Laser cooling – a technique for chilling atoms down to ultracold temperatures – has been extended to molecules for the first time. The advance could help make molecular quantum computers possible.
Cooling with lasers exploits a particular aspect of atomic behaviour. An atom will absorb a photon of a given frequency, transferring the energy to one of its electrons which is briefly kicked up to an excited state. When it drops back down, the atom emits a photon in a random direction and can then absorb another photon.
If the initial photons all arrive from a specific direction – along a laser beam – but the emitted photons leave the atom in random directions, the net result is that the atom travels more slowly towards the laser. Atoms or molecules travel more sluggishly at low temperatures, and so because the atom moves more slowly towards the laser, it is lower in temperature in this direction.
To approach absolute zero requires several thousand cycles of photon absorption and emission. There's a risk that during one cycle the atom might fall into a "dark state" that does not absorb the photons in the narrow frequency range emitted by the laser. It is for this reason that molecular laser cooling is difficult, says David DeMille of Yale University. Atoms have relatively simple sets of energy states making it easy to avoid a dark state. But molecules have more subtle energy states – the bonds between their atoms can stretch and squash into a range of vibrational and rotational states, some of which will be dark.
In the past decade, it has emerged that some molecules readily occupy relatively few vibrational states, and that there are ways to keep the molecule from spiralling into multiple rotational states. "The combination of these ideas is what led to this breakthrough," says Edward Shuman of DeMille's group.
He and his colleagues chose their molecule carefully so that it would fall into only three vibrational states even after more than 100,000 cycles. The best candidate was a polar molecule, more positively charged on one side and more negatively charged on the other, called strontium fluoride.
The team first cooled their molecules to about 4 kelvin using cold helium gas. They then used a filter to remove particularly energetic – or hot – molecules before introducing them to a 15-centimetre-long hall of mirrors which reflected the three cooling laser beams.
The main laser cooled the molecules, and the other two kicked molecules out of unwanted vibrational states so that they were always available to absorb the main laser's photons.
Hall of mirrors
By the time they reached the end of the hall, each molecule had crossed the laser beams roughly 75 times, absorbing and emitting 500 to 1000 photons. Consequently, in one direction the laser had all but frozen the atom's motion, chilling them down to a thousandth of a degree above absolute zero. Viewed from a different direction, however, the atoms retain the motion characteristic of 4 kelvin.
"This is a really complex problem experimentally and the DeMille group is very courageous to have kept hammering away at it till it was solved," says Lincoln Carr, a cold molecule researcher at the Colorado School of Mines in Golden. "It will be great to see this technique extended to other molecules besides strontium fluoride."
The team believes that molecular laser cooling could work in three dimensions with a larger array of lasers to cool the molecules in different directions. If so, the resulting slow-moving molecules could be loaded into an array to form the information register of a quantum computer.
Entanglement allows quantum bits, or qubits, to use quantum superposition to do multiple computations simultaneously. Polar molecules can become entangled through their electric dipoles, but their overall neutral charge allows them to withstand disturbances from outside electromagnetic fields, so a molecular quantum computer would be less prone to electrical interference than one with charged ions for qubits.
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