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arXiv:1311.2027 · Physical Review A

Rapid coherent control of population transfer in lattice systems

Shumpei Masuda, Stuart A. Rice

Published Mar 14, 2014• 32 citations
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Abstract

We derive the driving potential that accelerates adiabatic population transfer from an initial state to a target state in a lattice system without unwanted excitation of other states by extending to discrete systems the fast-forward theory of adiabatic transfer. As an example, we apply the theory to a model that describes a Bose-Einstein condensate in a quasi-one-dimensional optical lattice, and show that modulation of the tilting of the lattice potential can transfer the population of the Bose-Einstein condensate from site to site with high fidelity and without unwanted excitations.

DOI
10.1103/physreva.89.033621

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We derive the driving potential that accelerates adiabatic population transfer from an initial state to a target state in a lattice system without unwanted excitation of other states by extending to discrete systems the fast-forward theory of adiabatic transfer. As an example, we apply the theory to a model that describes a Bose-Einstein condensate in a quasi-one-dimensional optical lattice, and show that modulation of the tilting of the lattice potential can transfer the population of the Bose-Einstein condensate from site to site with high fidelity and without unwanted excitations.
A protocol is proposed for the rapid coherent loading of a Bose-Einstein condensate into the ground state of an optical lattice, without residual excitation associated with the breakdown of adiabaticity. The driving potential required to assist the rapid loading is derived using the fast-forward technique, and generates the ground state in any desired short time. We propose an experimentally feasible loading scheme using a bichromatic lattice potential, which approximates the fast-forward driving potential with high fidelity.
In the past two decades many schemes to actively control the dynamical evolution of a quantum system have been proposed and established. We focus attention on the use of assisted adiabatic processes to generate selected target states with high efficiency. Specific control protocols that fall in this category are sometimes called shortcuts to adiabaticity. We review two assisted adiabatic population transfer schemes, the counter-diabatic and fast-forward protocols, with the intent of calling attention to their conceptual basis and applications to real systems, specifically vibrational energy transfer in polyatomic molecules and manipulation of Bose-Einstein (BEC) condensates. The particular applications of the counter-diabatic and fast-forward protocols to improving the efficiency of population transfer between vibrational states of a polyatomic molecule that we have studied are (i) selective vibrational energy transfer in the nonrotating thiophosgene (SCCl2) molecule and (ii) enhancing the yield of HNC in the driven rotation free isomerization reaction HCN→CNH. A major component of our considerations concerns reduction of the degradation of population transfer efficiency within a subset of states that is generated by coupling of the subset of states to a manifold of background states. We have examined the influence of a manifold of background vibrational states on population transfer within a subset of embedded vibrational states, and on how that influence can be suppressed. We emphasize that any control protocol must be designed to minimize interference between the background states and excitations within the selected subset of states that are used to define initial and target states. We have also examined the application of the fast-forward protocol to the transport of population between sites of a BEC in an optical lattice. Through the applications we illustrate how the theoretical framework of an assisted adiabatic process depends on the character of the system studied, on the available control parameters, on the constraints imposed by the experimental setting, on the efficiency of population transfer (fidelity) to be achieved, and on the stability of the control process with respect to the variation of the external control fields.
We have performed a number of experiments with a Bose-Einstein condensate (BEC) in a one-dimensional optical lattice. Making use of the small momentum spread of a BEC and standard atom optics techniques, a high level of coherent control over an artificial solid-state system is demonstrated. We are able to load the BEC into the lattice ground state with a very high efficiency by adiabatically turning on the optical lattice. We coherently transfer population between lattice states and observe their evolution. Methods are developed and used to perform band spectroscopy. We use these techniques to build a BEC accelerator and a novel, coherent, large-momentum-transfer beam-splitter.
We show a method to accelerate quantum adiabatic transport of identical spinless particles interacting with each other by developing the preceding fast-forward scaling theory formed for one-particle systems [Masuda and Nakamura, Proc. R. Soc. A 466, 1135 (2010)]. We derive a driving potential which accelerates adiabatic dynamics of quantum systems composed of identical particles in order to obtain the final adiabatic states in any desired short time. We also exhibit an ideal rapid manipulation of dilute Bose gas in the ground state without energy excitation by using the fast-forward scaling theory.
In this thesis, I present an experimental study of quantum control techniques for transferring population between vibrational states of atoms trapped in an optical lattice. Results from a range of techniques are compared, including techniques tested previously in the same system. In the study of the Adiabatic Rapid Passage (ARP) technique, control of population transfer is realized through varying the chirp rate and modulation amplitude of a frequency-chirped sinusoidal displacement of the lattice. Meanwhile, dependence of population transfer on the chirp direction is observed, which is explained by a model of ARP in a 3-level system. In the study of the coherent control technique, interference between a one-phonon transition at 2\omega and a two-phonon transition at omega is experimentally demonstrated. The omega and 2\omega transitions are realized by sinusoidally displacing the optical lattice at omega and sinusoidally modulating the lattice depth at 2\omega, respectively. The branching ratio of transitions to the first excited state and to higher excited states is controlled by varying the relative phase between these two pathways. The highest measured branching ratio of 17\pm2 is achieved among all the experiments using this coherent control scheme. In the study of the GRadient Ascent Pulse Engineering (GRAPE) technique, a "pulse" involving both displacement and depth-modulation of the lattice is used to transfer population. This pulse is theoretically engineered with the GRAPE algorithm to optimize the fidelity between the first excited state and the final state, when the lattice Hamiltonian without gravity for a specific lattice depth is considered. The experimental result shows that there is almost no excitation into higher excited states during population transfer from the ground to the first excited state, even when this process is affected by gravity and inhomogeneous broadening in reality. By comparing all the techniques, the GRAPE technique is found to be the best in terms of increasing population transfer into the first excited state while reducing excitation into higher excited states. On the other hand, the ARP technique creates the highest normalized population inversion, a ratio of the difference to the sum of the ground and the first excited state populations.
Robust stimulated Raman exact passages are requisite for controlling nonlinear quantum systems, with wide applications, ranging from ultracold molecules to nonlinear optics to superchemistry. Inspired by shortcuts to adiabaticity, we propose the fast-forward scaling of stimulated Raman adiabatic processes with nonlinearity involved, describing the transfer from an atomic Bose-Einstein condensate to a molecular one by controllable external fields. The fidelity and robustness of atom-molecule conversion are shown to surpass those of conventional adiabatic passages, assisted by the fast-forward driving field. Finally, our results are extended to fractional stimulated Raman adiabatic processes for the coherent superposition of atomic and molecular states.