Physicists at the Massachusetts Institute of Technology have developed a technique that can align atoms (represented as spheres with arrows) closer together than before, down to 50 nanometers. The group plans to use this method to manipulate atoms into configurations that can produce the first purely magnetic quantum gates, a key building block of a new type of quantum computer. In this image, the magnetic interaction is represented by colored lines. Image courtesy of Li Du et al., Massachusetts Institute of Technology.
Proximity is key to many quantum phenomena because the interaction between atoms is stronger when particles are in close proximity. In many quantum simulators, scientists line up atoms as close together as possible to explore the singular state of matter and construct new quantum materials.
They usually do this by cooling the atoms to a resting state and then using lasers to position the particles at a distance of 500 nanometers, a limit set by the wavelength of the light. Now, physicists at the Massachusetts Institute of Technology have developed a technique that allows them to align atoms closer together, as small as 50 nanometers. For context, red blood cells are about 1,000 nanometers wide.
Physicists have demonstrated this new method in experiments with dysprosium, the most magnetic atom in nature. They used a new method to manipulate two layers of dysprosium atoms and precisely position the two layers between 50 nanometers. In this extreme proximity, the magnetic interaction is 1,000 times stronger than when the layers are separated by 500 nanometers.
A paper describing the work was published in the journal Science.
Scientists were able to measure two new effects caused by the proximity of atoms. Their enhanced magnetic force results in "thermalization", i.e., the transfer of heat from one layer to another, as well as synchronous oscillations between the layers. As the layers get farther and farther apart, these effects fade away.
"We've gone from positioning atoms to 50 nanometers apart, and you can do a lot of things," said Wolfgang Keitel, a John MacArthur professor of physics at MIT. "At 50 nanometers, atoms behave so differently that we're really entering a new state here.
Keitel and his colleagues say the new method could be applied to many other atoms to study quantum phenomena. For their part, the group plans to use the technique to manipulate atoms into configurations that can produce the first purely magnetic quantum gates, a key building block of a new type of quantum computer.
该研究的合著者包括第一作者和物理学研究生Li Du,以及Pierre Barral,Michael Cantara,Julius de Hond和Yu-Kun Lu,他们都是麻省理工学院 - 哈佛大学超冷原子中心,物理系和麻省理工学院电子研究实验室的成员。
图片说明:研究生 li Du(左)和 Yu-Kun Lu 调整激光系统的控制电子设备。 图片来源:Li Du et al
Peaks and troughs
To manipulate and arrange atoms, physicists typically first cool a cloud of atoms to a temperature close to absolute zero, and then use a laser beam system to encircle the atoms into an optical trap.
A laser is an electromagnetic wave with a specific wavelength (distance between the maximum values of the electric field) and frequency. The wavelength limits the smallest pattern that light can shape to typically 500 nanometers, the so-called optical resolution limit. Since the atoms are attracted to a specific frequency of lasers, the atoms will be at the peak point of the laser intensity. For this reason, existing techniques are limited in the distance at which atomic particles can be located and cannot be used to explore phenomena that occur at shorter distances.
"Conventional technology stays at 500 nanometers, not limited by atoms, but by the wavelength of light," Ketterle explains. "We have now found a new light technique where we can push that limit.
The team's new method, like the current technique, begins by cooling a cloud of atoms to about 1 micro-Kelvin, just one hair above absolute zero, at which point the atoms virtually stagnate. Physicists can then use lasers to move the frozen particles into the desired configuration.
Du and his collaborators then used two laser beams, each with a different frequency or color; and circular polarization, or the direction of the laser electric field. When two beams pass through a supercooled cloud of atoms, the atoms can rotate in opposite directions in the direction of polarization of either of the two lasers. The result is that the beam produces two identical sets of atoms, just with opposite spins.
Each laser beam forms a standing wave, a periodic pattern of electric field strength with a spatial period of 500 nanometers. Due to their different polarizations, each standing wave attracts and encloses one of two groups of atoms, depending on their spin. Lasers can be superimposed and tuned so that the distance between their respective peaks is as small as 50 nanometers, which means that the atoms attracted by the peaks of each laser will be the same 50 nanometers apart.
But in order to do this, the laser must be very stable and unaffected by all external noise, such as shaking or even breathing during experiments. The team realized that they could stabilize two lasers by guiding them via fibers, which were used to lock the beams in relation to each other.
"The idea of sending two beams through fiber means that the whole machine may shake violently, but the two laser beams remain absolutely stable with each other," Du said.
不同颜色的激光用于冷却和捕获镝原子。 图片来源：Li Du et al
Magnetic force at close range
As the first test of their new technique, the team used dysprosium atoms – a rare earth metal that is one of the strongest magnetic elements in the periodic table, especially at ultra-low temperatures. However, at the atomic scale, the magnetic interaction of the elements is relatively weak at a distance of 500 nanometers.
As with regular refrigerator magnets, the magnetic force of attraction between atoms increases with distance, and the scientists suspect that if their new technology could spacing dysprosium atoms to 50 nanometers, they might observe the emergence of other weak interactions between magnetic atoms.
"We can suddenly have magnetic interactions that used to be almost negligible, but now very powerful," Ketterle said.
The team applied their technique to dysprosium, first supercooling the atoms and then splitting them into two spin groups or layers by two lasers. They then directed the lasers through the fibers to stabilize them, and found that indeed, two layers of dysprosium atoms were attracted to their respective laser peaks, which effectively separated the atomic layers by 50 nanometers – the closest distance that any ultracold atomic experiment would be able to reach.
At such extremely close distances, the natural magnetic interaction of the atoms is significantly enhanced and is 1000 times stronger than when they are 500 nanometers apart. The team observed that these interactions led to two new quantum phenomena: collective oscillations, in which the vibration of one layer causes the other to vibrate synchronously; and thermalization, in which one layer transfers heat to another layer purely through magnetic fluctuations in atoms.
"Until now, heat between atoms could only be exchanged if they were in the same physical space and could collide," Du noted. "Now we have seen layers of atoms separated by a vacuum, which exchange heat through a fluctuating magnetic field.
The team's results introduce a new technique that can be used to locate multiple types of atoms in the vicinity. They also showed that atoms placed close enough could exhibit interesting quantum phenomena that could be exploited to construct new quantum materials and potentially provide magnetically driven atomic systems for quantum computers.
"We've really brought the super-resolution approach to the field, and it's going to be a versatile tool for doing quantum simulations," Ketterle said. "There are a lot of possible variants that we are working on.
更多信息:Li Du 等人,50 纳米尺度的原子物理学:偶极原子双层系统的实现,《科学》(2024 年)。 DOI: 10.1126/science.adh3023.www.science.org/doi/10.1126/science.adh3023
期刊信息： Science