Journal of Undergraduate Sciences FEATURES Slower than a Speeding Bullet Lene Hau gives the inside scoop on slowing light to 38 mph interview by Katrina Garcia Dr. Lene Hau of the Rowland Institute was recently in the national spotlight for slowing the speed of light by a factor of 20 million. Here, in an interview with JUS, she explains her research, talks about the ups and downs of the scientific endeavor, and offers her advice to aspiring scientists. Q: Please explain how you slowed down light. A: When you send light into glassglass has a refractive index of 1.5a vacuum has 1, sort of a reference point, right? That means, since the refractive index of glass is a little bit larger than 1.5, it means that light will slow down when it goes in there, but that factor of 1.5, so it slows down 50 percent. We slow it down [by a factor of] 20 million. To do that, we need a much more subtle effect, a much more interesting effect. And its really a quantum mechanical effect. (Dr. Hau goes on to explain the procedure that elicits this effect.) In the lab we have a fantastic atom refrigerator that al- lows us to get some really, really cold [sodium] atoms. First, we make what is called an optical molasses, a really Dr. Hau looks over the apparatus used to slow light. viscous medium set up by three pairs of counter-propa- gating laser beams, two just a little bit below the atomic resonance frequency. In this viscous medium we cool the atoms to a millionth of a degree above absolute zero. At but they get colder and colder and denser and denser that point we turn all the lasers off and then we turn on a also, if it is done right. But we actually dont wait for the hefty electromagnet. We have designed our own special hot atoms to leavewe kick them out. You simply apply electromagnet, called a 4-D magnet. Now once we cap- a radio frequency, and if you tune the frequency just right, ture these atoms in this electromagnet we can use the you can get the magnets of the hottest atoms to flip their fact that [sodium] atoms have a little magnetic dipole direction all of a sudden. If you flip them the wrong direc- moment. We can use this little magnetic dipole moment tionsouth and north poles will attract each other and to trap [the atoms in a changing magnetic field] so that we south and south will repel each otherall of a sudden can capture this cloud of atoms. Then we start cooling [they get kicked] out of the magnet[ic field]. And then the with what is called evaporative cooling. This part of the rest of the atomsthe cold onesare still trapped, and process is done in complete darknessit is totally dark in will collide and re-equilibrate at a lower temperature be- the lab. cause you got rid of the hot ones. And then you sweep to Evaporative cooling works similar to the way you cool a a little lower frequency and then you kick out the next set cup of coffee. The hot atoms will leave and the rest will of hot atoms. Thats evaporative cooling. Then if we collide and it will equilibrate at a lower temperature and sweep that radio frequency far enough down we get tem- then the next set of hot atoms will leave and so forth. You peratures of just a few billionths of a degree above abso- get a colder and colder cloud with fewer and fewer atoms lute zero, or nanokelvin. And if we cool down even further 10 JUS VOL 5 ISSUE 2 Journal of Undergraduate Sciences FEATURES we get a very odd state of matter called a Bose-Einstein is moving through our condensate. So, we create a little condensate. piece of condensate moving through our bigger conden- You cant use old Newtonian mechanics to describe the sate, and its moving very slowlyright now, 17 meters internal structures of atoms. We know that very well now; per second. we have to use quantum mechanics, which was devel- Its not becauseand many people misunderstand this oped in this century. When atoms get really cold you also its not that were creating a humongously big refractive have to describe the motion of the whole atom, the whole index in our medium. Our refractive index, curiously translation of motion, not just the internal structure, with enough, is actually 1. Its exactly the same as it is in quantum mechanics. You cant vacuum. At resonance, its 1. think of them as billiard balls We use our light-pulse on reso- and use old Newtonian me- nance, so its tuned to a reso- chanics. So each atom has a nance frequency in the atom. wavelength associated with it, Our refractive index, and thats the de Broglie wavelength. the important part, has a very [When this wavelength] be- steep variation. It varies very comes comparable to the in- rapidly around resonance. If terparticle distance then you you go a little bit away from can imagine how these atoms resonance, the refractive index will lock together and behave will not be 1 any more. There sort of like one super-atom in will be a very steep variation a totally correlated fashion. as a function of frequency. That is the Bose-Einstein con- Thats what were using. Its densate. Once we have done that steep variation of the re- that [created the Bose-Einstein fractive index that we set up condensate], then were ready in our medium. to slow the light down. So now we have 2 to 10 Q: What was your first sig- million atoms in our atom nificant scientific experience cloud, cigar shaped, typically that made you want to go into 100 to 300 microns long and science? maybe 10 to 30 microns wide. Then we send in the laser A: I always loved math and beam that will illuminate this that was sort of my first expo- atom cloud from the side. The sure to something that could atoms and the laser field will Lene Hau and team at home in the lab be related to physics. And I mix, entangle, and create a real really liked that from first grade entangled state for this system. and onwards. And then we By choosing the right intensity and the right frequency for started to have physics in junior high. That physics teach- that coupling laser field, we can tune the optical proper- ing, I would say, was pretty bad. But then I entered high ties of this mixed medium to do exactly what we want. school and thats a very special thing in Denmark. High With the right combination of cold atoms and that laser school teachers have university degrees they get field with the right intensity and frequency, we send the Masters degrees. All of a sudden we got math, and in light particles in along the long cigar direction of the cloud particular physics, taught at a very high level. In particular and then we simply measure how long it takes these light it was my encounter with the Bohr model that struck me particles to go through our medium. as absolutely fascinating. Bohr, a Danish physicist, created [Our measurements show that we] slow light down by a a new way of thinking with a paper back in 1913 where factor of 20 million. When you have a light pulse in he talked about how we should think about the atom as vacuumour light pulses are typically, say, two-and-a- having discrete energy levelsnot a continuum. He sort half microseconds long in timeand if you stand from the of kicked off this whole new way of thinking about the side and look at it from space, that would be 750 meters world in terms of quantum mechanics, a way to think dif- long. And then when it goes inside our medium, it basi- ferently about how atomic systems behave differently than cally compresses by that same factor of 20 million, so it our normal macroscopic world, and how we have to use becomes a little, localized 40-micron long light pulse that quantum mechanics and these discrete energy levels JUS VOL 5 ISSUE 2 11 Journal of Undergraduate Sciences FEATURES [when describing them]. That was just absolutely fascinat- ing. And then, of course I chose to go into mathematics A: Oh yeah! That was absolutely, totally fantastic, be- and physics. cause we started to see our first sign that we had slowed In Denmark, at least at the time when I took my de- the light down in the spring of 98. We started to see [that gree, you go to the university and you pick two topics the light] was delayed a little bit...wow! And then, of course, [and that is all you study]. I picked mathematics and phys- youre afraid. Gee, what if somehow somebody moved ics. And then you decide after approximately four years if the knob; its an artifact. It could be all kinds of things. you want to get a Masters in physics or a Masters in math- And then you want to do a test, for example, by turning ematics and whichever you dont choose you then have a that extra laser off, because if we turn that off the system Bachelors in. I thought that I wanted to do mathematics gets totally opaque. We cant get any light, so that was until I had quantum mechanics.