Imperfect chips pave the way for new quantum technology

This discovery is published in journal Science.

Messy accounts, noise on the line or production errors: Disorder is, in many respects, considered an evil. This also applies within photonics, and researchers worldwide have put considerable effort into perfecting optical chips which, among other applications, can be used in quantum technology.

An optical chip can be used to manipulate information in the form of light, and the functionalities are integrated in a few thousandths of a millimetre. Up until now, a major problem has, however, been the fact that nanometre-scale imperfections are inevitable during optical chip production. So far, it has been the general conviction that this reduces or simply destroys functionality, and that this has hampered the possibility of upscaling optical chips to larger and more complex circuits.

Disorder as a valuable resource

A group of physicists from DTU Fotonik has now turned this notion upside down and demonstrated that imperfection in the form of disordered structures on optical chips may actually be an advantage: The disordered structures on an optical chip may be used to capture, for example, light waves.

The research group has demonstrated that when the light is captured on the imperfect optical chip, the interaction of light with matter (an atom) is increased approximately 15 times. The discovery allows the production of a new type of optical chips where disorder is utilised as a valuable resource instead of being considered a limitation. It may potentially be used to develop efficient miniature lasers, solar cells and sensors and to pave the way for a completely new quantum information technology, including quantum computers.

Optical chips with ordered structures

On optical chips based on photonic crystals, a structure of holes is normally etched, and so far the aim has been to achieve a regular and ordered structure. Even though modern nanotechnological techniques make it possible to fabricate very precise structures, a certain element of disorder is inevitable in any real system. There will thus be roughness and variations in the positioning of the holes of which a photonic crystal is made up. By changing the distance between the holes in the photonic crystal and omitting a row of holes, a waveguide is created, which can guide light in desired directions, thus providing new possibilities for taming light. A properly designed photonic crystal thus makes it possible to stop or capture light -- and even control the emission of light.

Optical chips with disordered structures

The researchers at DTU Fotonik have fabricated an optical chip where disorder has deliberately been introduced in the structure. Without disorder, the light will propagate along the waveguide, whereas the presence of disorder alters this picture completely. The light will thus be captured in the waveguide as it is scattered on the imperfections and subsequently interferes with other parts of the light wave. This way of localising light has proved surprisingly efficient, and in the experiment carried out at DTU Fotonik, the researchers succeeded in localising the light in the waveguide within a region smaller than 25 microns (one micron = one thousandth of a millimetre). In their experiment, the researchers used nanoscopic light sources inside the photonic crystal (the so-called quantum dots). A quantum dot can be seen as an artificial atom emitting exactly one photon at a time. The researchers have thus succeeded in making a 'box for photons', i.e. capturing and retaining the elementary constituent of the light: the photon.

Unbreakable messages and quantum computers

The ability to localise light is crucial for many applications, as light in many contexts is intractable: It propagates at a speed of almost 300,000 km/s, making it very useful for transmitting information for use in optical communication. Unfortunately, it also means that the interaction with matter is generally inefficient, which is a problem for a number of applications, e.g. in solar cells and optical sensors or within quantum information technology. The dawning quantum information technology promises fundamentally new ways of coding and processing information, using the laws of quantum mechanics. This can, among other things, be used to exchange 100% unbreakable messages or, ultimately, for a quantum computer which can perform a number of calculation tasks far more efficiently than even the supercomputers of today.

Research based on Nobel Prize winner's theory

The use of very disordered structures to capture light waves was predicted in theory by the US researcher Philip W. Anderson, who was awarded the Nobel Prize in physics back in 1977.

In the 1950s, Philip W. Anderson predicted that the transport of electrons may be suppressed in a highly disordered lattice. This phenomenon is called Anderson localisation. This is due to the fact that electrons in the world of quantum mechanics have wave properties, and that these waves can interfere, like other types of waves can be mixed, which is a well-known phenomenon by everyone who has been swimming in the breakers. Anderson's discovery has proved to be a universal phenomenon which not only applies to electrons, but to all other types of waves. Disorder can thus also be used to localise light waves -- i.e., capture light in a very small area.

In quantum information technology, it is crucial to have a very strong light-matter coupling at the most elementary level -- i.e., so that one photon interacts efficiently with one atom. Such an increased coupling is exactly what the researchers at DTU Fotonik have demonstrated, where a photon in an Anderson-localised cavity interacts with a quantum dot. The increased coupling results in the quantum dot emitting a photon more rapidly when its wave length matches that of the cavity (i.e. is in resonance). This is exactly what the researchers have observed, as shown in Figure 3, which shows that the quantum dot emits photons up to 15 times more rapidly under resonant conditions than under non-resonant conditions.

The research group behind the discovery

The research has been conducted at the Department of Photonics Engineering at the Technical University of Denmark by a research group consisting of postdocs Luca Sapienza, Søren Stobbe and David Garcia, PhD students Henri Thyrrestrup and Stephan Smolka as well as Associate Professor and group leader Peter Lodahl.