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

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

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

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

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