Chao Hang

Controllable entanglement of lights in a five-level system

Abstract We analyze the nonlinear optical response of a five-level system under a novel configuration of electro-magnetically induced transparency. We show that a giant Kerr nonlinearity with a relatively large cross-phase modulation coefficient that occurs in such system may be used to produce an efficient photon–photon entanglement. We demonstrate that such photon–photon entanglement is practically controllable and hence facilitates promising applications in quantum information and computation.

Three-way entanglement and three-qubit phase gate based on a coherent six-level atomic system

We analyze the nonlinear optical response of a six-level atomic system under a configuration of electromagnetically induced transparency. We show that the enhanced completely cross fifth-order nonlinearity generated in such a system can be used to produce efficient three-way entanglement and to realize a three-qubit quantum phase gate. We demonstrate that such phase gate can be transferred to an all-optical Toffoli gate, facilitating practical applications in quantum information and computation.

Weak-light ultraslow vector solitons via electromagnetically induced transparency

We propose a scheme to generate temporal vector optical solitons in a lifetime broadened five-state atomic medium via electromagnetically induced transparency. We show that this scheme, which is fundamentally different from the passive one by using optical fibers, is capable of achieving distortion-free vector optical solitons with ultraslow propagating velocity under very weak drive conditions. We demonstrate both analytically and numerically that it is easy to realize Manakov temporal vector solitons by actively manipulating the dispersion and self- and cross-phase modulation effects of the system.

Weak-light superluminal vector solitons in a room-temperature four-level active-Raman-gain medium

We propose a scheme to generate vector optical solitons in a four-level active-Raman-gain medium. We show that this scheme, which is fundamentally different from that using optical fibers and that via electromagnetically induced transparency, is capable of achieving robust vector optical solitons with superluminal propagating velocity. We demonstrate that such vector optical solitons can be generated at room temperature and with very low light intensity. We investigate also the interaction between two superluminal vector optical solitons and point out that their elastic and inelastic colliding properties can be used to design rapidly responding optical soliton switching and logic gates.

Quantum random walks in a coherent atomic system via electromagnetically induced transparency

We propose a scheme to realize the quantum random walk in a coherent five-level atomic system via electromagnetically induced transparency (EIT). From optical Bloch equations describing the dynamics of the electromagnetic field and atomic population and coherence, we show that two circular-polarized components of a probe field display different dispersion properties and hence acquire different phase-shift modifications when passing through atomic cells. We demonstrate that the quantum coherence and interference owing to the EIT effect result in a low absorption of the probe field and hence provide a possibility of realizing a many-step phase-shift quantum random walk. The scheme may be used to experimentally highlight the characteristics of quantum random walk and lead to a promising application for quantum computation.