Fundamental Problems in Quantum Physics

chaired by Angelo Bassi (TS), Catalina Oana Curceanu (LNF), Detlef Duerr (Mathematisches Institut LMU)
from to (Europe/Rome)
at Erice (Sicily) - Italy

This is the final meeting of the COST Action

"Fundamental Problems in Quantum Physics"

The conference will present a critical perspective of theoretical and experimental research in quantum physics: where are with at, with our understanding of the quantum world? What did we learn so far, and what are the most relevant open issues?

The topics addressed by the conference include:
- Quantum Foundations
- Mathematical Physics & QFT
- Quantum Interferometry
- Testing fundamental principles
- Quantum complex systems
- Quantum gravity
- Gravity and Cosmology

Within the Workshop a special event will be organized in honor of GianCarlo Ghirardi to celebrate his 80s birthday.


Go to day
  • Monday, 23 March 2015
    • 09:30 - 13:00 Quantum interferometry
      • 09:30 Exploring the limits and applications of the quantum superposition principle: An introduction to matter wave interferometry with atoms, (bio)molecules and nanoparticles 45'
        This talk introduces concepts and technologies in quantum interference experiments with massive particles, ranging from atoms and complex molecules today to dielectric nanoparticles in the future. Our work is motivated by the insight that the quantum superposition principle is at the heart of all quantum physics but that matter wave interference has not yet been tested experimentally in the regime of high masses. There is good reason to believe that established physics will persist in all regimes. But there is equally good reason to believe that the opposite is true.  
        We will introduce the basic concepts of matter-wave physics with reference to modern atom interferometry and discuss how to extrapolate these achievements to objects of substantially higher mass and complexity. This comprises challenges regarding advanced molecular beam sources, coherence preparation, particle detection and avoidance of decoherence. We will focus on advanced nanomechanical and optical beam splitting technologies as well as the development of novel sources for nanoparticles in the mass range of 10^7-10^8 amu. In this mass range it will be important to consider additional phase averaging and decoherence, as well as the possibility of consequences of continuous spontaneous localization models.  
        Speaker: Markus Arndt (University of Vienna)
        Material: Slides pdf file}
      • 10:15 Pulsed matter-wave interferometry for testing the mass limits of quantum mechanics 45'
        We demonstrate a matter-wave interferometer in the time domain (OTIMA) as a powerful tool for testing the validity of quantum theory for large particles [1,2]. The interferometer operates in the near-field regime and utilizes three pulsed standing laser wave gratings. These imprint a periodic phase pattern on to the traversing matter waves and the photo depletion probability is modulated periodically with the distance from the reflecting mirror. Depending on the particle’s ionization or fragmentation cross section and their optical polarizability the gratings thus act as absorptive masks and phase gratings with an exceptionally small grating period of less than 80nm [3,4]. The pulsed experimental scheme facilitates interference measurements in the time domain which offers high count rate, visibility and measuring precision [3]. Since the action of pulsed optical gratings is non-dispersive the experiment is well suited for interference experiments on an increasingly large mass scale in the quest for novel decoherence effects, such as continuous spontaneous localization [4]. Additionally, the ability to resolve fringe shifts as small as a fraction of the grating period opens up to measuring optical and electrical nanoparticle properties in the OTIMA interferometer with time domain enhanced precision [6]. Experiments with various organic clusters and monomers have demonstrated the functionality of the interferometer and serve as a motivation for exploring the wave-particle character of particles with masses up to 105 amu and beyond.
        [1] N. Dörre, J. Rodewald, P. Geyer, B. von Issendorff, P. Haslinger, and M. Arndt, Phys. Rev. Lett. 113, 233001 (2014).
        [2] N. Dörre, P. Haslinger, J. Rodewald, P. Geyer, and M. Arndt, J. Opt. Soc. Am. B 32, 114 (2014). 
        [3] S. Nimmrichter, P. Haslinger, K. Hornberger, and M. Arndt, New J. Phys. 13, 75002 (2011).
        [4] S. Nimmrichter, K. Hornberger, P. Haslinger, and M. Arndt, Phys. Rev. A 83, 43621 (2011). 
        [5] S. Eibenberger, X. Cheng, J. P. Cotter, and M. Arndt, Phys. Rev. Lett. 112, 250402 (2014).
