High-dimensional quantum cloning and applications to quantum hacking

TL;DR

This paper demonstrates the first experimental realization of optimal cloning for high-dimensional quantum states using symmetrization, with applications to quantum cryptography security analysis.

Contribution

It introduces a universal method for high-dimensional quantum cloning and characterizes its performance through quantum state tomography.

Findings

Successful cloning of high-dimensional photonic states achieved.

Cloning machine characterized via quantum state tomography.

Cloning attack tested on BB84 protocol to assess security.

Abstract

Attempts at cloning a quantum system result in the introduction of imperfections in the state of the copies. This is a consequence of the no-cloning theorem, which is a fundamental law of quantum physics and the backbone of security for quantum communications. Although such perfect copies are prohibited, a quantum state may be copied with maximal accuracy via various optimal cloning schemes. Optimal quantum cloning, which lies at the border of the physical limit imposed by the no-signalling theorem and the Heisenberg uncertainty principle, has been experimentally realized for low dimensional photonic states. However, an increase in the dimensionality of quantum systems is greatly beneficial to quantum computation and communication protocols. Nonetheless, no experimental demonstration of optimal cloning machines has hitherto been shown for high-dimensional quantum systems. Here, we perform optimal cloning of high-dimensional photonic states by means of the symmetrization method. We show the universality of our technique by conducting cloning of numerous arbitrary input states, and fully characterize our cloning machine by performing quantum state tomography on \emph{cloned} photons. In addition, a cloning attack on a Bennett and Brassard (BB84) quantum key distribution protocol is experimentally demonstrated in order to reveal the robustness of high-dimensional states in quantum cryptography.