Complete Population Transfer Research Articles

Ultrafast coherent population transfer in two- and three-level quantum systems using sub-cycle and single-cycle pulses

The interactions of sub-cycle and single-cycle pulses with two- or three-level quantum systems are studied, respectively. For the two-level quantum system, two cases in which the carrier frequency of pulses is in resonance and far from resonance with the atom are analysed. The ultrafast complete population transfer can be obtained. The sub-cycle pulse with a far off-resonant carrier frequency is found to be more suitable for the population transfer. The relation between the area of pulses and population transfer is clarified in the sub-cycle and single-cycle domain. For the Lambda-type three-level quantum system, more than 90% of population transfer can be achieved from one level to another. The scheme is insensitive to the variation of the laser parameters such as the peak Rabi frequency and the carrier frequency for both two- and three-level quantum systems.

Tunneling manipulation of superfluid Fermi gas in the adiabatic condition

In the adiabatic limit, we study Rosen–Zener transition of superfluid Fermi gas in a double well potential. We find that s-wave scattering length of the system can affect the quantum transition dramatically. At certain regime the complete population transfer between two wells is observed. On the contrary, the quantum transition could be completely blocked in other regime. We have derived analytical expressions for the transition probability. We also find that, in the adiabatic limit, the population difference between two modes always oscillates around its corresponding fixed point during Rosen–Zener transition process. The difference between population imbalance and the corresponding fixed point decreases with the adiabatic condition becomes stronger. The numerical simulation show a power law relation between them. It is verified by the Rosen–Zener transition for superfluid Fermi gas in the limited cases of both deep Bose-Einstein condensate regime and unitarity regime.

N–H Stretching Vibrations of Guanosine–Cytidine Base Pairs in Solution: Ultrafast Dynamics, Couplings, and Line Shapes

Dynamics and couplings of N-H stretching vibrations of chemically modified guanosine-cytidine (G·C) base pairs in chloroform are investigated with linear infrared spectroscopy and ultrafast two-dimensional infrared (2D-IR) spectroscopy. Comparison of G·C absorption spectra before and after H/D exchange reveals significant N-H stretching absorption in the region from 2500 up to 3300 cm(-1). Both of the local stretching modes ν(C)(NH(2))(b) of the hydrogen-bonded N-H moiety of the cytidine NH(2) group and ν(G)(NH) of the guanosine N-H group contribute to this broad absorption band. Its complex line shape is attributed to Fermi resonances of the N-H stretching modes with combination and overtones of fingerprint vibrations and anharmonic couplings to low-frequency modes. Cross-peaks in the nonlinear 2D spectra between the 3491 cm(-1) free N-H oscillator band and the bands centered at 3145 and 3303 cm(-1) imply N-H···O═C hydrogen bond character for both of these transitions. Time evolution illustrates that the 3303 cm(-1) band is composed of a nearly homogeneous band absorbing at 3301 cm(-1), ascribed to ν(G)(NH(2))(b), and a broad inhomogeneous band peaking at 3380 cm(-1) with mainly guanosine carbonyl overtone character. Kinetics and signal strengths indicate a <0.2 ps virtually complete population transfer from the excited ν(G)(NH(2))(b) mode to the ν(G)(NH) mode at 3145 cm(-1), suggesting lifetime broadening as the dominant source for the homogeneous line shape of the 3301 cm(-1) transition. For the 3145 cm(-1) band, a 0.3 ps population lifetime was obtained.

Realization of population inversion under nonadiabatic conditions induced by the coupling between vibrational modes via Raman fields

AbstractWe study the effects of coupling between molecular vibrational modes attendant the excitation of Raman transitions. Raman active vibrational modes are described by two two‐level systems (TLSs) with nondegenerate ground states. The population inversion is investigated under the condition of zero relative phase between initially populated states. We use two ultrafast laser pulses for the pump and the Stokes fields that have chirps the same in the magnitude and opposite in the sign. Within this model, the coupling induces nonadiabatic effects leading to a diverse population distribution. By introducing the time delay between the Stokes and the pump pulse, we realized a complete population transfer in both coupled TLSs. We demonstrated the population inversion in a more generic case of the averaged initial relative phase. © 2012 Wiley Periodicals, Inc.

