Quantum state can not be cloned perfectly. However, we can clone it as good as we can in quantum information processing, so far not yet a solid state quantum cloning machine is available. We present the first experimental realization of optimal phase quantum cloning machine in solid state system at room temperature. The involved three states are encoded physically in three levels of a single electron spin with two Zeeman sub-levels at a nitrogen-vacancy defect center in diamond. The preparation of input state and the phase quantum cloning transformation are controlled by two independent microwave fields. The average experimental fidelity reaches 85.2% which is the best experimental result and very close to theoretical optimal fidelity 85.4%.

    We discovered the controllable effects of quantum fluctuations on spin free-induction decay at room temperature. Fluctuations of local fields cause decoherence of quantum objects. Usually at high temperatures, thermal noises are much stronger than quantum fluctuations unless the thermal effects are suppressed by certain techniques such as spin echo. We present the discovery of strong quantum-fluctuation effects of nuclear spin baths on free-induction decay of single electron spins in solids at room temperature. The competition between the quantum and thermal fluctuations is controllable by an external magnetic field. These findings are based on Ramsey interference measurement of single nitrogen-vacancy center spins in diamond and numerical simulation of the decoherence, which are in excellent agreement.