When you think about the temperatures associated with “cold,” you probably imagine a cold winter day, or a block of ice (32 °F, 0 °C, or 273.15 K). This is downright balmy compared to the nanokelvin (10-9 K) temperatures physicists can regularly achieve in the lab. Now, things are about to get even chillier with a new technique that can reduce the entropy—and therefore temperature—of a cold gas to near-absolute zero by finely controlling the number and energy level of atoms.
At near-absolute-zero temperatures, atoms can be held in an optical lattice—formed by standing light waves, where the atoms sit in the troughs of the waves at low potential energy. At these temperatures, they lose most of their thermal fluctuations and begin to act like an ideal quantum system. Atoms held in an optical lattice can be used to simulate electrons trapped in a crystalline solid, so this quantum system can be helpful in studying important phenomena like quantum magnetism and high-temperature superconductivity. The atoms could also be used for quantum logic gates and registers (the working memory of quantum computers).
Unfortunately, to truly create an ideal quantum system, physicists have to reach temperatures extremely close to absolute zero, in the picokelvin (pK, 10-12 K) range. The current record for low temperature is 100 pK, but this wasn’t a gas held in an optical lattice.
Reducing the entropy
In order to reach these ultracold temperatures, a team of Harvard physicists led by professor Marcus Greiner developed a new technique that cools the gas in an optical lattice by reducing the entropy, which is inherently tied to the temperature. According to the third law of thermodynamics, the entropy of a system reaches a minimum at absolute zero, so by reducing the entropy to its lowest achievable value, the temperature can be reduced to just about absolute zero.
One way to think about entropy is as a measure of disorder—the more energetic a system is, the more disordered it is likely to be. In this system, disorder comes from extra atoms sitting in the optical lattice sites. If you can remove the extra atoms, you can make it more orderly and simultaneously reduce the entropy and temperature.
Professor Greiner’s team did this by using a newly discovered phenomenon they call orbital exchange blockade (OEB), which prevents the addition of atoms to a site. An atom occupying an energy level at a single site prevents any further atoms at that site from being excited to the same energy level through repulsive interactions.
This OEB thing sounds great, but how do you exploit it to cool the gas? By changing the intensity of one of the standing light waves that forms the optical lattice, the "depth" of the lattice sites can be modulated. If you match the frequency of this change to the energy gap between the ground and excited levels of the lattice site, you can excite atoms to make the jump.
However, if an atom is already sitting in that excited level, OEB prevents any additional atoms from reaching it, forcing everything else to remain at the ground level. The excited atoms can then be removed from the system, reducing the entropy. Importantly, the excitation frequency depends on the number of atoms at each level, so you can control the final number of atoms at lattice sites.
Demonstrating the blockade
The team demonstrated their technique with two experiments, using a gas of rubidium-87 atoms in a square optical lattice. In the first, they started with a known number of atoms at each site (between one and four) all at the ground energy level. Then, by modulating the frequency, they gradually removed all the extra atoms, finishing with only one in each lattice site—a minimal entropy configuration.
In the second experiment, instead of starting with a known number of atoms all at the ground level, they loaded the lattice with a random number per site, with some excited and some at the ground level. As before, by sweeping the frequency, they removed all the extra atoms.
In the experiments, the team ran into some limitations that prevented optimal cooling and therefore kept the final temperature higher than they'd like. The laser beams used to generate the optical lattice heated the gas slightly, for one thing, although this could be reduced by changing the wavelength used. In addition, inefficiencies in the atom excitation process limited the final entropy.
Even if this technique is never used to reach the picokelvin temperature goal, being able to remove entropy and finely control the number of atoms at specific lattice sites will surely come in handy for quantum computing. In fact, they effectively created the largest quantum register yet, with over 1,000 controllable sites. No wonder, then, that the team leader (Prof. Greiner) won a 2011 MacArthur Foundation “genius” grant for this work.