BergeFest 2014 Programme..

The Programme

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22 April, Tuesday
08:30 Registration
09:00 BergeFest'14 Organising Chairs
Welcome & Opening addresses
09:15 Serge Haroche, École Normale Supérieure, Paris, France
Controlling photons in cavities
Arrow Abstract

I will review recent experiments in Cavity QED in which we count trapped microwave photons non destructively and use quantum feedback methods to stabilize the photon number to a preset value. Further developments of these experiments will be briefly discussed.

10:00 Gerd Leuchs, Max-Planck Institute for Science of Light, Erlangen, Germany
Time reversal symmetry in optics
Arrow Abstract

There is a general recipe for achieving optimum coupling of light to resonant optical material systems, such as Fabry Perot resonators, super- and sub-wavelength antenna structures. The extreme case for the latter is a single atom, which will be treated in detail. This coupling between light and a single atom is probably the most fundamental process in quantum optics.

10:30 Coffee/Tea Break
11:00 Wilhelm Becker, Max-Born Institute of Berlin, Germany
Atoms in intense laser field: light at the end of the tunnel
Arrow Abstract

A rare-gas atom in an intense laser field is one of the simplest realizations of a highly nonlinear driven system. This has by now been investigated experimentally and theoretically for almost fifty years, but new effects continue to emerge. Strong-field laser-atom physics has brought about high-order harmonic generation (HHG) and, as a byproduct, attosecond science. However, HHG is quite complicated since it involves collective (many atoms) and propagation effects. Above-threshold ionization (ATI), on the other hand, i.e. ionization with absorption of many more laser photons than necessary, can be studied as a one-atom effect. I will be concerned with ATI and discuss recent novel effects and their theoretical description. The work horse of the latter is (besides numerical solution of the time-dependent Schrödinger equation) the "strong-field approximation" (SFA), which includes the binding

11:30 Michael Fleischhauer, University of Kaiserslautern
Rydberg polaritons: Photons with non-local interactions
Arrow Abstract

Coupling light to Rydberg atoms under conditions of electromagnetically induced transparency leads to the formation of strongly interacting quasi-particles called Rydberg polaritons. Rydberg polaritons behave like hard-sphere objects with non-local, repulsive interaction. On short length scales avoided volumes form that can contain at most one photon. The non-local repulsive interaction can furthermore lead to interesting many-body states of photons which in 1D can be described in terms of a Luttinger liquid theory. The latter predicts a non-classical Wigner crystal of photons as ground state of the moving-frame Hamiltonian. The formation of these crystals will be discussed and conditions for their existence derived.

12:00 Giovanna Morigi, Saarland University, Saarbrücken , Germany
Quantum reservoir engineering of photons and atoms
Arrow Abstract

12:30 Lunch
14:00 Andreas Buchleitner, University of Freiburg, Germany
Quantum statistics vs interference in many-particle scattering
Arrow Abstract

Bunching or anti-bunching of two indistinguishable particles passing through a balanced beam splitter is one of the hallmarks of quantum statistics in fundamental quantum optical applications. We consider the more general scenario of N particles passing through a beam splitter array, and analyse the resulting transmission signal in terms of multiparticle interference and of the controllable, distinguishability-induced quantum-to-classical transition. Also the complexity issue arising with increasing particle number will be addressed.

14:30 Christian Miniatura, CQT, NUS, Singapore
Momentum signatures of Anderson localization
Arrow Abstract

