[Show abstract][Hide abstract] ABSTRACT: We perform echo spectroscopy on ultracold atoms in atom-optics billiards to study their quantum dynamics. The detuning of the trapping laser is used to change the "perturbation", which causes a decay in the echo coherence. Two different regimes are observed: first, a perturbative regime in which the decay of echo coherence is nonmonotonic and partial revivals of coherence are observed in contrast with the predictions of random matrix theory. These revivals are more pronounced in traps with mixed dynamics as compared to traps where the dynamics is fully chaotic. Next, for stronger perturbations, the decay becomes monotonic and independent of the strength of the perturbation. In this regime no clear distinction can be made between chaotic traps and traps with mixed dynamics.
[Show abstract][Hide abstract] ABSTRACT: We perform spectroscopy on the ground-state hyperfine splitting of 85 Rb atoms trapped in far-off-resonance optical traps. The existence of a spatially dependent shift in the energy levels is shown to induce an inherent dephasing effect, which causes a broadening of the spectroscopic line and hence an inhomogeneous loss of atomic coherence at a much faster rate than the homogeneous one caused by spontaneous photon scattering. We present here a number of approaches for reducing this inhomogeneous broadening, based on trap geometry, additional laser fields, and novel microwave pulse sequences. We then show how hyperfine spectroscopy can be used to study the quantum dynamics of optically trapped atoms.
Journal of Optics A Pure and Applied Optics 07/2005; 7:103-125. · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The decay in the hyperfine coherence of optically trapped atoms due to interactions with the environment, is only partly suppressed by "echo" spectroscopy [M. F. Andersen et al. Phys. Rev. Lett. 90, 023001 (2003)], primarily due to dynamical (time-dependent) dephasing mechanisms. We demonstrate here an improved pulse sequence, for which the decay of coherence is reduced by a factor of 2.5 beyond the reduction offered by the "echo" spectroscopy. This reduction occurs when each dark period in-between pulses is shorter than the time scale over which substantial dephasing develops. The coherence time is then limited by mixing to other vibrational levels in the trap and, to a lesser extent, the lifetime of the internal states of the atoms. Long coherence time of trapped atoms have attracted in-terest over the last years, not only because of their possible use in high-precision spectroscopy, but mainly because many proposed quantum information processing schemes require manipulations of internal states of atoms or ions, and any dephasing or decoherence of these states will lead to loss of information. In the case of trapped atoms or ions many dif-ferent approaches have been tried for increasing their coher-ence time. For trapped ions a "dephasing free subspace" was used to minimize the dephasing induced by the environment . For neutral atoms and ions several "compensating" tech-niques have been demonstrated, in which the interaction causing the dephasing is cancelled by an additional interac-tion of opposite sign . Recently, three groups have re-ported the use of coherence echoes for investigating the co-herence of trapped alkali atoms and ions in a superposition state of two of their ground states [3–5]. In Ref.  the increased coherence time was used to study quantum dynam-ics of ultracold atoms, while in Refs. [3,5] the study was related to quantum information processing with single ions and atoms. In this work we investigate the limitations on the coher-ence time achieved by echo techniques for ultracold atoms trapped in an optical dipole trap. The echo scheme of Refs. [3–5] is based on a " /2--/ 2" pulse sequence (see Fig. 1), following the original spin-echo concept . When the dephasing rate is constant, the phases accumulated after the pulse completely cancel the dephasing during the first half. However, a nonlinear dephasing rate will result in a residual net dephasing and a reduced coherence time. We used a new echo technique in order to improve the coherence time by the use of additional pulses between the two / 2 pulses. It relies on the fact that, if the phase compensation mechanism achieved with the "echo" is divided into several time inter-vals, the phase mismatch will accumulate during one time interval only. The method has a strong similarity with the "decoupling" method used in quantum-information schemes in nuclear magnetic resonance (NMR), where a repeated fast refocusing completely stops the dynamics (see Ref.  and references therein). Any mechanism causing a change