In a paper published in Nature on July 15, 2020, Physicists of Max Planck Institute of Quantum Optics, Garching, Germany have revealed the engineering of a metamaterial (engineered material that has the properties not usually found in nature), which is the lightest mirror in the world so far. It is a single structured layer of a few hundred atoms. Since atoms are arranged at a much greater distance from each other than they are on the metal surface, they have very low density, making it light weighted. The mirror is only several tens of nanometers thin, which is a thousand times thinner than the width of a human hair. Therefore it cannot be visually recognized. But still, the reflection is so strong that even the pure human eye could perceive it. Since the apparatus is enormous and it counts over a thousand single optical components and weighs about two tons, it cannot be used as a commodity mirror that people can use daily. But the mirror holds a significant place in the quantum realm.
What made the lightest mirror possible?
Usually, we utilize highly polished metal surfaces or specially coated optical glasses to improve the performance of the mirrors in smaller weights. But, here the mechanism is different. Identical Rubidium – 87 atoms were arranged in a two-dimensional array of optical lattices by ultra-cooling. And this was done by laser cooling and evaporative cooling. Several rubidium atoms were cooled which began at room temperature. By directing enough photons at the rubidium, the tiny force of each one can collectively slow down the atoms. This is known as laser cooling. Then, the temperature of the atoms was again reduced to around 10 nano kelvin. A precise magnetic field was applied in one direction to isolate a single layer of atoms. This is called evaporative cooling.
The crucial properties that made the mirror possible are:
- The atoms were ordered in a regular pattern.
- Sub-wavelength spacing – The spacing between atoms was lower than the optical transition wavelength of the atoms.
These properties of the mirror lead to suppression of diffuse scattering of light, bundling the reflection into a one dimensional and steady beam of light. Because of the comparatively close and discrete distance between atoms, an incoming photon can bounce back and forth between the atoms more than once, before it is being reflected. These effects lead to an “enhanced cooperative response to the external field” which, in this case, is a very strong reflection. And the incident light does not see individual atoms but rather a single reflective surface.
The ordered arrays of atoms made by loading ultra cold atoms into optical lattices were mainly exploited to study quantum simulations of condensed matter models. But through the engineering of this mirror, it turns out to be a powerful platform to study the new quantum optical phenomena pointed out by Jun Rui, the first author of the paper.
Why is the mirror important?
- This is the first example of a system in which an ordered ensemble of atoms interacts as a collective with incident light. This new form of interaction opens up new fields and new applications in quantum information processing.
- Further research on this could deepen the fundamental understanding of the quantum theory of light-matter interaction, many-body physics with optical photons, and enable the engineering of more efficient quantum devices.
- Further research could also lead to better quantum memories.
- When the excited atom is placed in a quantum superposition state, the random state of this atom determines whether the mirror reflects or not. The undecided state of the atom is also reflected in the mirror and thus the light when interacts with the quantum mirror. The atom, mirror, and light are entangled. Such a quantum switchable mirror offers interesting new possibilities for the transmission of quantum information.
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