Carbon tube in the center is used as a photon source and superconducting nanowires as receivers complete part of the optical chip. (Image credit: W.Pernice/WWU)
Optical quantum computers could potentially be used for quantum simulation of highly complex systems, safe data encryption and ultrafast calculation of huge data volumes.
Professor Ralph Krupke of the KIT explains that to be able to use this technology in practice, it has to be accommodated in a minimum of space. Whole laboratory spaces are however currently often used in experiments to investigate how optical quantum technology can be applied.
In an article published in Nature Photonics, scientists describe how they have managed to place a complete quantum optical structure on a chip for the first time. With this development, one of the conditions for the use of photonic circuits in optical quantum computers has been met.
Scientists from Russia, Germany and Poland took part in the study. They were led by Professors Manfred Kappes, Ralph Krupke and Carsten Rockstuhl of the Karlsruhe Institute of Technology (KIT), and Wolfram Pernice of the Westphalian Wilhelm University of Münster (WWU).
The term light quanta refers to photons or light particles. This is where the term “quantum photonics” comes from. For the first time, scientists used nanotubes made of carbon with a diameter 100,000 times smaller than a human hair as a light source for their quantum photonic circuit. When excited by laser light, each carbon tube emits a single light particle.
Any material emitting single photons makes it attractive as an ultra-compact light source for optical quantum computers.
Physicist Wolfram Pernice admits that it is extremely difficult to accommodate the laser technology on a chip that is scalable. Scalability is defined as the ability to miniaturize components so that their number can be increased, and it is a precondition for any technology before it can be used in an optical quantum computer.
In the new chip, additional laser systems are no longer required since all elements on the chip are triggered electrically. This is significantly simpler than the optical excitation normally used. Ralph Krupke conducts research at the Institute of Materials Science of the Darmstadt Technical University and the KIT Institute for Nanotechnology. He sees the combination of a single photon waveguide, detector and source on a scalable chip as an important step for research. By doing this, the team has shown that single photons can be emitted by electric excitation of the carbon nanotubes, thereby eliminating a limiting factor that has thus far prevented the potential application of the technology.
The methodology used by the scientists consisted of them first determining whether the flow of electricity through carbon nanotubes would cause single light quanta to be emitted. To achieve this, they made use of carbon nanotubes as single photon sources, superconducting nanowires as detectors, and nanophotonic waveguides.
A single photon source was connected with two detectors each to one waveguide. Liquid helium was used to cool the structure. This arrangement allowed single light quanta to be counted. An electron beam-scribing device was then used to manufacture the chips.
The scientists’ work is purely theoretical and it is not yet clear if it will ultimately lead to practical applications.