World Quantum Day | You read that right! Lasers can really cool particles!

As early as in the middle school physics class, the physics teacher told us that the substances we come into contact with in life are usually composed of molecules or atoms, and atoms are the smallest units that maintain the chemical properties of substances . For example, the helium in the helium balloons sold in amusement parks is a monatomic molecule composed of helium atoms. At this time, the helium in the balloon looks like it is completely still, but the helium atoms inside it are always in a non-stop "thermal motion state", and as the ambient temperature rises, the thermal motion speed of these microscopic particles continues to increase.

You may not believe it, but even if this helium balloon is placed in the coldest temperature in Mohe (-53℃, which is about 220K), the helium atoms inside it are still in high-speed random thermal motion at a speed of more than 120 kilometers per hour. In other words, the thermal motion speed of these microscopic particles is comparable to that of cars on the highway!

Therefore, if scientists want to precisely control a single atom, they have to first cool the atom to a temperature close to absolute zero (about -273.15°C, or 0 K). Only in this way can the atom be kept still as much as possible. So, how can we cool atoms that move super fast to such a low temperature limit?

The answer is - laser! You read that right, a more accurate term would be " laser Doppler cooling ".

01Can light deflect the trajectory of atoms ? ——Wonderful photon scattering interaction

In our traditional impression, photons (the basic "particles" of light) move extremely fast and carry very little energy. Therefore, compared with atoms with larger mass, it seems very difficult for photons to interact with them and exchange energy, which is like an ant trying to shake a lead ball.

In fact, as early as 1933, physicist Otto Frisch used the light from a sodium vapor lamp to successfully deflect the trajectory of a beam of sodium atoms. Although the deflection of the atomic beam trajectory was only about 1 mm, it strongly proved that photons can transfer energy with atoms. However, it is not easy to complete this experiment of deflecting the atomic trajectory, which requires the emitted photons to have a strong enough scattering interaction with the atoms.

Schematic diagram of the interaction between atoms and photons (copyright image of the library, reprinting may cause copyright disputes)

Simply put, each atom has an uneven and specific "energy ladder" inside - an energy level structure, and there is also a specific energy difference between different energy levels. When this atom encounters a photon with a frequency of exactly , it will "eat" the photon without hesitation, thereby completing its own energy level transition. As the price of "gluttony", the atom will change its original speed of movement due to the collision that occurs during the absorption of photons. What's more interesting is that this "greedy" atom is prone to "indigestion" symptoms, and randomly "spits out" a new photon with the same frequency to the surroundings, thereby restoring its original energy level state. In fact, in the study of atomic physics, the above process inside the atom has a more professional name - spontaneous radiation .

Schematic diagram of "spontaneous radiation" of atoms

(Image source: drawn by the author)

When this atom encounters multiple photons of the same frequency in the same direction, it will continue to repeat this "spontaneous radiation" cycle. As time accumulates , the reaction force that this atom receives each time it randomly " spits out " new photons in different directions will almost be canceled out on average. This means that after completing multiple cycles, the atom as a whole only feels the accumulated collision force from "eating" a bunch of photons in the same direction multiple times. This continuous interaction force is enough to deflect the trajectory of the atom's motion.

In the early 20th century, physicists could only perform experiments on deflecting atomic trajectories because they could not obtain laser beams with higher energy density. In the 1970s, with the rapid development of laser technology, physicists began to try to use laser beams to interact with atoms, hoping to slow down high-speed atoms.

02 Let atoms fall into the swamp of photons - optical sticky

However, it is not easy to make an atom with a very fast initial velocity "eat" the oncoming photon smoothly. This is because from the atom's point of view, the oncoming photon has a higher frequency due to the " Doppler effect ", so it cannot "eat" the photon (which is inconsistent with its own energy level difference), which means that the "spontaneous radiation" cycle cannot be completed smoothly.

Atoms sense photons as having a higher frequency

(Image source: drawn by the author)

In fact, the "Doppler effect" mentioned here is not unfamiliar to us. For example, when a police car sirens closer, we will feel that the pitch of the siren is getting higher and higher, that is, the frequency of the sound waves received by our ears gradually increases; and when the police car sirens move away, the siren will become lower and lower accordingly, that is, the frequency of the sound waves we hear gradually decreases. This change in the frequency of radiation felt by the observer due to the relative motion between the wave source and the observer was first proposed by Austrian physicist Christian Doppler in 1842, and is therefore called the "Doppler effect."

Schematic diagram of the "Doppler effect" (copyright image of the library, reprinting and use may cause copyright disputes)

Therefore, if this atom with a very fast initial velocity wants to "eat" a photon with a frequency of exactly , then considering the above-mentioned " Doppler effect ", the frequency of the oncoming photon itself needs to be slightly less than in order to successfully complete the " spontaneous radiation " cycle. In this way, thanks to the continuous scattering interaction between "atom-photon", the originally fast-moving atom will reduce its own speed due to the interception of photons.