The concepts of random walks and Brownian motion were developed at the beginning of the 20th century after groundbreaking founding works on the kinetic theory of gases. They could successfully explain numerous statistical phenomena ranging from the transport of classical particles (Boltzmann's kinetic equation, Drude's electronic transport) in physics to the migration of mosquitoes in a forest in biology. The propagation of waves in complex media did not escape this description, as exemplified by Lord Rayleigh's pioneering works in acoustics (Rayleigh's law) or those of Chandrasekhar and Milne in astrophysics (radiative transfer theory). Within this framework, a wave packet propagating in a disordered medium is multiply scattered in all directions by the heterogeneities of the medium. The scattered partial waves acquire random phases and, on average, the interference between these partial waves are smoothed out although the in itial wave packet can be perfectly coherent. In turn, the memory of the initial propagation direction is rapidly lost and the wave packet spreads diffusively in space, just like the swarm of mosquitoes in a forest.<br><br>This powerful and fruitful description of propagation in a random medium was nevertheless questioned in the late 50s when Anderson showed that disorder could bring wave transport to a halt, an interference phenomenon known as strong localization. In fact, we now know that phase coherence is not completely scrambled even far from the localization regime as exemplified by weak localization corrections to the diffusion constant, universal conductance fluctuations and by the celebrated coherent backscattering (CBS) phenomenon. The latter manifests itself as an interference peak in the momentum distribution of the wave packet centered in the direction opposite to the initial propagation direction. Even if the CBS effect has been thoroughl y studied, both experimentally and theoretically, its fate in the strong localization regime was largely unknown. This is what we have studied in a recent paper [PRL 109, 190601, (2012), arXiv :1204.3451]. We have performed a numerical study of the time evolution of the momentum distribution of a quasi-monochromatic matter wave packet evolving in a speckle potential. Much to our surprise, we have found that a new multiple scattering interference effect is triggered by strong localization in the forward direction. It manifests itself as a coherent forward peak (CFS) which starts to grow when strong localization sets in and it raises on the time scale of the Heisenberg time of the system. In the end, both CBS and CFS appear as twin peaks. I will present and explain these results in as much simple terms as possible.

15:00 Coffee/Tea Break
15:30 Jaroslav Řeháček, Palacký University, Olomouc, Czech Republic
Quantum tomography of beams: Wavefront sensing reveals optical coherence. Springer Lecture Notes in Physics lecture.
Arrow Abstract

We apply methods of quantum-state estimation to external degrees of freedom of light. We reinterpret operation of wavefront sensors in a quantum framework, as a simultaneous unsharp measurement of position and momentum. By resorting to the versatile tools of quantum estimation, we show that wavefront sensing can provide complete information on the coherence properties of the signal. This indicates that classical methods employed hitherto do not fully exploit the potential of the registered data. We confirm our predictions with an experimental characterization and 3D imaging of partially coherent vortex beams and comment on the field of view, dynamical range and resolution of the proposed beam tomography. [1] B. Soklasa, L. Motka, J. Rehacek, Z. Hradil, and L.L. Sánchez-Soto, Nature Communications doi:10.1038/ncomms4275.

16:00 Zdeněk Hradil, Palacký University, Olomouc, Czech Republic
What can we learn from tomography in optics
Arrow Abstract

Quantum tomography is a method inspired by quantum-state estimation for inferring the quantities which cannot be measured directly. In the talk we will consider the analogies between optics and quantum mechanics in order to assess the ultimate limitations and resolution for optical observations. As an interesting by-product we will show the optical problems where quantum mechanical formalism and statistical treatment could be advantageous, for example quantum interferometry, image processing or optical resolution.

16:30 End of session

23 April, Wednesday
09:00 Helmut Rauch, Vienna University of Technology, Austria
Neutrons reveal quantum phenomena
Arrow Abstract

Neutrons are appropriate particles for testing quantum phenomena with massive particles. Perfect crystal interferometry demonstrated its capacity to uncover several quantum phenomena [1]. The wave-particle duality has been studied by means of the Englert complementarity relation and pre- and post-selection measurements showed how quantum complete measurements make quantum physics less mystic. The influence of nuclear, electromagnetic and gravitational interactions has been investigated over the last decades. Many experiments dealt with the transition from a quantum to a classical behavior. Magnetic noise fields may destroy the interference pattern and multi-photon exchange may occur but proper post-selection can restore the whole quantum state [2]. The question whether the Compton frequency and proper time effects can be investigated and how this can be used to combine quantum and relativity effects means will be discussed [3]. Quantum contextuality experiments show how different degrees of freedom are entangled and how quantum outcomes are intrinsically coupled causing more correlations than classical physics does.

