Narrow-line laser cooling by adiabatic transfer

Abstract

We propose and demonstrate a novel laser cooling mechanism applicable to particles with narrowlinewidth optical transitions. By sweeping the frequency of counter-propagating laser beams in a sawtooth manner, we cause adiabatic transfer back and forth between the ground state and a longlived optically excited state. The time-ordering of these adiabatic transfers is determined by Doppler shifts, which ensures that the associated photon recoils are in the opposite direction to the particle's motion. This ultimately leads to a robust cooling mechanism capable of exerting large forces via a weak transition and with reduced reliance on spontaneous emission. We present a simple intuitive model for the resulting frictional force, and directly demonstrate its efficacy for increasing the total phase-space density of an atomic ensemble. We rely on both simulation and experimental studies using the 7.5 kHz linewidth 1 S 0 to 3 P 1 transition in 88 Sr. The reduced reliance on spontaneous emission may allow this adiabatic sweep method to be a useful tool for cooling particles that lack closed cycling transitions, such as molecules.

Frequency Measurements of Superradiance from the Strontium Clock Transition

“…Cooling into the lattice is performed by the narrow linewidth 7.5-kHz transition at 689 nm, following a similar procedure as detailed in Ref. [51]. Typically, the measured temperature of the atoms in the lattice is around 12 μK ≈ 65E rec .…”

Simulations of sawtooth-wave adiabatic passage with losses

“…As was found in Refs. [5][6][7][8][9], SWAP cooling does not achieve a temperature as low as that from Doppler cooling; for the range plotted in Fig. 1, the asymptotic temperature was more than ∼ 10X the Doppler temperature for ev- eryω r .…”

“…However, not all atoms, molecules, or nano-scale objects can be effectively laser cooled with known techniques, which motivates the search for new cooling methods. Reference [5] described a method for laser cooling called Sawtooth-Wave Adiabatic Passage (SWAP) based on chirping counter-propagating light waves. This method was proposed for cooling narrow linewidth transitions.…”

Laser cooling with adiabatic transfer on a Raman transition

“…Recently, a new mechanism called Sawtooth Wave Adiabatic Passage (SWAP) cooling was observed using a narrow-linewidth optical transition in 88 Sr atoms [25], and a related deflection force was previously reported in He atoms [26]. The SWAP technique has now been used for slowing Dy [27] and for creating faster, denser Sr magneto-optical traps (MOTs) [28,29].…”

“…At the end of the sweep, optical pumping is briefly applied to transfer atoms erroneously remaining in bñ | back to añ | . In comparison to the work in strontium [25], this is equivalent to being able to set Γ≈0 during the frequency sweep, but then setting Γ up to the lifetime of the intermediate state for a very brief period of time in between sweeps, potentially offering a different degree of freedom for optimizing cooling. Figure 2 shows the experimental setup for demonstrating 1D Raman SWAP cooling in 87 Rb.…”