Nano-waveguide sends single photons in the right direction
Researchers from the FOM Foundation, Delft University of Technology and Eindhoven University of Technology have succeeded in bundling single photons, the carriers of quantum information, into a compact beam. With this technique quantum communication can be made twenty times more efficient. The researchers placed their photon source in a self-developed nano-waveguide with a tapered point. As a result of this nearly all of the photons moved in a single line along the nano-waveguide. Using a special mirror the researchers also ensured that the photons moved in the same direction towards the tapered nano-waveguide tip. The researchers published their results yesterday online in Nature Communications.
Quantum computer research recently received extensive news coverage with the possible discovery of Majorana fermions. Progress is also being made in the area of transferring quantum information. Worldwide physicists are trying to develop new methods to exchange information between quantum bits, the memory elements of quantum computers. This communication proceeds via single photons (light particles). The more targeted single photons that can be sent from A to B, the more efficient the information transfer.
Light beams
The photons come from very small artificial light sources several nanometres in size, the so-called quantum dots. A quantum dot emits light particles in different directions just like a tiny light bulb. That is not very efficient, as then few photons reach the target destination. The solution is to bundle them into a small beam - a sort of laser beam. The researchers had done that previously by placing the quantum dots in a nanowire but then they only achieved an efficiency of one to two percent.
By changing the shape of the nanowire the physicists have now succeeded in achieving an efficiency of more than 42 percent. The wire is a sort of nano-waveguide where the quantum dot is not at a random position but exactly in the middle. This position proved to be essential. The researchers constructed the nano-waveguide in stages: first the quantum dot in the core of the wire and then the wire was built up layer by layer to form a shell around this.
Mirror
The design of the nano-waveguide ensures that all photons are emitted in the same direction along the nanowire. As the photons are transmitted both upwards and downwards, the researchers attached a gold mirror to the underside of the wire, as a result of which the photons emitted downwards were reflected back upwards. The tapering point of the nano-waveguide is the last crucial aspect in the design. This point ensures that no undesired reflections take place on the surface between the semiconductor and the air. Using this technique the researchers could bundle more than twenty times as many photons than they could in previous experiments.
Contact
Dr. Michael Reimer
Reference
M.E. Reimer, G. Bulgarini, N. Akopian, M. Hocevar, M. Bouwes Bavinck, M.A. Verheijen, E.P.A.M. Bakkers, L.P. Kouwenhoven, and V. Zwiller. Bright single-photon sources in bottom-up tailored nanowires. Nature Communications 3, 737 (2012) doi: 10.1038/ncomms1746.
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