Date: 24 November 2011
Venue: CQT Seminar Room, S15-03-15
Speaker: Elisabeth Giacobino, Laboratoire Kastler Brossel, Ecole Normale Supérieure, Université Pierre et Marie Curie, CNRS
Media: Video 
Abstract:
Superfluidity, the ability of a quantum fluid to flow without friction, is one of the most spectacular phenomena occurring in degenerate gases of interacting bosons. Since its first discovery in liquid helium-4, superfluidity has been observed in quite different systems, and recent experiments with ultra-cold trapped atoms have explored the subtle links between superfluidity and Bose–Einstein condensation.
In semiconductor microcavities, exciton-polaritons, which are mixed light-matter quasi-particles arising from the strong coupling between photons and excitons, have been shown to form Bose-Einstein condensates. It has been predicted that polaritons should behave as a novel quantum fluid, with unique properties stemming from their intrinsically non-equilibrium nature and their very low mass (~10-4 times that of the electron, inherited from their photonic component). This has stimulated the quest for an experimental demonstration of superfluidity effects in polariton systems.
I will present our recent results, demonstrating superfluid motion of polaritons, which manifests itself as the suppression of scattering from defects when the flow velocity is slower than the speed of sound in the fluid. Cerenkov-like wake patterns, vortices and dark solitons are also observed when the flow velocity is varied.
The experimental findings are in quantitative agreement with predictions based on a generalized Gross–Pitaevskii theory, and establish microcavity polaritons as a system for exploring the rich physics of non-equilibrium quantum fluids.
Date: 6 October 2011
Venue: CQT Seminar Room, S15-03-15
Speaker: Alán Aspuru-Guzik, Harvard University, USA
Media: Video
Abstract:
In this talk, I overview some of the aspects that intersect quantum information science and problems in chemistry. In particular, I will describe the simulation of chemical dynamics and electronic structure using quantum computers, both algorithms and experimental implementations. I will discuss the use of quantum adiabatic or annealing devices for solving lattice heteropolymer models associated with protein folding.
Date: 11 Aug 2011
Venue: CQT Seminar Room, S15-03-15
Speaker: Michael Fleischhauer,Universität Kaiserslautern, Germany
Media: Video
Abstract:
Slow-light polaritons are quasi-particles generated in the interaction of light with multi-level atoms driven by an external laser close to a Raman resonance. Their dispersion relation can be controlled to a large extend, representing massive Schroedinger particles on the one hand or multi-component, i.e. spinor objects with a Dirac-like spectrum on the other. In the latter case "relativistic" length and energy scales can be widely tuned, making relativistic effects accessible in the lab. Making use of the tunability of the mass the delocalization transition of the random-mass Dirac model with off-diagonal disorder can be experimentally observed.
In the second part of the talk the prospects to create strong interactions between dark-state polaritons using Rydberg atoms will be discussed. The dipole-dipole coupling between atoms in a Rydberg state leads to a strong and long-range interaction between polaritons, as well as to a blockade phenomenon. This interacion can give rize to interesting many-body phenomena, such as two-particle correlations which are much stronger than possible for pointlike interacting particles, crystallization of photons or quantum Hall states.
Date: 28 July 2011
Time: 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: By Boris Altshuler, Columbia University, USA
Media: Video
Abstract:
Localization of the eigenfunctions of quantum particles in a random potential was discovered by P.W. Anderson more than 50 years ago. In spite of its respectable maturity and intensive theoretical and experimental studies this field is far from being exhausted. Anderson localization was originally discovered in connection with spin relaxation and charge transport in disordered conductors. Later this phenomenon was observed for light, microwaves, sound, and more recently for cold atoms. Moreover, it became clear that the domain of applicability of the concept of localization is much broader. For example, it provides an adequate framework for discussing the transition between integrable and chaotic behavior in quantum systems. We will discuss current understanding of the Anderson localization and its manifestation in different physical situations. We will illustrate the main idea by several examples from adiabatic quantum computation to many-body statistical mechanics. We will demonstrate that physics of disordered many-body quantum systems can be described in the framework of the Anderson Localization.
