Interferometric Laser Cooling of Atomic Rubidium

Abstract

We report the 1-D cooling of 85 Rb atoms using a velocity-dependent optical force based upon Ramsey matter-wave interferometry. Using stimulated Raman transitions between ground hyperfine states, 12 cycles of the interferometer sequence cool a freely-moving atom cloud from 21 µK to 3 µK. This pulsed analog of continuous-wave Doppler cooling is effective at temperatures down to the recoil limit; with augmentation pulses to increase the interferometer area, it should cool more quickly than conventional methods, and be more suitable for species that lack a closed radiative transition.The laser cooling of atomic gases has revolutionized experimental atomic physics [1] and raised the prospect of a range of atomic quantum technologies [2,3]. However, traditional Doppler cooling [4,5] relies upon the velocity-dependence of a single narrow radiative transition, and spontaneous emission to reset the atomic state. The cooling force is limited to a half photon-impulse per excited-state lifetime and, as many impulses are needed, requires a transition that can be closed by a few repump lasers. Doppler cooling has thus so far been limited to a handful of atomic elements and molecules [6][7][8][9].In [10], Weitz and Hänsch proposed a mechanism that could extend laser cooling to a wider range of species, by replacing the continuous wave (CW) excitation of conventional Doppler cooling with the broadband laser pulses of Ramsey matter-wave interferometry, and interleaving inversion pulses to eliminate the dependence upon the internal state energies. The interference signal, and hence the impulse imparted, were thus determined only by the particle's kinetic energy; manifold transitions could be accessed and, while spontaneous emission remained the entropy-removing mechanism, various schemes [11][12][13] could increase the impulse per spontaneous event. With a drive towards efficient pulsed schemes for molecular cooling [14][15][16] supported by improved mode-locked laser technologies, interferometric cooling appears a promising and flexible tool.The idea of a pulsed Ramsey analog to CW Doppler cooling has until now remained untested. In this letter, we report the first experimental demonstration of 1-D interferometric cooling of a cloud of already ultracold Rb atoms. Our long-lived quasi-two-level system, comprising the two 5S 1/2 ground hyperfine states of 85 Rb between which we drive stimulated Raman transitions, in principle allows cooling to the recoil limit, and we show that with just 12 cycles of the interferometric cooling sequence the atom cloud is cooled from 21 ± 2 µK to 3.2 ± 0.4 µK. Relaxation after each cycle is achieved by rapid pumping and decay of the single-photon 5S 1/2 -5P 3/2 transition, and the cooling rate is therefore limited mainly by the time needed for interferometric resolution of the different velocity classes.The Raman interferometric cooling mechanism is as follows. Two π/2 laser pulses, separated by a dwell time τ , act upon a two-level atom |Ψ = c 1 |1 + c 2 |2 as the beamsplitter and combiner of a R...

Fluorescence double resonance optical pumping spectrum and its application for frequency stabilization in millimeter scale vapor cells

“…The spontaneous decay rate from 5P 3/2 to 5S 1/2 state is: p G =2 6.06MHz. 3,4,5 The spontaneous decay rate from 4D 5/2 to 5P 3/2 state is: p G =2 1.97MHz. where C ik is the Clebsch-Gordan coefficients which can be calculated according to, e.g.…”

“…Laser spectroscopy employing miniaturized vapor cells is attracting growing attention, primarily for its potential applications in compact frequency standards [1][2][3], laser cooling [4][5][6][7], magnetometry [8][9][10] and optical isolation [11], to name a few. In order to obtain a highly resolved spectral lines in the transition from one excited state to another, various optical pumping methods, such as the optical-optical double resonance (OODR) technique, have been successfully used [12].…”

Velocimetry, Cooling and Rotation Sensing by Cold-Atom Matterwave Interferometry

“…This process, which is not in principle limited to Raman transitions, shows similar cooling rates and limiting temperatures to conventional Doppler cooling, but allows adjustment of the capture range for higher temperatures without saturating the transition for lower velocities; Raman transitions and pulsed repumping allow sub-Doppler temperatures to be reached without the usual Sisyphus mechanisms. Using stimulated Raman transitions between the ground hyperfine states, we have demonstrated the 1D cooling of a freely moving cloud of 85 Rb atoms from 21 to 3 K [6]. As originally proposed [5], interferometric cooling includes a third laser pulse to invert the atomic population between the two interferometer pulses.…”

Laser cooling with adiabatic transfer on a Raman transition

“…However, standard Doppler cooling is limited in both final temperature and maximum force by the linewidth Γ and wavelength λ ≡ 2π/k of the available optical transitions -properties that are provided by nature and not under control of the experimentalist. Understanding methods and limitations for removing entropy from a system is of fundamental interest and continues to be widely explored [13][14][15][16][17][18][19].…”

“…After each photon absorption event, a molecule needs to return to the initial state via spontaneous emission for Doppler cooling to work, but it may be lost instead due to the abundance of accessible vibrational and rotational degrees of freedom [20,21]. This challenge has motivated the development of alternative techniques that rely on reducing the role of spontaneous emission relative to Doppler cooling, including Sisyphus cooling of molecules [18,23], bichromatic force slowing and cooling [16,24], and interferometric cooling [19].…”