Optimal pure state cloning and transposition are complementary channels

Vanessa Brzić, Dmitry Grinko, Michał Studziński, Marco Túlio Quintino (Mar 26 2026).

Abstract: State cloning and state transposition are fundamental transformations which, despite being desirable, cannot be perfectly implemented in quantum theory. In this work, we determine the optimal approximation for transforming $N$ qudits into $K$ copies of their transposition and prove the optimal fidelity for pure input states. We further show that the optimal qudit (N!\to!K)transposition map is the complementary channel of the optimal universal symmetric (N!\to!N!+!K)quantum cloning machine on pure states, implying that both tasks can be simultaneously realised by the same quantum operation and attain the same optimal fidelity. We then present an explicit quantum circuit that simultaneously implements optimal (N!\to!K)transposition and (N!\to!N!+!K)cloning and discuss its gate efficiency. Lastly, we investigate mixed-state (N!\to!1)qudit transposition and find the optimal performance in terms of white noise visibility, yielding the optimal structural physical approximation of state transposition in the multicopy regime.

Arxiv: https://arxiv.org/abs/2603.23628