Quantum Addition Circuits and Unbounded Fan-Out

We first show how to construct an O(n)-depth O(n)-size quantum circuit for addition of two n-bit binary numbers with no ancillary qubits. The exact size is 7n-6, which is smaller than that of any other quantum circuit ever constructed for addition with no ancillary qubits. Using the circuit, we then propose a method for constructing an O(d(n))-depth O(n)-size quantum circuit for addition with O(n/d(n)) ancillary qubits for any d(n)=Ω(log n). If we are allowed to use unbounded fan-out gates with length O(n^c) for an arbitrary small positive constant c, we can modify the method and construct an O(e(n))-depth O(n)-size circuit with o(n) ancillary qubits for any e(n)=Ω(log* n). In particular, these methods yield efficient circuits with depth O(log n) and with depth O(log* n), respectively. We apply our circuits to constructing efficient quantum circuits for Shor's discrete logarithm algorithm.

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