M. R. Tarbutt

Improved measurement of the shape of the electron

The electron is predicted to be slightly aspheric, with a distortion characterized by the electric dipole moment (EDM), de. No experiment has ever detected this deviation. The standard model of particle physics predicts that de is far too small to detect, 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 de that should be detectable. 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/c2 (where c is the speed of light) is difficult to reconcile with the absence of an electron EDM at the present limit of sensitivity. The size of the EDM is also intimately related to the question of why the Universe has so little antimatter. If the reason is that some undiscovered particle interaction breaks the symmetry between matter and antimatter, this should result in a measurable EDM in most models of particle physics. Here we use cold polar molecules to measure the electron EDM at the highest level of precision reported so far, providing a constraint on any possible new interactions. We obtain de = (−2.4 ± 5.7stat ± 1.5syst) × 10−28e cm, where e is the charge on the electron, which sets a new upper limit of |de| < 10.5 × 10−28e cm with 90 per cent confidence. This result, consistent with zero, indicates that the electron is spherical at this improved level of precision. Our measurement of atto-electronvolt energy shifts in a molecule probes new physics at the tera-electronvolt energy scale.

Slowing heavy, ground-state molecules using an alternating gradient decelerator

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.

Measurement of the electron's electric dipole moment using YbF molecules: methods and data analysis

We recently reported a new measurement of the electrons electric dipole moment using YbF molecules (Hudson et al 2011 Nature 473 493). Here, we give a more detailed description of the methods used to make this measurement, along with a fuller analysis of the data. We show how our methods isolate the electric dipole moment from imperfections in the experiment that might mimic it. We describe the systematic errors that we discovered, and the small corrections that we made to account for these. By making a set of additional measurements with greatly exaggerated experimental imperfections, we find upper bounds on possible uncorrected systematic errors which we use to determine the systematic uncertainty in the measurement. We also calculate the size of some systematic effects that have been important in previous electric dipole moment measurements, such as the motional magnetic field effect and the geometric phase, and show them to be negligibly small in the present experiment. Our result is consistent with an electric dipole moment of zero, so we provide upper bounds to its size at various confidence levels. Finally, we review the prospects for future improvements in the precision of the experiment.

Alternating gradient focusing and deceleration of polar molecules.

Beams of polar molecules can be focused using an array of electrostatic lenses in alternating gradient (AG) configuration. They can also be accelerated or decelerated by applying an appropriate high-voltage switching sequence to the lenses. AG focusing is applicable to molecules in both low-field- and high-field-seeking states and is particularly well suited to the problem of decelerating heavy molecules and those in their ground rotational state. We describe the principles of AG deceleration and set out criteria to be followed in decelerator design, construction and operation. We calculate the longitudinal and transverse focusing properties of a decelerator, and exemplify this by 2D-imaging studies of a decelerated beam of metastable CO molecules.

Design for a fountain of YbF molecules to measure the electron's electric dipole moment

We propose an experiment to measure the electric dipole moment (EDM) of the electron using ultracold YbF molecules. The molecules are produced as a thermal beam by a cryogenic buffer gas source, and brought to rest in an optical molasses that cools them to the Doppler limit or below. The molecular cloud is then thrown upward to form a fountain in which the EDM of the electron is measured. A non-zero result would be unambiguous proof of new elementary particle interactions, beyond the standard model.

Submillikelvin Dipolar Molecules in a Radio-Frequency Magneto-Optical Trap.

We demonstrate a scheme for magneto-optically trapping strontium monofluoride (SrF) molecules at temperatures one order of magnitude lower and phase space densities 3 orders of magnitude higher than obtained previously with laser-cooled molecules. In our trap, optical dark states are destabilized by rapidly and synchronously reversing the trapping laser polarizations and the applied magnetic field gradient. The number of molecules and trap lifetime are also significantly improved from previous work by loading the trap with high laser power and then reducing the power for long-term trapping. With this procedure, temperatures as low as 400  μK are achieved.

Molecules cooled below the Doppler limit

Magneto-optical trapping and sub-Doppler cooling of atoms has been instrumental for research in ultracold atomic physics. This regime has now been reached for a molecular species, CaF. 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 molecules1, and the extension of laser cooling to molecules has begun2,3,4,5,6. A magneto-optical trap (MOT) has been demonstrated for a single molecular species, SrF7,8,9, 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 arrays10 for quantum simulation11, launched into a molecular fountain12,13 for testing fundamental physics14,15,16,17,18, and used to study collisions and chemistry19 between atoms and molecules at ultracold temperatures.

A search for varying fundamental constants using hertz-level frequency measurements of cold CH molecules

Many modern theories predict that the fundamental constants depend on time, position or the local density of matter. Here we develop a spectroscopic method for pulsed beams of cold molecules, and use it to measure the frequencies of microwave transitions in CH with accuracy down to 3 Hz. By comparing these frequencies with those measured from sources of CH in the Milky Way, we test the hypothesis that fundamental constants may differ between the high- and low-density environments of the Earth and the interstellar medium. For the fine structure constant we find Δα/α=(0.3±1.1) × 10−7, the strongest limit to date on such a variation of α. For the electron-to-proton mass ratio we find Δμ/μ=(−0.7±2.2) × 10−7. We suggest how dedicated astrophysical measurements can improve these constraints further and can also constrain temporal variation of the constants.

A jet beam source of cold YbF radicals

We have developed a pulsed supersonic beam of slow, cold YbF molecular radicals with an intensity of 1.4 × 109 YbF molecules per steradian per pulse in the X2 Σ+ (v = 0, N = 0) ground state. The translational and rotational temperatures of the beam are equal. The lowest temperature produced was 1.4 K and the slowest centre-of-mass velocity was 290 K. We show that YbF can be made either by ablating Yb metal into a fluorine-bearing carrier gas or by ablating solid precursors into a pure inert carrier gas. This source is suitable for injecting a molecule decelerator and for high-resolution laser-rf-double-resonance studies such as the measurement of the electron electric dipole moment.

Versatile high resolution crystal spectrometer with x-ray charge coupled device detector

A family of Johann configuration curved crystal spectrometers has been designed to share the basic engineering features of compactness, modularity, facility of alignment and focus, and incorporation of solid-state charge coupled device detector arrays. These detectors have intrinsically low noise, useful energy resolution, two-dimensional position sensitivity, and readout modes that are programmable. The spectrometers, although relatively compact, with a Rowland circle diameter in the range 0.5–2 m, can still have sufficient resolving power, dispersion, and throughput to be invaluable in high resolution studies of atomic and plasma sources. This article discusses the basic design features and performance of these doubly dispersive spectrometers and illustrates their versatility by applications to studies of a wide range of laboratory x-ray sources such as line emission from highly ionized atoms in the extended plasmas of Tokamaks and nearly point plasmas produced by focused laser irradiation of solids and...