        Speaker: Jonas Rodewald (University of Vienna)
        Material: Slides pdf file}
      • 11:00 Coffe Break 30'
      • 11:30 Talbot interferometry with a point-like source: rationale, technology, & experiment 45'
        We are building a matterwave interferometer for 10^6 amu particles with 200 nm separation to test the superposition principle for massive particles [1] . A successful demonstration would correspond to a macroscopicity [2] mu=18 and would begin to constrain CSL lambda_CSL<1.4 · 10^r -11 Hz. I will outline the theoretical approach, describe the rationale for our design choices, and review the experimental progress so far. For particles in free-flight, our target mass appears to be a practical maximum for earth-bound experiments; I will briefly mention the MAQRO consortium which seeks to put an experiment like ours into space [3] 
        [1] Bateman, Nimmrichter, Hornberger, Ulbricht, Nat. Comm. 5, 4788 (2014)
        [2] Nimmrichter & Hornberger, Phys. Rev. Lett. 110,160403 (2013)
        [3] Kaltenbaek et al., Exp. Astro. 34, 123 (2012).
        Speaker: James Bateman (University of Southampton)
        Material: Slides pdf file}
      • 12:15 Quantum Walks Disprove Theories Based on Classical Trajectories 45'
        We report on a stringent test of the nonclassicality of the motion of a massive quantum particle, which propagates on a discrete lattice. Measuring temporal correlations of the position of single atoms performing a quantum walk, we observe a 6σ violation of the Leggett-Garg inequality. Our results rigorously excludes (i.e., falsifies) any explanation of quantum transport based on classical, well-defined trajectories. We use so-called ideal negative measurements—an essential requisite for any genuine Leggett-Garg test—to acquire information about the atom’s position, yet avoiding any direct interaction with it.
        C. Robens, W. Alt, D. Meschede, C. Emary, and A. Alberti, Ideal Negative Measurements in Quantum Walks Disprove Theories Based on Classical Trajectories, Phys. Rev. X 5, 011003 (2015).
        Speaker: Andrea Alberti (University of Bonn)
        Material: Slides pdf file}
    • 13:00 - 14:30 Lunch
    • 14:30 - 18:00 Mathematical Physics & QFT
      • 14:30 Fermi-Pasta-Ulam chains and their approach to thermal equilibrium 45'
        In 1953 Fermi, Pasta, and Ulam, technically supported by Tsingou, run pioneering numerical simulations on nonlinearly coupled oscillators. They wanted to test whether for long times the statistics is well described by the microcanonical ensemble, in particular confirm equipartition of energy. (The numerical findings were not too encouraging). In my talk, I will explain our current understanding of the issue raised by FPU.
        Speaker: Herbert Spohn (TU Munich)
        Material: Slides pdf file}
      • 15:15 Invariant Gibbs measures for Hamiltonian PDEs 45'
        In the this talk, we discuss the construction and related problems of the invariant Gibbs measures for Hamiltonian PDEs. We briefly go over the construction of the invariant Gibbs measures on the circle due to Bourgain '94. Then, we will describe the construction on the real line, concentrating on the defocusing nonlinear Schroedinger equation. 
        Speaker: Tadahiro Oh (University of Edinburg)
        Material: Slides pdf file}
      • 16:00 Coffee Break 30'
      • 16:30 Dirac's Class Two problems and recent developments in external field QED 45'
        To date classical and quantum electrodynamics is missing a well-defined equation of motion. I will give a brief overview of the main obstacles and report on recent developments to overcome one of those in the model of external field QED. In particular, I will discuss how an evolution for the Dirac sea subject to an external electromagnetic four-potential can be constructed that is free of singularities.
        Speaker: Dirk Deckert (LMU Munich)
        Material: Slides pdf file}
      • 17:15 Effective dynamics of a tracer-particle coupled to the ideal Fermi gas in the high-density limit 20'
        A tracer-particle coupled to the ideal Fermi gas in two spatial dimensions is considered. We prove that the time evolution of the system converges to the free dynamics in the high-density limit. Eventually, we explain why this can be understood in the sense of a mean field result despite the absence of a weak coupling parameter.
        Speaker: David Mitrouskas (LMU Munich)
      • 17:35 Quantum jumps and spikes 20'
        Quantum systems subjected to a continuous monitoring can be described by a diffusive stochastic differential equation. However, in the limit where the measurement rate becomes large, the system evolution starts to be effectively discontinuous with Poisson like jumps between the eigenvectors of the measure. The limiting regime can be precisely described and the transition rates computed. However, the transition to the jumpy regime is not trivial and power law fluctuations that we have called "quantum spikes" subsist in the limit. The objective of this short talk is to explain this phenomenon. If time permits, I will discuss the link with a classical toy model for measurement and I will briefly hint at the possible foundationnal applications of quantum spikes to collapse models.
