MIT physicists have improved the cooling techniques used to cool atoms into Bose-Einstein condensates. The new method is faster than the conventional one and conserves a large fraction of the original atoms. In research published in the journal Science, the team shows how it cooled a cloud of 2,000 rubidium atoms from room temperature to 1 mK, and from that generated a condensate of 1,400 atoms, conserving 70 percent of the original cloud.
“People are trying to use Bose-Einstein condensates to understand magnetism and superconductivity, as well as using them to make gyroscopes and atomic clocks,” says Vladan Vuletić, the Lester Wolfe Professor of Physics at MIT and senior author of the paper. “Our technique could start to speed up all these inquiries.”
The first Bose-Einstein condensates were successfully produced in 1995 by researchers in Colorado and by Wolfgang Ketterle and colleagues at MIT. Since then, scientists have conventionally created Bose-Einstein condensates through a combination of laser and evaporative cooling, a slow and inefficient process wherein more than 99 percent of the atoms in the original cloud are lost.
The process generally begins by shining laser beams from several directions on a cloud of atoms. The photons in the beam act as tiny ping pong balls, bouncing off much larger, basketball-sized atoms, and slowing them down a little in each collision. The laser’s photons also act to compress the cloud of atoms, limiting their motion and cooling them in the process.
There’s a limit to how much a laser can cool atoms, however, as there is less room for photons to scatter when a cloud becomes more dense; instead, they start to generate heat. And so, scientists typically turn off the light and switch to evaporative cooling, a process Vuletić says is “like cooling a coffee cup—you just wait for the hottest atoms to escape.”
The drawback is that evaporative cooling is a slow process that ultimately removes more than 99 percent of the original atoms in order to retain the atoms that are cold enough to turn into Bose-Einstein condensates. “In the end, you have to start with more than 1 million atoms to get a condensate consisting of only 10,000 atoms,” Vuletić says. “That’s a small fraction and a big drawback.”
Vuletić and his colleagues decided to research whether there was a method to get around the initial limitations of laser cooling, and discovered a way to cool atoms into condensates using laser light from start to finish—a much faster, atom-conserving approach that he describes as a “longstanding dream” among physicists in the field. “What we invented was a new twist on the method to make it work at high [atomic] densities,” Vuletić says.
The researchers employed conventional laser cooling techniques to cool a cloud of rubidium atoms down to just above the point at which atoms become so compressed that photons start to heat up the sample, and then switched over to a method known as Raman cooling, using a set of two laser beams to cool the atoms further. The team tuned the first beam so that its photons, when absorbed by atoms, turned the atoms’ kinetic energy into magnetic energy. The atoms, in response, slowed down and cooled further, while still maintaining the original total energy.
The team then aimed a second laser at the much-compressed cloud, tuned in such a way that the photons, when absorbed by the slower atoms, removed the atoms’ total energy, cooling them even further. “Ultimately the photons take away the energy of the system in a two-step process,” Vuletić says. “In one step, you remove kinetic energy, and in the second step, you remove the total energy and reduce the disorder, meaning you’ve cooled it.”
By removing the atoms’ kinetic energy, he says, one is essentially doing away with their random motions and transitioning the atoms into more of a uniform, quantum behavior resembling Bose-Einstein condensates. These condensates can ultimately take form when the atoms have lost their total energy and cooled sufficiently to reside in their lowest quantum states.
To reach this point, the researchers found they had to go one step further to completely cool the atoms into condensates. To do so, they needed to tune the lasers away from atomic resonance, meaning that the light could more easily escape from the atoms without pushing them around and heating them. The atoms become almost transparent to the photons, according to Vuletić, meaning that incoming photons are less likely to be absorbed by atoms, triggering vibrations and heat. Instead, every photon bounces off just one atom. “Before, when a photon came in, it was scattered by, say, 10 atoms before it came out, so it made 10 atoms jitter. If you tune the laser away from resonance, now the photon has a good chance of escaping before hitting any other atom. And it turns out by increasing the laser power, you can bring back the original cooling rate.”
Switching to the new processes after the initial round of laser cooling took the rubidium atoms from 200 mK to 1 mK in just 0.1 seconds, 100 times faster than the conventional method of switching from laser to evaporative cooling.
“When I was a graduate student, people had tried many different methods just using laser cooling, and it didn’t work, and people gave up. It was a longstanding dream to make this process simpler, faster, more robust,” Vuletić says. “So we’re pretty excited to try our approach on new species of atoms, and we think we can get it to get it to make 1,000-times-larger condensates in the future.”