Experimental linear-optical implementation of a multifunctional optimal qubit cloner

TL;DR

This paper reports the first experimental implementation of a versatile quantum cloning device capable of optimally cloning qubits across multiple regimes with high fidelity, advancing quantum communication and cryptography applications.

Contribution

The work introduces a multifunctional, optimal qubit cloning device that works across several regimes, including universal, phase-covariant, and mirror phase-covariant cloning, with near-maximal fidelity.

Findings

Achieved 97.5% of the theoretical maximum fidelity.

Demonstrated versatility across multiple cloning regimes.

Potential applications in quantum cryptography and quantum networks.

Abstract

We present the first experimental implementation of a multifunctional device for the optimal cloning of one to two qubits. Previous implementations have always been designed to optimize the cloning procedure with respect to one single type of a priori information about the cloned state. In contrast, our "all-in-one" implementation is optimal for several prominent regimes such as universal cloning, phase-covariant cloning, and also the first ever realized mirror phase-covariant cloning, when the square of the expected value of Pauli's Z operator is known in advance. In all these regimes the experimental device yields clones with almost maximum achievable average fidelity (97.5% of theoretical limit). Our device has a wide range of possible applications in quantum information processing, especially in quantum communication. For instance, one can use it for incoherent and coherent attacks against a variety of cryptographic protocols, including the Bennett-Brassard 1984 protocol of quantum key distribution through the Pauli damping channels. It can be also applied as a state-dependent photon multiplier in practical quantum networks.