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A laser-cooled molecular fountain to measure the electron EDM

Final Report Summary - EEDM (A laser-cooled molecular fountain to measure the electron EDM)

The aim of this project was to build an instrument that prepares a cloud of YbF molecules, cools them with laser light to μK temperature, and uses them to detect the electric dipole moment of the electron (eEDM).

As a first step in producing ultracold YbF molecules, we developed a cryogenic buffer-gas source that delivers ten times more YbF molecules than the previous best beam, with four times lower velocity. We invented the laser cooling techniques required to reach micro-Kelvin temperatures using CaF molecules, which have more favourable Franck-Condon factors than YbF. We demonstrated the use of laser light to slow the molecules, then optimized the frequency-chirp to make an intense, cold, velocity-controlled beam. To catch these molecules in a magneto-optical trap (MOT), we developed new ideas that address the difficulties arising from the many levels of a molecule, and went on to fill a MOT with CaF. The temperature was much higher than originally expected (too high for the proposed eEDM experiment) so we had to work out why. Once understood, we cooled molecules below the Doppler temperature for the first time, using a blue-detuned molasses.

The MOT and molasses exert frictional forces, but ultimately the molecules need to be in a conservative trap, or in free flight, to make the precise measurements that are planned. We therefore developed a far-off-resonance microwave trap and also used a (conventional) magnetic trap. We loaded the ultra-cold CaF molecules into the magnetic trap and showed a long lifetime for controlled quantum coherences in the trap.

Having developed the essential physics, we used this knowledge to laser cool a beam of YbF – the molecule of interest for the eEDM experiment – to below 100 μK in the transverse direction. We are now three quarters of the way through building a slow YbF beam suitable for measuring the eEDM. This will be very sensitive to magnetic field noise arising from thermal currents in the electric field plates. We have therefore developed and tested a new field plate design that greatly reduces this noise. We have also invented a new type of decelerator that combines optical pumping with the Zeeman shift to produce a Sisyphus force, and we will slow the YbF that way.

Referring to the original work plan, we have completed most of the sub-projects as anticipated, but the fountain of YbF is now a slow horizontal YbF beam. We have completed the state control and transverse cooling that this beam requires. Most of the steps toward making the eEDM measurement are complete. We have demonstrated microwave and laser control of the internal YbF states, as required to close the vibrational and rotational leaks. This has increased the signal:noise ratio in the current eEDM apparatus by a factor of 36. With the benefit of this improvement, a new eEDM measurement is under way in our old apparatus at the statistical noise level approaching 10^{-29} When the experiment moves to the new apparatus, this will be below 10^{-30} as anticipated in the original proposal. We are taking data but have not yet obtained a new eEDM result, mainly because the mastery of molecular laser cooling took much longer than we had anticipated. However, the depth of understanding that we have acquired now opens up new possibilities for laser cooling a range of other molecules and this has been an unexpected benefit of the programme.