光为什么能让原子变冷？

2026 年 10 月 22 日，法国物理学家克劳德·科昂－塔努吉去世，享年 93 岁。1997 年，他与朱棣文、威廉·菲利普斯共同获得诺贝尔物理学奖，获奖工作是发展用激光冷却和捕获原子的方法。[1]

On 22 October 2026, the French physicist Claude Cohen-Tannoudji died at the age of 93. In 1997, he shared the Nobel Prize in Physics with Steven Chu and William D. Phillips for developing methods to cool and trap atoms with laser light. [1]

光照在身上会暖，激光还可以切割金属。它怎么能用来制冷？

Light warms our skin, and a laser can cut metal. How can it also be used for cooling?

光也能给原子刹车

Light can put the brakes on atoms

想象一团稀薄气体，原子朝不同方向运动，有的快，有的慢。扣除气体整体的漂移后，剩下的无序运动与温度相联系。让这种运动减弱，气体就会变冷。

Imagine a dilute gas with atoms moving in different directions, some fast and some slow. After subtracting the overall drift of the gas, the remaining disordered motion is related to its temperature. Reduce that motion, and the gas becomes colder.

光子携带动量。原子吸收一个光子，就会受到一次微小的推动。如果光迎着原子的运动方向而来，这次推动便可能让它减速。难的是，原子四处运动，同一束光也可能让另一些原子加速。

Photons carry momentum. When an atom absorbs a photon, it receives a tiny push. If the light comes towards the moving atom, that push can slow it down. The difficulty is that atoms move in every direction, and the same beam can speed up other atoms.

多普勒效应提供了办法。让两束光相向传播，把频率调得略低于原子的共振频率。对于运动的原子，迎面而来的光频率显得更高，更接近共振，也就更容易被吸收。两边的作用不再相等，合力便阻碍原子的运动。这就是多普勒冷却的基本思路。[2]

The Doppler effect offers a way forward. Send two beams in opposite directions, with their frequency tuned slightly below the atom’s resonance. To a moving atom, the oncoming light appears higher in frequency and closer to resonance, making it more likely to be absorbed. The two beams no longer act equally, and their net force opposes the atom’s motion. This is the basic idea of Doppler cooling. [2]

但吸光之后，原子还会自发发出光子，受到随机反冲。冷却与随机扰动相互竞争，简单的多普勒理论因此预言了一个温度下限。[2]

After absorbing light, however, the atom spontaneously emits a photon and receives a random recoil. Cooling competes with these random disturbances, so simple Doppler theory predicts a lower temperature limit. [2]

原子比预想的更冷

Atoms became colder than expected

20 世纪 80 年代末，菲利普斯团队却测到了低于这个下限的温度。实验结果提醒人们，原子的内部结构还藏着简单模型没有考虑的冷却机制。[2]

In the late 1980s, Phillips’s team measured temperatures below that limit. The results showed that the internal structure of atoms could support cooling mechanisms missing from the simple model. [2]

科昂－塔努吉与让·达利巴尔等研究者发展的理论，把光的偏振、原子的内部状态和运动联系起来。其中一种机制，有个来自希腊神话的名字——西西弗斯冷却。[3]

Theories developed by Cohen-Tannoudji, Jean Dalibard and other researchers connected the polarization of light with the atom’s internal states and motion. One mechanism takes its name from Greek mythology: Sisyphus cooling. [3]

让原子反复爬坡

Making an atom climb the hill again

在合适的光场里，原子的势能随位置起伏。可以把它想象成一条山路。原子爬坡时，动能转化为势能，速度降低；如果始终沿同一条路走，下坡时又会把能量取回来。

In a suitable light field, the atom’s potential energy varies with position. Think of it as a hilly road. As the atom climbs, kinetic energy becomes potential energy and its speed falls. If it stayed on the same road, it would regain that energy on the way down.

关键在于，原子有不同的内部状态，对应不同的势能曲线。就像两条错开的山路，一条在当前位置到了高处，另一条恰好较低。[3]

The key is that different internal states correspond to different potential-energy curves. Imagine two offset roads: where one is high, the other is low. [3]

吸收和自发发射光子，可以使原子改变内部状态，这个过程称为光抽运。在合适条件下，原子更容易在较高势能处转移到当地势能较低的状态。它仍在原来的位置，却换了一条“路”，随后又要爬坡。

Absorption and spontaneous emission can change the atom’s internal state through a process called optical pumping. Under suitable conditions, an atom at relatively high potential energy is more likely to transfer to a state with lower potential energy at the same position. It has not changed position, but it has changed “roads” and will have to climb again.

爬坡消耗动能，光抽运使它换到较低的势能曲线，散射光带走能量。 反复发生的循环，让原子的无序运动进一步减弱。[4]

Climbing uses kinetic energy; optical pumping transfers the atom to a lower potential-energy curve; scattered light carries energy away. Repeated cycles can further reduce the atom’s disordered motion. [4]

这幅图景有所简化。真实的跃迁有概率，并非每到峰顶必然发生；光子的随机反冲也依然存在，所以这个过程不能无限制地降温。[4]

This picture is simplified. Real transitions are probabilistic and do not necessarily occur at each hilltop. Random photon recoil also remains, so the process cannot cool without limit. [4]

读到这里，可以试着拿掉“换状态”这一步。原子仍会上坡、下坡，却失去了这个循环中把能量交给光的环节。这个对照说明，冷却需要势能起伏与状态转换共同配合。

Try removing the change of internal state. The atom still travels uphill and downhill, but this cycle no longer transfers its mechanical energy to the light. The comparison shows why both the changing potential and the state transitions matter for cooling.

科昂－塔努吉的同事在讣告中，还记起了他条理清楚的黑板和手写讲义。[1] 今天再读他的演讲，仍能沿着那些能级图，跟上一个原子怎样在光里慢下来。

In their obituary, Cohen-Tannoudji’s colleagues also remembered his carefully organized blackboards and handwritten lecture notes. [1] Reading his lectures today, we can still follow those energy-level diagrams and understand how an atom slows down in light.