Selective excitation,coherent control,and attosecond spectrochronography of electron subshells in atomic systems

Optical field waveforms with an ultrabroad spectrum and a tailored phase are shown to enable selective excitation, coherent control, and attosecond spectrochronography of electron subshells in many-electron atomic systems. Analysis of the evolution of the density matrix of electron subshells in an atomic system driven by an ultrashort light pulse shows that the interference of different quantum pathways of electron dynamics plays a key role in the buildup of the nonlinear-optical response of such a system. Our analysis suggests a method whereby the attosecond dynamics of individual electron subshells in atoms can be coherently controlled with ultrashort laser pulses.     

Temporal coherent control of superfluorescent pulses

We demonstrate that collective atomic interferences can be investigated by measuring the superfluorescence (SF) time delay. A pair of broadband (≈20 nm), ultrashort (≈80 fs), collinear pulses with a variable delay coherently excites rubidium (Rb) atoms. The generated superfluorescent pulses at 420 nm on the cascade transition are recorded by a picosecond streak camera. Both intensity and SF time delay of the 420 nm pulse are altered as the delay between input pulses varies. In particular, the SF time delay of the normalized 420 nm pulse exhibits oscillations with different periods. This can be understood in terms of atomic and quantum interferences due to two possible two-photon excitation pathways through the intermediate levels (Rb D-lines).     

Manipulating light pulses via dynamically controlled photonic band gap

When a resonance associated with electromagnetically induced transparency in an atomic ensemble is modulated by an off-resonant standing light wave, a band of frequencies can appear for which light propagation is forbidden. We show that dynamic control of such a band gap can be used to coherently convert a propagating light pulse into a stationary excitation with nonvanishing photonic component. This can be accomplished with high efficiency and negligible noise even at the level of few-photon quantum fields thereby facilitating possible applications in quantum nonlinear optics and quantum information.     

Deterministic and storable single-photon source based on a quantum memory

A single-photon source is realized with a cold atomic ensemble (87Rb atoms). A single excitation, written in an atomic quantum memory by Raman scattering of a laser pulse, is retrieved deterministically as a single photon at a predetermined time. It is shown that the production rate of single photons can be enhanced considerably by a feedback circuit while the single-photon quality is conserved. Such a single-photon source is well suited for future large-scale realization of quantum communication and linear optical quantum computation.     

Test of the all-optical control of wave-particle duality of cavity photons by ordinary photodetection

The principle of complementarity refers to the ability of quantum entities to behave as particles or waves under different experimental conditions. We present a proposal for the experimental observation of the ultrafast all-optical control of the wave-particle duality of light. The device is constituted by a three-level quantum emitter strongly coupled to a microcavity (MC) and can be realized by exploiting a great variety of systems, ranging from atomic physics and semiconductor quantum dots to intersubband polaritons and Cooper pair boxes. The wavelike or particlelike behavior of MC photons can be probed by simply measuring the cavity output photon rate after excitation with pairs of phase-locked weak pulses with precise arrival times.     

Coherent control of stationary light pulses

We present a detailed analysis of the recently demonstrated technique to generate quasi-stationary pulses of light [M. Bajcsy, A.S. Zibrov, M.D. Lukin, Nature (London) 426 (2003) 638] based on electromagnetically induced transparency. We show that the use of counter-propagating control fields to retrieve a light pulse, previously stored in a collective atomic Raman excitation, leads to quasi-stationary light field that undergoes a slow diffusive spread. The underlying physics of this process is identified as pulse matching of probe and control fields. We then show that spatially modulated control-field amplitudes allow us to coherently manipulate and compress the spatial shape of the stationary light pulse. These techniques can provide valuable tools for quantum nonlinear optics and quantum information processing.     

Population transfer between two quantum states by piecewise chirping of femtosecond pulses: theory and experiment

We propose and experimentally demonstrate the method of population transfer by piecewise adiabatic passage between two quantum states. Coherent excitation of a two-level system with a train of ultrashort laser pulses is shown to reproduce the effect of an adiabatic passage, conventionally achieved with a single frequency-chirped pulse. By properly adjusting the amplitudes and phases of the pulses in the excitation pulse train, we achieve complete and robust population transfer to the target state. The piecewise nature of the process suggests a possibility for the selective population transfer in complex quantum systems.     