        Speaker: Antoine Tilloy (Laboratoire de Physique Théorique, Ecole Normale Supérieure)
        Material: Slides pdf file}
  • Tuesday, 24 March 2015
    • 09:30 - 13:00 Testing fundamental principles
      • 09:30 Testing the properties of the QED vacuum, a review. 45'
        In this talk I will review past and current experiments on the QED vacuum. The fluctuations of QFT vacuum are one of the hallmarks of field theory, and they carry with them a host of important implications. Photon-photon scattering -- in all its incarnations -- is a powerful tool to study these fluctuations in QED. Moreover, interactions of photons with yet-to-be detected pseudoscalar particles can provide precious hints of physics beyond the Standard Model, and lead to interesting reflections on the quantum nature of light.
        Speaker: Edoardo Milotti (University of Trieste)
        Material: Slides pdf file}
      • 10:15 Pauli Exclusion Principle tested with VIP2 - J. Marton for the VIP2 Collaboration 45'
        The Pauli Exclusion Principle (PEP) is one of the most fundamental pillars of physics and it has tremendous consequences - from the stability of matter to atomic and subatomic systems. According to many observations PEP must be valid to an extremely high degree and no violations were found up to now. On the other hand a simple explanation of PEP is missing. Numerous experimental investigations were performed to search for a tiny violation in different systems. The experiment VIP2 at the Gran Sasso underground laboratory is designed to test the PEP for electrons with high sensitivity by searching for forbidden X-ray transitions in copper atoms. VIP2 aims to improve the PEP violation limit obtained with our preceding experiment VIP by orders of magnitude. The experimental method, comparison of different PEP tests based on different assumptions and the developments for the VIP2 setup will be presented.
        Speaker: Johann Marton (Stefan Mayer Institute, Vienna)
        Material: Slides pdf file}
      • 11:00 Coffee Break 30'
      • 11:30 Weak interactions: What foundational lessons can be learnt? 45'
        Four (known) forces are ruling our Universe; the weak interaction is the only one violating discrete symmetries. This talk presents a surprising relation between the violation of such symmetries and various foundational concepts of quantum mechanics for spin-less (mesons) and spin particles (hyperons). These systems show also an unexpected and puzzling relation to another big open question: Why is our Universe dominated by matter, why did the antimatter slip off the map?
        Speaker: Beatrix Hiesmayr (University of Vienna)
      • 12:15 A new upper bound on collapse models parameters from spontaneous radiation emission. 45'
        Collapse models are phenomenological models where the collapse of the wave function is described through a non linear interaction with an external classic noise. Because of the interaction with this noise, collapse models make different predictions compared to Quantum Mechanics. In the Continuous Spontaneous Localization (CSL) collapse model, the strength of the noise effects is measured by the collapse rate lambda. Nowadays, the best upper bound on lambda comes from the study of the spontaneous radiation emission from Germanium.
        In the first part of the talk we review the calculation of the spontaneous radiation emission in the CSL model. In particular, we discuss about some problems which arise when perturbation theory is applied to the lowest perturbative order. 
        In the second part of the talk we compare this theoretical prediction with new experimental data. As a result, the current upper bound on lambda is improved by a few orders of magnitude.
        Speakers: Kristian Piscicchia (LNF-Frascati), Sandro Donadi (University of Trieste)
        Material: Slides pdf filedown arrow
    • 13:00 - 14:30 Lunch
    • 14:30 - 18:00 Quantum foundations
      • 14:30 Quantum Physics without Quantum Philosophy 45'
        Quantum philosophy, a peculiar twentieth century malady, is responsible for most of the conceptual muddle plaguing the foundations of quantum physics. When this philosophy is eschewed, one naturally arrives at Bohmian mechanics, which is what emerges from Schro ̈dinger’s equation for a nonrelativistic system of particles when we merely insist that “particles” means particles. 
