Physics of Trapping Atoms and Biomolecules
One of the fundamental problems of studying gas atoms is that they move too quickly, but many methods have now been developed to cool these atoms and trap them for later observation. Joining us to discuss these methods is Professor Steven Chu from Stanford University. Professor Chu’s work spans many realms of physics from observing individual biomolecules to the laser trapping of atoms, techniques for which he was awarded the Nobel Prize in Physics in 1997. He was recently visiting as one of the many prestigious Hitchcock lecturers.
Steve Chu (SC) joins Charles Lee (CL) to discuss cooling atoms and holding on the biomolecules.
CL: Professor Chu, thank you very much for joining us today.
SC: My pleasure.
CL: I think our audience is probably very fascinated in this work on how you actually cool atoms using lasers. Can you explain maybe a little bit how that is done?
SC: Well, the idea is that you want to reduce the average velocity of these atoms down to a very very low speed and that’s what we really mean by cooling the temperatures and measuring the average motional energy of these atoms. So, one does this by actually shining light on the atoms. As the light scatters from the atoms, you can actually cool them down. And then the trick is that you have to arrange the light to preferentially scatter off of photons opposing the motion and this is done simply by tuning the frequency of the light so that when the atoms are moving towards this laser beam, it has a frequency shift called the Doppler shift that actually shifts it more into resonance.
CL: I see, so what is it about the atom then that is able to take on this energy or be slowed by this particular frequency of light?
SC: Well, every atom absorbs at a certain frequency band and you have to think of the light as actually having little bits of momentum of light. Each particle of light, called photon, when it scatters off an atom, the atom will ricochet by just a tiny amount. But if you then have tens of thousands of these little ricochet recoils — even a lot of BBs aimed at a bowling bowl rolling down an alley — could slow it, stop it and turn it around.
CL: Do you need more than one laser then to keep the atom in place?
SC: Well, you need more than one laser beam and so what we do is split the laser into several pieces and then we use a bunch of mirrors, so I often say that the magic of cooling atoms is done with mirrors.
CL: I recall that you describe this as optical molasses. Why is that?
SC: Molasses has a particular property: if you put something in molasses, your finger for example, and you want your finger to go to the right, to the left, what happens is no matter which way it wants to go, it feels this viscous goo dragging it down, trying to slow it up and stop it. And that is very much like atoms in these laser beams. No matter which way it wants to go, it feels a force slowing it down and trying to stop it.
CL: And once you’ve cooled these actual atoms using these laser beams, how do you actually contain them within a particular region of space?
SC: Well, there are several ways. One way is to use magnetic fields or alternating electric fields, namely, even light. If you take a laser beam and you focus the laser beam very tightly, you create in a region of space, a place where there is very high electric field. The electric fields are due to the light beams and in a region of high electric field, as long as the electrons on the atom can keep in sync with the driving electric field, so if the electric field points for example, then the electrons relative to the nucleus of the atom then pointing in the direction where it wants to go into a region of high electric field. So, its exactly the same principle as a charged rod you use to pick up a little piece of paper.
CL: I see, so it’s just being attracted to this field?
SC: It’s attracted to a region of high field. And so, a simple focused laser beam is a region of high field.
CL: Intriguing, so how then did you study these molecules once they were in place?
SC: The remarkable thing of atoms once they are very cold and they are moving very slowly, we are talking about speeds on the scale of one centimeter per second. About the speed of an insect walking. And so what we can do now is reduce the speed of these atoms from supersonic jet planes to that speed. Once they’re moving that slowly, it’s easy to use very feeble forces to push them around and to hold them like for example this focused laser beam.
CL: What sort of results have come out of studying atoms at this really low temperature? What have we learned?
SC: First, it was a bit of a surprise. The initial theory of how these atoms should behave in these light beams turned out to be incomplete and the real explanation was much more complicated, but it allowed one to get atoms ten times colder than what we thought initially, so we had a deeper understanding of how light interacts with atoms. But beyond that, I would say one has to look at this as a new tool, a new technique. You got atoms, they are very cold, you can do with them what you want. So, the first things that came out were atomic clocks. In my lab when I first joined Stanford, we said okay, now that we’ve finally got these cold atoms, can we make a better clock? An atomic clock is something where you have two energy levels on an atom and what you want to do is you want to use those energy levels that are very well defined by nature to be a reference source. So imagine you have some microwave oscillator and you try to keep that microwave oscillator tuned to the energy levels of the atom, and of course the microwave oscillator is a thing that we manufacture and when we manufacture things, they are prone to drift, they are prone to all sorts of inaccuracies. But it’s okay because we are allowed to adjust that microwave oscillator so it’s always tuned in resonance with the atom. That’s the basic idea of the clock and we were able to make a better clock between two groups, both connected with cooling and trapping technologies. Within five years, it became the new standard for the world. Other applications include making atom interferometers. That’s a thing where you split up the atom quantum mechanically, so don’t think of it as splitting up the atom as in nuclear fission, but splitting it up quantum mechanically means the atom could be in one region of space and at the same time in another region of space. And then you bring them back together to have very sensitive tool for making measurements, rotations, accelerations, and so on.
CL: Would this also be useful for looking at the information transfer between the two different states as well?
