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
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
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.

Date: 19 November 2009
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Nicholas Bigelow, The Institute of Optics, University of Rochester

Title: Sculpting a spinor condensate: from Skyrmions to quantum memory
Abstract:
The spin and orbital angular momentum of a spinor Bose-Einstein condensate can be controlled using optical fields that carry orbital angular momentum. I will describe how this process works and our experimental progress using this approach to manipulate the BEC wavefunction. In particular, I will describe the creation of complex spin textures, coreless vortices and Skyrmions. I will also describe the application of this work to the realization of a robust memory for quantum information processing.

Date: 29 October 2009
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15

Speaker: Jean Dalibard, Laboratoire Kastler Brossel
Title: Exploring Flatland with cold atoms
Abstract:
In his world-famous novel "Flatland" published in 1884, the English writer Edwin Abbott imagined a social life in a two-dimensional world. With a very original use of geometrical notions, E. Abbott produced a unique satire of his own society. Long after Abbott's visionary allegory, Microscopic Physics has provided a practical path for the exploration of low-dimensional worlds. With the realization of quantum wells for example, it has been possible to produce two-dimensional gases of electrons. The properties of these fluids dramatically differ from the standard three-dimensional case, and some of them are still lacking a full understanding. During the last decade, a novel environment has been developed for the study of low-dimensional phenomena. It consists of cold atomic gases that are confined in tailor-made electromagnetic traps. With these gases, one hopes to simulate and understand more complex condensed-matter systems. The talk will discuss some aspects of this research, both from an experimental and a theoretical perspective. Connections with other domains of many-body physics, such as the Quantum Hall phenomenon, will also be addressed.

Date: 3 September 2009
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15

Speaker: Lieven Vandersypen, Kavli Institute of Nanoscience, Delft University of Technology
Title: Coherence and control of single electron spins in quantum dots
Abstract:
Individual electron spins isolated in semiconductor quantum dots are natural two-level quantum systems that could form the basis of a quantum information processor. Using a fully electrical approach, it is now possible to initialize, coherently manipulate and read out the spin state of a single electron in a quantum dot, and to couple it coherently to the spin of an electron in a neighbouring dot. Furthermore, we have come to a quantitative understanding of the timescales and mechanisms by which the spin loses phase coherence. Ongoing work focuses on integrating all buildings blocks in a single experiment, and on either control or elimination of the electron spin environment, in particular the nuclear spins in the quantum dot host material. This should permit using entangled spins as a new resource for quantum information processing.

References
R. Hanson, L.P Kouwenhoven, J.R. Petta, S. Tarucha, and L.M.K. Vandersypen, Spins in few-electron quantum dots, Reviews of Modern Physics 79, 1217 (2007)
I. T.Vink, K. C. Nowack, F. H. L. Koppens, J. Danon, Yu. V. Nazarov, and L. M. K. Vandersypen, Locking electron spins into magnetic resonance by electron-nuclear feedback, arXiv:0902.2659
F.H.L. Koppens, K.C. Nowack, and L.M.K. Vandersypen, Spin echo of a single electron spin in a quantum dot, Phys. Rev. Lett. 100, 236802 (2008)
K.C. Nowack, F.H.L. Koppens, Yu.V. Nazarov and L.M.K. Vandersypen, Coherent control of a single electron spin with electric fields, Science 318, 1430 (2007)
F.H.L. Koppens, C. Buizert, K.J. Tielrooij, I.T. Vink, K.C. Nowack, T. Meunier, L.P. Kouwenhoven, and L.M.K. Vandersypen, Driven coherent oscillations of a single electron spin in a quantum dot, Nature 442, 766-771 (2006)