        Speaker: Nino Zanghi (University of Genova)
        Material: Slides pdf file}
      • 15:15 Collapse models, the quantum-classical transition, and experimental searches 45'
        Physics at the quantum-classical border is one of the crucial fields of today’s research, both theoretical and experimental. One of the challenging questions is to understand if, and under which conditions, quantum linearity fails when the size and complexity of the system increases [1,2]. The exploration of this question has substantial consequences because if the quantum superposition principle fails beyond a certain scale (e.g., a mass scale), then it necessitates the modification of quantum theory (which may even result in a better scenario for unifying quantum theory with gravity [3]). Collapse models are one possible way among many, to modify the standard quantum theory in a fully-consistent way [1]. Collapse model assumes a universal noise field that, when acting on matter, introduces non-linear effects on the dynamics, which explains the collapse of the wave function. In other words, one can derive the random localization of the wave function at the end of a measurement, of course with the correct quantum probabilities. The strength of the collapse process scales with the size of the system, thus the wave function of microscopic systems can be superimposed, while macroscopic objects are always well-localized [1]. Recently, there has been rapid experimental progress in revealing quantum features such as particle-wave duality for large objects with tiny de Broglie wavelength of only a few hundred femtometer [4]. This progress provides the possibility to search for tests of collapse models. In this regard, quite a few new experimental schemes have been proposed. Most proposals are based on the natural idea of creating a macroscopic quantum superposition in space, in order to test the superposition principle. Creating macroscopic superpositions is very difficult, source of formidable technological challenges. We propose an alternative approach by measuring the fluctuating properties of light interacting with a system (e.g., a molecule or an optomechanical oscillator) [5]. Here, the collapse manifests as an extra broadening and shift in the spectral density. The most important advantageous of this new approach is that here there is no need for the preparation of a quantum superposed state. We speculated that the corresponding experimental realization is well within reach. 
        [1] A. Bassi and G. C. Ghirardi, Phys. Rep. 379, 257 (2003). A. Bassi, K. Lochan, S. Satin, T. P. Singh, and H. Ulbricht, Rev. Mod. Phys. 85, 471 (2013). 
        [2] M. Arndt and K. Hornberger, Nat. Phys. 10, 271 (2014). 
        [3] R. Penrose, Found. Phys. 44, 557 ( 2014) 
        [4] T. Juffmann, H. Ulbricht, M. Arndt, Reports on Progress in Physics 76 (8), 086402 (2013). 
        [5] M. Bahrami, A. Bassi, and H. Ulbricht, Phys. Rev. A 89, 032127 (2014). M. Bahrami, M. Paternostro, A. Bassi, and H. Ulbricht, Phys. Rev. Lett. 112, 210404 (2014).
        Speaker: Mohammad Bahrami (University of Trieste)
        Material: Slides pdf file}
      • 16:00 Coffee Break 30'
      • 16:30 Wave Functions on Space-Time 45'
        In non-relativistic quantum mechanics, a wave function depends on one time variable. A suitable relativistic generalization is a wave function that depends on a separate time variable for each particle, i.e., for N particles, it depends on N space-time points. Such a function is called a multi-time wave function. The study of these wave functions and their evolution equations is interesting from many perspectives, e.g., from a non-rigorous formal perspective as well as from a rigorous mathematical physics one. It is furthermore relevant for "standard" quantum (field) theory (i.e., on the level of wave functions, operators and quantum fields), as well as for the foundations of quantum mechanics, e.g., for Bohmian Mechanics, GRW and many worlds. In this talk I present an overview of the recent work of R. Tumulka and myself on the subject. The main questions I discuss are how and what kind of interaction can be implemented in the framework of multi-time wave functions and how these wave functions are related to quantum field theory and the Tomonaga-Schwinger approach.
        Speaker: Sören Petrat (IST - Vienna)
        Material: Slides pdf file}
      • 17:15 Dissipative models of spontaneous wave-function collapse 45'
        Collapse models explain the absence of quantum superpositions at the macroscopic scale, while giving practically the same predictions as quantum mechanics for microscopic systems [1,2,3]. A well-known problem of the original models is the steady and unlimited increase of the energy induced by the collapse noise. In this talk, I discuss two recently introduced collapse models [4,5], which guarantee a finite energy during the entire system's evolution, while preserving the specific features any collapse model must have. The first model is a generalization of the Ghirardi-Rimini-Weber model [2,4]: here, the wavefunction undergoes instantaneous localization processes, distributed in time according to a Poisson process. In the second model [3,5], which also applies to identical particles, the collapse noise modifies continuously the wavefunction, inducing a diffusive localization process. We define new localization operators, which depend on the momentum of the system and thus introduce dissipation in the dynamics, leading to an exponential relaxation of the energy to a finite value. Such a finite value is naturally associated with a finite temperature of the collapse noise and, remarkably, the models are effective even in the presence of a low temperature noise.
