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A quantum gate array can be programmed to evaluate the expectation value of any operator

Juan Pablo Paz·Augusto Roncaglia
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Abstract

A programmable gate array is a circuit whose action is controlled by input data. In this letter we describe a special--purpose quantum circuit that can be programmed to evaluate the expectation value of any operator $O$ acting on a space of states of $N$ dimensions. The circuit has a program register whose state $|Ψ(O)>_P$ encodes the operator $O$ whose expectation value is to be evaluated. The method requires knowledge of the expansion of $O$ in a basis of the space of operators. We discuss some applications of this circuit and its relation to known instances of quantum state tomography.

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Juan Pablo Paz, Augusto Roncaglia. 2003-06-20. A quantum gate array can be programmed to evaluate the expectation value of any operator. https://doi.org/10.1103/physreva.68.052316

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