Entangled photons on demand

Harnessing the counterintuitive rules of quantum mechanics could revolutionize the way information is sent and processed. Rather than handle data via billions of bits representing either a '1' or "0" with a transistor, quantum computers — which are based on the ability of elementary particles such as photons to process data — can potentially outperform classical computers for certain tasks and offer new ways to encrypt information.

Researchers at Tohoku University in Sendai, Japan, have now demonstrated a source of 'entangled' photons using a gallium arsenide quantum dot — semiconductor nanostructures that confine an electron in 3D. The energy levels of quantum dot can be tailored to only emit photons with desired properties, in this case pairs of entangled photons such that a measurement of one instantaneously affects the state of the other.

Thanks to the different behaviour of the confining potentials in gallium arsenide, the team found that when an electric field is applied across the device, troublesome fine-structure splits of energy levels were avoided unlike in other indium-gallium-arsenide structures. As a result, these quantum dots emit photons at shorter wavelengths of about 750nm, which is close to the optimum wavelength for standard silicon photon detectors.

"Our GaAs quantum dot is more suitable for optical quantum computation schemes compared to that of previous In(Ga)As devices," says lead author Mohsen Ghali, who is currently based at the physics department of Kafrelsheikh University in Egypt. "It allows applications as a source of two-photon coincidence with high visibility and high degree of entanglement, which may be of great interest to both the science and engineering communities."

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