        [1] A. Bassi, K. Lochan, S. Satin, T.P. Singh, and H. Ulbricht, Rev. Mod. Phys. 85, 471 (2013)
        [2] G.C. Ghirardi, A. Rimini, and T. Weber, Phys. Rev. D 34, 470 (1986)
        [3] G.C. Ghirardi, P. Pearle, and A. Rimini, Phys. Rev. A 42, 78 (1990)
        [4] A. Smirne, B. Vacchini and A. Bassi, Phys. Rev. A 90, 062135 (2014)
        [5] A. Smirne and A. Bassi, arXiv:1408.6446 (2014)
        Speaker: Andrea Smirne (University of Ulm)
        Material: Slides pdf file}
  • Wednesday, 25 March 2015
    • 09:30 - 13:00 Quantum complex system
      • 09:30 Quantum effects in complex materials: Determinism, statistics, structure 45'
        During the last couple of years, due to tremendous progresses on the experimental side, there has been an intensified debate on the possible role of quantum mechanics in the context of biophysics - i.e. in systems which are rightfully characterized as widely open, disordered, noisy, “complex”. Inspired by some analogies between paradigmes of biological and technological light-energy conversion, also a possible contribution of quantum science to enhanced, green energy technologies is under discussion. The talk will attempt to summarize the current state of the debate, and to provide a perspective on the challenging problems ahead.
        Speaker: Andres Buchleitner (University of Freiburg)
        Material: Slides pdf file}
      • 10:15 Sensitized upconversion with sunlight 45'
        It is well known that the efficiency of photovoltaic cells can benefit from upconversion of sub-bandgap light. Upconversion by triplet-triplet-annihilation is a very promising mechanism for this application. It employs two molecular species, one of which emits higher energy photons via fluorescence upon triplet-triplet-annihilation, whereas the second species sensitizes this process by supplying excitations for the annihilation process. These sensitizers absorb low energy photons with an efficiency that enables successful upconversion already with incoherent sunlight. In my talk, I will give a short introduction to the topic, characterize the transport phenomena in state-of-the-art upconversion materials and present ways to further improve the efficiency of the upconversion process.
        Speaker: Jochen Zimmermann (University of Freiburg)
        Material: Slides pdf file}
      • 11:00 Coffee Break 30'
      • 11:30 Quantum complexity in biological and material systems - Where do we stand? 45'
        An overview will be given of our current understanding of quantum complexity and the quantum-classical boundary in biological and material systems. In particular, we will discuss (i) whether "quantum-driven" functionality in these high-dimensional, structured systems is a robust feature or rather an accidental occurrence, (ii) to what extent quantum entanglement "scales up" into these systems that are at the border between the microscopic and macroscopic world, (iii) whether specific environments could act so as to protect low-dimensional subsystems from decoherence, and (iv) to what extent effective descriptions of these systems can be found. 
        Speaker: Irene Burghardt (University of Frankfurt)
        Material: Slides pdf file}
      • 12:15 Quantum dynamics of large systems - challenges and possible solutions 45'
        Quantum dynamics is ubiquitous in chemistry; it underlies photochemistry, electron transfer processes, nonradiative transitions, spectroscopy and much more. However, elucidating the quantum dynamics of complex systems remains a central challenge. 
        The problem has two aspects to it. One is the cost of solution of the time dependent Schrödinger equation which typically grows exponentially with the dimensionality of the system. Not less problematic is the fact that quantum dynamics is nonlocal in general and depends on the global potential surface which is rather difficult to generate, for systems with many degrees of freedom. In this talk I will review the main methodologies available for solution of the quantum dynamics and the promise of further progress based on the use of on-the-fly semiclassical methods.
        Speaker: Eli Pollack (Weizmann Institute - Rehovot)
        Material: Slides pdf file}
    • 13:00 - 14:30 Lunch
  • Thursday, 26 March 2015
    • 09:30 - 13:00 Quantum gravity
      • 09:30 String Theory and Quantum Gravity 45'
        Well known problems arise when trying to construct a quantum theory of gravity. After introducing string theory, we will review how string theory addresses these problems (and other problems in theoretical physics), and at which cost.
