The Lightest Mirror in the World

A mirror from the quantum world: Unlike individual atoms, an ensemble of atoms that reflects light quite well thanks to its collective behaviour can be illuminated directly with a laser beam. This is symbolized here with a mirror standing in the vicinity of an optical experiment and reflecting laser light. The actual mirror of the Garching physicists is not visible to the naked eye. (Source: C. Hohmann & D. Wei)

The lightest mirror in the world sheds new light on quantum phenomena. Physicists at the Max Planck Institute of Quantum Optics in Garching constructed the mirror by arranging only 200 atoms in a two-dimensional optical lattice consisting of inter­fering laser beams. The quantum mirror is the first example of a system in which an ordered ensemble of atoms interact as a collec­tive with incident light. This new form of inter­action between light and matter opens up a new field in basic research as well as new appli­cations in quantum infor­mation processing.

Mirrors are usually made of polished metal surfaces or specially coated glass so that they can reflect light as well as possible. Physicists from the Max Planck Institute have now shown for the first time that even a single layer of around 200 atoms can reflect light remarkably well. Because the atoms are arranged at a much greater distance from each other than they are in a metal surface, they have a very low density. The researchers have thus essen­tially constructed the lightest mirror in the world. With a diameter of about seven micrometres and a thickness of a mere ten nano­meters – which corresponds to the range of motion of the atoms in the optical lattice – the mirror itself is much too small to be seen with the naked eye. Its reflection, on the other hand, is visible to the naked eye because the atomic ensemble reflects light parti­cularly effec­tively.

Even though the actual mirror is extremely small, the instruments used to create the mirror fill an entire laboratory – as is the case for most quantum optical experiments. Over one thousand individual optical components, which together weigh around two tonnes, are needed to capture the atoms of the mirror in an optical lattice of crossed laser beams, generate the flashes of light to be reflected, and analyse the reflected light. The new material will therefore not replace conventional mirrors any time soon. But quantum physicists will be able to conduct many more experiments with it.

“The results are quite exciting for us”, says Jun Rui, a scientist at the Max Planck Institute of Quantum Optics. “This is because photons that strike our mirror create corre­lations between atoms. This mechanism has often been neglected in quantum optics”. Only through this interplay of atoms does the ensemble act on the incident light as a collective and not as a collection of indi­vidual particles. The collec­tive behaviour occurs when the distances between the atoms are smaller than the wavelength of the light that the mirror should reflect. Meta­phorically speaking, the incident light does not see individual atoms but rather a single reflec­tive surface. “This is the first time we have observed this collective behaviour in atoms held in an optical lattice”, says David Wei, a doctoral researcher at the Max Planck Institute of Quantum Optics who was also involved in the study. Such arrange­ments were therefore suitable for inves­tigating new quantum optical phenomena.

The collective interaction between the atomic ensemble and light opens up many new possi­bilities for basic research. The Max Planck physicists also have ideas about how their quantum mirror could be used for the processing – and especially the trans­mission – of quantum infor­mation. It has been suggested that the mirror be equipped with a special switch. The mirror can theo­retically be switched back and forth between two states by energe­tically exciting an atom in the ensemble. In one of these states, it reflects light; in the other, it becomes translucent.

However, the mirror of the physicists cannot yet be switched in such a way that it clearly does or does not reflect light. The researchers are therefore working on using sophis­ticated laser excitation to bring it into a super­position of the two states. In such a super­position state, it reflects light only with a certain proba­bility. It is precisely such states that make the processing of quantum information more efficient than classical infor­mation processing. This is because they combine two or more opposing properties.

When the researchers have placed the excitable atom in a quantum super­position state, the random state of this atom determines whether or not the mirror reflects. The undecided state of the atom is also reflected in the mirror and thus in the light when it interacts with the quantum mirror. Physicists say that atom, mirror, and light are entangled. “Such a quantum-switchable mirror offers interes­ting new possi­bilities for the trans­mission of quantum information – like the kind a quantum computer would output”, says Wei. Whether in quantum tech­nology or in basic research, physicists now hope to shed light on the prospects opened up by the quantum mirror. (Source: MPQ)

Reference: J. Rui et al.: A subradiant optical mirror formed by a single structured atomic layer, Nature 583, 369 (2020); DOI: 10.1038/s41586-020-2463-x

Link: Quantum Many-Body-Systems, Max Planck Institute of Quantum Optics MPQ, Garching, Germany

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