Optimal cloning of unitary transformations

TL;DR

This paper establishes the theoretical limits of cloning quantum unitary transformations, demonstrating an optimal method that surpasses previous strategies and has implications for quantum cryptography.

Contribution

It introduces the first optimal universal cloning protocol for unitary transformations, utilizing quantum channels with memory, advancing quantum information processing capabilities.

Findings

Optimal cloning outperforms measure-and-reprepare strategies

Quantum channels with memory enable superior cloning fidelity

Applications include enhanced two-way quantum cryptographic protocols

Abstract

After proving a general no-cloning theorem for black boxes, we derive the optimal universal cloning of unitary transformations, from one to two copies. The optimal cloner is realized by quantum channels with memory, and greately outperforms the optimal measure-and-reprepare cloning strategy. Applications are outlined, including two-way quantum cryptographic protocols.