Stefan Truppe

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.

An intense, cold, velocity-controlled molecular beam by frequency-chirped laser slowing

Using frequency-chirped radiation pressure slowing, we precisely control the velocity of a pulsed CaF molecular beam down to a few m s–1, compressing its velocity spread by a factor of 10 while retaining high intensity: at a velocity of 15 m s–1 the flux, measured 1.3 m from the source, is 7 × 105 molecules per cm2 per shot in a single rovibrational state. The beam is suitable for loading a magneto-optical trap or, when combined with transverse laser cooling, improving the precision of spectroscopic measurements that test fundamental physics. We compare the frequency-chirped slowing method with the more commonly used frequency-broadened slowing method.

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

Abstract Experiments with cold molecules usually begin with a molecular source. We describe the construction and characteristics of a cryogenic buffer gas source of CaF molecules. The source emits pulses with a typical duration of 240 s, a mean speed of about 150 m/s, and a flux of molecules per steradian per pulse in a single rotational state.

Characteristics of a magneto-optical trap of molecules

We present the properties of a magneto-optical trap (MOT) of CaF molecules. We study the process of loading the MOT from a decelerated buffer-gas-cooled beam, and how best to slow this molecular beam in order to capture the most molecules. We determine how the number of molecules, the photon scattering rate, the oscillation frequency, damping constant, temperature, cloud size and lifetime depend on the key parameters of the MOT, especially the intensity and detuning of the main cooling laser. We compare our results to analytical and numerical models, to the properties of standard atomic MOTs, and to MOTs of SrF molecules. We load up to molecules, and measure a maximum scattering rate of s−1 per molecule, a maximum oscillation frequency of 100 Hz, a maximum damping constant of 500 s−1, and a minimum MOT rms radius of 1.5 mm. A minimum temperature of 730 μK is obtained by ramping down the laser intensity to low values. The lifetime, typically about 100 ms, is consistent with a leak out of the cooling cycle with a branching ratio of about . The MOT has a capture velocity of about 11 m s−1.

A high quality, efficiently coupled microwave cavity for trapping cold molecules

We characterize a Fabry?P?rot microwave cavity designed for trapping atoms and molecules at the antinode of a microwave field. The cavity is fed from a waveguide through a small coupling hole. Focussing on the compact resonant modes of the cavity, we measure how the electric field profile, the cavity quality factor, and the coupling efficiency, depend on the radius of the coupling hole. We measure how the quality factor depends on the temperature of the mirrors in the range from 77 to 293 K. The presence of the coupling hole slightly changes the profile of the mode, leading to increased diffraction losses around the edges of the mirrors and a small reduction in quality factor. We find the hole size that maximizes the intra-cavity electric field. We develop an analytical theory of the aperture-coupled cavity that agrees well with our measurements, with small deviations due to enhanced diffraction losses. We find excellent agreement between our measurements and finite-difference time-domain simulations of the cavity.

MEASUREMENT OF THE LOWEST MILLIMETER-WAVE TRANSITION FREQUENCY OF THE CH RADICAL

The CH radical is an important constituent of stellar atmospheres, interstellar gas clouds and is of fundamental importance to interstellar chemistry. Furthermore, it offers a sensitive way to test the hypothesis that fundamental constants measured on earth may differ from those observed in other parts of the universea. Here, we present a measurement of the lowest millimeter-wave transition of CH, near 535 GHz, with an accuracy of 0.6 kHzb, an improvement of nearly two orders of magnitude compared to the previous best rest frequencies. We drive the millimeter-wave transitions using the 54th harmonic of a frequency synthesizer phase-locked to a 10 MHz GPS frequency reference. Using ALMA this transition has recently been observed in the absorber PKS 1830-211 at a redshift of z = 0.89c. As pointed out by de Nijs et al.d a very robust and sensitive means to search for variations in fundamental constants could be obtained by observing the lowest millimeter-wave transition of CH along with the two Λ-doublets at 3.3 and 0.7 GHz, all from the same interstellar gas cloud.