Polylogarithmic-depth controlled-NOT gates without ancilla qubits

Abstract

Abstract Controlled operations are fundamental building blocks of quantum algorithms. Decomposing n-control-NOT gates (C n (X)) into arbitrary single-qubit and CNOT gates, is a crucial but non-trivial task. This study introduces C n (X) circuits outperforming previous methods in the asymptotic and non-asymptotic regimes. Three distinct decompositions are presented: an exact one using one borrowed ancilla with a circuit depth $$\Theta (\log {(n)}^{3})$$ Θ ( log ( n ) 3 ) , an approximating one without ancilla qubits with a circuit depth $${{{{{{{\mathcal{O}}}}}}}}(\log {(n)}^{3}\log (1/\epsilon ))$$ O ( log ( n ) 3 log ( 1 / ϵ ) ) and an exact one with an adjustable-depth circuit which decreases with the number m≤n of ancilla qubits available as $${{{{{{{\mathcal{O}}}}}}}}(\log {(n/\lfloor m/2\rfloor )}^{3}+\log (\lfloor m/2\rfloor ))$$ O ( log ( n / ⌊ m / 2 ⌋ ) 3 + log ( ⌊ m / 2 ⌋ ) ) . The resulting exponential speedup is likely to have a substantial impact on fault-tolerant quantum computing by improving the complexities of countless quantum algorithms with applications ranging from quantum chemistry to physics, finance and quantum machine learning.