December 9, 2021

Beyond Going Long

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The first demonstration of the quantum effect that makes matter invisible

Researchers at MIT have demonstrated a strange quantum effect that has been predicted for decades: if you make a gas cloud dense and cold enough, it can become invisible. The team used lasers to compress and cool lithium gas to densities and temperatures low enough to scatter less light. The researchers say that if they could cool the cloud even closer to absolute zero (minus 273.15 degrees Celsius), it would become completely invisible. This strange effect is the first concrete example of a quantum mechanical process called Pauli blocking.

The new technique could be used to develop light-suppressing materials to prevent information loss (quantum decoherence) in quantum computers. This prohibition is based on the Pauli exclusion principle, which was first formulated by the well-known Austrian physicist Wolfgang Pauli in 1925. Pauli assumed that the so-called fermion particles (protons, neutrons and electrons) in the same quantum state cannot coexist in the same space.

Since there are only a limited number of energy states at the quantum level, this forces electrons to be stored in atoms in envelopes of higher energy levels, which orbit around the atomic nucleus. At the same time, it keeps the electrons of individual atoms apart, because according to a 1967 article co-authored by the well-known physicist Freeman Dyson, all atoms would collapse together without the exclusion principle, with a massive release of energy.

The principle of exclusion also applies to atoms in a gas. Atoms in a gas cloud usually have a lot of room to skip, which means that while fermions can be associated with the Pauli exclusion principle, there are so many unoccupied energy levels they can jump into that this principle doesn’t hamper their motion significantly. If you send a photon into a relatively warm gas cloud, any atom it encounters will be able to interact with it and absorb its momentum, reverting to a different energy level and scattering the photon away. But if you cool the gas, the situation will change.

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Atoms lose energy, fill in the lowest available states, and form a type of matter called a Fermi sea. Particles now surround each other, and cannot move to higher energy levels or go down to lower levels. They are stacked in packages like concertgoers sitting in a completely sold out perspective and can no longer interact with the light. If you send light into the system, Pauli will block it and the light will simply turn into a straight line. An atom can only scatter a photon if it can absorb the force of its impact by moving to another “chair”.

If all the “seats” were occupied, it would not have the ability to absorb the impact and scatter the photon. This will make the corn transparent, the researchers explained. But to enter a cloud of atoms in this state is very difficult. This requires not only extremely low temperatures, but also compressing the atoms to a high density, which is a very difficult task. After trapping the gas in an atomic trap, the scientists directed a laser beam at it. The photons in the laser beam are tuned to only collide with atoms moving in the opposite direction, slowing the atoms down and thus cooling them. The researchers froze the lithium cloud to 20 microkels, which is just above absolute zero.

Then they used a second, precisely focused laser to compress the atoms to a standard density of about a billion atoms (a unit followed by 15 zeros) per cubic centimeter. Then, to see their cooled atoms convincing, the physicists shined a third and final laser beam — carefully calibrated so as not to alter the temperature or density of the gas — onto their atoms and used a highly sensitive camera to count the number of scattered photons.

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As their theory predicted, cooled and compressed atoms scatter 38 percent less light than those at room temperature. Two independent teams also cooled two other gases, potassium and strontium, to demonstrate this effect as well. All three papers showing Pauli’s ban were published in the Journal on November 18 Science. After the researchers demonstrated the Pauli retardation effect, they could eventually use it to develop materials that transmit light.


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