Physicists build mirrors from just 1,000 atoms

But don’t expect to see your face in one – this is for making optical circuits and quantum computers. Cathal O’Connell reports.

A conceptual drawing of how optical circuits may look in future computers.

Mirror, mirror in mid-air. Two independent teams of physicists have created the world’s most ethereal mirrors – made of just 1,000 or 2,000 atoms suspended in a vacuum.

The mirrors are held in space like beads on a string. By controlling the spacing between the atoms, the physicists could make the strings reflect up to 75% of the light shone on them.

The reflectivity can be switched rapidly on or off, just by applying a few bursts of light – so the new mirrors could be useful for controllably bouncing light around optical circuits. And because the atoms interact with one another as well as the light, the set-up might be useful for linking quantum bits (or “qubits”) together in a quantum computer.

The mirrors were created by two independent groups, one in France and the other in Denmark. Both are described in the current issue of Physical Review Letters.

In a regular mirror, such as a polished metal surface, light is reflected because it interacts with the cloud of unattached electrons floating free in the metal, causing them to wobble. These wobbling electrons then re-emit the light. (These electrons are also the reason metals conduct electricity, so that’s the connection between shininess and conductivity.)

But the new mirrors use something called Bragg reflection, which is a bit different.

As Australia-born British physicist William Lawrence Bragg discovered in 1912, light waves scattering off layers in a crystal are reinforced at certain angles – those where neighbouring light waves return in lock step.

Building on this work, just three years later, Bragg and his dad, William Henry Bragg, won the Nobel Prize for physics for using X-rays to figure out the structure of crystals. This kicked off the whole field of X-ray crystallography – instrumental a few decades later in unravelling the double helix structure of DNA.

But when the spacing between the crystal layers is just right, the scattering angle is 90 degrees and so the crystal strongly reflects the light back where it came from – a special case known as Bragg reflection.

A schematic showing a Bragg diffraction – the usual scattering of light that occurs when two beams with identical wavelength and phase approach a crystalline solid and are scattered off two different atoms within it. The lower beam traverses an extra length of 2dsinθ. But when the angle θ of the light hitting the surface is 90°, the light is reflected straight back the way it came – a Bragg reflection.

Whereas regular mirrors can reflect any visible wavelength (that’s why mirrors appear to have no “colour” of their own), a Bragg mirror only reflects one wavelength. So don’t expect to see yourself in one.

But that’s no limitation for communications technology, or optical circuits, which involve shuttling around light of a single wavelength.

In 2011, a German team managed to turn a cloud of cold atoms into a Bragg mirror. They crisscrossed beams of lasers to arrange the atoms of the cloud into a lattice with just the right spacing. Although they achieved 80% reflection, they needed 10 million atoms to do it.

Now two teams have dramatically reduced the number of atoms needed to make a useful mirror. Instead of simply shining a beam of light into a cloud of atoms (as the German group did), the teams transmit light along microscopically thin optical fibres. Atoms precisely positioned next to the fibre do the reflecting.

When light travels along very thin optical fibres, some of the light spills out forming a so-called evanescent field – you can picture the field as a glowing halo around the fibre.

Because the light is intensely confined in this halo, the interaction with any nearby atoms is very strong. This means only 1,000 or so atoms are needed to achieve a reflection, versus tens of millions for the cloud situation.

The groups created their strings of single atoms by holding them in place using a laser beam running parallel to the fibre, and just a few hundred nanometres away, via the optical tweezers effect.

Each string was evenly spaced with atoms every few hundred nanometres and was about one millimetre long.

This mirror switching mechanism could be very useful for making optical switches in light-based circuitry.

The physicists then sent another beam of light along the fibre – and this one interacted with the string of atoms through the halo of its evanescent field. When the spacing between the atoms was tuned just right, the Bragg condition applied – and much of the light was reflected back along the fibre in the opposite direction.

The Danish team could reflect about 10% of the light using a string of 1,300 caesium atoms. While the French team reflected 75% using 2,000 atoms, also of caesium. The increased reflectivity achieved by the French group was not just a factor of more atoms in a row, they also had better control over the positions of their atoms.

The mirror could be rapidly disassembled and reassembled simply by knocking the atoms out of their ordered state, and then replacing them. This mirror switching mechanism could be very useful for making optical switches in light-based circuitry.

Red light sent through an optical fibre is visible in the fibre segment that is just a few hundred nanometres in diameter in this Danish experiment.
J. Appel / University of Copenhagen

The atoms also interact with one another via the light field, and over quite a long range. This kind of interaction could be used to simulate less tangible quantum interactions, or even for linking quantum bits (or “qubits”) together in quantum computers.

All these applications will need stronger interactions between the light and the atoms – in effect a reflectivity much closer to 100%.

Both teams have some tricks up their sleeves to achieve this, such as using longer sections of very thin fibre, or by reshaping the surface of the fibre to increase the interaction.

As Wolfgang Ketterle, a physicist in quantum optics at the Massachusetts Institute of Technology told the American Physical Society, these works represent “a major advance in engineering and controlling how atoms scatter light”.

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Cathal O'Connell is a science writer based in Melbourne.