Date: 26 May 2011
Time: 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: By Luca Turin, BSRC Fleming, Vari, Greece
Media: Video
Abstract:
Our sense of smell is a extraordinarily good at molecular recognition: we can identify tens of thousands of odorants unerringly over a wide concentration range. The mechanism by which this happens do so is still hotly debated. One view is that molecular shape governs smell, but this notion has turned out to have very little predictive power. Some years ago I revived a discredited theory that posits instead that the nose is a vibrational spectroscope, and proposed a possible underlying mechanism, inelastic electron tunneling. In my talk I will review the history and salient facts of this problem and describe some recent experiments that go some way towards settling the question.
Date: 24 March 2011
Time: 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Tom Gallagher, University of Virginia, USA
Abstract:
Experiments probing the dipole-dipole interactions in a frozen Rydberg gas made from 300μK Rb in a magneto optical trap are described. Resonant energy transfer measurements, in which the energy of an initial pair of atoms is tuned into resonance with the energy of a final state pair exhibit widths in excess of the simplest binary interaction estimates. We attribute the larger than expected widths to the existence of many body interactions and the fact that since the detection favors close pairs of atoms. In contrast, recent measurements of the dipole-dipole broadening of ns-np microwave transitions show less broadening than the simplest estimates and cusped lineshapes. These observations can be understood by considering several factors; the reduction, even to zero, of the dipole-dipole energy shifts due to the spin-orbit interaction, the strengths of the allowed microwave transitions, and the fact that many of the atoms are in low density regions of the trap. While the gas is frozen on a 1μs time scale, the attractive force associated with the dipole-dipole interaction leads to ionizing collisions on a 5-10 μs time scale, which can in turn lead to the spontaneous evolution to a plasma. There are, however, open questions related to the evolution to a plasma.
Date: 10 February 2011
Time: 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Jeremy O'Brien, University of Bristol
Abstract:
Quantum information science aims to harness uniquely quantum mechanical properties to enhance measurement and information technologies, and to explore fundamental aspects of quantum physics. Of the various approaches to quantum computing [1], photons are particularly appealing for their low-noise properties and ease of manipulation at the single qubit level [2]. Encoding quantum information in photons is also an appealing approach to quantum communication, metrology (eg. [3]), measurement (eg. [4]) and other quantum technologies [5]. However, the implementation of optical quantum circuits with bulk optics has reached practical limits. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturisation and scalability [6]. Here we report high-fidelity silica-on-silicon integrated optical realisations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate [7]. We have demonstrated controlled manipulation of up to four photons on-chip, including high-fidelity single qubit operations, using a lithographically patterned resistive phase shifter [8]. We have used this architecture to implement a small-scale compiled version of Shor's quantum factoring algorithm [9] and demonstrated heralded generation of tuneable four photon entangled states from a six photon input [10]. We have combined waveguide photonic circuits with superconducting single photon detectors [11]. We describe complex quantum interference behaviour in multi-mode interference devices with up to eight inputs and outputs [12], and quantum walks of correlated particles in arrays of coupled waveguides [13]. Finally, we give an overview of our recent work on fundamental aspects of quantum measurement [14,15] and single photon sources [16,17].
[1] T. D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, and J. L. OBrien, Nature 464, 45 (2010).
[2] J. L. O'Brien, Science 318, 1567 (2007).
[3] T. Nagata, R. Okamoto, J. L. O'Brien, K. Sasaki, and S. Takeuchi, Science 316, 726 (2007).
[4] R. Okamoto, J. L. O'Brien, H. F. Hofmann, T. Nagata, K. Sasaki, and S. Takeuchi, Science 323, 483 (2009).
[5] J.L.O'Brien, A.Furusawa, and J.Vuckovic, Nature Photon. 3, 687 (2009).
[6] A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, and J. L. O'Brien, Science 320, 646 (2008).
[7] A. Laing, A. Peruzzo, A. Politi, M. R. Verde, M. Halder, T. C. Ralph, M. G. Thompson, and J. L. O'Brien, Appl. Phys. Lett. 97, 211109 (2010)
[8] J. C. F. Matthews, A. Politi, A. Stefanov, and J. L. O'Brien, Nature Photon. 3, 346 (2009).
