Photon emitting 'light

Leiden University

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A Ph.D. candidate at has developed an innovative technique for creating the elementary building blocks of a future quantum computer or internet in a more controlled manner, opening up a potential solution to many of the challenges along the road to this long-sought technology.

Petr Steindl’s doctoral thesis, which he defended last week as the final step in his Ph.D. program at Leiden University in Germany, explores a new technique for generating photons using quantum dots and microcavities.

“Simply speaking, a quantum dot is a little island of semiconducting material,” Steindl said in a Leiden University statement. “Because it is only a few nanometers in size it feels quantum effects, just like an atom.”

Sometimes called artificial atoms, quantum dots offer a more controllable way to explore quantum phenomena, making them ideal for the task of emitting single photons from a material.

To do this, Steindl put this semiconducting ‘island’ in a microcavity, which is a hole only a few nanometers across so that it only allows precise wavelengths of light to pass through it.

“You can imagine this cavity as two mirrors facing each other,” Steindl said. “Laser light bounces back and forth between them. The quantum dot does not like interacting with light, but the optical cavity makes it more likely because the laser passes the dot many times.”

This light eventually interacts with electrons in the quantum dot, and this is where things get interesting for quantum computer researchers.

“The resonant laser excites an electron in the quantum dot from its ground energy state to a higher one,” Steindl said. “When it falls back to the ground state, the quantum dot emits a single photon. The microcavity conveniently directs this photon toward the rest of our setup.”

Separating the photon from the laser is challenging since it is of the same wavelength as the laser, but that can also be addressed according to Steindl.

“The challenge, however, is to separate this photon from the laser light. It has the same wavelength as the laser but a slightly different polarisation. You can exploit that property to isolate the photon.”

Single photons can then be used in all kinds of other technology, especially in quantum computing applications where single photons can have powerful quantum effects.

“We know that single photons are useful for security and authentication,” Steindl said. “For example, you can send two identical single photons from different locations on a beam splitter. If these photons arrive in an altered state or not simultaneously, you know there was an eavesdropper.”

“I find it totally amazing to build these light structures,” Steindl added. “The fact that it is possible to do this at all is mind-boggling. That we can understand physics at such a deep level. Although it is fascinating, the potential for quantum applications almost seems like a side-effect to me.”