Date: 16 April 2009
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Jörg Schmiedmayer, Atominstitut der Österreichischen Universitäten, TU-Wien
Abstract:
AtomChips aim at the miniaturization and integration of quantum optics and atomic physics on to a single chip, analogous to electronic circuits. It combines the best of both worlds: The perfected manipulation techniques from atomic physics with the capability of nanofabrication. AtomChips promise to allow coherent manipulation of matter waves on the quantum level by using high spatial resolution electro magnetic potentials from structures on the atom chip or by employing adiabatic radio frequency (RF) or micro wave (MW) potentials [1]. The talk will give an overview of the recent advances in the concepts, fabrication and experimental realization of AtomChips by illustrating the many different tasks that can be performed using ultra cold or Bose-Einstein condensed (BECs) atoms manipulated on the chip. These range from measuring magnetic and electric fields with unprecedented sensitivity by observing the density modulations in trapped highly elongated 1d BECs [2], to fundamental studies of the universal properties in low dimensional systems like non equilibrium dynamics and coherence decay [3] or signatures of thermal and quantum noise [4] in one dimensional super fluids. This work was supported by the European Union integrated project SCALA, the DIP the FWF and the Wittgenstein Prize.
[1] T. Schumm et al. Nature Physics, 1, 57 (2005); S. Hofferberth et al. Nature Physics, 2, 710 (2006)
[2] St. Wildermuth et al. Nature 435, 440 (2005); S. Aigner et al. Science 319, 1226 (2008)
[3] S. Hofferberth et al. Nature 449, 324 (2007) [4] S. Hofferberth et al. Nature Physics, 4, 489 (2008)

Date: 2 April 2009
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Serge Haroche, ENS and Collège de France
Abstract:
Cavity QED experiments are described, in which a beam of Rydberg atoms is used to manipulate and probe non-destructively microwave photons trapped in a very high Q superconducting cavity. We realize ideal quantum non-demolition (QND) measurements of photon numbers, observe the radiation quantum jumps due to cavity relaxation and prepare non-classical fields such as Fock and Schrödinger cat states. Combining QND photon counting with a homodyne mixing method, we reconstruct the Wigner functions of these non-classical states and, by taking snapshots of these functions at increasing times, obtain movies of the decoherence process. We also observe that the coherent evolution of the field in the cavity is frozen when we measure non-destructively its photon number and we realize in this way a simple demonstration of the quantum Zeno effect. These experiments open the way to the implementation of quantum feedback procedures aimed at preserving over long time intervals the quantum coherence of non-classical states of radiation in a cavity.

Date: 26 March 2009
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Patrick Hayden, McGill University, Canada
Abstract:
I'll discuss information retrieval from evaporating black holes, assuming that the internal dynamics of a black hole is unitary and rapidly mixing. Instead of locking away information for a near eternity, black holes can reveal their information in Hawking radiation remarkably quickly. The resulting estimate of a black hole's information retention time, based on speculative dynamical assumptions, is just barely compatible with the black hole complementarity hypothesis. The reason these conclusions hold is that typical local quantum circuits generate highly efficient quantum error-correcting codes. (Joint work with John Preskill)

Date: 12 February 2009
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Ian Walmsley, University of Oxford, UK.
Abstract:
Photonic quantum technologies are largely based on the effects of quantum interference coupled with the nonlinearities that can be induced by measurements. This means that attention must be paid to the character of the individual light particles that are used to build up large-scale entanglement. In turn, this requires the development of sources and detectors that can verify the quantum state of the light field in all its degrees of freedom. I shall discuss how "vacuum engineering" can be used to prepare pure-state single photon wavepackets using nonlinear optics and conditional detection. The efficacy of such methods of preparation is predicated on the ability to understand what it is that the detector is measuring, and I shall illustrate a procedure for fully characterizing a quantum detector using tomography, thereby verifying the measurement operations for the device.

Date: 5 February 2009
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Wolfgang Behr, University of Zurich, Switzerland
Abstract:
Once premodern European and Arabic cipherers started to leave the more pedestrian "physical" approaches to steganography behind, techniques of encryption in political, military, religious or private contexts consisted mainly in the transformative manipulation of a plaintext and the devision of adequate decoding keys. They were thus by and large tantamount to the evolution of ever more powerful mapping algorithms, and dependent on increasingly sophisticated insights into the distributional properties of letters and lexical roots occuring in alphabetically represented written language.

In China, the topic of information secrecy has been discussed since the pre-imperial period. State institutions maintaining certain types of government information secret and laws regulating the punishment of its leakage were of great complexity already during the Qin and Han dynasties, and reached unsurpassed bureaucratic intricacies under the Ming. Nevertheless, actual coding procedures seem to have developed quite slowly throughout imperial Chinese history, in the public as well as the private sectors. After a short overview of the earliest anecdotal evidence related to the disguising of information in early Chinese military texts, my talk will concentrate on three techniques, which illustrate the difficulty of pre-modern encryption on the basis of a logographic writing system representing a largely monosyllabic, tonal language. These are (1) the so-called "character verification" method, described in the Northern Song Essentials from the Military classics by Zeng Gongliang (999-1078), which maps numbered military commands onto sequences of characters appearing in a memorized pentasyllabic poem, serving as the key; (2) a system inspired by the phonological categories of the Middle Chinese rhyme dictionary tradition, and ascribed to the famous anti-Japanese Ming general Qi Jiguang (1528-1588); (3) homophonic substitution and “synthanalytic” character manipulation in poetry and edicts, recorded, e.g. in documents from the Taiping rebellion.

