December 7: CQT Annual Symposium  The Famous, The Bit and The Quantum By Martin Plenio and Mario Szegedy 

Oct 7: The past of a quantum particle in our manyworlds universe Lev Vaidman, Tel Aviv University 

Sept 9: Where is Quantum Mechanics Likely to Break Down? Daniel M. Greenberger, City University of New York 

Aug 20: Quantumlimited measurements: One physicist's crooked path from
quantum optics to quantum information Carlton M. Caves, University of New Mexico 

Aug 10: Space Clocks and Fundamental Tests By Christophe Salomon, Laboratoire Kastler Brossel, France 

May 27: Computing with Quantum Knots: Majorana Fermions, NonAbelian Anyons, and Topological Quantum Computation By Sankar Das Sarma, University of Maryland 

April 22: Quantum Optics in Wavelength Scale Structures By John Rarity, University of Bristol Note: Postponed to later date. 

March 18: One World Versus Many By Adrian Kent, Centre for Quantum Computation, University of Cambridge 

February 24: Ensemble encoding of quantum registers for quantum computing and communication By Klaus Mølmer, Lundbeck Foundation Theoretical Center for Quantum System Research, University of Aarhus 
Date: 7 October 2010
Time: 4pm  5pm
Venue: CQT Seminar Room, S150315
Speaker: Lev Vaidman, Tel Aviv University
Abstract:
I will argue that if we want to believe that physics of today can explain everything, we have to accept existence of parallel worlds in our Universe. I will explain how to see an object without light and why an attempt prove, without light, that an object is absent, fails. This will demonstrate that Wheeler's approach to the past of a quantum particles does not explain the weak trace it leaves and that the proper description of the past of a quantum particles requires addition of a quantum state evolving backward in time.
Date: 9 September 2010
Time: 4pm  5pm
Venue: CQT Seminar Room, S150315
Speaker: Daniel M. Greenberger, City University of New York
Abstract:
Quantum theory is very robust and nobody knows where it will break down. However, there are certain structural weaknesses that offer clues as to where
it may ultimately falter. One is its total disconnect to gravity at a very fundamental level (a physical level, not due to mathematical problems like
nonlinearity, etc).
Even at the level of the equivalency principle there is a basic
incompatibility, which can be traced back to the lack of a fundamental length
scale (independent of the Planck length, which is important but not relevant
here). We believe such a scale exists and will be ultimately responsible for
the breakdown of the theory as we know it today.
Date: 12 August 2010
Time: 4pm  5pm
Venue: CQT Seminar Room, S150315
Speaker: Carlton M. Caves, University of New Mexico
Abstract:
Quantum information science has changed our view of quantum mechanics. Originally viewed as a nag, whose uncertainty principles restrict what we can do, quantum mechanics is now seen as a liberator, allowing us to do things, such as secure key distribution and efficient computations, that could not be done in the realistic world of classical physics. Yet there is one area, that of quantum limits on highprecision measurements, where the two faces of quantum mechanics remain locked in battle. Using my own career as a convenient backdrop, I will trace the history of quantumlimited measurements, from the use of nonclassical light to improve the phase sensitivity of an interferometer, to the modern perspective on how quantum entanglement can be used to improve measurement precision, and finally to how to do quantum metrology without entanglement by using a nonlinear interferometer.
Speaker Bio:
Carlton M. Caves is a Distinguished Professor in the Department of
Physics and Astronomy at the University of New Mexico. He received
the PhD in Physics from the California Institute of Technology in
1979. He worked at Caltech as a postdoctoral Research Fellow through
1981 and as a Senior Research Fellow in Theoretical Physics from 1982
through 1987. From 1988 till 1992 he was Associate Professor of
Electrical Engineering and Physics at the University of Southern
California. He moved to to UNM as Professor of Physics and Astronomy
in 1992. He was awarded the 1990 Einstein Prize of the Society for
Optical and Quantum Electronics for his work on nonclassical light.
He is the author of over 120 scientific papers on topics in
gravitation theory, quantum optics, nonlinear dynamics, and quantum
information science. His present research is concentrated on quantum
metrology and quantum information theory. He is a Fellow of the
American Physical Society and the American Association for the
Advancement of Science.
Date: 10 August 2010
Time: 4pm  5pm
Venue: CQT Seminar Room, S150315
Speaker: Christophe Salomon, Laboratoire Kastler Brossel, France
Abstract:
We will first review the recent progress in atomic clocks operating in the microwave and optical domain of the electromagnetic spectrum. The second of the SI system of units is realized today with an accuracy of 3 1016 by a number of laser cooled atomic fountains worldwide. Optical clocks have recently reached a frequency stability and accuracy in the
1018 range [1] opening new perspectives for time keeping and fundamental tests.
We will then present the status of the ACES mission of the European Space Agency scheduled for flight to the International Space Station from 2013 to 2015 [2]. ACES will
embark a laser cooled cesium clock designed for microgravity operation (PHARAO ), an active hydrogen maser (SHM), and a high precision time transfer system operating in the microwave domain. This microwave link (MWL) will enable frequency comparisons
between the space clocks and a network of ground based clocks belonging to worldwide metrology institutes and universities. The link is designed for obtaining a relative frequency resolution of 1017 after a few days of measurement duration for intercontinental comparisons. In 20092010, all elements of the flight payload have successfully passed the Engineering Model tests and flight models are under construction. We will present the latest measurement results and flight model designs.
In a second part we will describe tests of fundamental physical laws using ultrastable clocks in space and on the ground that are planned for the ACES mission. An improved measurement of Einstein's gravitational redshift will be made at the two parts per million level. By comparing clocks of different nature at the 1017/year level, new limits will be obtained for the time variation of the fundamental constants of physics such as the fine