Selective and efficient control of population transfer in the presence of an equally populated initial doublet

We present a way to realize selective, robust, and efficient population transfer in a ladder-type four-level system with an equally populated doublet in the initial state. By merging stimulated Raman adiabatic passage and chirped adiabatic passage techniques, we show that selective and complete population transfer from either of the initial doublet levels to the final state can be achieved with chirped pump and Stokes pulse pairs when the spectral resolution constraint of a frequency swept pulse holds (the energy separation of doublet levels should be much larger than the pulse bandwidth). We further demonstrate that the spectral resolution of selective population transfer can be enhanced significantly by using pump pulse pair and Stokes pulse due to temporal quantum interference between the sequential transitions in the time domain, which can also be well followed as a result of the frequency-domain formation of a comblike structure of the pulse pair spectrum. The proposals take into account the case of population transfer in realistic atomic and molecular systems, and may have applications in quantum coherent control of systems with equally populated initial manifolds.

Gaussian and sinc-shaped few-cycle-pulse-driven ultrafast coherent population transfer inΛ-like atomic systems

We report and propose a simple scheme to achieve efficient and fast coherent population transfer (CPT) by utilizing either a nonlinearly chirped Gaussian shaped few-cycle laser pulse or an unchirped sinc-shaped few-cycle laser pulse. The proposed scheme is shown to be fairly robust against the variation of the laser parameters such as temporal pulse width, chirp rates, carrier envelope phases and Rabi frequencies. We find that compared to the so-called stimulated Raman adiabatic passage (STIRAP) technique, our scheme for complete population transfer with few-cycle Gaussian shaped laser pulses requires less pulse area.

Complete population transfer in a three-state quantum system by a train of pairs of coincident pulses

A technique for complete population transfer between the two end states $|1\ensuremath{\rangle}$ and $|3\ensuremath{\rangle}$ of a three-state quantum system with a train of $N$ pairs of resonant and coincident pump and Stokes pulses is introduced. A simple analytic formula is derived for the ratios of the pulse amplitudes in each pair for which the maximum transient population ${P}_{2}(t)$ of the middle state $|2\ensuremath{\rangle}$ is minimized, ${P}_{2}^{\mathrm{max}}={\mathrm{sin}}^{2}(\ensuremath{\pi}/4N)$. It is remarkable that, even though the pulses are on exact resonance, ${P}_{2}(t)$ is damped to negligibly small values even for a small number of pulse pairs. The population dynamics resembles generalized $\ensuremath{\pi}$ pulses for small $N$ and stimulated Raman adiabatic passage for large $N$ and therefore this technique can be viewed as a bridge between these well-known techniques.

Dynamic transition from complete population transfer to self-induced transparency

We explore the connection between adiabatic rapid passage, a technique that allows for robust complete population transfer (CPT) in a resonant absorber, and self-induced transparency solitons. We employ the auxiliary differential equation finite-difference time-domain method to fully resolve the dynamics of femtosecond light pulses propagating through an absorbing two-level medium. Using the example of linearly chirped Gaussian pulses we find that the pulses achieving robust CPT for a single two-level system are also pulses that will transform into solitary waves when incident on a resonantly absorbing medium. At the entrance into the absorber strongly chirped, adiabatic pulses completely transfer the electronic population from the ground to the upper state. As the pulses suffer absorption they undergo a dynamic transition and quickly split into an off-resonant radiation part and one or several unchirped solitary pulses that may carry away some of the initially achieved population inversion. This spectral and temporal transformation process limits the spatial depths of the region of CPT in a dense absorber regardless of the initial pulse amplitude. We also show that the frequency-domain pulse area, defined as the spectral amplitude at the resonance frequency, allows for a better distinction between the regimes of $2\ensuremath{\pi}$-pulse and $0\ensuremath{\pi}$-pulse formation than time-domain measurements.

