# Focus: Metamirror Generates Interference at a Distance

Physics 8, 64
A proposed metasurface made of tiny gold antennas could act as either a flat mirror or a concave, focusing mirror, depending on the radiation pattern of the source, which could lead to new ways to control quantum systems.

The field of quantum optics relies on the ability to precisely control quantum interactions between light and matter, typically at scales corresponding to the wavelength of light or less. Researchers now propose a design, using well-established technology, that could allow manipulation of quantum effects at distances of one hundred wavelengths or more. Devices using the technique could have many applications, including improvement of the efficiency of lasers or photocells and manipulation of quantum information carried by photons.

The ability to fine-tune the interactions of photons and atoms is of use in a variety of applications. In an atom with two or more excited states, for example, controlled quantum interference between the photons it emits and the states that emit them can change the populations of those states and the rates at which they decay. Through such manipulations, researchers could improve the efficiency of lasers and photocells [1]. Another use, says Pankaj Jha of the University of California, Berkeley, might be in quantum computing and information processing, where interference could control the flow of quantum bits within an array of trapped atoms, ions, or other particles.

Typically, however, different excited states in the same atom are “orthogonal” to each other, meaning that a photon emitted by one state cannot interfere with the other. One way to induce interference is to make the atom’s electromagnetic environment anisotropic, so that it alters a photon’s properties and allows it to interact with an orthogonal state [2]. This might be done by placing the atom in a nanoengineered “photonic” structure that guides photons in carefully designed ways. But such methods require extraordinarily precise placement of the emitting atom and are easily disturbed by short-range forces that arise through undesirable quantum effects between the atom and the structure.

Jha and his colleagues propose a new way to create an anisotropic environment. They have designed a surface that reflects a single atom’s orthogonal photons in different ways, allowing quantum interference with the emitting states to occur. Moreover, the design of the “mirror” makes it possible to place the anisotropic region many wavelengths away from the surface, far from disturbing influences.

The theorists propose a patterned metasurface consisting of gold rectangles (nanoantennas) of subwavelength sizes placed in a specific pattern on a gold slab. A thin layer of nonconducting material (they suggest magnesium fluoride) separates the nanoantennas from the slab. For a wavelength of 894 nanometers, corresponding to a widely used emission frequency of cesium, the overall reflectivity of the metasurface is better than $80%$, according to their calculations.

The metasurface pattern contains a repeating arrangement of five nanoantennas of different sizes, which leads to the anisotropy in reflection. The researchers assumed that the atom emits radiation in a so-called dipole pattern, which is weakest at the atom’s “poles” and strongest around the “equator.” For a dipole lying parallel to the metasurface and oriented in a specific direction—call it $x$—the metasurface acts like a simple plane mirror. But for a dipole oriented along the $y$ direction, the metasurface acts like a concave mirror, reflecting light back to a focal point. The reflected photon, concentrated in intensity and distorted by the curved mirror, can then interfere both with the state that emitted it and with a second, orthogonal state. The anisotropic environment created by the metasurface thus makes interference possible between two orthogonal states of a single emitter.

The team’s calculations show that appropriate design of the metasurface pattern can create an anisotropic environment as much as 100 wavelengths from the surface and with a size of about one wavelength. That’s a large enough region that current technology allows an atom to be placed within it, Jha says. Bill Barnes of the University of Exeter, UK, says that creating a surface with a response that depends on dipole orientations and exerts effects many wavelengths from the surface is “very different from previous work.” He adds that the new scheme will prompt him and likely others to think about how it might be used.

This research is published in Physical Review Letters.

–David Lindley

David Lindley is a freelance writer in Alexandria, Virginia, and author of Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science (Doubleday, 2007).

## References

1. M. O. Scully, K. R. Chapin, K. E. Dorfman, M. B. Kim, and A. Svidzinsky, “Quantum Heat Engine Power Can Be Increased by Noise-Induced Coherence,” Proc. Natl. Acad. Sci. U.S.A. 108, 15097 (2011)
2. G. S. Agarwal, “Anisotropic Vacuum-Induced Interference in Decay Channels,” Phys. Rev. Lett. 84, 5500 (2000)

## Related Articles

Atomic and Molecular Physics

### Focus: The Coolest Molecular Ion Beams

Two research teams have created the coldest molecular ion beams ever, putting molecules in their ground states of rotation and providing improved experimental stand-ins for interstellar gas clouds. Read More »

Quantum Physics

### Viewpoint: The Thermodynamic Cost of Measuring Time

A simple model of an autonomous quantum clock yields a quantitative connection between the clock’s thermodynamic cost and its accuracy and resolution. Read More »

Quantum Physics

### Synopsis: Quantum Sensing of Magnetic Fields

A new design for an atomic magnetometer utilizes so-called quantum nondemolition measurements to detect very weak magnetic-field signals. Read More »