Date: 4 March, 2005 (Paulo’s birthday)

Who: Paulo Mendonca (UQ)

Seminar type: Journal club

• Wojciech H. Zurek. “Decoherence and the Transition from Quantum to Classical - Revisited”. quant-ph/0306072 (2003).
• Wojciech H. Zurek. “Decoherence, einselection and the quantum origins of the classical”. Rev. Mod. Phys.. 75, 715 (2003).
• Wojciech H. Zurek. “Pointer basis of quantum apparatus: Into what mixture does the wave packet collapse?” Phys. Rev. D 24, 1516 (1981).
• Wojciech H. Zurek. “Environment-induced superselection rules” Phys. Rev. D 26, 1862 (1982).
• Jeffrey Bub. “Interpreting the Quantum World”. Cambridge: Cambridge University Press (1997).

Date: February 25th, 2005

Who: Timothy Ralph

Abstract:
I will discuss a linear optics quantum computation scheme that employs an incremental parity encoding approach and is tolerant to photon loss errors. The scheme employs a circuit model but uses techniques from cluster state computation and achieves comparable resource usage. To illustrate the technique I will describe a quantum memory which is fault tolerant to photon loss.

Suggested reading:

• A. J. F. Hayes, A. Gilchrist, C. R. Myers, T. C. Ralph, J. Opt. B: Quantum Semiclass. Opt. 6, 533 (2004)
• T .C. Ralph, A. Hayes, A. Gilchrist, quant-ph/0501184

I’m pleased to announce that the new QiSci homepage/blog is currently under development. All personal pages have been added. Please check your details for correctness and let me know if any changes or additions are necessary. If your profile is missing a photo, homepage or arXiv link, or any other information, please let me know.

Date: 18 February, 2005

Who: David Poulin (UQ)

Seminar type: Research talk

Abstract:
I will present a unified approach to quantum error correction, called operator quantum error correction. I will first review the well known techniques — the standard error correction model, the method of decoherence-free subpaces, and the noiseless subsystem method. Then, I will show how the latter scheme admits a generalization that is not restricted to unital maps, i.e. to the commutant of the interaction algebra. The unified approach is obtained by combining this generalized noiseless subsystem method to active error correction, and it incorporates all known schemes. Finally, I will demonstrate that the quantum error correction condition from the standard model is a necessary condition for all known methods of quantum error correction.

Joint work with David Kribs and Raymond Laflamme.

Reference: quant-ph/0412076

Date: 11 February, 2005

Who: Dominic Berry (UQ)

Seminar type: Research talk

Abstract:
A quantum channel is a completely positive and trace preserving map on quantum states, and models communication channels. The capacity for communication of classical information is often quantified by the Holevo capacity, which is the maximum Holevo information at the output of the channel. It is well known that four states in the ensemble are sufficient to achieve the capacity for qubit channels; however, four states are not always necessary. In particular, if the channel is unital (it is invariant on the maximally mixed state), two orthogonal states are sufficient. For more general qubit channels, the capacity may be achieved for two non-orthogonal inputs, three states, or four states may be required. I will present a systematic method of distinguishing between the different cases, as well as efficient methods of calculating the capacity.