Quantum Information Processes In Cavity QED And Opto-mechanical Systems

Quantum information science has attracted a lot of attentions since the invention of Shor’s algorithm. The thesis focuses on realizing the quantum information processes and optical devices in optical cavity system, such as cavity QED systems and opto-mechanical systems. Here the quantum information processes include the quantum state transfer, quantum logic gates and generating quantum entangled states.In the first part of the thesis, schemes to realize quantum information processes in cavity QED systems are discussed. We consider the systems containing two remote cavities, which are connected by an optical fiber. (Multi) Atoms are trapped in the cavities and couple with the cavity modes, which resonantly couple with the fiber mode. If the atoms resonantly and collectively interact with the local cavity fields but there is no direct interaction be-tween the atoms, we show that an ideal quantum state transfer and highly reliable quantum swap, entangling, and controlled-Z gates can be deterministically realized between the dis-tant cavities. We find that the operation quantum information processes can be greatly speeded up, and the effects of spontaneous emission of atoms and photon leakage out of cavity can also be greatly diminished as number of the atoms in the cavities increases. We also show that an effective squeezing reservoir can be engineered in the system under appro-priate conditions. Then we show that a two-qubit geometric CPHASE gate and entangling gate between the atoms in the two cavities can be implemented through adiabatically ma-nipulating the engineered reservoir. This scheme that combines engineering environment with decoherence-free space and geometric phase quantum computation together has the remarkable features:a CPHASE gate with arbitrary phase shift is implemented by simply changing the strength and relative phase of the driving fields, and entangling gate can generate stable extremely entangled two-qubit state without measurement.In the second part of the thesis, non-classical light source and cooling scheme in opto- mechanical systems are discussed. We propose a scheme to produce continuous variable en-tanglement between phase-quadrature amplitudes of two light modes in an opto-mechanical system. For proper driving power and detuning, the entanglement is insensitive with bath temperature and Q of the mechanical oscillator. Under realistic experimental conditions, we find that the entanglement could be very large even at room temperature. The noise from laser phase fluctuation sets a major technical obstacle to cool the nano-mechanical oscillators to the quantum region. We propose a cooling configuration based on the opto-mechanical coupling with two cavity modes to significantly reduce this phase noise. After optimization of the cavity parameters, we show through simple arguments that the intrinsic cooling limit of the opto-mechanical oscillator is set by Tenv/Q, where Tenv is the environ-ment temperature and Q is the mechanical quality factor. We also discuss detection of the phonon number when the mechanical oscillator is cooled near the quantum region and specify the required conditions for this detection.