|Laser light produced in a layered gallium arsenide (GaAs) semiconductor nanostructure results in entangled photon pairs. (Image: Rolf Horn)|
Innsbruck, Austria--Moving closer to the ultimate goal in quantum optics of placing entire systems on semiconductor chips, Austrian and Canadian scientists, led by Gregor Weihs of the University of Innsbruck and the University of Waterloo (Waterloo, ON, Canada), have demonstrated a viable source of entangled photon pairs in a semiconductor.1 Applications in quantum cryptography and quantum computing rely on the production of such entangled photon pairs.
Gallium arsenide nanostructure
The team, which also includes researchers from the University of Toronto, used gallium arsenide (GaAs), which has nonlinear characteristics that make a variety of physical phenomena technologically useful. In the material, laser photons can be split into pairs of photons of lower energy. These photon pairs are entangled, meaning they have a common quantum state.
However, using these pairs for information processing is a problem. Because the laser photons and the entangled photon pairs travel at different speeds through the transparent material, the pairs mostly mutually annihilate, explains Gregor Weihs. "The yield of photon pairs is therefore extremely low," he says.
To get around this problem, the researchers fabricated a nanostructure consisting of layers with different refractive indices. At the layer boundaries, light reflections direct the photons so that they travel at the same speed through the material, preventing mutual annihilation (if everything is set up properly). This greatly increases the efficiency of the photon source.
A future quantum chip
Producing such well-defined nanostructures is no easy task; as a result, the physicists reported that their experiment still has high power losses. "The effect, however, is so efficient that we get, even under these conditions, a very good signal," says Weihs.
Now Weihs and his team aim to develop a photon source in which the polarization of the photons can be entangled -- a property very useful for quantum information processing. "We have always dreamed of such an integrated source of photons in which we send in an electrical pulse and obtain entangled photons at the output," says Weihs. "So in our experiments I'll be able to replace the complex structures that now take up half a laboratory bench." The effort could eventually lead to compact, fully integrated quantum-optical components that make quantum information processing available for everyday use.
1. Rolf Horn et al., Phys. Rev. Lett. 108, 153605 (2012) DOI: dx.doi.org/10.1103/PhysRevLett.108.153605.