Inspired by this Doppler cooling scheme, in 1982, William Phillips' team from the National Institute of Standards and Technology (NIST) successfully reduced the speed of sodium atoms, which were originally moving in a certain direction, from an average thermal motion speed of 3,600 kilometers per hour to about 144 kilometers per hour for the first time (according to the velocity distribution relationship in thermodynamic statistics, the sodium atoms were cooled to about 70 mK, or 0.07 K).

The deceleration of a certain directional atom only needs to consider the movement in a single direction, while the cooling of the entire atomic group requires the simultaneous deceleration in six directions in three-dimensional space: front, back, up, down, left, and right. This requires three pairs of counter-propagating laser beams to act simultaneously. In 1985, the Chu Stevens team at Bell Labs in the United States used three pairs of counter-propagating laser beams to irradiate a vapor group of sodium atoms and successfully cooled a group of sodium atoms at the intersection of the three pairs of lasers. At this time, the temperature of the atomic group was as low as the limit temperature of Doppler cooling (about 0.00024 K), and this special atomic group state is also called " optical molasses ".

Although this "optical sticking" technology can efficiently cool the atomic clusters, in theory it can only hinder the movement of the atomic clusters (similar to trapping atoms in a swamp of photons), so it does not really imprison the atomic clusters (the lifetime of the atomic clusters can only be stable at the order of seconds). This means that in order to stably imprison the atomic clusters in three-dimensional space for a long time, another additional interaction in space pointing to the intersection of the lasers is required .

03Magneto -optical trap: the perfect combination of optical sticky mass and static magnetic field

In 1987, Chu's group collaborated with Pritchard's group at MIT to experimentally use a scheme combining optical adhesion with a static magnetic field with a gradient distribution in space to successfully achieve the cooling and trapping of atomic clusters. This atomic trap that combines a gradient static magnetic field with optical adhesion is also called a " magneto-optical trap (MOT) ."

Schematic diagram of the "magneto-optical trap" (copyright image of the library, reprinting and use may cause copyright disputes)

Specifically, the magnetic field at the intersection of three pairs of counter-propagating laser beams in the magneto-optical trap is zero, and the average scattering of the atomic clusters at the center of the potential well is also zero. By precisely controlling the gradient distribution of the static magnetic field in three-dimensional space, the atoms at the edge of the potential well can be restrained by the reverse magnetic field and will not escape outward.

In other words, the magneto-optical trap uses optical adhesion to calm the atoms on the one hand , and uses the gradient magnetic field to push the atomic clusters to the center of the potential well on the other hand, thereby achieving a combined " cooling + trapping " effect on the atomic clusters.

Thanks to the invention of magneto-optical trap technology, physicists have the opportunity to achieve long-term stable trapping of microscopic particles, thus providing the possibility for precise control of microscopic particles and promoting the development of quantum information technology. It is also because of their outstanding contributions to laser cooling and trapped atoms that Chu and William Phillips shared two-thirds of the 1997 Nobel Prize in Physics.

Conclusion

However, the speed of atoms after Doppler cooling is still not zero, and there is a temperature limit of its own, also known as the " Doppler temperature limit ". This is because for atoms, although the recoil effects in multiple spontaneous radiations are averaged out, atoms are always absorbing photons and spontaneous radiation, which makes the atoms in a state of random walking and cannot really come to a complete standstill.

Generally speaking, after Doppler cooling, the temperature limit of atoms is in the order of several hundred μK (microkelvin ). To further reduce the cooling limit of atoms, it is necessary to introduce more powerful "sub-Doppler cooling" in addition to completing Doppler cooling.

So, how did physicists use their imagination to successfully lower the temperature of atoms to the μK or even nK (nanoKelvin) level in experiments? Let’s explore the mystery of “sub-Doppler cooling” in the next article!

References

[1] (Otto Frisch) Frisch R. Experimenteller nachweis des Einsteinschen strahlungsrückstoβes[J]. Zeitschrift für Physik, 1933, 86(1- 2): 42-48.

[2] Phillips WD, Metcalf H. Laser deceleration of an atomic beam[J]. Physical Review Letters, 1982, 48(9): 596-599.

Prodan J, Phillips WD, Metcalf H. Laser production of a very slow monoenergetic atomic beam[J]. Physical Review Letters, 1982, 49(16): 1149-1153.

[3] (Zhu Diwen) Chu S, Hollberg L, Bjorkholm J, et al. Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure[J]. Physical Review Letters, 1985, 55(1): 48-51.

[4] (Pritchard: MOT) Raab EL, Prentiss M, Cable A, et al. Trapping of neutral sodium atoms with radiation pressure[J]. Physical Review Letters, 1987, 59(23): 2631-2634.

[5] (Sub-Doppler cooling) Lett PD, Watts RN, Westbrook CI, et al. Observation of atoms laser cooled below the Doppler limit[J]. Physical Review Letters, 1988, 61(2): 169-172.

Author: Luan Chunyang, PhD, Department of Physics, Tsinghua University

Reviewer: Luo Huiqian, Researcher, Institute of Physics, Chinese Academy of Sciences

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.