        Speaker: Luis Alday (University of Oxford)
        Material: Slides pdf file}
      • 10:15 The AdS/CFT duality 45'
        The AdS/CFT duality brought the holographic principle to center stage in string theory. This correspondence relates theories of gravity on curved spaces to ordinary quantum field theories on the boundaries of those spaces. Due to its nature, it provides a tool for studying strongly interacting field theories, where perturbative methods are not applicable. In this talk I will discuss this duality and present some examples of its realisation.
        Speaker: Agnese Bissi (University of Oxford)
        Material: Slides pdf file}
      • 11:00 Coffee Break 30'
      • 11:30 Lessons for Quantum Foundations from Quantum Gravity 45'
        My title could seem like a joke: what could quantum theory -- with its 90 years of success covering a huge range of scales and an enormous variety of phenomena -- possibly learn from quantum gravity, a theory that doesn't exist yet?  I will argue that one approach to the problem of quantum gravity, in which spacetime is postulated to be fundamentally discrete or atomic, points in a direction that could have implications for our understanding of all quantum systems.
        Speaker: Fay Dowker (Imperial College London)
        Material: Slides pdf file}
      • 12:15 Cosmological and Dynamical Aspects of Quantum Gravity with Torsion 45'
        In the context of concrete models of torsionful geometries of quantum gravity, in particular the ones inspired from string theory, I discuss the role of torsionful backgrounds in the early Universe in providing scenarios for the observed matter/antimatter asymmetry. In particular, within the context of Kalb-Ramond torsion that arises due to the antisymmetric tensor field in the spectrum of string-inspired models, which in four space-time dimensions is equivalent to a sort of axion field, I discuss the role of constant (Lorentz-violating) backgrounds of the torsion for Leptogenesis and subsequent Baryogenesis. The effective field theory corresponding to such situations has the form of some terms in the so called Standard Model Extension of Kostelecky and collaborators. I also discuss the role of quantum fluctuations of the torsion, and argue that, under certain circumstances, it may also be held responsible for dynamical generation of chiral Majorana neutrino masses in novel mechanisms beyond seesaw, proceeding through chiral anomalous graphs in the pertinent field theory.
        Speaker: Nick Mavromatos (Kings College London)
        Material: Slides pdf file}
    • 13:00 - 14:30 Lunch
    • 14:30 - 18:00 Ghirardi Fest - 80 years in science
      • 14:30 Nonlocality and Ontology: What Bell and Einstein, Bohm, and Ghirardi showed. 45'
        I will briefly discuss three issues of fundamental physical significance for our quantum world, issues on which GianCarlo Ghirardi has worked long and hard and with great success: quantum nonlocality, the measurement problem, and the importance of local beables.
        Speaker: Shelly Goldstein (Rutgers University)
        Material: Slides pdf file}
      • 15:15 Correlations beyond entanglement 45'
        Separable states may have a kind of correlation which cannot be captured by standard measures of entanglement. These states do not violate Bell inequality and yet they can give computational power to quantum computers which surpass any classical computer. We discuss these states and in particular we focus on a well-known subclass of them called Werner states. We point out an intriguing property of these states and relate it to the impossibility of a universal NOT operation in quantum mechanics. 
        Speaker: Vahid Karimipour (Sharif University)
        Material: Slides pdf file}
      • 16:00 Coffee Break 30'
      • 16:30 The experimental test of GRW: interferometry and beyond 45'
        Recent progress in technological developments allow to explore the quantum properties of very complex systems, bringing the question of whether also macroscopic systems share such features, within experimental reach. It seems feasible to generate for instance quantum superposition states of particles of mass of one million amu (atomic mass unit) [1]. The interest in such experiments is increased by the fact that, on the theory side, many suggest that the quantum superposition principle is not exact, departures from it being the larger, the more macroscopic the system [2]. We will put a special emphasis on possible tests of the GRW model, which provides the most challenging parameter set. We will report on new proposals to experimentally test the superposition principle with non-interferometric methods. Testing the superposition principle intrinsically also means to test suggested extensions of quantum theory, so-called collapse models. Such collapse models predict a heating effect, which results in a random motion of any isolated particle in space. The heating can be monitored using optomechanical systems even in the classical regime, if competing sources of heat can be reduced sufficiently. We will emphasise levitated optomechanical systems and discuss the possibility to test the heating effect by detecting the motion of the particle in space [4] as well as in the frequency domain where heating is manifested as spectral broadening [5]. We will show quantitative calculations for experiments in both regimes and compare to the state-of-the-art. We will also report on the status of our experiments on optical levitation and parametric feedback cooling of the 3d motion of dielectric nanoparticles at high vacuum conditions.
