创新背景

当高度稀释的气体冷却到极低的温度时，会显示出奇怪的性质。因此，一些气体会形成玻色-爱因斯坦凝聚物，所有原子的量子态都束聚于一个单一的量子态。有一些气体则会表现得像具有周期性结构的无摩擦流体的状态。物理学家希望能在冷却由极性分子组成的气体时发现特别多样化和揭示性的量子物质形式。但将分子气体冷却到超低温却非常难。

创新过程

慕尼黑大学的量子物理学家Immanuel Bloch和马克斯·普朗克量子光学研究所领导的团队基于微博的旋转场使用钠钾分子气体找到了一种可以使分子气体冷却到低温的方法。

团队使用冷却蒸发的方式来冷却气体，类似与一杯热咖啡的冷却过程。咖啡中水分子不断碰撞导致部分动能交换，两个特别高能的分子碰撞会让其中一个变得足够快从而被蒸出来，另一个分子则保持较少的能量。气体冷却使用和咖啡冷却同样的方式，可以冷却到几个纳开尔文，相当于比零下273.15℃高十亿分之度。
与未结合的原子相比，分子的结构非常复杂，分子在碰撞过程中会粘到一起，人为干预控制它们的运动并不容易。且极性分子就像微小的磁铁，彼此会卡在一起，在实验中非常容易丢失。如果气体由分子组成的话，这些分子必须在非常低的温度下进一步稳定。

研究人员应用一种专门准备的电磁场来克服障碍，将其作为分子的能量屏蔽器来防止它们粘在一起。屏蔽能量的磁场由强大的旋转微波场创造，磁场使分子以更高的频率旋转，如果两个分子过于接近，彼此就可以交换动能。同时，分子也会以同样的方式对齐，使彼此相互排斥并迅速分离。

研究在含有钠钾分子气体的光学陷阱下放置一个螺旋天线，用以产生满足需求性质的微波场，分子互锁的速度有所减少。在微波场影响下，分子之间形成了远距离的强烈电相互作用，平均每个分子大约500次碰撞，比没有旋转微波场时候的碰撞频率高了许多，足以通过蒸发将气体冷却到接近绝对零度。

三分之一秒后温度就达到了21纳开尔文左右，远低于临界费米温度，标志分子气体温度到达极限，低于这个极限，量子效应将主导气体的行为，出现奇怪的现象。研究人员表示，研究达到了迄今为止极性分子气体的最低温度。如果改进实验装置的技术，它们可以达到更低的温度。新的冷却技术简单有效，可以集成到大多数具有超冷极性分子的实验设置中。

创新关键点

利用旋转微波场制造电磁场，避免分子互相粘合，提高分子碰撞频率以实现蒸发冷却，将气体冷却到接近绝对零度。

创新价值

为研究超流体和超固体等特殊物质状态开辟新的途径，有助于研究超冷极性分子，或可帮助量子计算机用超冷分子储存数据。

The rotating microwave field creates an electromagnetic field to cool the molecular gas

A team led by quantum physicist Immanuel Bloch and the Max Planck Institute for Quantum Optics at the University of Munich found a way to cool the molecular gas to low temperatures using a microblog-based rotation field using sodium-potassium molecular gases.

The team used cooling evaporation to cool the gas, similar to the cooling process with a cup of hot coffee. The constant collision of water molecules in coffee leads to partial kinetic energy exchange, and the collision of two particularly energetic molecules will make one of them fast enough to be steamed out, and the other molecule will maintain less energy. Gas cooling is used in the same way as coffee cooling and can be cooled to several Nakelvins, equivalent to parts per billion degrees higher than minus 273.15 °C.

Compared to unbound atoms, the structure of the molecules is very complex, the molecules stick together during the collision, and it is not easy to control their movement with human intervention. And polar molecules are like tiny magnets that get stuck together and are very easy to lose in experiments. If the gas is composed of molecules, these molecules must be further stabilized at very low temperatures.

The researchers applied a specially prepared electromagnetic field to overcome obstacles, acting as energy shields for molecules to prevent them from sticking together. The magnetic field shielding the energy is created by a powerful rotating microwave field that causes the molecules to rotate at higher frequencies, and if two molecules are too close together, kinetic energy can be exchanged with each other. At the same time, the molecules are aligned in the same way, making them mutually exclusive and rapidly separating.

The study placed a spiral antenna under an optical trap containing sodium and potassium molecular gas to produce a microwave field that meets the nature of demand, and the speed of molecular interlocking has been reduced. Under the influence of the microwave field, strong electrical interactions are formed between molecules over long distances, with an average of about 500 collisions per molecule, which is much higher than the collision frequency when there is no rotating microwave field, enough to cool the gas to near absolute zero by evaporation.

After a third of a second the temperature reaches about 21 nakelvin, well below the critical Fermi temperature, marking the molecular gas temperature reaching its limit, below this limit, the quantum effect will dominate the behavior of the gas, a strange phenomenon. The researchers say the study reached the lowest temperature of a polar molecular gas to date. If the technology of experimental apparatus is improved, they can reach lower temperatures. The new cooling technology is simple and effective and can be integrated into most experimental setups with ultracold polar molecules.