Date: 7 October 2010
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15
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, S15-03-15
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 non-linearity, 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, S15-03-15
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 high-precision 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 quantum-limited 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, S15-03-15
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 10-16 by a number of laser cooled atomic fountains worldwide. Optical clocks have recently reached a frequency stability and accuracy in the 10-18 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 10-17 after a few days of measurement duration for intercontinental comparisons. In 2009-2010, 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 ultra-stable clocks in space and on the ground that are planned for the ACES mission. An improved measurement of Einstein's gravitational red-shift will be made at the two parts per million level. By comparing clocks of different nature at the 10-17/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, S15-03-15
Speaker: Sankar Das Sarma, University of Maryland
Abstract:
I will discuss the revolutionary new concept of topological quantum computation, which is fault-tolerant 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 non-local topological correlations in the underlying physical system. The key idea is non-Abelian statistics of the quasiparticles (called 'anyons' as opposed to fermions or bosons), where the space-time 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 non-Abelian anyons may exist. Among the most significant possibilities are certain even-denominator fractional quantum Hall states, exotic chiral p-wavesuperconductors, 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 non-Abelian 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, S15-03-15
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 non-linear 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 light-storage times we can achieve strong coupling which is a form of entanglement between the atom-cavity system and single photons. When we couple light in and out of these structures we will see strong non-linear 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 non-linear 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, S15-03-15
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 many-worlds 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 many-worlds theories to empirical data. I review some interesting and ingenious attempts in this direction by Wallace, Greaves-Myrvold and others, and explain why they don't work.

Date: 24 February 2010
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Klaus Mølmer, Lundbeck Foundation Theoretical Center for Quantum System Research, University of Aarhus
Media: Video   PDF

Abstract:
In the conventional lay-out of a register for quantum computing, all qubits are associated with individual two-level quantum systems. Individual addressing and interaction with these systems permit one-bit gates, while a controllable pair-wise interaction between systems is needed to accomplish two-bit gates.
We present a different approach that encodes a multi-bit quantum register in the collective internal state populations of an ensemble of identical multi-level 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 multi-mode photonic device.
Other schemes for collective encoding of quantum registers in hybrid quantum systems will be briefly discussed.