09:30 Jun Suzuki, The University of Electro-Communications, Tokyo, Japan
Wave-particle duality in quantum estimation theory
Arrow Abstract

Wave-particle duality for a qubit is discussed in the framework of quantum estimation theory. Two parameters are introduced for qubit states representing fringe visibility and which-way information. We analyze trade-off relations for mean square errors when estimating these two parameters within i.i.d. setting. Trade-offs are then compared based on several known precision bounds such as SLD Cramer-Rao bound, RLD Cramer-Rao bound, and n1 bound (Nagaoka-Hayashi-Gill-Masser bound). We also investigate the optimal measurement which attains the n1 bound and discuss its properties.

10:00 Lev Vaidman, Tel Aviv University, Israel
Bohr, von Neumann, Wheeler, Aharonov, Bohm and Englert: Where was the photon which passed through an interferometer?
Arrow Abstract

Various approaches to a situation in which a photon is present not where you would expect it to be are discussed.

10:30 Coffee/Tea Break
11:00 Daniel Greenberger, The City College Of New York, USA
Testing the uncertainty relation between mass and proper time
Arrow Abstract

11:30 Dagomir Kaszlikowski, CQT, NUS, Singapore
The notorious “quantum non-locality” in a different way.
Arrow Abstract

We show that the outcomes of measurements on correlated quantum systems that are spatially separated can be compressed much more e fficiently than their classical counterparts. We show this on an example of bit strings generated by singlet correlations that we compress using Huffman coding. We then draw general conclusions on compressibility of quantumly correlated strings using Kolmogorov complexity.

12:00 Hans Briegel, University of Innsbruck and IQOQI, Austria
Towards quantum artificial intelligence
Arrow Abstract

No Abstract

12:30 Lunch
14:00 Ulrike Herzog, Humboldt-University Berlin, Germany
Optimal strategies for discriminating qubit states
Arrow Abstract

Quantum state discrimination is the task to determine the actual state of a quantum system, prepared with a given prior probability in one of N given states. Since non-orthogonal states cannot be distinguished perfectly, discrimination strategies have been developed that are optimized with respect to various criteria. The measurements performing the strategies are often generalized measurements, involving in many cases a certain probability of getting an inconclusive outcome. In this talk, after a brief review of different strategies for state discrimination, two strategies for the optimal discrimination between N mixed qubit states are considered. First, we investigate the measurement that maximizes the overall probability of correct results when the probability of inconclusive results is fixed at a given value, which corresponds to minimum-error discrimination if the fixed value is equal to zero. Second, we study the measurement where the given states are discriminated with maximal confidence for each guess while the overall probability of inconclusive results is kept as small as possible. The general properties of the optimal qubit-state measurements are investigated and examples are given.

14:30 Thomas Durt, Institute FRESNEL, Marseille, France
Would a quantum system succumb to its own gravitation?
Arrow Abstract

It is not fully clear yet how gravity works in the quantum regime. Self-gravitational effects are known to be negligible at the scale of ``elementary'' quantum objects (particles, atoms, molecules). They are likely to be measurable at the mesoscopic scale however. Required experimental implementations are nearly ideal (high vacuum, low temperature and microgravity environment). New theoretical tools are also necessary in order to simulate the evolution of such objects, which is intrinsically non-linear and non-local. As we aim to show, experiments are difficult but not impossible to implement.

15:00 Coffee/Tea Break (End of session)

24 April, Thursday
09:00 Reinhard Werner, Leibniz Universität Hannover, Germany
Bohmian encounters
Arrow Abstract

Abstract to be advised

09:30 Ruediger Schack, Royal Holloway, University of London Egham, UK
QBism, or what quantum mechanics says about the world
Arrow Abstract

By taking seriously Asher Peres's dictum "Unperformed experiments have no results", QBism arrives at a consistent interpretation of quantum mechanics that gives an explicit role to the perceiving subject. QBism is radical in that it treats quantum mechanics not as a theory directly about the world, but as a tool that any agent can use to organize his personal experience. In QBism there is no tension between quantum mechanics and special relativity---QBism has no room for nonlocality. This talk introduces QBism, contrasts it with other interpretations of quantum mechanics and explains how QBism avoids the difficulties that have plagued quantum foundations for the last 90 years.

