One of the most fundamental laws of nature, the so-called no-cloning theorem, states that an unknown quantum state cannot be perfectly copied. This fact has an imminent impact on quantum information processing. For instance, it allows designing inherently secure cryptographic protocols or assures the impossibility of superluminal communication. Although perfect quantum copying is impossible, one can still investigate how well such an operation can be approximated within the limits of laws of physics. The first design of an optimal cloning machine was suggested by Bužek and Hillery in 1996.
The cloner is called optimal when it gives the best results allowed by quantum mechanics. Moreover universal cloning (UC) should operate equally well for all possible input qubit states (e.g. polarization of a photon). In contrast, limiting the cloning operation to a specific subset of input qubit states, one can achieve a more precise cloning operation.
We successfully constructed a cloning device capable to work optimally in several cloning regimes just by changing the device parameters. We tested the cloner in three prominent regimes, see figure. We used tight time correlated photon pairs generated in a crystal of LiI03 and wave plates (WP) to prepare appropriate polarization state of signal and ancilla photon qubit. The cloning operation is realized by interference of the two photons on a polarization-dependent beam splitter (PDBS) followed by polarization state filtration in interferometers composed of two beam dividers (BD). Using methods of quantum tomography and state estimation we were able to precisely characterize the cloning operation for different a priori information about the cloned state. The most important result of our experiment is the verification that properly built linear-optical qubit cloner can operate close to theoretical limit in all three tested cloning regimes.