Quantum-control spectroscopy with exact state selectivity

A method of exact state-selective spectroscopy is introduced, based on quantum control through four specific short laser pulses. The exact conditions for the two pairs of ultrafast pulses are set by the feedback control for selective excitation to one specific resonance state while the other state is destructively interfered as the shadow pair, leading to a state-selective spectrum.     

Optimum conditions for superradiant damping

The conditions are investigated under which cooperative or superradiant effects are greatest in an inhomogeneously broadened atomic system that is excited by a coherent light pulse. The coupled non-linear atomic equations of motion are solved numerically for excitation pulses of various areas. It is shown that the absolute intensity of the response decreases strongly with decreasing pulse are below π, but that the relative superradiant contribution increases with decreasing pulse area. The reasons for this are discussed, and it is suggested that excitation by a pulse in the neighborhood of π/2 may represent an optimum compromise for the observation of superradiance.     

A quantum computer in the scheme of an atomic quantum transistor with logical encoding of qubits

A scheme of a multiqubit quantum computer on atomic ensembles using a quantum transistor implementing two qubit gates is proposed. We demonstrate how multiatomic ensembles permit one to work with a large number of qubits that are represented in a logical encoding in which each qubit is recorded on a superposition of single-particle states of two atomic ensembles. The access to qubits is implemented by appropriate phasing of quantum states of each of atomic ensembles. An atomic quantum transistor is proposed for use when executing two qubit operations. The quantum transistor effect appears when an excitation quantum is exchanged between two multiatomic ensembles located in two closely positioned QED cavities connected with each other by a gate atom. The dynamics of quantum transfer between atomic ensembles can be different depending on one of two states of the gate atom. Using the possibilities of control for of state of the gate atom, we show the possibility of quantum control for the state of atomic ensembles and, based on this, implementation of basic single and two qubit gates. Possible implementation schemes for a quantum computer on an atomic quantum transistor and their advantages in practical implementation are discussed.     

Selective preparation of the maximum coherent superposition state in four-level atoms

We demonstrate that the maximum coherent superposition state can be selectively prepared using a sequence of pulse pairs in lambda-type atomic systems,with the final level as a doublet.In each pair,the Stocks pulse comes before the pump pulse,with their back edges overlapping.Numerical results indicate that by tuning the interval of the adjacent pulse pairs,the selective maximum coherent superposition state preparation between the initial and one of the final levels can be achieved.The phenomenon is caused by the accumulative property of the pulse sequence.     

A Noise-Robust Pulse for Excitation Transfer in a Multi-Mode Quantum Memory

Multi-mode quantum memory is a basic element required for long-distance quantum communication,as well as scalable quantum computation.For on-demand readout and long storage times,control pulses are crucial in order to transfer atomic excitations back and forth into spin excitations.Here,we introduce noise-robust composite pulse sequences for high-fidelity excitation transfer in multi-mode quantum memory.These pulses are robust to the deviations in amplitude and the detuning parameters of realistic conditions.We show the efficiency of these composite pulses with a typical rare-earth ion-doped system.This approach could be applied to a variety of quantum memory schemes.     

Multichannel selective femtosecond coherent control based on symmetry properties

We present and implement a new scheme for extended multichannel selective femtosecond coherent control based on symmetry properties of the excitation channels. Here, an atomic nonresonant two-photon absorption channel is coherently incorporated in a resonance-mediated (2+1) three-photon absorption channel. By proper pulse shaping, utilizing the invariance of the two-photon absorption to specific phase transformations of the pulse, the three-photon absorption is tuned independently over an order-of-magnitude yield range for any possible two-photon absorption yield. Noticeable is a set of "two-photon dark pulses" inducing widely tunable three-photon absorption.     

