Dec 18 2018
If it is possible to produce a single photon, instruct it how to spin, and give directions to it on where to go, then what is created is a basic element for next-generation computers that work with light rather than wires.
Defects in exotic, two-dimensional materials known as transition-metal dichalcogenides may be just what scientists need to advance quantum computing. Theoretical models by scientists at Rice University have predicted how particular 2D materials could be modified to produce photons with custom polarization. (Image credit: Sunny Gupta)
As shown by various laboratories, that seems feasible with atom-thick materials. Currently, researchers at Rice University have gained insights into the mechanism by which two-dimensional (2D) materials can be controlled to synthesize the desired photons.
This month, it was reported by the Rice lab of materials theorist Boris Yakobson that when pre-arranged imperfections are added to atom-thick materials such as molybdenum disulfide, they turn out to be perfectly capable of emitting single photons on demand in either left or right polarization.
The discovery, which was achieved through first-principle simulations, has been described in Nano Letters journal, published by the American Chemical Society.
The photons arise out of designer defects in the 2D lattice that add their own unique electronic characteristics to semiconducting materials. Considering molybdenum disulfide—their best prospect thus far—a tinge of rhenium in just the perfect spot results in a configuration of atoms that have energy states sitting comfortably inside and are isolated from the natural band gap of the material.
As soon as they are in place, it would be possible to align the magnetic moments of atoms in the defect with a polarized magnet. When they are excited with light, they are brought to a higher energetic state; however, the band gap is sufficiently large that the energy can go only in one way: out, as a desired single photon.
Atoms that make up the defect have magnetic moments that can be random, but a magnetic field can bring them to a particular quantum state, either up or down. After that, if you shine light on the defect, it goes from its ground state to an excited state and emits a desirable single photon, with specific polarization. That makes it a bit, which will be useful in quantum information processing.
Boris Yakobson, Materials Theorist, Rice University.
“The defect’s optical transition lies in the optical fiber telecommunication band, which is ideal for integration into photonic circuits,” added Sunny Gupta, who is a Rice graduate student and lead author of the study.
All of the 2D prospects designed by Yakobson, Gupta, and alumnus Ji-Hui Yang are dichalcogenides—semiconductors incorporating chalcogens and transition metals. They also designed boron nitride, tungsten diselenide, tungsten disulfide, zirconium disulfide, diamane (2D diamond, which labs are beginning to produce), and, for comparison, 3D diamond.
One of the advantages we argue here relative to 3D materials is that extraction of the photon is much easier, because the material is basically transparent and there is so little thickness. Photons are not so easy to extract from 3D materials, because they may get stopped by internal reflections, or be refracted, or just dissipate in the material. But 2D materials are more open and the photon is produced near the surface, making its extraction for utility easier.
Boris Yakobson, Materials Theorist, Rice University.
No one yet knows where that photon goes. “We know the material can produce a photon of well-defined polarization and energy, and we suspect for good reasons that its direction is also well-defined, yet only probabilistically,” stated Yakobson. “But we don’t want to go too far with the theories before somebody tries to make it in the lab.”
The study was supported by the Army Research Office and the Office of Naval Research. Supercomputing resources were offered by the National Science Foundation-supported DAVinCI cluster at Rice, administered by the Center for Research Computing and procured in partnership with Rice’s Ken Kennedy Institute for Information Technology, the Department of Defense High Performance Computing Modernization Program, and the Department of Energy Office of Science-supported National Energy Research Scientific Computing Center.