Molecules cooled below the Doppler limit

Abstract

Magneto-optical trapping and sub-Doppler cooling have been essential to most experiments with quantum degenerate gases, optical lattices, atomic fountains and many other applications. A broad set of new applications await ultracold molecules 1 , and the extension of laser cooling to molecules has begun 2-6 . A magneto-optical trap (MOT) has been demonstrated for a single molecular species, SrF 7-9 , but the sub-Doppler temperatures required for many applications have not yet been reached. Here we demonstrate a MOT of a second species, CaF, and we show how to cool these molecules to 50 µK, well below the Doppler limit, using a three-dimensional optical molasses. These ultracold molecules could be loaded into optical tweezers to trap arbitrary arrays 10 for quantum simulation 11 , launched into a molecular fountain 12,13 for testing fundamental physics [14][15][16][17][18] , and used to study collisions and chemistry 19 between atoms and molecules at ultracold temperatures.We first focus on the MOT, which is likely to become a workhorse for cooling molecules just as it is for atoms. Previously, only SrF had been trapped this way. For SrF, two types of MOT have been developed, a d.c. MOT where the lifetime was short and the temperature high 7,8 , and a radiofrequency (rf) MOT where longer lifetimes and lower temperatures were achieved 9,20 . In the rf MOT, optical pumping into dark states is avoided by rapidly reversing the magnetic field and the handedness of the MOT laser. It has been suggested that the detrimental effects of dark states can also be avoided in the d.c. MOT by driving the cooling transition with two oppositely polarized laser components, one red-and one bluedetuned 21 . We use this dual-frequency technique to make a d.c. MOT of CaF and find that it works just as well as the rf MOT. Thus, we demonstrate a MOT of a second molecular species, which is important for applications of ultracold molecules, and also verify the effectiveness of this new scheme. Figure 1 illustrates the experiment, which is described in more detail in Methods. A pulse of CaF molecules produced at time t = 0 is emitted from a cryogenic buffer gas source, then decelerated by frequency-chirped counter-propagating laser light, and finally captured in the MOT between t = 16 and 40 ms. Figure 2a shows the molecules in the MOT, imaged on a charge-coupled device (CCD) camera by collecting their fluorescence. We estimate that there are (1.3 ± 0.3) × 10 4 molecules in this MOT (see Methods), with a peak density of n = (1.6 ± 0.4) × 10 5 cm −3 . These are similar to the best values achieved for SrF 20 . To determine the MOT lifetime, we fit the decay of its fluorescence to a single exponential. Figure 2b shows this lifetime as a function of the scattering rate. The lifetime is typically 100 ms and decreases with higher scattering rate, suggesting loss by optical pumping to a state not addressed by the lasers. We do not see the precipitous drop in lifetime observed at low scattering rate in the d.c. MOT of SrF 9 . To watch the molecules ...

A buffer gas beam source for short, intense and slow molecular pulses

“…The flux diminishes after running the source continuously for about 24 h, but is easily recovered by going through a warm-up and cool-down cycle that Downloaded by [Imperial College London Library] at 07:21 16 November 2017 takes about 12 h. The source has run for many months without requiring maintenance. It has been an invaluable starting point for making cold, slow, velocity-controlled beams by frequency-chirped laser slowing (35), and for loading a magneto-optical trap of CaF (37). As well as these applications, the source may be useful for spectroscopic studies and for loading Stark and Zeeman decelerators.…”

“…This is the design we used for our recent experiments on laser slowing and magneto-optical trapping of CaF molecules (35,37). The length of the cell is extended by 10 mm, and we have added a conical aperture with an inner diameter of 3.5 mm at the exit.…”

“…Recently, buffer-gascooled beams of a few molecular species have been slowed to low velocity using radiation pressure (31)(32)(33)(34)(35), and CONTACT S. Truppe s.truppe09@imperial.ac.uk The underlying research materials for this article can be accessed at Zenodo (https://doi.org/10.5281/zenodo.995663) and may be used under the Creative Commons CCZero license. these slow molecules have been captured and cooled to low temperatures in magneto-optical traps (36)(37)(38). The high intensity and low velocity of the beams produced by the buffer gas method was crucial to the success of these experiments.…”

Precision Measurement of Time-Reversal Symmetry Violation with Laser-Cooled Polyatomic Molecules

“…Trapped molecular ions have shown great power in EDM searches [3], primarily due to their long coherence time of ∼1 s. Neutral species offer the ability to increase the number of trapped molecules much more easily and essentially without limit compared to ions, while retaining strong robustness against systematic errors. Here we show that laser-cooled and trapped polyatomic molecules offer a combination of features not available in other systems, including long lifetimes, robustness against systematic errors, and scalability, and present a feasible approach to access PeV-scale BSM physics.A very promising route to trapping EDM-sensitive molecules is direct laser cooling and trapping from cryogenic buffer gas beams (CBGBs), which has advanced tremendously in the last few years [4][5][6][7][8][9][10][11]. The molecules that have been cooled so far posses an electronic structure that makes them amenable to laser cooling, but also precludes the existence of Ω doublets, such as the 3 Δ 1 molecular state used in the two most sensitive electron EDM measurements [2,3].…”

“…A very promising route to trapping EDM-sensitive molecules is direct laser cooling and trapping from cryogenic buffer gas beams (CBGBs), which has advanced tremendously in the last few years [4][5][6][7][8][9][10][11]. The molecules that have been cooled so far posses an electronic structure that makes them amenable to laser cooling, but also precludes the existence of Ω doublets, such as the 3 Δ 1 molecular state used in the two most sensitive electron EDM measurements [2,3].…”