Optimal universal quantum circuits for unitary complex conjugation

TL;DR

This paper develops optimal quantum circuits that convert multiple calls of a unitary operation into its complex conjugate, achieving maximal fidelity and robustness, extending prior single-call results to multiple calls and broader transformations.

Contribution

It introduces the first optimal parallel quantum circuits for complex conjugation of unitary operations with multiple calls, generalizing previous single-call and specific cases.

Findings

Optimal circuits for any number of calls and dimensions

Achieves average fidelity of (k+1)/[d(d-k)]

Proven optimality in fidelity and robustness

Abstract

Let $U_d$ be a unitary operator representing an arbitrary $d$-dimensional unitary quantum operation. This work presents optimal quantum circuits for transforming a number $k$ of calls of $U_d$ into its complex conjugate $\bar{U_d}$. Our circuits admit a parallel implementation and are proven to be optimal for any $k$ and $d$ with an average fidelity of $\left\langle{F}\right\rangle =\frac{k+1}{d(d-k)}$. Optimality is shown for average fidelity, robustness to noise, and other standard figures of merit. This extends previous works which considered the scenario of a single call ($k=1$) of the operation $U_d$, and the special case of $k=d-1$ calls. We then show that our results encompass optimal transformations from $k$ calls of $U_d$ to $f(U_d)$ for any arbitrary homomorphism $f$ from the group of $d$-dimensional unitary operators to itself, since complex conjugation is the only non-trivial automorphisms on the group of unitary operators. Finally, we apply our optimal complex conjugation implementation to design a probabilistic circuit for reversing arbitrary quantum evolutions.