        [1] Bateman, J., S. Nimmrichter, K. Hornberger, and H. Ulbricht, Near-field interferometry of a free-falling nanoparticle from a point-like source, Nat. Com. 5, 4788 (2014). 
        [2] Bassi, A., K. Lochan, S. Satin, T.P. Singh, and H. Ulbricht, Models of Wave-function Collapse, Underlying Theories, and Experimental Tests, Rev. Mod. Phys. 85, 471 - 527 (2013). 
        [3] Bahrami, M., A. Bassi, and H. Ulbricht, Testing the quantum superposition principle in the frequency domain, Phys. Rev. A 89, 032127 (2014).
        [4] Bera, S., B. Motwani, T.P. Singh, and H. Ulbricht, A proposal for the experimental detection of CSL induced random walk, Sci. Rep. 5, 7664 (2015). 
        [5] Bahrami, M., M. Paternostro, A. Bassi, and H. Ulbricht, Non-interferometric Test of Collapse Models in Optomechanical Systems, Phys. Rev. Lett. 112, 210404 (2014).
        Speaker: Hendrik Ulbricht (University of Southampton)
        Material: Slides pdf file}
      • 17:15 Talk 28 45'
  • Friday, 27 March 2015
    • 09:30 - 13:00 Gravity and Cosmology
      • 09:30 Fundamental issues in gravity theory and its relation to Quantum Mechanics 45'
        I will discuss several conceptually interesting open problems in General Relativity, including those relating to Quantum Mechanics.
        Speaker: Domenico Giulini (University of Hannover)
        Material: Slides pdf file}
      • 10:15 Prospects of experimental tests of a fundamentally semi-classical gravitation theory 45'
        I discuss the hypothetical possibility of a theory in which only matter is quantised, but gravity is described by the classical theory of General Relativity, even at the fundamental level. From the most naive approach for such a theory, one obtains the Schrödinger-Newton equation as a non-relativistic limit. I present possibilities of experimental tests of such an alternative to Quantum Gravity.
        Speaker: Andre Grossardt (University of Trieste)
        Material: Slides pdf file}
      • 11:00 Coffee Break 30'
      • 11:30 Must space-time be singular? 45'
        According to Einstein's theory of general relativity space-time singularities such as a big bang may occur. Moreover, the Penrose-Hawking theorems showed that such singularities are actually generic. It has been believed that a quantum theory for gravity might avoid such singularities. The answer will of course depend on which approach to quantum gravity one considers. It will also depend on which version of quantum theory one adopts. In this talk, we will consider the question in the context of the Bohmian version of quantum mechanics, for the special case of a homogeneous and isotropic universe. According to the Bohmian approach there is an actual space-time metric. So one immediate virtue of this approach is that the question in our title is well-posed, having the same meaning as in Einstein's theory. We show that there is a non-zero probability for the space-time to be non-singular in the case of the Wheeler-DeWitt approach to quantum gravity, while it is one in the case of loop quantum gravity. This is in contrast to the consistent histories approach to quantum theory which predicts a singularity for the Wheeler-DeWitt approach with probability one.
        Speaker: Ward Struyve (LMU Munich)
        Material: Slides pdf file}
      • 12:15 Gravity related spontaneous decoherence: from Wheeler-Bekenstein-Hawking to optomechanics 45'
        The inception of a universal gravity-related irreversibility took place originally in quantum cosmology but it turned out soon that a universal non-unitary dynamics is problematic itself. Independent investigations of the quantum measurement postulate clarified that a non-unitary dynamics is of interest already in the non-relativistic context. An intricate relationship between Newton gravity and quantized bulk matter might result in universal non-relativistic violation of unitarity - also called spontaneous decoherence. The corresponding gravity-related spontaneous decoherence model is now on the verge of detectability in optomechanical experiments. It is also a toy-model of cosmic quantum-gravitational non-unitarity, illuminating that the bottle-neck of quantum-gravity is the quantum measurement postulate instead of quantum cosmology.
        Speaker: Lajos Diosi (Wigner Research Center for Physics)
        Material: Slides pdf file}
    • 13:00 - 14:30 Lunch
    • 14:30 - 16:00 MC meeting of the Cost Action