. These probabilities therefore do not exist in the world, but in the heads (and the calculations) of the users of the theory. When expressed in the language of POVMs and density matrices, the Born rule for determining the probabilities for experimental outcomes sometimes takes a particularly illuminating form, transforming our judgements about the results of one set of experiments into what the rules of quantum mechanics determine for the probabilities for any other experiment on the same system. I will review some of the mathematical formalism in which this can be expressed most elegantly and try to elucidate the contrasts with other interpretations of quantum mechanics, paying special attention to cosmological contexts.17:30-1810 Julien Serreau (APC Paris), Stability of de Sitter spacetime against infrared quantum scalar field fluctuations
We study the backreaction of superhorizon fluctuations of a light quantum scalar field on a classical de Sitter geometry by means of the Wilsonian renormalisation group. This allows us to treat the gravitationally amplified fluctuations in a nonperturbative manner and to analytically follow the induced renormalisation flow of the spacetime curvature as long wavelength modes are progressively integrated out. Unbounded loop corrections in the deep infrared are eventually screened by nonperturbative effects which stabilise the geometry.Thursday 20 September
Thursday 20 September
09:00-9:40 David Kaiser (MIT Cambridge, USA), Cosmic Bell Experiments: Testing Bell’s Inequality with Measurement Settings from Distant Astronomical Objects
Bell's inequality was originally derived under the assumption that there are no statistical correlations between the choices of measurement settings and anything else that can causally affect the outcomes of measurements on entangled particles. This assumption has come to be known as ``freedom of choice" (also referred to as ``measurement independence" and ``settings independence"). Though the freedom-of-choice loophole has received the least attention among the major loopholes in Bell tests, recent theoretical work indicates that the standard (quantum-mechanical) interpretation of Bell tests is most vulnerable to this particular loophole. In this talk I will describe recent ``Cosmic Bell" experiments that address the freedom-of-choice loophole by using real-time astronomical observation of distant objects to select measurement settings on Earth, thereby harnessing cosmological phenomena to test the foundations of quantum theory.09:45-10:25 Francesco Marino (CNR-National Institute of Optics, Italy), Testing Quantum Gravity Models in Optomechanical Experiments
Table-top cavity optomechanical experiments represent a promising platform for testing quantum mechanics and its interplay with gravity. Here we discuss a series of optomechanical experiments able to perform highly-sensitive measurements of the position of macroscopic quantum oscillators. These experiments have the potential of testing some general ideas in quantum-gravity phenomenology, ranging from deformed commutators at the Planck scale to quantum-gravity induced non-locality. We finally overview the present status of our experiment based on a nanometric membrane, which has been optically-cooled close to its quantum ground state.11:00-11h40 Jiri Minar (Nottingham University, UK), Open quantum dynamics and its application to cosmology
In this talk I will first review the concept of a dynamics of open quantum systems. I will discuss the superoperator formalism which, in certain regimes, allows for analytical determination of the dynamical properties, namely the metastable and steady states of the system and the associated timescales. I will demonstrate the application of the formalism on the example of two-photon--atom interactions in the micromaser setting. In the small coupling limit, it takes the form of a squeezing Hamiltonian bearing a resemblance to the ones used in the treatment of inflationary curvature perturbations. Going beyond this perturbative regime, I will describe the structure of the generated quantum states which are in general non-Gaussian and can posses high quantum Fisher information.I will then describe the open quantum dynamics of conformally-coupled scalar fields appearing in the models of scalar fifth-forces. Applying the influence functional approach, we derive evolution equations for the matrix elements of the reduced density operator of the matter sector. In particular, we employ a novel approach akin to the LSZ reduction that is more familiar to scattering-matrix theory. The resulting equations allow one to analyse the decoherence of the matter fields induced by the fifth-forces scalars. This provides a possible experimental probe, using for instance atom-interferometry, of the corresponding classes of screened modified theories of gravity.
