Quantum Computing Based On The Cavity Quantum Electronic Dynamics

Based on the principle of coherent superposition and quantum entanglement, quantum computation can solve certain problems much faster than on a conventional computer and thus is a very promising new research field. This matter of fact has triggered a lot of studies on the theoretical and practical aspects of quantum computing in the past years. Different physical systems, including cold trapped ions, cavity quantum electrodynamics (cavity QED), nuclear magnetic resonance, quantum dots, Josenphson junctions in superconducting circuits, and linear optics etc, have been proposed to implement quantum information. Among them, cavity QED, where atoms are strongly coupled to quantized electromagnetic fields inside a high-Q cavity, offers an almost ideal system for the implementation of distributed quantum computing.By virtue of the technique of the Stimulated Raman Adiabatic Passage (STIRAP) and based on the cavity QED, this thesis is mainly focused on presenting some proposals to implement nonlocal interaction between spatially distant quantum network nodes for distributed quantum compution.1. We present a scheme to prepare atomic cluster states. All atoms, except for the flying qubit—atom N, are confined in spatially separated cavities and atom N passes through all the cavities in sequence.In the scheme, under the conditions that all the Rabi frequencies of the cavities'mode are much larger than that of the laser fields, the population of the cavites being excited can be negligible. In addition, atomic transitions are largely detuned with cavity modes. So the decays of the cavities and the atomic spontaneous emissions can be effectively avoided.In order to calculate the influence of cavity decay, we solve exactly the system dynamics through numerical simulations. The numerical simulation shows that even in case that the decay rate equals the effective Rabi frequency, the fidelity of two-atom cluster states approximate to 0.98 under the condition that the ratio between the Rabi frequency of the cavity mode and the maximum Rabi frequency of the laser field is 10.2. We present a scheme to transfer three-dimensional quantum states between two atoms trapped in distant cavities connected by an optical fibre, whose modes are resonant with the cavity modes. The detuning of atom-cavity coupling can be any value.Performing an adiabatic passage along dark states, the fibre modes remain in the vacuum state and the atoms are always in the ground state. Under certain conditions the population of the cavites being excited can be negligible. So the decoherence due to the the decays of fibre and cavites and the atomic spontaneous emission can be effectively avoided.By virtue of quantum jump method, we discuss the effect of atomic spontaneous emissions and photon decay with some probability and some fidelity. The numerical results show:(1) the fiber decay rate has little influence on them; (2) the atomic spontaneous emissions and the cavity decay have similar effect on them in the resonant case; (3) in the large detuning case, the atomic spontaneous emission almost has no effect on them, and the main source of decoherence comes from the cavity decay; (4) under the same dissipation conditions, the successful probability and the transferring fidelity are smaller in the large detuning case than those in the resonant case.3. A scheme is proposed to implement distributed quantum computation in decoherence-free subspaces (DFSs) via adiabatic passage. The logical single-qubit is encoded in two atoms trapped in a single-mode cavity connected by an optical fiber.Our scheme is immune from the decoherence due to dephasing in virtue of encoding scheme and to spontaneous emission from excited states as the system in our scheme evolves along a dark state. Furthermore, the decoherence due to photon decay is greatly suppressed since the fiber mode remains in a vacuum state and the populations of the cavities'modes being excited can be negligible under certain condition.It is shown that the minimum fidelity of the resultant gate operation for an arbitrary input state could be over 0.97.4. We present a scheme for an arbitrary state controlled unitary gate between two nonlocal qubits, which makes the resultant gate suitable for distributed quantum computation. Two remote atomic qubits are separately trapped in two distant cavities connected by an optical fiber.Based on adiabatic passage, our scheme is immune to the decoherence due to spontaneous emission and to photon decay from the cavity modes and the fiber mode. It is shown that the minimum fidelity of the resultant gate operation for an arbitrary input state could be over 0.98.