Physical Realization Of Multi-Purpose Quantum Cloning Machine

The linearity of quantum mechanics makes it impossible to clone an arbitrary quantum state accurately, but with the development of quantum information, the approximate cloning becomes increasingly important. For example, it is unneces-sary for some quantum operations to be implemented with absolute accuracy. People have obtained the rigorous mathematical optimal fidelities of 1â†'2 universal quan-tum cloning,1â†'2 phase-covariant quantum cloning and 1â†'M universal quantum cloning in theory and their implementations also been achieved in some physical systems.Burkard and Loss pointed out that in quantum dots formed by GaAs/AlGaAs the local spin can be labeled and the spin interaction can be controlled by the metal gate electrode. In this dissertation, we propose a scheme of quantum cloning based on quantum dot system. Because of the easy maneuverability of quantum dot system and its stability to the external environment, we use the metal gate electrode to control the excess electron in quantum dot. In the implementations, we carry out a Fredkin gate to realize the 1â†'2 universal, and phase-covariant quantum cloning machines. Especially, both types of the cloning can be achieved by using the same process so we call it multi-purpose quantum cloning machine. We only need to perform the specific operations at a particular time to obtain two cloning machine. By numerical calculation, we find that its fidelity can achieve the optimal value. The proposed scheme is feasible due to the long decoherence time and the nice scalability of the quantum dot system.