Molecular Fountain

The resolution of any spectroscopic or interferometric experiment is ultimately limited by the total time a particle is interrogated. Here we demonstrate the first molecular fountain, a development which permits hitherto unattainably long interrogation times with molecules. In our experiments, ammonia molecules are decelerated and cooled using electric fields, launched upwards with a velocity between 1.4 and 1.9 m/s and observed as they fall back under gravity. A combination of quadrupole lenses and bunching elements is used to shape the beam such that it has a large position spread and a small velocity spread (corresponding to a transverse temperature of $<10\mu \mathrm{K}$ and a longitudinal temperature of $<1\mu \mathrm{K}$) when the molecules are in free fall, while being strongly focused at the detection region. The molecules are in free fall for up to 266 ms, making it possible, in principle, to perform sub-Hz measurements in molecular systems and paving the way for stringent tests of fundamental physics theories.

Figure 1

Experimental setup with simulated trajectories. (a) Schematic view of the top part of the vertical beam machine showing the end of the traveling wave decelerator and the quadrupole lens system. The quadrupole lens consists of four cylindrical rods suspended by two ceramic discs. Two ring electrodes bunch molecules in the $z$ direction. For a view of the inside, part of the quadrupole and the buncher have been cut in the figure. Molecules are ionized by a UV laser and imaged on a phosphor screen located behind a multichannel plate (MCP). The image is recorded using a charge coupled device (CCD) camera and a photo-multiplier tube (not shown). The red curves show a simulation of trajectories through the lens system for a beam launched with a velocity of $1.8\mathrm{m}/\mathrm{s}$. (b)–(g) Phase-space plots showing the acceptance of the setup in both the longitudinal [(b)–(d)] and transverse directions [(e)–(g)], at three different heights. Note that the axes of panel (g) are scaled by a factor of 10 compared to panels (e) and (f). The grey ellipses show the distribution of the packet of molecules at the exit of the decelerator.

Figure 2

Time-of-flight profiles of the rising and falling beams. Number of detected ammonia ions per shot as a function of time after opening the pulsed molecular beam valve for the rising (top panel) and falling (lower panel) beams. Each data point (open circles) is averaged for 400 (1000) shots for the rising (falling) beam. The red lines show fits to Lorentz profiles. Background signals of typically 0.7 ions per shot (rising beam) and 0.15 ions per shot (falling beam) have been subtracted.