Physicists make “sunglasses for atoms” using bizarre quantum effect
Researchers in Barcelona have developed a new, extremely selective
optical filter to protect atomic quantum memories, part of a proposed
quantum Internet, using one of the most paradoxical effects in quantum
physics. In the quantum network, particles of light (photons) would be
sent between distant locations, and stored in quantum memories
(collections of atoms), when they are not in use.
We all know the effect of sunglasses: Sunglasses block dangerous,
invisible UV light in order to protect our eyes, while they let pass
most of the visible light. In principle, this kind of filter could be
used to feed quantum memories the very specific color they can store
while protecting them from light that has not the right color. To have
an idea of the kind of selection we are talking about: Of the light
visible to the human eye, less than one millionth can be stored in an
atomic quantum memory. But there is a problem: it is hard to find a
filter that transmits only this extremely specific color and blocks
all the rest. However, it is easy to find a filter that absorbs a very
specific color.
To solve this problem, researchers led by Prof. Morgan Mitchell at
ICFO have used a paradoxical quantum effect known as “Interaction-Free
Measurement” or IFM. First predicted in the 1990s, IFM uses quantum
interference, in which a particle takes two paths at the same time, to
do something that would seem impossible: IFM can turn transmission
into absorption and vice versa. Through IFM, a collection of atoms can
therefore be made to transmit a specific color, instead of absorbing
it. The researchers took advantage of this and put the same atoms that
are used in a quantum memory into an IFM to implement a filter at the
wavelength at which the atoms are normally absorbing.
The researchers used these “sunglasses for atoms” - that select 1 part
out of 5 million of the visible spectrum - to create pure single
photons of an extremely well-defined color for the first time. These
filtered photons have all requirements that are essential in an
advanced quantum network.
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LW12091
Why bubbles don’t stick to walls
The next time you watch the bubbles glide elegantly up the side of your champagne glass, consider that you are in fact observing the result of a fundamental physical force, predicted by H. C. Hamaker in 1937, but only now conclusively demonstrated.
The Van der Waals force acts between all bodies, and is almost always attractive, often responsible for strong adhesion, such as the feet of geckos. However, for certain combinations of materials, the force can be a repulsion, which is very strong at small separations. It is possible that in the future it could be used to levitate objects and make devices which are experience no friction.
This work uses a tiny bubble as a probe to measure these forces. The bubble is repelled by solid surfaces in water, and in order to measure such small forces precisely, an Atomic Force Microscope is used. This device measures the movement of a tiny lever the size of a human hair to measure small forces very accurately.
It is found that, as predicted by theory, the strength of the force can be controlled by the material of which the surface is composed, with gold setting the standard for the biggest repulsion.
These results not only demonstrate the presence and magnitude of this unusual force for the first time, but also suggest that we may be able to design micro-scale devices for handling fluids with no friction
