Light cages could give quantum-information networks a boost

Physics

Atoms and light: the main image is an illustration of atoms entering the cage, which contains light. The insets are electron microscope images of the cage’s structure. (Courtesy: Flavie Davidson-Marquis, Julian Gargiulo, Esteban Gómez-López, Bumjoon Jang, Tim Kroh, Chris Müller, Mario Ziegler, Stefan A. Maier, Harald Kübler, Markus A. Schmidt & Oliver Benson)

A new on-chip device that is very good at mediating interactions between light and atoms in a vapour has been developed by researchers in Germany and the UK.  Flavie Davidson-Marquis at Humboldt University of Berlin and colleagues call their device a “quantum-optically integrated light cage” and say that it could be used for wide range of applications in quantum information technology.

Hybrid quantum photonics is a rapidly growing area of research that integrates different optical systems within miniaturized devices. One area of interest is the creation of devices for the control, storage and retrieval of the quantum states of light using individual atoms. This is usually done by integrating on-chip photonic devices with miniaturized cells containing warm vapours of alkali atoms. However, this approach faces challenges due to inefficient vapour filling times, high losses of quantum information near cell surfaces and limited overlaps between the wavelengths of light used in optical circuits and the wavelengths of atomic transitions.

Arrays of dielectric cylinders

Now, Davidson-Marquis and colleagues have addressed these issues by creating an on-chip light cage that is integrated onto an alkali vapour cell. Fabricated extremely precisely using a 3D laser nanoprinter, the pipe-like cage has a diameter of about 24 micron and is made from arrays of dielectric cylinders, arranged in a hexagonal pattern around a hollow core.

Unlike the fibres and planar waveguides used previously, this compact, easy-to-handle light-guiding structure can be accessed laterally – allowing alkali atoms to diffuse into and fill the core in minutes. Furthermore, by coating the cage with an alumina nanofilm, the researchers could precisely tune the wavelength of the transmitted light to that of the alkali atom transition. The coating also prevents the corrosive alkali vapour from damaging the polymer cage cylinders.

Transparent window

In experiments involving vapours of caesium atoms, the team observed the emergence of electromagnetically induced transparency (EIT) within their light cage. This is a useful quantum-optical effect that occurs when specific atomic transitions induced by a light beam create a window where light with a narrow range of wavelengths can propagate freely through an otherwise opaque vapour. EIT can also be used to “store” light signals within an atomic vapour.

The team envisages clear routes towards further improvements to their on-chip device. Owing to its long-term stability, ease of integration, and versatile production through 3D laser nanoprinting, the device could be interfaced with other silicon-chip-compatible devices such as waveguides for light modulation and frequency conversion. It can also be coupled to optical fibres.

In the future it should be possible to use EIT to create a broad range of transparent windows. This could be useful for creating highly compact storage devices for quantum information or systems that control the arrival times of photons in quantum networks. Potential applications of devices with these capabilities include optical switches, quantum memories and quantum repeaters.

The device is described in Light: Science and Applications.

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