11:45-12:25 Hendrik Ulbricht (Southampton University, UK), Prospects for using levitated optomechanics to test fundamental physics
We will present our experiments on levitating nanoparticles by optical and magnetic forces in vacuum. The goal of such experiments is to gain control, ultimately even coherent control on different degrees of freedom of the centre-of-mass motion of a trapped particle; including translation [1], rotation [2] and precession [3]. Towards this goal we develop new theoretical concepts for the preparation of motional states [4] and implement those as experimental manipulation techniques [5,6]. Some experiments are based on optical trapping and manipulation, while others make use of magnetic fields generated by a permanent magnet close to a type-1 superconductor. All those schemes have in common that they hold promise to be utilised as sensors of weak forces [7]. Such forces include gravity but also forces, which are based on magnetic moments of few electron and nuclear Spins. The ultimate goal of our experiments is to distinguish quantum from classical dynamical evolution in the macroscopic domain [8].[1] J.Vovrosh, M. Rashid, D. Hempston, J. Bateman, M. Paternostro and H. Ulbricht, J. Opt. Soc. Am. B 34, 1421-1428 (2017). [2] M. Toroš, M. Rashid, H. Ulbricht, arXiv:1804.01150 (2018). [3] M. Rashid, M. Toroš, A. Setter, and H. Ulbricht, arXiv:1805.08042 (2018).[4] M. Rashid, M. Toroš, H. Ulbricht, Quantum Measurement and Quantum Metrology 4, 17-25 (2017). [5] M. Rashid, T. Tufarelli, J. Bateman, J. Vovrosh, D. Hempston, M. S. Kim, H. Ulbricht, Phys. Rev. Lett. 117, 273601 (2016). [6] A. Setter, M. Toroš, J. F. Ralph, H. Ulbricht, Phys. Rev. A 97, 033822 (2018). [7] D. Hempston, J. Vovrosh, M. Toroš, M. Rashid, H. Ulbricht, Appl. Phys. Lett. 111, 133111 (2017). [8] Ralph, J. F., M. Toroš, S. Maskell, K. Jacobs, M. Rashid, A. J. Setter, H. Ulbricht, arXiv:1711.09635 (2017).14:00-14:40 Richard Holman (Minerva Schools San Francisco, USA), Oh, What an (En)Tangled Web we Weave...: Inflation and the space of Initial States
How well can we constrain the initial quantum state of metric perturbations sourced during inflation? We exhibit an interesting new class of quantum states that entangle the scalar metric perturbations $\zeta$ with other fields such as scalars as well as the tensor metric perturbations $h_{i j}$. These states are theoretically consistent, for inflation that lasts close to its minimum number of e-folds, give distinguishable signatures in the power spectrum and may be able to explain some long-standing anomalies in the CMB power spectrum. This argues for the construction of an effective theory of quantum {\em states} that could have enough power to single out a unique quantum state from the data.14:45-15:25 Tomislav Prokopec (Utrecht University, The Netherlands), Inflation, asymptotic safety and conformal symmetry
Based on “Inflation in an effective gravitational model and asymptotic safety,” Lei-Hua Liu, Tomislav Prokopec, Alexei A. Starobinsky, Phys.Rev. D98 (2018) no.4, 043505, arXiv:1806.05407 [gr-qc], and on a work in preparation.15:30-16:10 Vincent Vennin (APC Paris), Quantum decoherence during inflation
Decoherence may play a role in the quantum-to-classical transition of primordial cosmological fluctuations. But if it occurs in the early Universe, the interaction with the environment that gives rise to it also changes observable predictions such as the power spectrum from inflation and the amount of non Gaussianities. I will show how this opens up the possibility to observationally probe quantum decoherence.16:45-17:25 William H. Kinney (Stockholm University, Sweden), Eternal Inflation in Hilltop Models
Eternal inflation has long been known to be generic in large-field inflation models, where one or more scales in the problem exceed the Planck scale. I consider the case of small-field inflation, in the limit of hilltop-type potentials, where the region in field space admitting eternal inflation is exponentially small. In particular, I consider a simplified model of the hilltop in which the evolution of the inflaton is at all times dominated by quantum evolution instead of classical slow roll, and show that eternal inflation occurs even when all scales in the potential are arbitrarily far below the Planck scale. This provides a simple and clear demonstration that quantum effects at energy scales very small relative to the Planck scale can have an order-unity or larger effect on the global structure of spacetime.17:30-1810 Sebastien Renaux-Petel (IAP Paris), Inflation, Geometry and Stochasticity
The stochastic approach aims at describing the long-wavelength part of quantum fields during inflation by a classical stochastic theory. The link between its Langevin and Fokker-Planck descriptions depends on an implicit discretisation procedure, the two prominent ones being the Ito and Stratonovich prescriptions. We show the related difficulties of formulating a stochastic theory that maintains general covariance under field redefinitions, and study stochastic inflation in curved field spaces.Friday 21 September
Friday 21 September
09:00-9:40 Gerardo Adesso (Nottingham University, UK), Generic emergence of objectivity of observables in infinite dimensions
Quantum Darwinism posits that information becomes objective whenever multiple observers indirectly probe a quantum system by each measuring a fraction of the environment. It was recently shown that objectivity of observables emerges generically from the mathematical structure of quantum mechanics, whenever the system of interest has finite dimensions and the number of environment fragments is large. Despite the importance of this result, it necessarily excludes many systems of interest that are infinite-dimensional, including quantum field modes. Extending the study of Quantum Darwinism to infinite dimensions is a nontrivial task: we tackle it here by using a modified diamond norm, suitable to quantify the distinguishability of channels in infinite dimensions. We prove two theorems that bound the emergence of objectivity, first for finite energy systems, and then for systems that can only be prepared in states with an exponential energy cut-off. We show that the latter class of states includes any bounded-energy subset of single-mode Gaussian states.09:45-10:25 Mauro Paternostro (Queen's University Belfast, UK), Revealing quantumness without looking
Your prankster friend gave you a box into which, he says, there is a quantum system. He asks you to hold the box for him, and not to ruin the fragile quantum system that is inside. But you do not trust him and want to find out if he is telling the truth or not. How would you ascertain that the system within your friend’s box is indeed genuinely quantum?As preposterous as this situation might sound, it is not far from conditions routinely found in quantum labs: the direct revelation of the non-classical properties of a system is often either too disruptive for the system itself (if you measure it, you ruin it!), or simply technically difficult to realise (the system might be difficult to access, just like the one in your friend’s box).