Adiabatic Raman passage via two-photon resonant transitions in a five-level Λ system

We investigate a five-level Λ system for achieving the efficient population transfer between two ground-state levels without essentially populating other three intermediate excited-state levels. This five-level Λ system is found to have an approximate dark state characterized by very small (but not exactly zero) absolute eigen-values when we work in the parameter regime where all single-photon pump and Stokes transitions are well suppressed. Dynamically manipulating the approximate dark state in a proper way, we can either attain the complete population transfer from one ground-state level to another ground-state level or create the maximal coherence between the two ground-state levels. The dynamic process of a stimulated Raman adiabatic passage is implemented in fact by manipulating a two-photon resonant pump transition and a two-photon resonant Stokes transition, which is especially important when the two ground-state levels differ by Δm=±4 in the magnetic quantum number.

Controllability on relaxation-free subspaces: On the relationship between adiabatic population transfer and optimal control

We consider the optimal control problem of transferring population between states of a quantum system where the coupling proceeds only via intermediate states that are subject to decay. We pose the question whether it is generally possible to carry out this transfer. For a single intermediate decaying state, we recover the Stimulated Raman Adiabatic Passage (STIRAP) process which we identify as the global optimum in the limit of infinite control time. We also present analytic solutions for the case of transfer that has to proceed via two consecutive intermediate decaying states. We show that in this case, for finite power the optimal control does not approach perfect state transfer even in the infinite time limit. We generalize our findings to characterize the topologies of paths that can be achieved by coherent control under the assumption of finite power.

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.

The tunneling phenomena of the Fermi superfluid gases in unitarity by manipulating the Fermi-Fermi scattering length

In the present work, we investigate the tunneling dynamics of superfluid Fermi gas in a double-well potential in a unitarity regime by adjusting the Fermi-Fermi scattering length. The scattering length can affect the quantum transition dramatically. In a certain regime, the complete population transfer between two modes can be obtained. However, in some other regimes, the quantum transition can be completely blocked.

Nuclear coherent population transfer with X-ray laser pulses

Coherent population transfer between nuclear states using X-ray laser pulses is studied. The laser pulses drive two nuclear transitions between three nuclear states in a setup reminding of stimulated Raman adiabatic passage used for atomic coherent population transfer. To compensate for the lack of γ-ray laser sources, we envisage accelerated nuclei interacting with two copropagating or crossed X-ray laser pulses. The parameter regime for nuclear coherent population transfer using fully coherent light generated by future X-Ray Free-Electron Laser facilities and moderate or strong acceleration of nuclei is determined. We find that the most promising case requires laser intensities of 1017–1019 W/cm2 for complete nuclear population transfer. As relevant application, the controlled pumping or release of energy stored in long-lived nuclear states is discussed.

Optimal control of population and coherence in three-level Λ systems

Optimal control theory (OCT) implementations for an efficient population transfer and creation of maximum coherence in a three-level system are considered. We demonstrate that the half-stimulated Raman adiabatic passage scheme for creation of the maximum Raman coherence is the optimal solution according to the OCT. We also present a comparative study of several implementations of OCT applied to the complete population transfer and creation of the maximum coherence. Performance of the conjugate gradient method, the Zhu–Rabitz method and the Krotov method has been analysed.

Pythagorean coupling: Complete population transfer in a four-state system

Complete population transfer in a four-coupled-modes system is analyzed from a geometrical point of view. An analytical solution of the dynamics is written by the use of two distinct frequencies, the generalization of the single Rabi frequency of the two-state dynamics. We also present its visualization on two separate Bloch spheres with two independent torque equations. With this scheme we analytically derive the requirements for complete population transfer in a four-state quantum system. Interestingly, the solutions are found to be linked to fundamental number theory, whereas complete population transfer occurs only if the ratios between coupling coefficients exactly match a set of Pythagorean triples.

