‘Impossible’ quantum effects observed in atoms positioned closely together

In the realm of quantum physics, proximity plays a crucial role. As atoms interact more strongly when they are positioned closely together, scientists have long sought ways to arrange them as tightly as possible in quantum simulators.

These simulators allow researchers to explore exotic states of matter and build novel quantum materials. However, there has been a limit to how close atoms could be positioned…until now.

Atoms in very close quarters

Typically, scientists cool the atoms to a stand-still and use laser light to arrange them, but the wavelength of light has restricted the minimum distance between particles to around 500 nanometers.

Now, a team of physicists at MIT has developed a breakthrough technique that enables them to position atoms a mere 50 nanometers apart. To put this into perspective, a red blood cell measures about 1,000 nanometers in width.

Harnessing the power of dysprosium

The MIT team, led by Wolfgang Ketterle, the John D. MacArthur Professor of Physics, demonstrated their new approach using dysprosium, the most magnetic atom in nature.

By manipulating two layers of dysprosium atoms and precisely positioning them 50 nanometers apart, they observed magnetic interactions 1,000 times stronger than if the layers were separated by the previous limit of 500 nanometers.

“We have gone from positioning atoms from 500 nanometers to 50 nanometers apart, and there is a lot you can do with this,” says Ketterle. “At 50 nanometers, the behavior of atoms is so much different that we’re really entering a new regime here.”

Furthermore, the researchers were able to measure two new effects caused by the atoms’ proximity: thermalization, where heat transfers from one layer to another, and synchronized oscillations between the layers. These effects diminished as the layers were spaced farther apart.

Overcoming the optical resolution limit

Conventional techniques for manipulating and arranging atoms have been limited by the wavelength of light, which typically stops at 500 nanometers. This optical resolution limit has prevented scientists from exploring phenomena that occur at much shorter distances.

“Conventional techniques stop at 500 nanometers, limited not by the atoms but by the wavelength of light,” explains Ketterle. “We have found now a new trick with light where we can break through that limit.”

The team’s innovative approach begins by cooling a cloud of atoms to about 1 microkelvin, just above absolute zero, causing the atoms to come to a near-standstill.

They then use two laser beams with different frequencies and circular polarizations to create two groups of atoms with opposite spins.

Each laser beam forms a standing wave, a periodic pattern of electric field intensity with a spatial period of 500 nanometers.

By tuning the lasers such that the distance between their respective peaks is as small as 50 nanometers, the atoms gravitating to each laser’s peaks are separated by the same distance.

Stabilizing the lasers for atom positioning

To achieve this level of precision, the lasers must be extremely stable and resistant to external noise. The team realized they could stabilize both lasers by directing them through an optical fiber, which locks the light beams in place relative to each other.

“The idea of sending both beams through the optical fiber meant the whole machine could shake violently, but the two laser beams stayed absolutely stable with respect to each others,” says lead author and physics graduate student Li Du.

Observing strange quantum phenomena

By applying their technique to dysprosium atoms, the researchers observed two novel quantum phenomena at the extremely close proximity of 50 nanometers.

First is collective oscillation, where vibrations in one layer caused the other layer to vibrate in sync. Next is thermalization, where one layer transferred heat to the other purely through magnetic fluctuations in the atoms.

“Until now, heat between atoms could only by exchanged when they were in the same physical space and could collide,” notes Du. “Now we have seen atomic layers, separated by vacuum, and they exchange heat via fluctuating magnetic fields.”

Atom positions and quantum simulators

The team’s results introduce a new technique that can be applied to many other atoms to study quantum phenomena.

They believe their approach can be used to manipulate and position atoms into configurations that could generate the first purely magnetic quantum gate.

This would be a key building block for a new type of quantum computer.

“We are really bringing super-resolution methods to the field, and it will become a general tool for doing quantum simulations,” says Ketterle. “There are many variants possible, which we are working on.”

Positioning atoms for better quantum computers

In summary, the MIT team’s pioneering technique opens up a new frontier in quantum physics, enabling scientists to explore previously inaccessible phenomena and build novel quantum materials.

By positioning atoms a mere 50 nanometers apart, they have unlocked a realm where magnetic interactions reign supreme and quantum effects emerge in stunning clarity.

As researchers continue to refine and expand upon this approach, they inch closer to the development of purely magnetic quantum gates and the realization of cutting-edge quantum computers.

The future of quantum simulations looks brighter than ever, and the possibilities are limited only by the imagination of the scientists who dare to push the boundaries of what is possible.

The study’s co-authors include Pierre Barral, Michael Cantara, Julius de Hond, and Yu-Kun Lu, all members of the MIT-Harvard Center for Ultracold Atoms, the Department of Physics, and the Research Laboratory of Electronics at MIT.

The full study was published in the journal Science.

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