Graphene is fascinating stuff with promise for use in a seeming endless number of applications. This month researchers from the University of Vienna and Institute of Photonic Sciences in Barcelona have proposed graphene-based, two-photon quantum logic gates gate for use in universal quantum computing.

Optical approaches to quantum computing have long looked interesting because of the robustness and mobility of single photons but those approaches have been difficult to achieve. “We propose a universal two-qubit quantum logic gate, where qubits are encoded in surface plasmons in graphene nanostructures, that exploits graphene’s strong third-order nonlinearity and long plasmon lifetimes to enable single-photon-level interactions,” report the researchers led by Philip Walther at the University of Vienna.

Since its discovery a decade or so ago graphene — 2-D material consisting of a single layer of carbon atoms arranged in a honeycomb structure — has tantalized scientists. “It is about 100 times stronger than the strongest steel. It conducts heat and electricity very efficiently and is nearly transparent. Graphene also shows a large and nonlinear diamagnetism, even greater than graphite, and can be levitated by Nd-Fe-B magnets. Researchers have identified the bipolar transistor effect, ballistic transport of charges and large quantum oscillations in the material.”[i]

This latest work is presented in a Nature paper (Quantum computing with graphene plasmons) published last week and also briefly summarized on the Phys.org website yesterday.

“Our gate does not require any cryogenic or vacuum technology, has a footprint of a few hundred nanometers, and reaches fidelities and success rates well above the fault-tolerance threshold, suggesting that graphene plasmonics offers a route towards scalable quantum technologies,” write the researchers in their paper.

As the Phys.org article notes, it was only recently realized that nonlinear interactions can be greatly enhanced by using plasmons. In a plasmon, “light is bound to electrons on the surface of the material. These electrons can then help the photons to interact much more strongly.” However, plasmons in standard materials decay before the needed quantum effects can take place.

Here’s an excerpt from the Phys.org article describing the work.

“In their proposed graphene quantum logic gate, the scientists show that if single plasmons are created in nanoribbons made out of graphene, two plasmons in different nanoribbons can interact through their electric fields. Provided that each plasmon stays in its ribbon multiple gates can be applied to the plasmons which is 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, first author of the study.

“Their proposed scheme makes use of several unique properties of graphene, each of which has been observed individually. The team in Vienna 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 temperature it should readily lend itself to being scaled up, as is required for many quantum technologies.”

The researchers believe their work will indeed be applicable in many quantum information science applications: “By combining ideas from quantum optics with nanoplasmonics, our work opens up an entirely new and promising avenue in the search for single-photon nonlinearities. While we have studied the application of graphene nanoplasmonics to a quantum logic gate, this could also be used for deterministic optical implementations of quantum teleportation,cluster state generation,and single- photon sources,underlining the applicability of this platform.”

Link to Nature paper: https://www.nature.com/articles/s41534-019-0150-2.pdf

Link to Phys.org article: https://phys.org/news/2019-05-quantum-graphene-plasmons.html

Feature art: Schematic of a graphene-based two-photon gate. Credit: University of Vienna, created by Thomas Rögelsperger

[i]Graphene, https://en.wikipedia.org/wiki/Graphene