Compiling Conditional Quantum Gates without Using Helper Qubits

Abstract

We present a compilation scheme for conditional quantum gates. Our scheme compiles a multi-qubit conditional to a linear number of two-qubit conditionals. This can be done straightforwardly with helper qubits, but we show how to do it without using helper qubits and with much fewer gates than in previous work. Specifically, our scheme requires 1/3 as many gates as the previous best scheme without using helper qubits, which is essential for practical use. Our experiments show that several quantum-circuit optimizers have little impact on the compiled code from the previous best scheme, confirming the need for our new scheme. Our experiments with Grover's algorithm and quantum walk also show that our scheme has a major impact on the reliability of the compiled code.