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SYSA: Quantum state analysis and estimation

SYSA 2: Quantum state analysis and estimation II

SYSA 2.4: Invited Talk

Tuesday, March 14, 2006, 15:30–16:00, HVI

A consumer guide to quantum state preparators — •Reinhard Werner — Inst. Math.Phys, TU Braunschweig

One of the basic requirements for a quantum computer is that we can initialize it in a specific, usually pure quantum state. However, any physical state source will produce deviations form the desired state, and we need to employ methods of error correction to improve the quality of the initialization, especially at the beginning of a very long computation.

In this talk we ask: What promise do we need about the quality of the source so that we can guarantee any desired quality of initial states after error correction? In order to isolate this problem from the problem of improving on noisy gates, we assume that unitary operations can be performed exactly, but that the ancillas used for irreversible operations, including measurements, are also obtained from the noisy source under investigation.

A typical sufficient promise about the source would be exact independence of the successive states. However, this is hardly realistic in experimental situations. It would seem that some approximate version of independence would suffice, but we show that no promise made with finite accuracy will ever do. Possible alternatives for the premise of a threshold theorem (such as stationarity and entropy criteria) are discussed.