paper

Quantum Circuits with Unbounded Fan-out

arXiv:quant-ph/0208043 · doi:10.4086/toc.2005.v001a005

Abstract

We demonstrate that the unbounded fan-out gate is very powerful. Constant-depth polynomial-size quantum circuits with bounded fan-in and unbounded fan-out over a fixed basis (denoted by QNCf^0) can approximate with polynomially small error the following gates: parity, mod[q], And, Or, majority, threshold[t], exact[q], and Counting. Classically, we need logarithmic depth even if we can use unbounded fan-in gates. If we allow arbitrary one-qubit gates instead of a fixed basis, then these circuits can also be made exact in log-star depth. Sorting, arithmetical operations, phase estimation, and the quantum Fourier transform with arbitrary moduli can also be approximated in constant depth.

20 pages, 9 figures, STACS'2003. v3: rewritten from scratch, new co-author, everything put into constant depth (including quantum Fourier transform). v4: polished a lot

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