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: