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Physical Review A

Deceleration of heavy atoms with a continuous stimulated force

Jin Wei, Di Wu, Taojing Dong, Chenyu Zu, Yong Xia, Jianping Yin

Published Jan 10, 2025• 1 citation
Publisher

Abstract

Heavy atoms play a critical role in scientific research and applications by providing a better understanding of fundamental physical phenomena. The use of a slowed atomic beam to extend the interaction time is expected to enhance measurement sensitivity, resolution, and precision. Here, we theoretically demonstrate a rapid and short-distance deceleration of heavy atoms by using a continuous stimulated force with optical phase compensation and frequency chirping. The scheme effectively overcomes the spatial inhomogeneity of the stimulated force on atoms during deceleration. Our approach employs the numerical solutions of the optical Bloch equations for two-level atoms driven by time-dependent fields, and simulates atom trajectories using a three-dimensional Monte Carlo method. Concerning a buffer-gas-cooled radium atom beam with an initial velocity of 130 m/s, from our simulations it can be decelerated to 5 m/s within 5 cm distance using 40 mW laser power, achieving a deceleration force approximately 35 times stronger than the radiation force and covering an effective velocity range over eight times that of the radiation force. In addition, the deceleration of other heavy atoms, such as francium and thallium, also are applicable. This approach offers an efficient tool for rapidly slowing and cooling complex atoms and molecules, facilitating future precision measurements.

DOI
10.1103/physreva.111.013307

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•Aug 1, 2012•0 citations
Heavy atoms play a critical role in scientific research and applications by providing a better understanding of fundamental physical phenomena. The use of a slowed atomic beam to extend the interaction time is expected to enhance measurement sensitivity, resolution, and precision. Here, we theoretically demonstrate a rapid and short-distance deceleration of heavy atoms by using a continuous stimulated force with optical phase compensation and frequency chirping. The scheme effectively overcomes the spatial inhomogeneity of the stimulated force on atoms during deceleration. Our approach employs the numerical solutions of the optical Bloch equations for two-level atoms driven by time-dependent fields, and simulates atom trajectories using a three-dimensional Monte Carlo method. Concerning a buffer-gas-cooled radium atom beam with an initial velocity of 130 m/s, from our simulations it can be decelerated to 5 m/s within 5 cm distance using 40 mW laser power, achieving a deceleration force approximately 35 times stronger than the radiation force and covering an effective velocity range over eight times that of the radiation force. In addition, the deceleration of other heavy atoms, such as francium and thallium, also are applicable. This approach offers an efficient tool for rapidly slowing and cooling complex atoms and molecules, facilitating future precision measurements.
Polar heavy-atom molecules have been well recognized as promising candidates for precision measurements and tests of fundamental physics. A much slower molecular beam to increase the interaction time should lead to a more sensitive measurement. Here we theoretically demonstrate the possibility of the stimulated longitudinal slowing of heavy-atom molecules by the coherent optical bichromatic force with a constant beat phase. Taking the YbF meolecule as an example, we show that a rapid and short-distance deceleration of heavy molecules by a phase-compensation method is feasible with moderate conditions. A molecular beam of YbF with a forward velocity of 120 m/s can be decelerated below 10 m/s within a distance of 3.5 cm and with a laser irradiance for each traveling wave of 107.2W/cm2107.2\phantom{\rule{0.28em}{0ex}}{\mathrm{W}/\mathrm{cm}}^{2}107.2W/cm2. Our proposed slowing method could be a promising approach to break through the space constraint or the limited capture efficiency of molecules loadable into a magneto-optical trap in traditional deceleration schemes, opening the possibility for a significant improvement of the precision measurement sensitivity.
Abstract We propose a scheme for dual-species deceleration and trapping of a cold atom–molecule mixture by a frequency chirping stimulated force. We study the stimulated force exerted on MgF and Rb using optical Bloch equations based on a direct numerical solution for the time-dependent density matrix. We analyze the relationship between the frequency chirping rate and the number of MgF molecules and Rb atoms. In addition, we study the dynamical process of molecular deceleration and the effect of transverse diffusion. Monte–Carlo simulations show that buffer-gas-cooled MgF and Rb beams, with initial velocities of 200 m/s and 130 m/s respectively, can be decelerated to less than 10 m/s. This is achieved with laser powers of as low as 357 mW for MgF and 10 mW for Rb per traveling wave. The rapid deceleration minimizes molecular loss due to transverse diffusion during the deceleration process. The estimated number of molecules that can be trapped in a magneto-optical trap (MOT) is about 9.0 × 106, which is an order of magnitude larger than the number of MgF molecules decelerated by the spontaneous radiation force. The results offer a promising starting point for further studies of sympathetic cooling.
This work presents the proposed schemes for decelerating cold molecules created in pulsed supersonic expansions using pulsed optical lattices. The first scheme relies on the application of a decelerating optical lattice with a dipole-potential well depth of approximately 1 K, created by rapidly chirping medium-intensity fields in the 10 W/cm/sup 2/ range. This scheme is an optical analog of the Start decelerator that has been successfully used to slow a range of polar molecules. A significant fraction (about 10%) of the very heavy I/sub 2 /molecules in an Ar buffer gas is shown to be slowed by using this method over sub-microsecond time scales. In the second technique, an optical lattice with larger well depth in the 100 K range travelling at half the supersonic beam velocity is used to trap a significant fraction of the cold, high-velocity molecules. This technique is developed based on the property of periodic motion of the trapped molecules in the optical lattice; the molecules reverse their initial (relative) velocities in the lattice reference frame after half a period. Using this method the cold molecules can be transferred from high speed to zero velocity on nanosecond time scales. As an example, 33% of a CO molecule beam (1 K) with a velocity of 230 m/s is slowed utilising optical intensities of less than 10/sup 12/ W/cm/sup 2/.
The spontaneous decay rate $\ensuremath{\gamma}$ of excited atomic states limits the radiative force on atoms to $\ensuremath{\Elzxh}k\ensuremath{\gamma}/2.$ This bottleneck in the dissipative exchange of momentum between atoms and light can be overcome by coherent control of the momentum exchange interactions. The bichromatic force provides such control by using stimulated instead of spontaneous emission to produce the force, while at the same time exploiting spontaneous emission to enable a dissipative interaction. It is implemented with two counterpropagating light beams, each containing two frequencies, whose phases, amplitudes, and frequency difference are carefully chosen. We have made precise measurements of the extremely large magnitude and velocity range of this bichromatic force, and have shown that its velocity dependence near the edge of its range is suitable for atomic beam slowing and laser cooling. Our measurements have corroborated various models and calculations of this bichromatic force.
Over the past 30 years, optical manipulation of neutral atoms has been primarily performed with a monochromatic laser beam. The simplest tool for the control of atomic motion is the radiative force exerted by a monochromatic laser on a twolevel atom. The radiative force arises from absorption followed by spontaneous emission, and its magnitude is limited by the atom's excited state lifetime. The coherent momentum exchanges between light elds and atoms can be exploited to produce long-range optical forces much greater than the radiative force through the use of absorption-stimulated emission processes. Adiabatic Rapid Passage (ARP) is a long-existing method to invert the population of a two-level nuclear spin system. Its extension to the optical domain necessitates a frequency chirped light pulse to interact with a two-level atom via the dipole interaction. I will first present a numerical study of the properties of optical forces on moving atoms derived from purely stimulated processes produced by multiple ARP sequences. This will be followed by experimental observations of long-range ARP forces much larger than the radiative force in metastable helium. Sequences of properly timed laser pulses may be used for rapid deceleration of neutral atomic (or molecular) beams. | 171 pages