structure constant alpha and the ratio of electron to proton mass. The ability to compare microwave and optical clocks using the recently developed frequency comb technique opens a wide range of possibilities in clock comparisons. Finally a new kind of relativistic geodesy based on the Einstein effect will provide information on the Earth geoid, complementing the recent determination obtained by space geodesy methods.
References:
[1]C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, and T. Rosenband, Phys. Rev. Lett. 104, 070802 (2010)
[2] L. Cacciapuotti, and C. Salomon, Eur. Phys. J. Special Topics, 172, 57 (2009)
Date: 27 May 2010
Time: 4pm  5pm
Venue: CQT Seminar Room, S150315
Speaker: Sankar Das Sarma, University of Maryland
Abstract:
I will discuss the revolutionary new concept of topological quantum computation, which is faulttolerant at the hardware level with no need, in principle, of any quantum error correction protocols. Errors simply do not happen since the physical qubits and the computation steps are protected against decoherence by nonlocal topological correlations in the underlying physical system. The key idea is nonAbelian statistics of the quasiparticles (called 'anyons' as opposed to fermions or bosons), where the spacetime braiding of the anyons around each other, i.e. quantum 'knots', form topologically protected quantum gate operations. I will describe in details the status of the subject by discussing the theoretical principles guiding the experimental search for the appropriate topological phases of matter where such nonAbelian anyons may exist. Among the most significant possibilities are certain evendenominator fractional quantum Hall states, exotic chiral pwavesuperconductors, sandwich structures made from superconductors/semiconductors or superconductors/insulators, and suitable cold atomic systems. In the context, I will also discuss the race to find Majorana fermions in solid state systems, with the Majorana fermions being the simplest generic examples of nonAbelian objects in nature. I will explain how the subject of topological quantum computation synergistically brings together conformal field theory and advanced mathematics on one hand with materials science and quantum information on the other.
Date: 22 April 2010
Time: 4pm  5pm
Venue: CQT Seminar Room, S150315
Speaker: John Rarity, University of Bristol
Abstract:
The interaction between light and matter in optical structures that are at or below the wavelength scale could provide unprecedented performance in the storage of data, the switching of light and the generation of light of tailored properties.
For instance wavelength scale confinement can enhance nonlinear effects. An example is our experiments on four wave mixing effects in micron diameter optical fibres which provide novel sources of entangled photon pairs.
Also by creating nanoscale light traps, cavities, in wavelength scale periodic dielectric structures we can drastically modify light emission from 'atom like' objects (quantum dots, colour centres). In the case of long lightstorage times we can achieve strong coupling which is a form of entanglement between the atomcavity system and single photons. When we couple light in and out of these structures we will see strong nonlinear effects down to the single photon level. Thus we could make a low power switch at light levels orders of magnitude below existing technology. We can also expect to exploit the quantum properties as ultimately we have the single photon nonlinear element needed for quantum computation.
I will show a few example results to illustrate these ideas and discuss future plans now funded through an ERC advanced grant.
Date: 18 March 2010
Time: 4pm  5pm
Venue: CQT Seminar Room, S150315
Speaker: Adrian Kent, Centre for Quantum Computation, University of Cambridge
Media: Video
Abstract:
There is a compelling intellectual case for exploring whether purely
unitary quantum theory defines a sensible and scientifically adequate
theory, as Everett originally proposed. Many different
and incompatible attempts to define a coherent Everettian quantum theory
have been made over the past fifty years, suggesting this is a problem
about which humans are excellent at forming strong intuitions but very
bad at forming persuasive arguments.
In this talk I review recent work in this area. I argue that
considerable light is shed on the problem once one realizes that
manyworlds theories are just that  novel and distinct scientific
theories, not reinterpretations of standard quantum theory.
This forces us to reconsider from first principles whether (and if so
how) we can relate manyworlds theories to empirical data.
I review some interesting and ingenious attempts in this direction by
Wallace, GreavesMyrvold and others, and explain why they don't work.
Date: 24 February 2010
Time: 4pm  5pm
Venue: CQT Seminar Room, S150315
Speaker: Klaus Mølmer, Lundbeck Foundation Theoretical Center for Quantum System Research, University of Aarhus
Media: Video PDF
Abstract:
In the conventional layout of a register for quantum computing, all qubits
are associated with individual twolevel quantum systems. Individual
addressing and interaction with these systems permit onebit gates, while a
controllable pairwise interaction between systems is needed to accomplish
twobit gates.
We present a different approach that encodes a multibit quantum register in
the collective internal state populations of an ensemble of identical
multilevel quantum systems. This method establishes a linear relationship
between the number of bits and the internal state Hilbert space dimension.
It does not require experimental access to individual particles, but it
relies on an interaction that restricts the collective populations to the
(bit) values zero and unity.
We devote a detailed discussion to applications with neutral atoms
interacting via Rydberg excited states, and we show that, e.g., a small
cloud of cesium atoms may be operated as a universal quantum computer with
up to 14 bits and as a deterministic multimode photonic device.
Other schemes for collective encoding of quantum registers in hybrid quantum
systems will be briefly discussed.