I’ve been reading about laser-trapped cold atoms (6Li in particular, which is a fermion) and was amazed at the number of things to keep track of in the experiments, just to gain that degree of control of the atoms.

The techniques (more like element-specific exploitations) that I could vaguely understand individually were:

Can anyone explain how all of these things can be combined for a single experimental setup?
I would expect each of the above tricks would alter the energy levels of the atoms in its own way, so that successfully applying one would make the others completely not applicable. Maybe they’re at completely independent frequencies or directions, but I wasn’t able to tell just from reading a handful of abstracts / intro paragraphs of reviews.

eg. Wouldn’t the magnetic field applied for the BEC-BCS crossover somehow cancel some of the magnetic field in the MOT and make it less effective (or complete ineffective)?
eg. Wouldn’t the RF waves or optical lattices interfere with/decohere the laser used in the MOT?

  • $\begingroup$ This is hard to be answered by a non specialist, but the general answer is: we are talking quantum states, and quantum states have strange complicated quantum stabilities, as is obvious by the existence even of crystals, where several inputs enter, nevertheless clean lattices exist as the solution of the addition of the various field and bounds: distances, rotations, vibrations,electric fields,magneticfields, orbitals of electrons in individual molecules .... $\endgroup$
    – anna v
    Commented May 25, 2018 at 4:11
  • $\begingroup$ Lasers are really quantum mechanical tools, have a look at this video to see how when lasers are involved the whole macroscopic setup is one quantum mechanical state, youtube.com/watch?v=J4Ecq7hIzYU . You are describing a more complicated system but there is no reason that it could not work because interactions are not classical where lasers are involved. $\endgroup$
    – anna v
    Commented May 25, 2018 at 4:13
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    $\begingroup$ I think this question is a great place for a cold atom experimentalist to give an overview of how cold atom experiments are done practically. It has taken a lot of progress in optics to get to the point where such experiments are possible, and many (including myself) would love to learn about the pieces needed to make it work. $\endgroup$
    – KF Gauss
    Commented May 25, 2018 at 4:14
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    $\begingroup$ @annav I have the feeling the question is really about the experimental details, not just the fundamental physics. For example, BECs were possible in principle for a long time, but it took a lot of experimental tools to get to the point of bringing them to reality. $\endgroup$
    – KF Gauss
    Commented May 25, 2018 at 4:16
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    $\begingroup$ @user157879 lets hope someone in such experiments is reading this $\endgroup$
    – anna v
    Commented May 25, 2018 at 4:21

1 Answer 1


They definitely can interfere with each other! For example,

Wouldn’t the magnetic field applied for the BEC-BCS crossover somehow cancel some of the magnetic field in the MOT and make it less effective (or complete ineffective)?

is completely correct- it would be hard, if not impossible, to investigate such physics with atoms in a MOT.

One very helpful way that people get around this is by trapping atoms in an optical trap with light that is far from any atomic transition, sometimes called an optical dipole trap (1). This kind of trap uses the interaction between the atomic electric dipole and light to create a conservative trapping force, so unlike a MOT it doesn't automatically provide any cooling. Instead, it is often the endpoint of an elaborate cooling procedure, which begins with a MOT. One of the main advantages of such far-detuned fields is that the force they apply to a given atom is generally insensitive to the internal state, such as the spin. Therefore, one can manipulate the spin or change the magnetic fields to do something like BEC-BCS physics without affecting the trapping force. Most optical lattices are similarly created with far-detuned lasers.

So using far-detuned lasers can make things much easier to understand, but other manipulation techniques can certainly affect each other. For example, applying RF to manipulate spin can affect a Feshbach resonance, as can near-resonant light. These have been used experimentally to modify the properties of resonances (2), (3). So these different tools can interfere with each other, but sometimes this can actually be turned around and used to extend their powers. Of course, sometimes this interference doesn't work out for what you want to do, in which case you might have to be careful about what fields you have on at the same time. As another possibility, sometimes two fields will affect each other in a relatively benign way, such as the shifts in energy levels from one field causing a shift in the absolute location of a resonance for another field, but not much else. So effects can range from nonexistent to extremely important to significant but only requiring a calibration.

A separate issue, which probably isn't quite what you had in mind, is the question of electrical cross-talk between all the different laboratory components. This is a practical matter rather than a fundamental one, but it can also cause huge problems. For example, RF radiation used to manipulate atoms might also be picked up on a laser circuit as noise, and cause some unwanted modulation of the laser. A great deal of experimental effort goes into isolating these systems as much as possible.

Here is a typical example of how it might all come together:

An experimental cycle begins by collecting atoms in a MOT. These atoms are then transferred to an optical trap consisting of intersecting far-detuned laser beams, and cooled further (with loss of atoms) by evaporative cooling. At the end of this cooling process there is a degenerate gas held in this optical trap. Then the optical lattice is turned on such that the atoms are loaded into the ground band. At this point the experiment itself begins, where the gas is probed (perhaps using RF or Raman transitions or any number of other tools), and then ultimately measured in a destructive way by taking an image using on-resonant light.

  • $\begingroup$ Since you are knowledgable about this topic, could you expand your answer to give a broader overview of the techniques OP mentions (and others), and how things come together in cold atom experiment? This would make your answer a great resource for newbies like me. $\endgroup$
    – KF Gauss
    Commented May 29, 2018 at 6:21
  • $\begingroup$ @user157879 I have given a brief example that I hope is helpful. It does not attempt to answer your question in full because that would literally take a textbook (for example, global.oup.com/academic/product/…) and thus fall under the "too broad" category of questions for this site. If you have other specific questions, maybe you can ask them separately and I (and perhaps others) will be happy to take a crack at them. $\endgroup$
    – Rococo
    Commented May 30, 2018 at 1:21
  • $\begingroup$ Also, I'm a little puzzled by the downvote (assuming it wasn't from @user157879)... comments welcome. $\endgroup$
    – Rococo
    Commented May 30, 2018 at 1:24

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