In this talk I will illustrate a scheme based on quantum communication and the theory of quantum correlations, that allows you to "certify" the quantum nature of an inaccessible system. I will show how, besides its fundamental interest, the scheme is prone to verification in a number of experimental settings, including quantum optomechanics. Finally, I will conjecture that it can be used as a trojan horse to investigate the possible quantum nature of gravity -- for which I will describe a recent proposal for an experiment -- and biological processes.
The work presented in this talk is based on the following papers T. Krisnanda, M. Zuppardo, M. Paternostro, and T. Paterek, Phys. Rev. Lett. 119, 120402 (2017) S. Bose et al., Phys. Rev. Lett. 119, 240401 (2017) [see also Synopsis in Physics: https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.119.240402]T, Krisnanda, C. Marletto, V. Vedral, M. Paternostro, and T. Paterek, arXiv:1711.06485 (2017)
11:00-11h40 Angelo Bassi (Trieste University, Italy), Testing spontaneous wave function collapse: from the lab to the universe
Models of spontaneous wave function collapse (collapse models) propose to modify the Schrödinger equation by including nonlinear and stochastic terms, which describe the collapse of the wave function in space. These spontaneous collapses are “rare” for microscopic systems, hence their quantum properties are left almost unaltered. They add coherently in composite systems, giving rise to an amplification effect, so that macroscopic spatial superpositions of macro-objects are rapidly suppressed. I will review the main features of collapse models, and will present an update of the most promising ways of testing them in interferometric and non-interferometric experiments, showing the current lower and upper bounds on their parameters. will discuss ideas to connect the collapse of the wave function to gravity: The Diosi-Penrose model, Adler’s model and the Schroedinger-Newton equation. I will also show how collapse models can possibly shed a light on some cosmological problems.11:45-12:25 Alessio Belenchia (IQOQI Vienna, Austria), Quantum Superposition of Massive Objects and the Quantization of Gravity
In this talk we analyse a gedankenexperiment, previously considered in the literature, which involves quantum superpositions of charged and/or massive bodies. In the electromagnetic case, we show that the quantisation of electromagnetic radiation and vacuum fluctuations of the electromagnetic field both are essential for avoiding apparent paradoxes with causality and complementarity. We then analyse the gravitational version of this gedankenexperiment which was not correctly analysed in the previous literature. We show that the analysis of the gravitational case is in complete parallel with the electromagnetic case provided that gravitational radiation is quantised and that vacuum fluctuations limit the localisation of a particle to no better than a Planck length. This provides support for the view that (linearised) gravity should have a quantum field description, a relevant result in view of the growing interest in proposals for table-top experiments probing gravity-induce entanglement.14:00-14:40 Fay Dowker (Imperial College of London, UK), The Cosmological “Constant” Lambda as a Quantum Gravity Effect
By treating the whole universe as a quantum system, and using features of the causal set approach to quantum gravity, Sorkin predicted the magnitude of the cosmological ``constant'', Lambda, today should be of the order of the ambient matter density today. This prediction was verified in the late 1990’s and is the only prediction from quantum gravity that has been verified. I will review Sorkin’s argument which uses the path integral — or sum-over-histories — approach as the fundamental framework for quantum theory. I will review the state of play on models based on Sorkin’s original heuristic argument and their phenomenology, for example fits to the CMB and other data sets.14:45-15:25 Ward Struyve (LMU Munich, Germany), Must space-time be singular?
