Physicists from the University of Vienna and the Institute of Photonic Sciences have shown that graphene structures can be tailored to enable single photons to interact with one another. Their findings have led them to propose a new potential architecture for a two-photon logic gate for quantum computing. Logic gates — the devices that allow one photon to change the state of a second photon — will be needed in large numbers to build a photonic quantum computer. One way to achieve this is to build gates using a nonlinear material in which two photons can interact. However, standard nonlinear materials are too inefficient to be used for this purpose. Schematic of a graphene-based two-photon gate. Courtesy of University of Vienna/Thomas Rögelsperger. A team led by professor Philip Walther proposes to use graphene to create a quantum logic gate. The team further proposes to enhance the interaction of photons within the gate by creating plasmons in the graphene. In a plasmon, light is bound to electrons on the surface of the material. These electrons can help the photons to interact more strongly. Nonlinear interactions can be enhanced by using plasmons, and the peculiar configuration of the electrons in graphene leads to both a strong nonlinear interaction and plasmons that live for a long time. Plasmons in standard materials decay before the needed quantum effects can take place In their proposed graphene-based 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 own ribbon, multiple gates, which are required for quantum computation, can be applied to the plasmons. “We have shown that the strong nonlinear interaction in graphene makes it impossible for two plasmons to hop into the same ribbon,” said researcher Irati Alonso Calafell. The proposed universal two-qubit quantum logic gate, where qubits are encoded in surface plasmons in graphene nanostructures, exploits graphene’s strong third-order nonlinearity and long plasmon lifetimes to enable single-photon-level interactions. The team in Vienna is currently performing experimental measurements on a graphene-based system to confirm the feasibility of using its gate with current technology. Since the gate is small and operates at room temperature, it should readily lend itself to being scaled up, the team said. According to the researchers, the gate has achieved fidelities and success rates well above the fault-tolerance threshold, suggesting that graphene plasmonics offers a route toward scalable photonic quantum computers. The research was published in npj Quantum Information (https://doi.org/10.1038/s41534-019-0150-2).