This is a schematic of a graphene-based two-photon gate. Image: University of Vienna, created by Thomas Rögelsperger.
This is a schematic of a graphene-based two-photon gate. Image: University of Vienna, created by Thomas Rögelsperger.

Physicists from the University of Vienna in Austria and the Institute of Photonic Sciences in Barcelona, Spain, have shown that tailored graphene structures can allow single photons to interact with each other. As the physicists report in a paper in npj Quantum Information, this finding could lead to new designs for optical quantum computers.

Photons barely interact with the environment, making them a leading candidate for storing and transmitting quantum information. But this same feature makes it especially difficult to manipulate information encoded in photons.

In order to build a photonic quantum computer, one photon must change the state of a second photon. Such a device is called a quantum logic gate, and millions of logic gates will be needed to build a quantum computer. One way to achieve this is to use a so-called 'nonlinear material', in which two photons can simultaneously interact within the material. Unfortunately, standard nonlinear materials are far too inefficient for use in fabricating a quantum logic gate.

Recently, scientists realized that nonlinear interactions can be greatly enhanced by using plasmons, which are quasiparticles created when light binds with electrons on the surface of a material. These electrons can help the photons to interact much more strongly. In standard materials, however, plasmons decay before the needed quantum effects can take place.

In this new work, the team of physicists led by Philip Walther at the University of Vienna propose to create plasmons in graphene. This two-dimensional material, discovered barely a decade ago, consists of a single layer of carbon atoms arranged in a honeycomb structure. For this particular purpose, the peculiar configuration of the electrons in graphene leads to both an extremely strong nonlinear interaction and plasmons that persist for an exceptionally long time.

In their proposed graphene quantum logic gate, the physicists show that, if single plasmons are created in graphene nanoribbons, two plasmons in different nanoribbons can interact through their electric fields. Providing each plasmon stays in its ribbon, this means multiple gates can be applied to the plasmons, as required for quantum computation.

"We have shown that the strong nonlinear interaction in graphene makes it impossible for two plasmons to hop into the same ribbon" says Irati Alonso Calafell from the University of Vienna, who is first author of the paper.

This quantum logic gate makes use of several unique properties of graphene, each of which has been observed individually. The Vienna team is currently performing experimental measurements on a similar graphene-based system to confirm the feasibility of their gate with current technology. Since the gate is naturally small, and operates at room temperatures, it should readily lend itself to being scaled up for use in future quantum technologies.

This story is adapted from material from the University of Vienna, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.