SC: Yes, that’s yet another application that hasn’t really become practical, but just splitting the atom apart and bringing it back together allows you to measure acceleration so much more accurately, so much more quickly, that there’s research going on to actually make this into a useful tool of exploration for oil and minerals.
CL: This is certainly very fascinating work but I do want to move into the work that you are doing on watching individual biomolecules move. I’m curious, why did you approach this question?
SC: Well, in actual fact and all honesty, we were working with atoms and quite by accident, we first said okay, this is a lesson in standard electricity, we can use focused laser beams to hold onto atoms, but as a preliminary trial and much easier experiment, we can scale it up, can we use a focused laser beam to hold onto little plastic particles? These are one micro diameter spheres of polystyrene thrown into water and that was just a warm-up experiment. The water would act as the optical molasses, a viscous medium that wants to stop the beads. So that experiment worked very well and then we were emboldened to try on atoms and several months later, the experiment worked on atoms. Now in the meantime, the person whom did the trapping of the beads — we were collaborating with a fellow by the name of Art Ashkin — continued to play around with these beads. He discovered this quite by accident. In a typical day you go into your lab, you got beads in water, you push them around and see what they can do. One day, he comes in and he sees these other things in the water. To be sure this was done in New Jersey at Bell Labs and what he found was eventually bacteria started to grow in the water, and he was able to hold on to individual bacteria and move them around. If he turns off the light, they would swim away. He would go back and re-tract them and push them around and they would swim away again. Then he found that he not only could hold onto bacteria , he could hold onto individual virus particles. So when I got to Stanford, I said well this is really great and by then, you could reach inside a single celled organism and hold on to the nucleus of the organism and other people found that you could reach inside the nucleus and hold on to an individual chromosome of a live organism. And you don’t have to puncture the nucleus or the cell because it’s light penetrating through this. So I said, this sounds pretty good, why don’t we try to hold onto individual biomolecules? What type of experiments could one do in biology? I wasn’t really concerned at the moment, I said I’m sure there must be something. Well, let’s just see if we could do it, so in the late eighties, we succeeded in holding on to individual biomolecules. I was sort of moonlighting, I got a MD/PhD student at Stanford to teach me a little biochemistry and late at night working with him in the lab, we finally succeeded in gluing on these spheres. There was a little mistake, I was naive in biochemistry, but after I succeeded, I turned to my graduate students and said okay, I think this can be done, why don’t you perfect it. And then it took a year and a half to perfect. The technical details are not important, but in the end, what happened was here was again a new method or technique. You can hold onto individual molecules and that actually exploded as well in terms of its applications both in polymer physics and also in biophysics. One of the early applications that I helped some of my colleagues at Stanford do and also other people was that you could start to do an experiment where for example, you want to understand how muscles contract. Well, ultimately you can describe the tissues and what the structures are but ultimately it boils down to the essential ingredients of muscle contraction can be described as the action of one big molecule called myosin, the name is not important here. And it attaches to a long rod called actin and the unit of fuel that we use is ATP. When it burns one unit of ATP, this molecule can actually contract, actually change its shape and pull on this filament called actin. And we were able to arrange an experiment where you feed this one ATP molecule, it contracts and pulls on this rod and using an optical tweezer, you pull against the rod and you can actually measure the force of a single pull of a single molecule when it burns one unit of fuel. You can measure how far it moves when it burns this fuel. Now this has become literally a cottage industry because it turns out, that first of all, you begin to explore all the what we call molecular motors that induce motion on these other filaments. These are motors not only used in muscle contraction but they are also used in the transport of material within a cell. So if you want to move material from one end of the cell to the other, there’s a little motor, a little bag, you put the stuff in the bag and it trucks along this highway. Now it turns out that much of what happens in biology is forces and motions and distortions of molecules as they interact with other molecules. And this technique, along with two other techniques that were invented roughly within a five to ten year period, has enabled biologists and biophysicists to begin to look at interaction of molecules now based on what many of us believe are becoming to be the important way of looking at it, that biomolecules actual move, they change their shapes, and it’s those movements that change their shape. The structures that they take enables us to describe how they work in terms of what we call a mechanism. It’s a real physics/chemistry description of what it’s really doing. It goes from Step A to Step B, there’s a mystery and it is now seen as essentially physics.
CL: It seems like it’s removing all the inherent discrepancies that might result from a bulk study of a lot of molecules.
SC: When you mention bulk studies, that is typically experiment in a chemistry or biology lab, you work with a test-tube full of (what? now these days it’s an Eppendorf volume full of) these biomolecules. And it’s very difficult sometimes to find out the true behavior when you have an average. It’s like you have an average of the all students at a campus. Well, there is a lot of different types of students doing different types of things and so that’s all smeared into some one average. The average of all the students going about their business, taking different courses doesn’t really tell you what’s really happening.
CL: We’re running out of time here, but I’m just curious so is there anything else coming down the pipeline?
SC: Biophysicists and biologists have taken these single-molecule methods that were developed not only in my group but also in groups around the world. One of the leaders here, there have been several groups here that have been biophysical methods, like Carlos Bustamante’s group. So, one is using these methods to attack more complex problems and they seem to be working well. What’s coming down the pipe is really a deeper understanding of how life really works.
CL: Professor Chu, I just want to thank you very much for your time and joining us today.
SC: My pleasure.