Quantum leap in quantum encryption

Better light source developed for quantum cryptography

With a laser, electrons and "holes" are generated (arrow down). An acoustic wave pumps the charges in the direction of quantum dots. These are excited and emit single photons (arrow up). © PDI
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Berlin researchers have developed a new light source, with which they can send individual photons in high repetition rate and at precisely defined time intervals. As the scientists write in the journal Nature Photonics, they use surface acoustic waves.

With individual photons, the smallest "light particles", information can be encrypted to be eavesdropped. This is based on quantum mechanical laws, according to which you can not measure the properties of a photon - and thus it can not listen - without changing exactly these properties.

Quantum cryptography becomes more powerful

Conventional single-photon sources, which laser-excite atoms or semiconductor nanoparticles, called quantum dots, emit photons at a repetition rate of up to 80 megahertz. In order to make quantum cryptography even more efficient, one would like to achieve the highest possible repetition rates of the emitted photons. In addition, the individual light particles must be emitted in very uniform intervals.

Gallium arsenide chip with quantum dots

The researchers around Paulo Santos from the Paul Drude Institute for Solid State Electronics have now found a way to send photons with a ten times higher repetition rate and set their distances exactly the same. On a tiny gallium arsenic chip is a collection of quantum dots. At some distance from the points, the researchers use a laser to generate positive and negative electrical charges, ie electrons and "holes".

At the other end of the chip, an acoustic transducer sends out a surface wave - a kind of sound wave - that "pumps" the charge carriers in the direction of the quantum dots. The electrons are located in the valley of the wave, the "holes" on the wave crest. If both charges meet in a quantum dot, it is excited with the result that it sends out a photon. display

Double property of light used

Over the height of the wave peaks and valleys, the researchers can control which quantum dots are excited. Because these are slightly different and therefore have different emission energies. This also allows photons with different energies to be emitted.

To demonstrate that they are truly single photons, researchers use a technique that uses the dual nature of light to be both wave and particle. They direct the emitted light through a beam splitter to two detectors. If a single light particle is present, it can not divide, which is why only one of the two detectors receives a signal.

Interception of the photons is optimized

Before this system can be used for quantum cryptography, the researchers still face some challenges. Thus, the previous experiments at temperatures close to the absolute zero, which is not suitable for practical use.

In addition, scientists want to optimize the capture of the photons that are now emitted in all directions. By using a resonator, they want to ensure that the light particles preferably fly in one direction.

(idw - Forschungsverbund Berlin, 26.10.2009 - DLO)