Article,

Elementary gates for quantum computation

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Physical Review A, 52 (5): 3457--3467 (Mar 23, 1995)
DOI: 10.1103/physreva.52.3457

Abstract

We show that a set of gates that consists of all one-bit quantum gates (U(2)) and the two-bit exclusive-or gate (that maps Boolean values \$(x,y)\$ to \$(x,x y)\$) is universal in the sense that all unitary operations on arbitrarily many bits \$n\$ (U(\$2^n\$)) can be expressed as compositions of these gates. We investigate the number of the above gates required to implement other gates, such as generalized Deutsch-Toffoli gates, that apply a specific U(2) transformation to one input bit if and only if the logical AND of all remaining input bits is satisfied. These gates play a central role in many proposed constructions of quantum computational networks. We derive upper and lower bounds on the exact number of elementary gates required to build up a variety of two-and three-bit quantum gates, the asymptotic number required for \$n\$-bit Deutsch-Toffoli gates, and make some observations about the number required for arbitrary \$n\$-bit unitary operations.

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