Stark-induced adiabatic Raman passage for preparing polarized molecules

We propose a method based on Stark-induced adiabatic Raman passage (SARP) for preparing vibrationally excited molecules with known orientation and alignment for future dynamical stereochemistry studies. This method utilizes the (J, M)-state dependent dynamic Stark shifts of rovibrational levels induced by delayed but overlapping pump and Stokes pulses of unequal intensities. Under collision-free conditions, our calculations show that we can achieve complete population transfer to an excited vibrational level (v > 0) of the H(2) molecule in its ground electronic state. Specifically, the H(2) (v = 1, J = 2, M = 0) level can be prepared with complete population transfer from the (v = 0, J = 0, M = 0) level using the S(0) branch of the Raman transition with visible pump and Stoke laser pulses, each polarized parallel to the z axis (uniaxial π-π Raman pumping). Similarly, H(2) (v = 1, J = 2, M = ±2) can be prepared using SARP with a left circularly polarized pump and a right circularly (or vice versa) polarized Stokes wave propagating along the z axis (σ(±)-σ(∓) Raman pumping). This technique requires phase coherent nanosecond pulses with unequal intensity between the pump and the Stokes pulses, one being four or more times greater than the other. A peak intensity of ~16 GW/cm(2) for the stronger pulse is required to generate the desirable sweep of the Raman resonance frequency. These conditions may be fulfilled using red and green laser pulses with the duration of a few nanoseconds and optical energies of ~12 and 60 mJ within a focused beam of diameter ~0.25 mm. Additionally, complete population transfer to the v = 4 vibrational level is predicted to be possible using SARP with a 355-nm pump and a near infrared Stokes laser with accessible pulse energies.

Coherence generation and population transfer by stimulated Raman adiabatic passage and π pulse in a four-level ladder system

We propose a new scheme for achieving the complete population transfer and the optimal coherence generation between the ground state and the Rydberg state in a four-level ladder system by combining the STIRAP or fractional STIRAP technique and the π pulse technique. We consider, in particular, two different situations where spontaneous emission from the two highest states are neglected or not. Our numerical calculations show that the time width and the delay time of the π pulse are two critical parameters for attaining the maximal population transfer and coherence generation in this scheme.

Complete population transfer induced by transient nonadiabatic effects in a strongly detuned micromaser

We show that complete population inversion can be obtained in a micromaser even in the presence of large detuning. This spectacular effect is due to nonadiabatic transitions induced in the atomic system when the atom crosses the cavity. This effect is shown to depend strongly on the shape of the cavity mode and the axial atomic position.

Tunneling-induced coherent electron population transfer in an asymmetric quantum well

We propose an asymmetric double quantum wells structure with a common continuum and investigate the effect of resonant tunneling on the control of coherent electron population transfer between the two quantum wells. By numerically solving the motion equations of element moments, the almost complete electron population transfer from the initial subband to the target subband could be realized due to the constructive interference via flexibly adjusting the structure parameters.

Effects of a single fermion in a Bose Josephson junction

We consider the tunneling properties of a single fermionic impurity immersed in a Bose-Einstein condensate in a double-well potential. For strong boson-fermion interaction, we show the existence of a tunnel resonance where a large number of bosons and the fermion tunnel simultaneously. We give analytical expressions for the line shape of the resonance using degenerate Brillouin-Wigner theory. We finally compute the time-dependent dynamics of the mixture. Using the fermionic tunnel resonances as a beam splitter for wave functions, we construct a Mach-Zehnder interferometer that allows complete population transfer from one well to the other by tilting the double-well potential and only taking into account the fermion's tunnel properties.