Date: 12 January 2012, 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Wojciech Zurek, Los Alamos National Laboratory, USA
Media: Video

I discuss three insights into the transition from quantum to classical. I will start with (i) a minimalist (decoherence-free) derivation of preferred states. Such pointer states define events (e.g., measurement outcomes) without appealing to Born's rule. Probabilities and (ii) Born’s rule can be derived from the symmetries of entangled quantum states. With probabilities at hand one can analyze information flows from the system to the environment in course of decoherence. They explain how (iii) robust “classical reality” arises from the quantum substrate by accounting for objective existence of pointer states of quantum systems through redundancy of their records in the environment. Taken together, and in the right order, these three advances elucidate quantum origins of the classical.*  
*W. H. Zurek, Nature Physics 5, 181-188 (2009).

Date: 12 January 2012, 5.30pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Raymond Laflamme, IQC, Waterloo, Canada
Media: Video

The Achilles' heel of quantum information processors is the fragility of quantum states and processes.  Without a method to control imperfection and imprecision of quantum devices, the probability that a quantum computation succeed will decrease exponentially in the number of gates it requires. In the last fifteen  years, building on the discovery of quantum error correction, accuracy threshold theorems were proved showing that error can be controlled using a reasonable amount of resources as long as the error rate is smaller than a certain threshold. We thus have a scalable theory describing  how to control quantum systems. I will briefly review some of the assumptions of the accuracy threshold theorems and comment on recent experiments that have been done and should be done to turn quantum error correction into an experimental reality.

Date: 09 February 2012, 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Guido Burkard, Department of Physics, University of Konstanz, Germany
Media: Video

Carbon, in the form of graphene and carbon nanotubes, has recently emerged as an interesting alternative material for electronics. Here, we argue that carbon is also a unique material for a new type of electronics that is based on the electron spin rather than its charge, known as spintronics, and in particular for spin-based quantum computing [1]. Due to the low concentration of nuclear spins and relatively weak spin-orbit coupling, carbon-based structures allow for long coherence times, which is the primary figure of merit for the quality of a spin quantum bit (qubit). We discuss the formation of quantum dots, acting as electron “traps’’, in graphene and their potential use for quantum information processing. In diamond, the spin coherence of defect centers can persist even at ambient temperatures. After introducing this fascinating quantum system, we briefly present a particular mechanism for storing and retrieving quantum information in an atomic nucleus in diamond [2].

[1] B. Trauzettel, D. Bulaev, D. Loss, and G. Burkard, Nature Phys. 3, 192 (2007).
[2] G. D. Fuchs, G. Burkard, P. V. Klimov, and D. D. Awschalom, Nature Phys. 7, 789 (2011).

Date: 08 March 2012, 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Gershon Kurizki, Weizmann Institute of Science, Israel

I shall revisit the traditional formulation of the three laws of thermodynamics and the bounds they imply on thermodynamic observables , and argue that all of these have to be modified and reformulated when the system is enacted upon on time scales shorter than the bath memory time. Practical consequences related to work extraction and cooling will be demonstrated to be in stark contrast with existing schemes .

Date: 24 May 2012, 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Philippe Bouyer, Laboratoire Charles Fabry, France

The transport of quantum particles in non ideal material media (eg the conduction of electrons in an imperfect crystal) is strongly affected by scattering from impurities of the medium. Even for a weak disorder, semi-classical theories, such as those based on the Boltzmann equation for matter-waves scattering from the impurities, often fail to describe transport properties and full quantum approaches are necessary. The properties of the quantum systems are of fundamental interest as they show intriguing and non-intuitive phenomena that are not yet fully understood such as Anderson localization, percolation, disorder-driven quantum phase transitions and the corresponding Bose-glass or spin-glass phases. Understanding quantum transport in amorphous solids is one of the main issues in this context, related to electric and thermal conductivities.  

Ultracold atomic gases can now be considered to revisit the problem of quantum conductivity and quantum transport under unique control possibilities. Dilute atomic Bose-Einstein condensates (BEC) and degenerate Fermi gases (DFG) are produced routinely taking advantage of the recent progress in cooling and trapping of neutral atoms. Transport has been widely investigated in controlled potentials with no defects, for instance periodic potentials (optical lattices). Controlled disordered potentials can also be produced with various techniques such as the use of magnetic traps designed on atomic chips with rough wires, the use of localized impurity atoms, the use of radio-frequency fields or the use of optical potentials. This recently lead to the observation of the Anderson Localization of a BEC in 1D and 3D,and the study of diffusion properties during matter-wave transport.

Date: 26 July 2012, 4pm
Venue: CQT Seminar Room, S15-03-15
Speaker: Peter Hanggi, University of Augsburg 

This talk is aimed at highlighting issues that relate of doing thermodynamics and statistical physics of finite size systems. This theme gained considerable importance in view of fascinating advances in nanotechnology and system biology. While the fathers of thermodynamics developed the famous 5 Laws of Thermodynamics (incl. a “–1_Law),  having in mind macroscopic systems, these grand concepts need to be inspected anew in view of the fact that the fluctuations grow with decreasing size to a level where they even may play the dominant role. This holds true particularly for the inter-relationships between fluctuations and, importantly, the measurements of work, heat & heat flow and thermodynamic equilibrium quantifiers such as (non-fluctuating) free energy changes or production of thermodynamic entropy. -- Subtleties occur in equilibrium thermodynamics for small systems, such as the role of finite size for quantities like (possibly negative-valued) canonical heat capacitance in presence of strong system-environment coupling.  

I further elaborate on recent, timely results for nonequilibrium classical and quantum fluctuation relations. An attempt is made to outline pitfalls and still open issues (relativistic and others)   together with those inherent difficulties that likely do emerge with the experimental validation scenarios of such relations.  Finally, a connection between Fluctuation Relations, nonlinear Response and its feasible relation with the ever-lasting intriguing challenge in detecting the origin of an "Arrow of Time" will be elucidated.   

This presentation is based on joint work with Michele Campisi, Gert-Ludwig Ingold and Peter Talkner, all at the University of Augsburg. -- Own pertinent recent works that apply for the theme of this talk are:   

[1] G. L. Ingold, P. Hänggi, and P. Talkner
Specific heat anomalies of open quantum systems
Phys. Rev. E 79, 061105 (2009)
 
[2]   M. Campisi, P. Hanggi, and P. Talkner
Colloquium: Quantum fluctuation relations: Foundations and applications
Rev. Mod. Phys. 83, 771--791 (2011);
Addendum and Erratum: Quantum fluctuation relations: Foundations and applications
Rev. Mod. Phys. 83, 1653 (2011).
 
[3] M. Campisi and P. Hanggi
Fluctuation, Dissipation and the Arrow of Time
Entropy 13, 2024--2035 (2012).