Experimental asymmetric phase-covariant quantum cloning of polarization qubits

TL;DR

This paper demonstrates two optical methods for asymmetric phase-covariant quantum cloning of polarization qubits, achieving high fidelities surpassing universal cloning limits through interference and polarization filtering.

Contribution

Introduces two novel optical implementations of asymmetric phase-covariant cloning machines with tunable asymmetry and high fidelities.

Findings

High cloning fidelities above universal cloning limit

Effective control of cloning asymmetry using polarization filtering

Successful realization of two different optical cloning schemes

Abstract

We report on two optical realizations of the $1 \to 2$ asymmetric phase-covariant cloning machines for polarization states of single photons. The experimental setups combine two-photon interference and tunable polarization filtering that enables us to control the asymmetry of the cloners. The first scheme involves a special unbalanced bulk beam splitter exhibiting different splitting ratios for vertical and horizontal polarizations, respectively. The second implemented scheme consists of a balanced fiber coupler where photon bunching occurs, followed by a free-space part with polarization filters. With this later approach we were able to demonstrate very high cloning fidelities which are above the universal cloning limit.