Quantum Phase Gate Based On Cavity Quantum Electrodynamics

Quantum information science is developed through combination of quantum mechanics and information science, and is an interdisciplinary one which is mainly pertinent to preparation, transmission, storage, manipulation and reading of quantum states. Development of the cavity quantum electrodynamics (Cavity QED), concentrating on quantum computation, has reached a level that interaction process between atoms and light fields can be manipulated at will. This makes it possible that the evolution of atoms - optical field coupling system can be controlled purposely by changing the way of interaction among atoms. Therefore, cavity QED is regarded as the most promising in all the physical systems which are proposed for realization of quantum information processing and even quantum computers.This research work aims at the physical realization of quantum phase gate based on cavity QED systems. First of all, a basic model concerns the interaction between quantized light field and atoms, is given and the widely used approximation method - effective Hamiltonian method is discussed in the calculation of mismatch, by ommiting fast shocks items., And then two-bit quantum phase gate scheme is proposed in making use of atom - cavity mode resonance interaction and non-resonance interaction, respectively., Last but not the least, a controlled phase gate for multi-bit scheme is proposed by utilizing an atom as a fixed bit and a photon as a flying bit.In the resonance scheme, since the interaction between cavity mode and atoms is in a resonant state, the operation time of the phase gate is pretty short. Moreover, the resonant interaction between atoms and cavity field can occur without involvement of external field and this makes it easier to be implemented experimentally. Ground states in the phase gate pass through the same damping process, thus the phase gate is insensitive to atomic spontaneous emission and cavity photon leakage. Consequently, this scheme is of a high fidelity. In the non-resonance scheme, the two excited states of atoms can be adiabatically eliminated, so Raman coupling between two ground states occur in the gate operation, the two atoms are in the ground state and the cavity mode is only in weakly excited state. As the result, the gate operator is insensitive to the atomic spontaneous radiation, the cavity loss and cavity mode in the presence of thermal photons. In the controlled phase gate of the multi-bit scheme, the set of atomic states are used as control bits, input and output pulses of a single photon polarization state as controlled bits. When all the atoms are in f , the input and output pulses have a ? phase change. When the atoms in the other states, the input and output pulses do not change the relative phase. Numerical simulation shows that the atomic system is not restricted by the Lame-Dicke conditions, and the impact of atomic spontaneous emission on the gate operation in this scheme is negligible, so the program has high fidelity.