“Abandon every hope, all you who enter.”
Like the entrance to hell in Dante’s Inferno, this gate is one-way only. Physicists have created a laser barrier that lets atoms through only in one direction — mimicking the “demon,” proposed by James Clerk Maxwell in a thought experiment, that selectively opens or closes a microscopic gate to let some atoms through but not others.
The method could cool gases of atoms or molecules to temperatures close to absolute zero, making new kinds of experiments possible. “It’s a very nice demonstration of the one-way barrier concept,” comments Gabriel Price of the University of Texas at Austin. The laser barrier concept was first set forth in 2005 by Austin’s Mark Raizen and his collaborators as a way to cool gases to extremely low temperatures.
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Normally, collecting a gas into a smaller volume ends up increasing the gas’s temperature, while letting the gas expand lowers it. But the laser barrier stuffs the gas into a smaller volume with only a minute increase in its temperature, Daniel Steck of the University of Oregon in Eugene and his colleagues report in the June 20 Physical Review Letters.
Researchers could then let it expand and get colder than the temperature at which it started, even approaching temperatures just above absolute zero.
Steck’s team first trapped a gas of rubidium atoms using laser light, essentially creating a box with walls made of electromagnetic fields.
The researchers then added two laser beams, parallel and next to each other. The beams cut through the trap, dividing it into halves. The beam on the left acted as a barrier, while the beam on the right played the role of the demon.
The researchers tuned the barrier beam to a frequency that would make it interact with the outermost electron in each rubidium atom. When those electrons were in their lowest-energy state, the beam would let the atoms through. But if the electrons were in a slightly higher-energy orbit — an “excited state” — the beam created a repulsive force that made the atoms bounce back.
Initially, no atoms were excited. Atoms approaching the beams from the left would go through the “barrier” unimpeded. But atoms approaching from the right would first have to cross the demon beam. That beam kicked (the technical term is “pump”) the atoms’ outermost electrons into their higher-energy state. Thus, they were turned back when they reached the barrier beam.
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The pumping beam acted like a demon closing a gate only when atoms tried to cross from right to left. “Eventually, all atoms will get stuck on one side,” Steck says. Unlike Dante’s gate, though, this one leads to a colder place.
While other cooling methods only work on particular elements, the barrier technique could cool a wide variety of atoms and molecules down to less than one ten-thousandth of a kelvin, Steck says. Cooling matter allows physicists to study exotic states of matter, and could be helpful for building new kinds of atomic clocks.
A similar cooling trap was described in the March 7 Physical Review Letters. A Texas team that included Raizen and Price achieved a similar result with a more complicated setup that included a magnetic as well as an optical trap, and a barrier beam in between.
In 1871, Maxwell described the thought experiment of an all-knowing demon that could control which particles could cross a gate between two containers. Such an entity would have been able to sort particles — based on their velocity, for example — and thus put more order into the universe.
But, asked Maxwell, doesn’t the second law of thermodynamics say that the total amount of disorder in the universe is to forever increase? In fact, physicists have demonstrated that in order to lower the disorder inside the containers, any such demon will always have to produce disorder elsewhere.
In Steck’s case, the demon’s secret is rather subtle. Like all lasers, Steck’s pumping beam is an orderly arrangement of photons, all traveling in the same direction. And a photon increases the energy level of a rubidium atom by scattering off of it. “But the scattered photon goes in a random direction,” Steck observes. So while the atoms get a little more order in their lives, the pumping laser ends up with a little less.