Quantum Fan-out is Powerful

@article{Hyer2005QuantumFI,
  title={Quantum Fan-out is Powerful},
  author={Peter H\oyer and Robert Spalek},
  journal={Theory of Computing},
  year={2005},
  volume={1},
  pages={81-103}
}
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 ) can approximate with polynomially small error the following gates: parity, mod[q], And, Or, majority, threshold[t], exact[t], 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… CONTINUE READING
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