Yong Fan Chen

Coherent Optical Memory with High Storage Efficiency and Large Fractional Delay

A high-storage efficiency and long-lived quantum memory for photons is an essential component in long-distance quantum communication and optical quantum computation. Here, we report a 78% storage efficiency of light pulses in a cold atomic medium based on the effect of electromagnetically induced transparency. At 50% storage efficiency, we obtain a fractional delay of 74, which is the best up-to-date record. The classical fidelity of the recalled pulse is better than 90% and nearly independent of the storage time, as confirmed by the direct measurement of phase evolution of the output light pulse with a beat-note interferometer. Such excellent phase coherence between the stored and recalled light pulses suggests that the current result may be readily applied to single photon wave packets. Our work significantly advances the technology of electromagnetically induced transparency-based optical memory and may find practical applications in long-distance quantum communication and optical quantum computation.

Low-light-level photon switching by quantum interference

We report an experimental demonstration of low-light-level photon switching by quantum interference in laser-cooled 87Rb atoms. A resonant probe pulse with an energy per unit area of one photon per lambda2/2pi propagates through the optically thick atoms. Its energy transmittance is greater than 63%, or a loss of less than e(-1), because of the effect of electromagnetically induced transparency. In the presence of a switching pulse with an energy per unit area of 1.4 photons per lambda2/2pi, the energy transmittance of the same probe pulse becomes less than 37%, or e(-1). This substantial reduction of probe transmittance caused by switching photons may lead to potential applications in single-photon-level nonlinear optics and manipulation of quantum information.

Demonstration of the Interaction between Two Stopped Light Pulses

This study reports the first experimental demonstration that two light pulses were made motionless and interacted with each other through a medium. The scheme with motionless light pulses maximizes the interaction time and can achieve a considerable efficiency even below single-photon level. To demonstrate the enhancement of optical nonlinear efficiency, the experiment in this study used the process of one optical pulse switched by another based on the effect of electromagnetically induced transparency. Moving light pulses activate switching at an energy per area of 2 photons per atomic absorption cross section as discussed in [Phys. Rev. Lett. 82, 4611 (1999)]. This study demonstrates that motionless light pulses can activate switching at 0.56 photons per atomic absorption cross section, and that the light level can be further reduced by increasing the optical density of the medium. The result of this work enters a new regime of low-light physics.

Low-light-level all-optical switching

We propose and experimentally demonstrate an all-optical switch that utilizes the light-storage technique. The energy of the retrieved probe pulse is reduced to 10% due to the presence of the switching pulse with one photon per atomic cross section. A single photon switched by another is feasible in this scheme.

Large cross-phase modulations at the few-photon level

We demonstrate an efficient cross-phase modulation (XPM) based on a closed-loop double-Λ system. The property of the double-Λ medium can be controlled by changing the phases of the applied optical fields. This phase-dependent XPM scheme can achieve large phase modulations at low-light intensities without requiring cavities or tightly focusing laser beams. With this scheme, we observe a π-level phase shift with two pulses, both consisting of eight photons in cold rubidium atoms. Such a novel scheme provides a simple route to generate strong interactions between photons and may have potential applications in all-optical quantum signal processing.

Low-light-level four-wave mixing by quantum interference

We observed electromagnetically induced transparency-based four-wave mixing (FWM) in the pulsed regime at low light levels. The FWM conversion efficiency of

Manipulating the retrieved frequency and polarization of stored light pulses

3.8(9)%

Enhancement of exchange coupling between GaMnAs and IrMn with self-organized Mn(Ga)As at the interface

was observed in a four-level system of cold

Coherence properties of amplified slow light by four-wave mixing.

{}^{87}\mathrm{Rb}

Effect of nickel addition on microstructure and properties of Ti-Co-Ni alloys

atoms using a driving laser pulse with a peak intensity of