A crew of physicists from MIT has efficiently cooled molecules in a fuel of sodium potassium (NaK) to a temperature of 500 nanokelvins, creating ultracold molecules.
The air round us is a chaotic superhighway of molecules whizzing by way of house and always colliding with one another at speeds of a whole lot of miles per hour. Such erratic molecular habits is regular at ambient temperatures.
However scientists have lengthy suspected that if temperatures have been to plunge to close absolute zero, molecules would come to a screeching halt, ceasing their particular person chaotic movement and behaving as one collective physique. This extra orderly molecular habits would start to kind very unusual, unique states of matter — states which have by no means been noticed within the bodily world.
Now experimental physicists at MIT have efficiently cooled molecules in a fuel of sodium potassium (NaK) to a temperature of 500 nanokelvins — only a hair above absolute zero, and over one million occasions colder than interstellar house. The researchers discovered that the ultracold molecules have been comparatively long-lived and steady, resisting reactive collisions with different molecules. The molecules additionally exhibited very sturdy dipole moments — sturdy imbalances in electrical cost inside molecules that mediate magnet-like forces between molecules over giant distances.
Martin Zwierlein, professor of physics at MIT and a principal investigator in MIT’s Analysis Laboratory of Electronics, says that whereas molecules are usually full of power, vibrating and rotating and shifting by way of house at a frenetic tempo, the group’s ultracold molecules have been successfully stilled — cooled to common speeds of centimeters per second and ready of their absolute lowest vibrational and rotational states.
“We’re very near the temperature at which quantum mechanics performs a giant position within the movement of molecules,” Zwierlein says. “So these molecules would now not run round like billiard balls, however transfer as quantum mechanical matter waves. And with ultracold molecules, you may get an enormous selection of totally different states of matter, like superfluid crystals, that are crystalline, but really feel no friction, which is completely weird. This has not been noticed up to now, however predicted. We’d not be removed from seeing these results, so we’re all excited.”
Zwierlein, together with graduate pupil Jee Woo Park and postdoc Sebastian Will — all of whom are members of the MIT-Harvard Heart of Ultracold Atoms — have published their results in the journal Physical Review Letters.
Sucking away 7,500 kelvins
Each molecule consists of particular person atoms which might be bonded collectively to kind a molecular construction. The only molecule, resembling a dumbbell, is made up of two atoms linked by electromagnetic forces. Zwierlein’s group sought to create ultracold molecules of sodium potassium, every consisting of a single sodium and potassium atom.
Nevertheless, because of their many levels of freedom — translation, vibration, and rotation — cooling molecules straight could be very tough. Atoms, with their a lot easier construction, are a lot simpler to sit back. As a primary step, the MIT crew used lasers and evaporative cooling to chill clouds of particular person sodium and potassium atoms to close absolute zero. They then basically glued the atoms collectively to kind ultracold molecules, making use of a magnetic discipline to immediate the atoms to bond — a mechanism referred to as a “Feshbach resonance,” named after the late MIT physicist Herman Feshbach.
“It’s like tuning your radio to be in resonance with some station,” Zwierlein says. “These atoms begin to vibrate fortunately collectively, and kind a sure molecule.”
The ensuing bond is comparatively weak, creating what Zwierlein calls a “fluffy” molecule that also vibrates fairly a bit, as every atom is bonded over a protracted, tenuous connection. To carry the atoms nearer collectively to create a stronger, extra steady molecule, the crew employed a method first reported in 2008 by teams from the College of Colorado, for potassium rubidium (KRb) molecules, and the College of Innsbruck, for non-polar cesium (Ce2) molecules.
For this method, the newly created NaK molecules have been uncovered to a pair of lasers, the massive frequency distinction of which precisely matched the power distinction between the molecule’s preliminary, extremely vibrating state, and its lowest attainable vibrational state. By means of absorption of the low-energy laser, and emission into the high-energy laser beam, the molecules misplaced all their obtainable vibrational power.
With this technique, the MIT group was in a position to carry the molecules right down to their lowest vibrational and rotational states — an enormous drop in power.
“In phrases of temperature, we sucked away 7,500 kelvins, identical to that,” Zwierlein says.
Of their earlier work, the Colorado group noticed a big disadvantage of their ultracold potassium rubidium molecules: They have been chemically reactive, and basically got here aside after they collided with different molecules. That group subsequently confined the molecules in crystals of mild to inhibit such chemical reactions.
Zwierlein’s group selected to create ultracold molecules of sodium potassium, as this molecule is chemically steady and naturally resilient in opposition to reactive molecular collisions.
“When two potassium rubidium molecules collide, it’s extra energetically favorable for the 2 potassium atoms and the 2 rubidium atoms to pair up,” Zwierlein says. “It seems with our molecule, sodium potassium, this response just isn’t favored energetically. It simply doesn’t occur.”
Of their experiments, Park, Will, and Zwierlein noticed that their molecular fuel was certainly steady, with a comparatively lengthy lifetime, lasting about 2.5 seconds.
“Within the case the place molecules are chemically reactive, one merely doesn’t have time to check them in bulk samples: They decay away earlier than they are often cooled additional to look at fascinating states,” Zwierlein says. “In our case, we hope our lifetime is lengthy sufficient to see these novel states of matter.”
By first cooling atoms to ultralow temperatures and solely then forming molecules, the group succeeded in creating an ultracold fuel of molecules, measuring one thousand occasions colder than what may be achieved by direct cooling strategies.
To start to see unique states of matter, Zwierlein says molecules should be cooled nonetheless a bit additional, to all however freeze them in place. “Now we’re at 500 nanokelvins, which is already unbelievable, we find it irresistible. An element of 10 colder or so, and the music begins taking part in.”
This analysis was supported partly by the Nationwide Science Basis, the Air Pressure Workplace of Scientific Analysis, the Military Analysis Workplace, and the David and Lucile Packard Basis.
Publication: Jee Woo Park, et al., “Ultracold Dipolar Gasoline of Fermionic 23Na40Okay Molecules in Their Absolute Floor State,” Phys. Rev. Lett. 114, 205302, 18 Might 2015; doi:10.1103/PhysRevLett.114.205302
PDF Copy of the Research: Two-Photon Pathway to Ultracold Ground State Molecules of 23Na40K
Picture: Jose-Luis Olivares/MIT