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Magneto-optical trapping of a diatomic molecule

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Laser cooling and trapping are central to modern atomic physics. The most used technique in cold-atom physics is the magneto-optical trap (MOT), which combines laser cooling with a restoring force from radiation pressure. For a variety of atomic species, MOTs can capture and cool large numbers of particles to ultracold temperatures (less than ∼1 millikelvin); this has enabled advances in areas that range from optical clocks to the study of ultracold collisions, while also serving as the ubiquitous starting point for further cooling into the regime of quantum degeneracy. Magneto-optical trapping of molecules could provide a similarly powerful starting point for the study and manipulation of ultracold molecular gases. The additional degrees of freedom associated with the vibration and rotation of molecules, particularly their permanent electric dipole moments, allow a broad array of applications not possible with ultracold atoms1. Spurred by these ideas, a variety of methods has been developed to create ultracold molecules. Temperatures below 1 microkelvin have been demonstrated for diatomic molecules assembled from pre-cooled alkali atoms2,3, but for the wider range of species amenable to direct cooling and trapping, only recently have temperatures below 100 millikelvin been achieved4,5. The complex internal structure of molecules complicates magneto-optical trapping. However, ideas and methods necessary for creating a molecular MOT have been developed6,7,8,9,10,11 recently. Here we demonstrate three-dimensional magneto-optical trapping of a diatomic molecule, strontium monofluoride (SrF), at a temperature of approximately 2.5 millikelvin, the lowest yet achieved by direct cooling of a molecule. This method is a straightforward extension of atomic techniques and is expected to be viable for a significant number of diatomic species6,7. With further development, we anticipate that this technique may be employed in any number of existing and proposed molecular experiments, in applications ranging from precision measurement12 to quantum simulation13 and quantum information14 to ultracold chemistry15.

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Figure 1: Experimental set-up.
Figure 2: Magneto-optical trapping of SrF.
Figure 3: Measurement of MOT properties.

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We thank E.R. Edwards for contributions towards the construction of the experiment. We acknowledge funding from AFOSR (MURI), ARO, and ARO (MURI). E.B.N. acknowledges funding from the NSF GRFP.

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All authors contributed to the experiment, the analysis of the results and the writing of the manuscript.

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Correspondence to D. J. McCarron.

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Extended data figures and tables

Extended Data Figure 1 Relevant energy levels and transitions in SrF.

a, Vibrational branching in SrF. Solid upward arrows denote transitions driven by the MOT lasers. Spontaneous decays from the A2Π1/2(v′ = 0) state (solid wavy arrows) and A2Π1/2(v′ = 1, 2) states (dashed wavy arrows) are governed by the vibrational branching fractions b0v, b1v and b2v, as shown. b, Optical addressing scheme for the SrF MOT. c, Energy levels of the X2Σ(v = 0, N = 1) state versus B. Energy levels are labelled by their mF value with mF = 2 (red lines), mF = 1 (orange lines), mF = 0 (green), mF = −1 (blue) and mF = −2 (purple).

Extended Data Figure 2 Slowing laser spectra.

a, Scale diagram showing the frequency extent of the , and slowing lasers (vertical red bars) relative to the four SR/HF manifolds of the X2Σ(v = 0, 1, 2; N = 1) states of SrF (horizontal black bars to right). The relative splittings of the four SR/HF levels in the X2Σ(N = 1) state are the same to within ∼1 MHz for v = 0, 1, 2 (ref. 31). The dashed lines mark the centres of the N = 1 SR/HF levels for the labelled velocity, and the level structure shown corresponds to v = 0 m s−1. b, Optimized spectral profiles of the three slowing lasers. The upper x axis shows velocity for a Doppler shift equivalent to the frequency labelled on the lower x axis. The light is modulated via a fibre EOM with fmod = 3.5 MHz. The and lasers are each modulated by passing through two bulk EOMs with resonant frequencies at ∼40 MHz and ∼9 MHz.

Extended Data Figure 3 Molecular beam longitudinal velocity.

Shown are examples of slowed (grey circles) and unslowed (black squares) velocity profiles of the molecular beam detected upstream from the trapping region at . These profiles are for the optimized slowing conditions that produce the largest MOT, as discussed in the main text.

Extended Data Figure 4 MOT dependence on laser frequency.

Shown is LIF in the trapping region versus detuning when Δ00 and are varied together (top), when is varied alone (middle) and when Δ10 is varied alone (bottom). As expected (and typically observed for atomic MOTs), the SrF MOT operates over a fairly narrow range of negative detuning values for the trapping lasers but requires only that the repump lasers be sufficiently near resonance. Error bars, standard error for multiple scans across each detuning range (14 scans for top and middle; 24 scans for bottom).

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Barry, J., McCarron, D., Norrgard, E. et al. Magneto-optical trapping of a diatomic molecule. Nature 512, 286–289 (2014).

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