Implementing the fanout operation with simple pairwise interactions
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It has been shown that, for even n, evolving n qubits according to a Hamiltonian that is the sum of pairwise interactions between the particles, can be used to exactly implement an (n+1)-qubit fanout gate using a particular constant-depth circuit arXiv:quant-ph/0309163. However, the coupling coefficients in the Hamiltonian considered in that paper are assumed to be all equal. In this paper, we generalize these results and show that for all n, including odd n, one can exactly implement an (n+1)-qubit parity gate and hence, equivalently in constant depth an (n+1)-qubit fanout gate, using a similar Hamiltonian but with unequal couplings, and we give an exact characterization of which couplings are adequate to implement fanout via the same circuit. We also investigate pairwise couplings that satisfy an inverse square law and give planar arrangements of four qubits that (together with a target qubit) are adequate to implement 5-qubit fanout.
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