Advancing quantum information society
The ATESIT project focused on teleportation and entangled state information technology aiming to develop new methods for generation and exploitation of different types of two- and multi-particle entangled states. These could be further employed in quantum communication protocols. Furthermore, the project work concentrated on fundamental protocols such as universal quantum cloning, demonstrating their scalability. Researchers aimed to produce an operational universal optimum quantum cloning machine, but there were certain limitations. Due to the linearity aspect of quantum mechanics it was difficult to 'clone', that is to reproduce, an arbitrary quantum state perfectly. In addition, the realisation of a universal NOT gate that may flip any qubit into an orthogonal one, was unachievable. The cloning effect in quantum optics relates to the photon amplification process in an Optical Parametric Amplifier (OPA). Thereby, identical photons that are generated in an arbitrary quantum polarisation state are further injected into the amplifier on the input mode. The amplifier on the output 'cloning mode' provides 'clones' of the input qubit. On the output 'anticloning (AC) mode' the OPA realises a quantum NOT gate that performs the operation to flip a qubit. Based on this process, the Universal NOT (U-NOT) transformation, that is the performance of the best possible approximation of an anti-unitary operation using a universal quantum machine, was demonstrated. The U-NOT gate is highly associated with the quantum estimation of an unknown state. The system that was employed in the experimental demonstration was a quantum self-injected optical parametric amplifier (QI-OPA) of entangled photon states. The universal cloning method can be used in the design of new algorithms and protocols. Quantum cloning allows redistribution of the initial information content into many parts and forms the basis of quantum cryptography. Quantum cryptography bases its security on the impossibility to clone unknown quantum states and optimal quantum cloning is considered as the best eavesdropping attack on some quantum cryptography protocols.
