How to decelerate a molecule


When it comes to shuttling individual atoms about, physicists have made giant strides in cooling, trapping and even collimating them into matter wave beams. These kinds of tricks are already being used for matter-wave interferometry on chips.

But if you want to do the same kinds of things with molecules, you’re out of luck. There are two problems. First, molecules are much harder to slow down and trap in decent quantities. And second, they are much more difficult to ID. Atoms are usually identified by the light emitted by electronic transitions, which are usually in the visible part of the spectrum. In most molecules, however, these transition are in the UV and so much harder to access.

Now Samuel Meek and friends from the Fritz-Haber Institute, a Max Plank Institute in Berlin, have tackled one of these problems by building a molecular decelerator on a chip. The device consists of an array of electrodes that create an electric field with a local minimum, or well, that polar molecules tend to fall into. The well can be moved along the array.

Decelerating molecules is then a matter of matching the velocity of the well to that of the incoming molecules and then rapidly slowing it down. Meek and co say that in this way they have halved the kinetic energy of carbon monoxide molecules by slowing them from 360m/s to 240m/s.

That’s impressive and the team reckons that with a little tweaking, the chip will be able to bring the CO molecules to a standstill.

Strangely, nobody has given much thought to what you can do with stationary CO molecules. One option is to use them to store qubits for quantum computing but there seem to be few other ideas.

Which means there’s a good opportunity here for a creative thinker to make a mark.

Ref: A Stark Decelerator on a Chip

One Response to “How to decelerate a molecule”

  1. Samuel Meek says:

    I was glad to see my work mentioned here. However, I thought I should try to provide a bit of context to the work that is not mentioned in the paper itself.

    First of all, this is not the first time that polar molecules have been decelerated using electric fields. This has been done in 1999 in by Bethlem et al [Phys. Rev. Lett. 83, 1558] and in many experiments since then. A review is available in S. Y. T. van de Meerakker et al, Nature Physics 4, 595 (2008), and an example of such an experiment can be found on the arXiv at

    Such deceleration experiments are performed by letting the molecules fly from regions of low electric field to regions of high electric field. During this process, molecules in the relevant quantum states lose kinetic energy. If the molecules were to fly back to a low field region, they would regain this kinetic energy. Before this can happen, however, the electric field is switched so that the molecule that was at high field before is at low field afterward. In the process, the molecule permanently loses a bit of energy. The energy lost by switching once isn’t enough to stop the molecule, so generally, the process has to be repeated about 100 times. Such decelerators generally use fields produced by electrodes 4 mm apart with a voltage difference of 40 kV.

    The new decelerator presented in this paper uses exactly the same forces as the big decelerators. Its method of operation, however, is quite different. One possible analogy would be to consider catching a baseball without a glove. Even though it is easy to hold a ball in your hand, it is very difficult to catch the ball with your bare hand; most likely, the ball will bounce off and probably hurt your hand in the process. However, if you move your hand backwards to match the velocity of the ball as it arrives, you can decelerate it slowly and successfully catch it. In the case of the molecules, if we would try to load fast molecules into a stationary well, the well would not be strong enough to contain them, and they would fly out again. If we instead move the well to match the initial velocity of the molecules and decelerate it slowly once the molecules are in the moving trap, we can remove the kinetic energy slowly and thus decelerate the molecules. Because this is all implemented on a much smaller length scale, the potentials applied are only about 200 V instead of 40 kV as in the previous devices.

    As to what we will do with the CO molecules once we bring them to standstill, the short answer is: not much. The problem is that CO can only be manipulated easily in a metastable state that only lives for a few milliseconds. After they decay, they no longer see the electric field minima as traps. As a result, metastable CO molecules would not be suitable for quantum computation. The advantage of using metastable CO is that it has 6 eV of internal energy, so it is easy to detect, making it well suited characterizing a new decelerator. Though this short lifetime is a limitation, it is presents an opportunity: by trapping metastable CO molecules in a trap at rest, it is possible to precisely measure their lifetime. This has actually been done with a large decelerator, and the paper can be found on the arXiv here:

    In the end, if we want to consider experiments that require long storage times, we would use another polar molecule, such as ND_3, OH, or NH. These molecules can be manipulated in their electronic ground states and can thus be trapped for longer periods of time.