In a concluding discussion, the inhibitive role of the non-alphabetic writing system in the development of formalized coding procedures will be weighed against the influence, which the “non-development of probabilistic thinking“, i.e. the “lack of an exploratory serious playfulness“ (Mark Elvin) in China might have brought about.

Date: 20 January 2009
Time: 4pm - 5pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Seth Lloyd, Massachusetts Institute of Technology, USA
Abstract:

Venue: CQT Seminar Room, S15-03-15

Title:
PHOTON CORRELATIONS IN MULTI-CAVITY OPEN SYSTEMS

Abstract:
The photon-blockade effect, where nonlinearities at the single-photon level alter the quantum statistics of light emitted from a cavity, has been observed in cavity quantum electrodynamics experiments with both atomic and solid-state systems. Now that single cavity QED is experimentally well established, it is time to investigate the richness of periodic arrays of cavities displaying single-photon nonlinear behavior.
In particular, we review our theoretical work on the intriguing possibilities offered by measuring photon correlations in tunnel-coupled multi-cavity arrays, taking into account continuous coherent driving and dissipation. For systems of increasing complexity, it will be shown that photon-correlation measurements would provide a unique probe of the crossover from the linear (i.e.
classical) to the strongly correlated photon regime, which can be identified even in the presence of a dynamical equilibrium established from the balance of driving and losses.

Venue: CQT Seminar Room, S15-03-15

Title:
Revisiting cavity quantum electrodynamics with quantum dots and semiconducting cavities
(When decoherence becomes a resource)

Abstract:
Thanks to technological progresses in the field of solid-state physics, a wide range of quantum optics experiments previously restricted to atomic physics, can now be implemented using quantum dots (QDs) and semi-conducting cavities. Still, a QD is far from being an isolated two-level atom. As a matter of fact, solid state emitters are intrinsically coupled to the matrix they are embedded in, leading to decoherence processes that unavoidably broaden any transition between the discrete states of these artificial atoms. At the same time, very high quality factors and ultra small modal volumes are achieved for state of the art cavities. These new conditions open an unexplored regime for cavity quantum electrodynamics (CQED) so far, where the emitter's linewidth can be of the same order of magnitude, or even broader than the cavity mode one. In this kind of exotic regime, unusual phenomena can be observed. In particular, we have shown [1] that photons spontaneously emitted by a QD coupled to a detuned cavity can efficiently by emitted at the cavity frequency, even if the detuning is large; whereas if the QD is continously pumped, decoherence can induce lasing [2]. These effects clearly show that decoherence, far from being a drawback, is a fundamental resource in solid-state cavity quantum electrodynamics, offering appealing perspectives in the context of advanced nano-photonic devices.

[1] "Pure emitter's dephasing : a resource for advanced single photon sources" PRA 79, 053838 (2009).
[2] "Controlling the dynamics of a coupled atom-cavity system by pure dephasing : basics and applications in nanophotonics", PRB 81, 245419 (2010).

Venue: CQT Seminar Room, S15-03-15

Title:
Stability and decay of Bloch oscillations in Bose-Einstein condensates with time-dependent atom-atom interactions

Abstract: Bose-Einstein condensates in tilted optical lattices allow the observation of Bloch oscillations. Generically, the interaction leads to dephasing and to the decay of the wave packet. By means of Feshbach resonances, however, experimentalists can tune the s-wave scattering length to zero or modulate it in time. We investigate the effect of such time-managed interactions on Bloch oscillations. Additionally to the noninteracting case and a solitonic solution, we find an infinite family of modulations that preserve the Bloch oscillating wave packet [1]. In these cases, the stability follows from a time-reversal argument. In the unstable cases, we employ a collective-coordinates ansatz and a stability analysis, in order to quantify the decay of the Bloch oscillations. In particular we show that in presence of external perturbations, an additional modulation of the interaction can enhance the lifetime of the Bloch oscillation [2].

[1] Gaul et al. PRL 102, 255303 (2009)
[2] Díaz et al. PRA 81, 051607R (2010)