EFFECTIVE PURE STATES FOR BULK QUANTUM COMPUTATION

In bulk quantum computation one can manipulate a large number of indis tinguishable quantum computers by parallel unitary operations and meas ure expectation values of certain observables with limited sensitivity . The initial state of each computer in the ensemble is known but not pure. Methods for obtaining effective pure input states by a series of manipulations have been described by Gershenfeld and Chuang (logical labeling) [Science 275. 350 (1997)] and Cory et al. (spatial averaging ) [Proc. Natl. Acad. Sci. USA 94, 1634 (1997)] for the case of quantum computation with nuclear magnetic resonance. We gives different techn ique called temporal averaging. This method is based on classical rand omization, requires no ancilla quantum bits, and can be implemented in nuclear magnetic resonance without using gradient fields. We introduc e several temporal averaging algorithms suitable for both high-tempera ture and low-temperature bulk quantum computing and analyze the signal -to-noise behavior of each. Most of these algorithms require only a co nstant multiple of the number of experiments needed by the other metho ds for creating effective pure stales.