[9] A. Politi, J. C. F. Matthews, and J. L. O'Brien, Science 325, 1221 (2009).
[10] J. C. F. Matthews, A. Peruzzo, D. Bonneau, and J. L. O'Brien, arXiv:1005.5119
[11] C. M. Natarajan, A. Peruzzo, S. Miki, M. Sasaki, Z. Wang, B. Baek, S. Nam, R. H. Hadfield, and J. L. O'Brien, Appl. Phys. Lett. 96, 211101 (2010).
[12] A. Peruzzo, A. Laing, A. Politi, T. Rudolph, and J. L. O'Brien, Nature Comm. in press; arXiv:1005.5119
[13] A. Peruzzo, M. Lobino, J. C. F. Matthews, N. Matsuda, A. Politi, K. Poulios, X.-Q. Zhou, Y. Lahini, N. Ismail, K. Worhoff, Y. Bromberg, Y. Silberberg, M. G. Thompson, and J. L. O'Brien, Science 329, 1500 (2009)
[14] A. Laing, T. Rudolph, and J. L. O'Brien, Phys. Rev. Lett. 102, 160502 (2009).
[15] X-Q Zhou, TC Ralph, P Kalasuwan, M Zhang, A Peruzzo, BP Lanyon, and JL OBrien arXiv:1006.2670
[16] J. P. Hadden, J. P. Harrison, A. C. Stanley-Clarke, L. Marseglia, Y.-L. D. Ho, B. R. Patton, J. L. OBrien, and J. G. Rarity, Appl. Phys. Lett. 97, 241901 (2010)
[17] C. Xiong, G. D. Marshall, A. Peruzzo, M. Lobino, A. S. Clark, D.-Y. Choi, S. J. Madden, C. M. Natarajan, M. G. Tanner, R. H. Hadfield, S. N. Dorenbos, T. Zijlstra, V. Zwiller, M. G. Thompson, J. G. Rarity, M. J. Steel, B. Luther-Davies, B. J. Eggleton, J. L. O’Brien arXiv:1011.1688
Date: 9 February 2011
Time: 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Antonio Acin, ICFO, Barcelona (Spain)
Abstract:
Randomness is an intriguing concept which has fascinated, and keeps fascinating, many different communities, from Philosophy to Physics and Mathematics. On the other hand, randomness has also become a useful resource, as random numbers are used, for instance, in cryptographic applications, gambling or the simulation of physical and biological systems. Up to now, any of the existing solutions for randomness generation had to face the following problems:
(i) certification: how can one prove that the obtained symbols are truly random?
(ii) privacy: how can one be sure that the generated symbols are also random to any other external observer and
(iii) device-independence: how do imperfections in the devices used in the generation process affect the randomness of the generated symbols?
We provide a novel formalism for randomness generation which solves all these problems: using the non-local correlations of entangled quantum states, it is possible to generate certifiable, private and device-independent randomness.
Date: 11 January 2011
Time: 5pm
Venue: Ngee Ann Kongsi Auditorium (Level 2), Singapore Management University
Speaker: Charles H. Bennett of IBM Research USA
Abstract:
The information revolution is largely based on what a physicist would call a classical view of information, assuming that it can be copied freely and is not disturbed by observation. Quantum effects in information processing, which prevent the information in microscopic objects like atoms or photons from being observed or copied accurately, were long regarded as a mere nuisance, but are now known to make possible feats such as quantum cryptography, quantum teleportation and dramatic computational speedups. Although progress toward a practical quantum computer is slow, other surprising quantum informational effects continue to be discovered, and quantum cryptographic systems are already available commercially. Most importantly, the quantum approach has led to a more coherent and powerful way of thinking about how physical objects interact and influence one another, and how that interaction can be used to compute, communicate, and protect privacy. This talk will avoid mathematical complications and instead aim to explain central quantum concepts like entanglement, which at first sight seem counterintuitive.