Chao Zhuang

Experimental Demonstration of the Effectiveness of Electromagnetically Induced Transparency for Enhancing Cross-Phase Modulation in the Short-Pulse Regime.

We present an experiment using a sample of laser-cooled Rb atoms to show that cross-phase modulation schemes continue to benefit from electromagnetically induced transparency (EIT) even as the transparency window is made narrower than the signal bandwidth (i.e., for signal pulses much shorter than the response time of the EIT system). Addressing concerns that narrow EIT windows might not prove useful for such applications, we show that while the peak phase shift saturates in this regime, it does not drop, and the time-integrated effect continues to scale inversely with EIT window width. This integrated phase shift is an important figure of merit for tasks such as the detection of single-photon-induced cross-phase shifts. Only when the window width approaches the systems dephasing rate γ does the peak phase shift begin to decrease, leading to an integrated phase shift that peaks when the window width is equal to 4γ.We present an experiment using a sample of laser-cooled Rb atoms to show that cross-phase modulation schemes continue to benefit from electromagnetically-induced transparency (EIT) even as the transparency window is made narrower than the signal bandwidth (i.e., for signal pulses much shorter than the response time of the EIT system). Addressing concerns that narrow EIT windows might not prove useful for such applications, we show that while the peak phase shift saturates in this regime, it does not drop, and the time-integrated effect continues to scale inversely with EIT window width. This integrated phase shift is an important figure of merit for tasks such as the detection of single-photon-induced cross phase shifts.

Coherent control of population transfer between vibrational states in an optical lattice via two-path quantum interference.

We demonstrate coherent control of population transfer between vibrational states in an optical lattice by using interference between a one-phonon transition at 2ω and a two-phonon transition at ω. The ω and 2ω transitions are driven by phase- and amplitude-modulation of the lattice laser beams, respectively. By varying the relative phase of these two pathways, we control the branching ratio between transitions to the first excited state and those to the higher states. Our best result shows a branching ratio of 17±2, which is the highest among coherent control experiments using analogous schemes. Such quantum control techniques may find broad application in suppressing leakage errors in a variety of quantum information architectures.

Coherence Freeze in an Optical Lattice Investigated Via Pump-Probe Spectroscopy

Motivated by our observation of fast echo decay and a surprising coherence freeze, we have developed a pump-probe spectroscopy technique for vibrational states of 85Rb atoms in an optical lattice to gain information about the memory dynamics of the system. We monitor the time-dependent changes of frequencies experienced by atoms and characterize the probability distribution of these frequency trajectories. We show that the inferred distribution, unlike a naive microscopic model of the lattice, correctly predicts the main features of the observed echo decay.

Persistent Current of Bi2223/Ag Closed Coil in 77 K

Closed superconducting coils made from Bi2223/Ag multi-filamentary tapes were successfully fabricated. By measuring the magnetic flux density at the center of the coil, we were able to observe the persistent current mode of superconducting closed coil in liquid nitrogen. The persistent current measured was 1.39 A and remained unchanging for more than 500 hours after several hours decay. It was estimated from the decay curve that the upper limit resistance of the closed-loop coil was in the order 10-13 Omega of . Current-voltage characteristic was derived form the decay curve and different models were applied in fitting the experiment data..

AC analysis of a high temperature superconductor coil with short circuits

The dynamic property of a high temperature superconductor (HTS) coil which possesses short circuits between windings was investigated by magnetic measurement. Based on a general lock-in technique, the net magnetomotive force (mmf) provided by a short circuit coil carrying ac current is given in two parameters: the imaginary part of the mmf accounting for the power dissipation in the coil, and the real part of the mmf relating to the flux and attractive force in the magnetic circuit. These two terms are given in a broad range of current amplitude A and frequency f, which cover the main operational region for typical controllability requirements and other dynamic applications. An equivalent circuit model based on the assumption that the short circuits are formed by a group of closed circuits located coaxially with the coil was proposed. The parameters of the circuit are determined by fitting the experimental results in some specific cases. The feasibility and efficiency of the equivalent circuit concerning its applications at low and high frequency are discussed respectively.

Observing the Onset of Effective Mass

The response of a particle in a periodic potential to an applied force is commonly described by an effective mass, which accounts for the detailed interaction between the particle and the surrounding potential. Using a Bose-Einstein condensate of (87)Rb atoms initially in the ground band of an optical lattice, we experimentally show that the initial response of a particle to an applied force is in fact characterized by the bare mass. Subsequently, the particle response undergoes rapid oscillations and only over time scales that are long compared to those of the interband dynamics is the effective mass observed to be an appropriate description. Our results elucidate the role of the effective mass on short time scales, which is relevant for example in the interaction of few-cycle laser pulses with dielectric and semiconductor materials.

Observing the onset of effective mass of a Bose-Einstein condensate in an optical lattice

Here, we report on the first experimental observation of effective mass dynamics using a Bose-Einstein condensate of Rubidium-87 atoms initially in the ground band of an optical lattice. By studying the response of the particles to an abruptly applied force, we show that, while the long-time behaviour is described by the effective mass, the initial response is indifferent to the presence of the lattice. Our results represent an important contribution towards a full understanding of the ultrafast response of electrons in a solid, relevant for example in the interaction of few-cycle laser pulses with dielectric and semiconductor materials.

Weak-value amplification of low-light-level cross phase modulation

We report on our experimental progress towards observing weak-value amplification of low-light-level cross-phase modulation which will be the first observation of a weak measurement relying on true entanglement between distinct systems.

Increasing the giant Kerr effect by narrowing the EIT window beyond the signal bandwidth

We experimentally show that giant EIT-based Kerr nonlinearities may make use of EIT windows narrower than the signal bandwidth, allowing for experiments with short signal pulses to benefit from this enhancement, e.g. for QND measurements.

Increasing The Giant Kerr Effect By Narrowing The EIT Window Beyond The Signal Bandwidth

We experimentally show that giant EIT-based Kerr nonlinearities continue to benefit from narrowing the EIT window even as the signal bandwidth comes to exceed this transparency width