The electron is predicted to be slightly aspheric [1], with a distorsion characterised by the electric dipole moment (EDM), d e . No experiment has ever detected this deviation. The standard model of particle physics predicts that d e is far too small to detect [2], being some eleven orders of magnitude smaller than the current experimental sensitivity. However, many extensions to the standard model naturally predict much larger values of d e that should be detectable [3]. This makes the search for the electron EDM a powerful way to search for new physics and constrain the possible extensions. In particular, the popular idea that new supersymmetric particles may exist at masses of a few hundred GeV is difficult to reconcile with the absence of an electron EDM at the present limit of sensitivity [4,2]. The size of the EDM is also intimately related to one 1

The most sensitive measurements of the electron electric dipole moment de have previously been made using heavy atoms. Heavy polar molecules offer a greater sensitivity to de because the interaction energy to be measured is typically 10 3 times larger than in a heavy atom. We report the first measurement of this kind, for which we have used the molecule YbF. Together, the large interaction energy and the strong tensor polarizability of the molecule make our experiment essentially free of the systematic errors that currently limit de measurements in atoms. Our first result de = (−0.2 ± 3.2) × 10 −26 e cm is less sensitive than the best atom measurement, but is limited only by counting statistics and demonstrates the power of the method.

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 ...

We have studied the deflection of ground-state sodium atoms passing through a micron-sized parallel-plate cavity by measuring the intensity of a sodium atomic beam transmitted through the cavity as a function of cavity plate separation. This experiment provides clear evidence for the existence of the Casimir-Polder force, which is due to modification of the ground-state Lamb shift in the confined space of a cavity. Our results confirm the magnitude of the force and the distance dependence predicted by quantum electrodynamics. PACS numbers: 42.50.Wm, 32.70Jz, 42.50.Lc Physicists have long been intrigued by the idea that the electromagnetic vacuum interacts with charged particles to produce observable effects. The first experimental verification of this idea was the discovery [1] that the 251/2 and 2P\/2 states of hydrogen are not degenerate. Crudely speaking, the degeneracy is split by the ac Stark effect due to the interaction with the vacuum. Energy shifts of this type are now well established and are generally known as Lamb shifts. The vacuum field in the vicinity of a conducting plate is different from that of free space. In particular, at a distance L from the plate, the spatial distribution, polarization, and spectral density of the vacuum field are substantially altered for frequencies below ~~c/L because of the boundary conditions imposed by the plate. The first discussion of a physical effect due to this modification of the vacuum dates back to 1948 and the seminal work of Casimir [2]. Casimir and Polder [3] discussed the interaction of a neutral atom with a plane conducting plate and showed that the modified vacuum gives rise to a spatially varying Lamb shift whose gradient corresponds to an attractive long-range force. Similar long-range forces are found between any pair of neutral objects, the most famous example being perhaps the Casimir force between two conducting plates. We refer to any such force on an isolated atom as a Casimir-Polder force.Although some quantitative measurements exist on the long-range forces between macroscopic dielectrics [4], the Casimir force has been studied only qualitatively [5], and the Casimir-Polder interaction has eluded detection altogether [6]. Recent experiments on the Rydberg states of helium [7] have yielded precise measurements of the long-range interaction between the Rydberg electron and the He + core, but have not yet reached the point of testing the Casimir-Polder interaction [8], known in that system as K r 'et. In the experiment reported here we have probed the vacuum field in a parallel-plate cavity using a beam of ground-state sodium atoms. Since the vacuum field varies with position, the atoms experience a Casimir-Polder force which pushes them towards the cavity walls. We have used the deflection of the beam to demonstrate the existence of this force and to confirm quantitatively the strength predicted by quantum electrodynamics.For a spherical ground-state atom (3s sodium) between parallel ideal mirrors, the position-dependent atom-cavity interacti...

We demonstrate slowing and longitudinal cooling of a supersonic beam of CaF molecules using counter-propagating laser light resonant with a closed rotational and almost closed vibrational transition. A group of molecules are decelerated by about 20 m/s by applying light of a fixed frequency for 1.8 ms. Their velocity spread is reduced, corresponding to a final temperature of about 300 mK. The velocity is further reduced by chirping the frequency of the light to keep it in resonance as the molecules slow down.Comment: 6 pages, 6 figure

We demonstrate one-dimensional sub-Doppler laser cooling of a beam of YbF molecules to 100  μK. This is a key step towards a measurement of the electron's electric dipole moment using ultracold molecules. We compare the effectiveness of magnetically assisted and polarization-gradient sub-Doppler cooling mechanisms. We model the experiment and find good agreement with our data.

We have decelerated a supersonic beam of 174YbF molecules using a switched sequence of electrostatic field gradients. These molecules are 7 times heavier than any previously decelerated. An alternating gradient structure allows us to decelerate and focus the molecules in their ground state. We show that the decelerator exhibits the axial and transverse stability required to bring the molecules to rest. Our work significantly extends the range of molecules amenable to this powerful method of cooling and trapping.

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