Optimal universal and phase-covariant quantum cloning machines with quantum-dot spins in cavity QED

Schemes for implementing a 1 -> 2 optimal universal quantum cloning machine (UQCM) and a 1 -> 3 ancilla-free optimal phase-covariant quantum cloning machine (PCCM) are proposed by manipulating the transverse spin-spin interactions of quantum dots mediated by a vacuum single-mode cavity. The interactions among different quantum dots are selectively controlled by the classical laser fields. These quantum cloning machines are more robust due to the very long spin decoherence time of quantum dots. Furthermore, the experimental implementation is simplified in comparison with those using atoms as qubits. The quantum dots are embedded in the cavity simultaneously; hence the troubles caused by the qubits passing through multiple cavities are effectively avoided.