10:00 Ulrich Mohrhoff, Sri Aurobindo International Centre of Education, Pondicherry, India
How to manifest a world
Arrow Abstract

Quantum theory’s irreducible empirical core is a probability calculus. While it presupposes the events to which (and on the basis of which) it serves to assign probabilities, and therefore cannot account for their occurrence, it has to be consistent with it. It must make it possible to identify a system of observables whose values are real per se, as against “real by virtue of being indicated by the values of observables that are real per se.” What makes this possible is that the spatiotemporal differentiation of the physical world is incomplete: the existence of a real-valued spatiotemporal background is an unrealistic idealization. This is shown by applying a novel interpretive principle to (interfering) alternatives that involve distinctions between regions of space. By applying it to alternatives that involve distinctions between things, this principle also makes it safe to claim that the macroworld is brought into being by a progressive differentiation of a single, intrinsically undifferentiated substance. By entering into reflexive spatial relations, this substance gives rise to (i) what looks like a multiplicity of relata if the reflexive quality of the relations is not taken into account, and (ii) what looks like a substantial expanse if the spatial quality of the relations is reified. If there is a distinctly quantum domain, it is a non-spatial and non-temporal dimension across which the transition from the unity of this substance to the multiplicity of the macroworld takes place. Instead of being constituents of the manifested world, subatomic particles, atoms, and molecules are instrumental in its manifestation.

10:30 Coffee/Tea Break
11:00 Kwek Leong Chuan, CQT, NUS, Singapore
Quantum metrology with photonic polarization gears
Arrow Abstract

Quantum metrology bears a great promise in enhancing measurement precision. Recently, we demonstrate that NOON-like photonic states using angular momentum states could be set up with a photonic gear based on a super-resolving Malus law. We show that this e ect leads to single-photon angular measurements with the same precision of polarization- only quantum strategies with m photons, but robust to photon losses. In our recent study, we also combine the gear e ect with the quantum enhancement due to entanglement, thus exploiting the advantages of both approaches. The high gear ratio m boosts the current state of the art of optical non-contact angular measurements by almost two orders of magnitude.

11:30 Ady Mann, Technion-Israel Institute of Technology, Isreal
Quantum mechanical retrodiction through an extended mean king problem
Arrow Abstract

A variant of the mean king problem is used to allow a retrodiction of the results of past measurements. In this version the king doesn't tell Alice his measurement basis, but instead both the king and Alice repeat their measurements. This allows Alice to deduce both the basis and the outcome of the king's first measurement.

12:00 Karol Życzkowski, Jagiellonian University, Crakow, Poland
Stronger entropic uncertainty relations
Arrow Abstract

Entropic uncertainty relations in a finite-dimensional Hilbert space are investigated. We provide two new bounds for the sum of an arbitrary number of M entropies characterizing outcomes of M measurements of an N-dimensional state taken in M arbitrary basis.
One of the new bounds is shown to be stronger for any state than the bound obtained independently by two groups in 2013 with help of the majorization techniques. The other complementary result improves the 2013 bound of Coles and Piani, stronger than the well-known bound of Maassen and Uffink. Results obtained can be applied for various problems in quantum information theory, including the description of multipartite entanglement, estimation of mutual information and capacity of quantum channels.

^* A joint work with Zbigniew Puchala and Lukasz Rudnicki

12:30 Lunch
14:00 Free Afternoon

25 April, Friday
09:15 Roy Glauber, Harvard University, Cambridge, MA, USA
Some Recollections of Los Alamos -- and the Nuclear Era
Arrow Abstract

Personal experiences, feelings, and interpretations, regarding wartime science

10:30 Coffee/Tea Break
11:00 Kazimierz Rza̧żewski, Center for Theoretical Physics of the Polish Acadamy of Science, Warsaw, Poland
Classical fields and quantum measurement of Bose-Einstein condensate
Arrow Abstract

We analyze a process of physical splitting of the Bose-Einstein condensate and the mutual coher- ence of the two subsystems. Within the classical fields approximation we show that this coherence is degraded if atoms interact and if we account for the sufficiently long observation time. We also show, that upon recombination, when the barrier is removed, the coherence across the sample is re- stored. The coherence is not fully degraded if the splitting potential remains sufficiently penetrable.