in the internal quantum state of an atom, will clearly cause a drop in the ensemble coherence. On the other hand, effects that lead to dephasing but conserve the internal quantum state of the atom such as Rayleigh scattering of far detuned photons , slow fluctua-tions of the trap laser power and slow dynamics, will not cause an immediate decoherence but will lead to a dephasing that grows in time. The "multiple-" technique presented here can suppress dephasing from these mechanisms. We study the coherence of the two magnetically insensi-tive ground states of 85 Rb atoms trapped in a far-off-resonance optical dipole trap (FORT). These two levels (5S 1/2 , F =2,m F =0, denoted 1, and 5S 1/2 , F =3,m F =0, denoted 2) are separated by the hyperfine energy splitting E HF = HF with HF =2 3.036 10 9 s −1 . Since the dipole potential is inversely proportional to the trap laser detuning
Physical Review A 07/2004; 70:013405. · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the coherence properties of thermal atoms confined in optical dipole traps where the under-lying classical dynamics is chaotic. A perturbative expression derived for the coherence of the echo scheme of Anderson et al. [Phys. Rev. Lett. 90, 023001 (2003)] shows that it is a function of the survival probability or fidelity of eigenstates of the motion of the atoms in the trap. The echo coherence and the survival probability display "system specific" features, even when the underlying classical dynamics is chaotic. In particular, partial revivals in the echo signal and the survival probability are found for a small shift of the potential. Next, a "semiclassical" expression for the averaged echo signal is presented and used to calculate the echo signal for atoms in a light sheet wedge billiard. Revivals in the echo coherence are found in this system, indicating that they may be a generic feature of dipole traps.
Physical Review A 06/2004; 69:063413. · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The loss in coherence in a trapped-atoms "echo" experiment is suppressed by a different pulse sequence. The coherence time is then limited by mixing to other vibrational levels and the lifetime of the internal states
[Show abstract][Hide abstract] ABSTRACT: We investigate the dephasing of ultra cold 85Rb atoms trapped in an optical dipole trap and prepared in a coherent superposition of their two hyperfine ground states by interaction with a microwave pulse. We demonstrate that the dephasing, measured as the Ramsey fringe contrast, can be reversed by stimulating a coherence echo with a pi pulse between the two pi / 2 pulses, in analogy to the photon echo. We also demonstrate that "echo spectroscopy" can be used to study the quantum dynamics in the trap even when more than 10(6) states are thermally populated and to study the crossover from quantum to classical dynamics.
[Show abstract][Hide abstract] ABSTRACT: We discuss a recently demonstrated type of microwave spectroscopy of trapped ultra-cold atoms known as "echo spectroscopy" [M.F. Andersen et. al., Phys. Rev. Lett., in press (2002)]. Echo spectroscopy can serve as an extremely sensitive experimental tool for investigating quantum dynamics of trapped atoms even when a large number of states are thermally populated. We show numerical results for the stability of eigenstates of an atom-optics billiard of the Bunimovich type, and discuss its behavior under different types of perturbations. Finally, we propose to use special geometrical constructions to make a dephasing free dipole trap.
Communications in Nonlinear Science and Numerical Simulation 12/2002; · 2.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a novel method for reducing the inhomogeneous frequency broadening in the hyperfine splitting of the ground state of optically trapped atoms. This reduction is achieved by the addition of a weak light field, spatially mode-matched with the trapping field and whose frequency is tuned in-between the two hyperfine levels. We experimentally demonstrate the new scheme with Rb 85 atoms, and report a 50-fold narrowing of the rf spectrum.
Physical Review A 10/2002; 66:045401. · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the effects of curving trajectories by applying external force fields on a particle in a billiard. We investigate two special cases: a constant force field and a parabolic potential. These perturbations change the stability conditions and can lead to formation of elliptical orbits in otherwise hyperbolic billiards. We demonstrate these effects experimentally with ultra-cold atoms in atom-optic billiards.
Journal of Physics B Atomic Molecular and Optical Physics 04/2002; 35(9):2183. · 2.03 Impact Factor