According to Einstein's theory of general relativity space-time singularities such as a big bang typically occur. It is believed that a quantum theory for gravity may eliminate such singularities. Whether this is indeed the case may of course depend on which approach to quantum gravity, for example the Wheeler-DeWitt theory or loop quantum gravity. But it may also depend on the approach to quantum mechanics itself, for example the standard quantum approach, consistent histories or Bohmian mechanics. For the case of mini-superspace models, it has been claimed in the context of standard quantum theory that the Wheeler-DeWitt theory does not eliminate the singularities, while loop quantum gravity does. However, the analysis is plagued by a number of conceptual problems: the measurement problem, the problem of time and the problem of what exactly it means to have a singularity. In my talk, I will explain the Bohmian approach to the Wheeler-DeWitt theory and loop quantum gravity and how this approach solves these conceptual problems. I will show for mini-superspace models that in the Wheeler-DeWitt theory the answer to the question of singularities depends on the wave function and the initial metric, and that in loop quantum gravity there are no singularities.TBA15:30-16:10 Nishant Agarwal (University of Massachusetts Lowell, USA), Open system dynamics in the early Universe
The inflationary Universe can be treated as an open quantum system, with long wavelength or bath modes coupled to short wavelength or system modes in the presence of interactions. On tracing out bath modes we find that the evolution of system modes is non-Hamiltonian and non-Markovian, and the bath can therefore modulate quantum coherences in the system. We also find that a Markovian limit exists and may be relevant at certain times depending on properties of the bath. I may also briefly discuss next steps in calculating correlators in this fully quantum framework, to connect our results to cosmological observables.16:45-17:25 Tommi Markkanen (Imperial College London, UK), De Sitter stability and Coarse Graining
The exponentially expanding de Sitter solution is frequently encountered in cosmology due to its significance for the late and early Universe. However, its stability in a quantised theory has for a long time been the source of debate. In this talk I discuss this issue in the framework of semi-classical gravity. Based on a first principle calculation, I argue that when one considers the experiences of a local observer, the quantum back reaction destabilises de Sitter space leading to a gradual continuous increase of the dS horizon. This implies that the cosmological 'constant' decays over time, with flat space as the asymptotic late time solution. For inflation this mechanism allows the construction of models whose initial conditions arise dynamically as attractor solutions without fine-tuned couplings. arXiv:1703.06898 and arXiv:1712.07977.17:30-1810 Sayantan Choudhury (Max Planck Institute for Gravitational Physics, Potsdam, Germany), Quantum entanglement in De Sitter cosmology: A study using stringy Axion with Bunch Davies and alpha vacua
In this work, we study the phenomena of quantum entanglement by computing de Sitter entanglement entropy from Von Newmann measure. For this purpose, we consider a bipartite quantum field theoretic setup in presence of axion originating from Type II B string theory. We consider the initial vacuum to be CPT invariant non-adiabatic alpha vacua state under SO(1,4) ismometry, which is characterized by a real one-parameter family. To implement this technique we use a $S^2$ which divide the de Sitter into two exterior and interior sub-regions. First, we derive the wave function of axion in an open chart for alpha vacua by applying Bogoliubov transformation on the solution for Bunch-Davies vacuum state. Further, we quantify the density matrix by tracing over the contribution from the exterior region. Using this result we derive entanglement entropy, Renyi entropy and explain the long-range quantum effects in primordial cosmological correlations by computing mean square vacuum fluctuations. Compared to the usual approach followed in (primordial) cosmology here we use most elegant and technically correct methods to compute the expression for the power spectrum of vacuum fluctuation. For this computation, we consider two causally separated region ({\bf L} and {\bf R}) in open hyperbolic chart of the de Sitter space-time. We start our analysis with three approaches -1. field operator expansion (FOE) technique with the quantum entangled state, 2. reduced density matrix (RDM) formalism with mixed quantum state and 3. non-entangled state (NES) method. For FOE and RDM we are tracing out the physical information from of one of the regions ({\bf R} ) and for NES we consider the Hilbert space is only described by region {\bf L} in de Sitter hyperbolic open chart. For massless ($\nu=3/2$) axion field we get the exact scale invariant feature of the power spectrum on small scale from this three formalism at the leading order which is consistent with the finding of the observational probes for early universe cosmology. Additionally, we due to quantum entanglement we get different quantum correction terms in the power spectrum for these thee formalisms at the next to leading order. Such correction terms serve the purpose of the breaking of degeneracy among the outcomes of the FOE, RDM and NES formalisms in the superhorizon limit. On the other hand, for massive ($\nu\neq 3/2$) axion filed we get a slight deviation from scale invariance and exactly quantify the spectral tilt of the power spectrum in small scales. Apart from that, for massless ($\nu=3/2$) and massive ($\nu=3/2$) axion field, we find distinguishable features of the power spectrum for the FOE, RDM, and NES on the large scales, which is the outcome of quantum entanglement. We also find that such large-scale effects are comparable to or greater than the curvature radius of de Sitter space. We also provide a comparison between the results obtained from Bunch-Davies vacuum and the generalized-alpha vacua, which implies that the amount of quantum entanglement and the long-range effects are larger for non zero value of the parameter alpha. Most significantly, our derived results for alpha vacua provides the necessary condition for generating non zero entanglement entropy in primordial cosmology.