11:30 Marek Kus, Center for Theoretical Physics of the Polish Acadamy of Science, Warsaw, Poland
Geometry of multiparticle entanglement
Arrow Abstract

Abstract to be advised

12:00 Miguel Orszag, Pontificia Universidad Catolica de Chile, Santiago, Chile
Propagation and distribution of quantum correlations
Arrow Abstract

We study the propagation and distribution of quantum correlations through two chains of atoms inside cavities joined by optical fibers. We found, under different initial conditions either propagation or multiple entanglement in cavity QED.

12:30 Saverio Pascazio, Universita di Bari, Italy
Typicality in the interference of Bose-Einstein condensates Typicality in the interference of Bose-Einstein condensates
Arrow Abstract

Interference fringes are observed when two independently prepared Bose-Einstein condensates are released and overlap. At first sight, this phenomenon seems to be in contrast with commonly accepted wisdom on Young-type double-slit interference experiments from independent sources: there, no interference can be observed, as the relative phase between the two wave functions is not constant (this is what is meant, after all, by “independent” sources). Today most authors agree that this phenomenon in Bose-Einstein condensates can be ascribed to "measurement-induced interference", according to which the phase of the condensate's wave function is established by measurement. These ideas are explained in review papers and even in textbooks, to signify the fundamental importance of the problem and its correct interpretation. In this talk, I would like to contribute to this discussion. I will show that interference is typical, in the sense that most wave functions, randomly sampled out of a suitable sector of the Hilbert space that describes the Bose-Einstein condensate, display interference.

13:00 Lunch
14:00 Alexander Ling, CQT, NUS, Singapore
Entangled Photon Sources for Space
Arrow Abstract

A space-based network of entangled photon sources and detectors can beam photons down to ground based receivers, enabling quantum key distribution on a global scale. We propose that the first steps to such a network can be performed using nanosatellites. We present our efforts to develop complete entangled photon systems that are compatible with nanosatellites. Results from near-space demonstrations will be presented.

14:30 Enrique Solano, University of the Basque Country, Bilbao, Spain
Quantum Theatre of Impossible Physics
Arrow Abstract

I will briefly review the main goals of quantum simulations as our quantum theatre in current quantum technologies, ranging from aesthetics and the exchange of knowledge between different fields to the possibility of beating classical computers. I will explain then how it is possible to simulate unphysical operations, as complex conjugation, time-reversal, and others. Furthermore, I will show how this leads to the concept of embedding quantum simulators, allowing the simplified measurement of physical quantities that otherwise would require full tomography. Finally, I will give some examples on how to apply these concepts in current quantum technologies as trapped ions and superconducting circuits.

15:00 Coffee/Tea Break
15:30 Luigi Amico, University of Catania
Local response of topological order to an external perturbation
Arrow Abstract

I elaborate on the idea that topologically ordered ground states can be distinguished through a specific property related to the notion of LOCC, known as differential local convertibility. Such a property can be expressed in terms of the response of the Renyi entropies to an external perturbation. Here, I will discuss results for 1d and 2d systems displaying topological order in the ground state. They hold experimental promise because the subsystem size can be independent of the correlation length, in addition we discuss their implications for quantum information processing tasks using topologically ordered states. Incidentally, I will discuss the implications of such findings in the context of universal quantum simulators.

16:00 Heinz Siedentop, Ludwig-Maximilians-Universität München, Germany
Large Atoms and Their Momentum Density
Arrow Abstract

We show that the momentum density of large atoms tends to the minimizer of Englert's momentum energy functional.

16:30 Berge's talk