Query and Depth Upper Bounds for Quantum Unitaries via Grover Search

Abstract

We prove that any $n$-qubit unitary can be implemented (i) approximately in time $\tilde{O}\left(2^{n/2}\right)$ with query access to an appropriate classical oracle, and also (ii) exactly by a circuit of depth $\tilde{O}\left(2^{n/2}\right)$ with one- and two-qubit gates and $2^{O(n)}$ ancillae. The proofs involve similar reductions to Grover search. The proof of (ii) also involves a linear-depth construction of arbitrary quantum states using one- and two-qubit gates (in fact, this can be improved to constant depth with the addition of fanout and generalized Toffoli gates) which may be of independent interest. We also prove a matching $\Omega \left(2^{n/2}\right)$ lower bound for (i) and (ii) for a certain class of implementations.