Jiepeng Zhang

All-optical switching at ultralow light levels

We report an experimental demonstration of all-optical switching at ultralow light levels in coherently prepared Rb atoms. A signal light pulse is switched on and off by a control light pulse at different frequencies in a four-level atomic system based on multiphoton interferences. We observed a switching efficiency of 55% with the signal and control light pulses containing approximately 20 photons each, corresponding to a control energy density of approximately 10(-5) photons per atomic cross section lambda(2)/(2pi).

Slow light with cavity electromagnetically induced transparency

We report an experimental observation of slow light propagation in cold Rb atoms exhibiting cavity electromagnetically induced transparency (EIT). The steep slope of the atomic dispersion manifested by EIT reduces the light group velocity. The cavity filtering and feedback further contribute to the slowdown and delay of the light pulse propagation. A combination of the cavity and the EIT atomic system significantly improves the performance of the slow light propagation. A propagation time delay of approximately 200 ns was observed in the cavity and Rb EIT system, which is approximately 70 times greater than the time delay calculated for the light pulse propagation through the same Rb EIT system without the cavity.

Copropagating superluminal and slow light manifested by electromagnetically assisted nonlinear optical processes.

We report an experimental observation of nonlinear optical gain and loss assisted by electromagnetically induced transparency (EIT) and simultaneous superluminal and subluminal light propagations. Two circular components of a linearly polarized light initiate third-order nonlinear processes in which one circular component is attenuated while the other component is amplified in a three-level EIT system. Near the atomic resonance, the attenuated circular component experiences steep normal dispersion and propagates with a slow group velocity, while the amplified circular component experiences steep anomalous dispersion and propagates with a superluminal group velocity.

Optical switching mediated by quantum interference of Raman transitions.

We report an experimental study of quantum interference between two-photon Raman transitions and demonstration of the phase control of light attenuation/transmission in cold Rb atoms. By varying the phase and frequency of a weak control laser, either constructive interference or destructive interference between the two-photon Raman transitions in a three-level Lambda system can be manipulated. The interference enables absorptive switching of one field by another field at low light levels.

Suppressing normal mode excitation by quantum interference in a cavity-atom system

Collective coupling of multiple atoms with a cavity mode produces two normal modes that are separated in energy by Vacuum Rabi splitting. We show that the normal mode excitation of the cavity-atom system can be suppressed by coupling a control laser to the atomic system from free space. The control laser splits the normal mode of the cavity-atoms system and opens two excitation channels. The destructive quantum interference between the two channels renders the cavity-atoms system opaque to the light coupled to the cavity-atom system. We demonstrate suppression of the normal mode excitation by the destructive quantum interference in an experiment using cold Rb atoms confined in an optical cavity.

Collective coupling of atoms with cavity mode and free-space field

We report an experimental study of the collective coupling of three-level atoms with a cavity mode and a free-space laser field. The measurement of the cavity transmission with a weak probe field coupled into the cavity mode reveals three spectral peaks: two sidebands and a central peak, which is produced by the coherent interaction of the free-space field with individual atoms and the collective interaction of the multiple atoms with the cavity mode. The experimental results agree with a simple calculation based on the classical light transmission through a cavity containing multiple atoms coherently driven by a free-space laser.

Backward four-wave mixing in a four-level medium with electromagnetically induced transparency

We analyze backward four-wave mixing in a four-level system exhibiting electromagnetically induced transparency (EIT). We show that EIT suppression of the linear absorption leads to the resonantly enhanced four-wave mixing process and the conversion efficiency near 100% may be achieved in a dense four-level medium with negligible decoherence decay.

Manipulating atomic coherence and interference in a coupled atom-cavity system

Collective coupling of multiple atoms with a cavity mode produces two normal modes that are separated in energy by Vacuum Rabi splitting. We show that quantum coherence and interference can be produced by a control laser that couples the atoms confined in the cavity mode from free space, which leads to suppression of the normal mode excitation, or polariton excitation of the cavity-atom system. The control laser splits the normal mode of the cavity-atoms system and opens two excitation channels. The destructive quantum interference between the two channels renders the cavity-atoms system opaque to the light coupled into the cavity mode. We demonstrate suppression of the normal mode (polariton) excitation by the destructive quantum interference in an experiment with cold Rb atoms confined in an optical cavity.

Atomic coherence and interference in a coupled atom–cavity system

When a coherently prepared atomic medium is confined in an optical cavity, the atomic coherence and interference manifested by electromagnetically induced transparency (EIT) can be manipulated and enhanced by the collective atom–cavity coupling and the cavity feedback. We present an experimental study of the light transmission through an optical cavity coupled with coherently prepared cold Rb atoms. In the frequency domain, the cavity transmission spectrum exhibits spectral peaks representing the cavity normal modes associated with the multi-atom vacuum Rabi splitting and the intra-cavity dark state manifested by the combined EIT effects of the absorption suppression and the steep normal dispersion. In the time domain, a light pulse coupled into the cavity mode propagates with a slow group velocity and the propagation time delay is significantly increased by the cavity feedback.

Vacuum Rabi Splitting and Intracavity Dark State in a Cavity-Atoms System

We report an experimental study of a cavity-atom composite system and observation of the intracavity dark state and the multi-atom vacuum Rabi splitting.