飞秒激光脉宽对非绝热耦合分子波包运动的影响研究

利用含时波包法研究了强飞秒泵浦-探测激光场中激光脉宽对非绝热耦合NaI分子波包运动的影响.发现波包的振荡周期随脉宽增长而增大,而振荡幅度随脉宽增长而减小.非绝热效应引起的波包在交叉区域的分裂情况影响各态布居.脉宽增长,NaI分子的激发概率增大,而解离概率减小.研究表明调节激光场脉宽可实现对波包运动的控制从而控制态布居的选择性分布.研究结果可以为实验上实现分子的光控制以及量子调控过程提供一定的参考.     

Quantum interference in atomic vapor observed by four-wave mixing with incoherent light

Pairs of pulses from an incoherent source are used to investigate the time-resolved four-wave mixing response of atomic rubidium when a two-photon resonance is involved in the nonlinear process. By varying the relative polarization of the pulse pairs, we are able to select the quantum pathways and clearly distinguish optical and quantum interferences.     

二阶量子路径的相干控制

研究了有_N个中间能级连接初态和目标态的情况下量子路径的控制策略．以_N=3的量子体系为例,详细阐述了针对弱的宽频场的4_N分块控制方案,其中分块的边界频率依赖于量子体系的共振频率．此策略利用了路径幅度中共振和非共振项的相干效应,可通过只调节2_N个相位变量实现．     

Enhancement of coherent population transfer in a double Λ-type four-level system via a train of pulse pairs

We propose a flexible way to significantly enhance population transfer efficiency with a train of time-separated pump-Stokes pulse pairs when the non-adiabatic coupling between two degenerate adiabatic states exists in a double Λ-type four-level system, where the pump and Stokes pulses in each pair can be applied in either counterintuitive or intuitive order. It is shown that the needed Rabi frequency for achieving complete population transfer can be reduced dramatically with the increase of number of pump-Stokes pulse pairs, which results from temporal constructive quantum interference between the sequential transitions and subsequent coherent accumulation; moreover, an arbitrary coherent superposition between the two lower states can be realized by suitably tuning the Rabi frequency and the time delay between each pump-Stokes pair. The method may find applications in control of chemical reactions, quantum optics, and quantum information processing.     

Spectra induced by interactions of atoms with ultrashort electromagnetic pulses

An ultrashort pulse of an electromagnetic field incident on an atom shakes the atom and gives rise to various electron transitions in it. These processes are accompanied by the reemission of the incident ultrashort pulse. This paper studies the relation between the spectra of reemitted photons of an ultrashort pulse and the transitions of atomic electrons into particular states. The obtained partial reemission spectra can allow one to relate direction patterns to the probability of the excitation of an atom into different states.     

Quantum State Transfer Between Any Pair of Qubits in a Quantum Network via Optical Fibers

We propose scheme for transferring quantum state between any pair of nodes in a quantum network. Each node consists of an atom and a cavity, with the atom acting as the quantum bit. Any two adjacent nodes are connected by an optical fiber. During the operation neither the atomic system nor the fibers are excited, which is important in view of decoherence. Under certain conditions, the probability that the cavities are excited is negligible. The method has an inherent robustness against the fluctuation perturbations in the classical control parameters and the randomness in the atomic position. The scheme can be generalized to implement quantum phase gate between any two remote qubits.     

Pulsed and monochromatic excitation of semiconductor quantum wells under the conditions of quantum confinement

The reflectance and absorbance of light by quantum wells whose width is comparable to the light wavelength have been calculated. The difference in the refractive indices of the materials of the quantum well and the barriers has been taken into account. Pulsed irradiation with an arbitrary shape of the exciting pulse has been considered, and the existence of two closely spaced discrete excitation levels has been assumed. This pair of levels can correspond to two magnetopolaron states in a quantizing magnetic field directed perpendicular to the plane of the quantum well. The ratio between the magnitudes of nonradiative and radiative dampings of electronic excitations is arbitrary. The final results have been obtained without invoking the approximation in which the Coulomb interaction of electrons and holes is negligible.