Scientists set a record for the lowest temperature - just one billionth of a degree above absolute zero

Researchers at the Max Planck Institute for Quantum Optics have developed a new cooling technique for molecular gases - one that can cool polar molecules to 21 nanokelvin (10-9 Kelvin), just two billionths of a degree above absolute zero, setting a new low temperature record. This new method of cooling polar molecular gases to near absolute zero paves the way for studying the quantum effects of "exotic matter".

01Cooling a gas to ultra-low temperatures based on a rotating microwave field

The technique used by the experimental team is based on a rotating microwave field, which helps stabilize the collisions between molecules during the cooling process by means of an "energetic shield". In this way, the Max Planck researchers succeeded in cooling the sodium-potassium molecular gas to two parts per billion above absolute zero. By doing so, they have set a new record for low temperatures. In the future, this new technology will allow the creation, and exploration, of many forms of quantum matter that until now were not available experimentally.

A closer view of the main vacuum chamber for the sodium-potassium molecule experiment. Four high-voltage copper wires in the center are led to an ultra-high vacuum glass chamber where the supercooled polar molecules are created.

When the highly dilute gas is cooled to very low temperatures, strange properties become apparent. As a result, some gases form so-called "Bose-Einstein condensates": a type of matter in which all atoms move in unison; another example is a "supersolid": a state in which matter behaves like a frictionless fluid with a periodic structure. .

Physicists hope to discover particularly diverse and revealing forms of quantum matter when cooling gases composed of polar molecules. They are characterized by a non-uniform charge distribution: unlike free atoms, they can rotate, vibrate, attract or repel each other; however, it is difficult to cool molecular gases to ultra-low temperatures.

A group of researchers at the Max Planck Institute for Quantum Optics has now found a simple and effective way to overcome this obstacle.

02Evaporative cooling - "like cooling a cup of coffee"

In their experiments, the researchers used a gas composed of sodium-potassium (NaK) molecules that were confined in an optical trap by a laser beam. To cool the gas, the team relied on a method that has long proven effective for cooling unbound atoms - "evaporative cooling.

This method works similarly to the familiar process that causes a hot cup of coffee to cool down," said Xin-Yu Luo, director of the Laboratory of Ultracold Polar Molecules in the Quantum Many-Body Systems Division at the Max Planck Institute for Quantum Optics. In coffee, water molecules constantly collide, thereby exchanging some of their kinetic energy. If two particularly energetic molecules collide, one of them can be fast enough to escape the coffee (it is evaporated out of the cup), while the other molecule remains with less energy, and this is how the coffee gradually cools down. In the same way, a gas can be cooled to 21 nanokelvins: just two parts per billion above absolute zero (-273.15 degrees Celsius)."

Xin-Yu Luo, leader of the Quantum Many-Body Systems Research Group

"However, if the gas is composed of molecules, these molecules must be further stabilized at very low temperatures." The reason for this, Luo says, is that the structure of molecules is much more complex compared to unbound atoms. As a result, controlling their motion during collisions is difficult: these molecules may stick together during collisions. Moreover, "polar molecules behave like tiny magnets that can snap together. In this case, they are useless in the experiment." explains Andreas Schindewolf, who conducted the research in Xin-Yu Luo's group.

In recent years, these difficulties have proven to be a huge obstacle to research.

03Dedicated electromagnetic microwaves: keeping molecules separated

To overcome this obstacle, the researchers used a trick: an additional specially prepared electromagnetic field was applied to act as an "energy shield" for the molecules - which prevents them from sticking together.

Andreas Schindewolf

Andreas Schindewolf explains, "We created this energy shield using a powerful, rotating microwave field, which causes the molecules to rotate at a higher frequency. If two molecules get too close to each other, they can thus exchange kinetic energy; but at the same time, they align themselves against each other in such a way that they repel each other and quickly separate again."

To create a microwave field with the desired properties, the researchers placed a helical antenna under an optical trap containing a gas of sodium-potassium molecules. "As a result, the rate at which the molecules become 'interlocked' (interlocked) is reduced by more than an order of magnitude; moreover, under the influence of the magnetic field, strong and long-range electrical interactions between the molecules are formed. As a result, they collide much more frequently than in the absence of a rotating microwave field: about 500 times per molecule on average. This is enough to cool the gas to near absolute zero by evaporation." Xin-Yu Luo said.

04Exciting times: breakthrough for ultracold polar molecules

A sodium laser system that generates yellow light for laser cooling and imaging of sodium atoms.

After only a third of a second, the temperature reaches 21 nano-Kelvin. This is well below the critical "Fermi temperature". It marks a limit below which quantum effects dominate the behavior of gases and "exotic states" of matter begin to emerge.

"The temperature we have reached is the lowest of any polar molecular gas to date." Luo is pleased to say that Max Planck's researchers believe they can reach even lower temperatures by making technical improvements to the experimental setup.

The results could have far-reaching implications for the study of quantum effects and quantum matter.

Immanuel Bloch

Immanuel Bloch, director of quantum many-body systems at the Max Planck Institute for Quantum Optics, said: "Because the new cooling technique is so simple, it can also be integrated into most experimental setups with ultracold polar molecules, so the method should soon be widely used and help lead to even more new discoveries. Microwave-assisted cooling not only opens up a range of new ways to study particular states of matter, such as superfluids and supersolids, it could also be useful in quantum technologies: for example, in a quantum computer, data could perhaps be stored by ultracold molecules."

Xin-Yu Luo also said, "This is a truly exciting time for researchers studying ultracold polar molecules."

Link：

https://www.mpg.de/19035150/0728-qopt-a-nanokelvin-microwave-freezer-for-molecules-153540-x1?c=2249