Formed within wells in a stack of two different layered materials – a monolayer semiconductor and an anti-ferromagnetic crystal – the chiral quantum light emissions rise out of the material and could be used for quantum information and communication applications. Image: Los Alamos National Laboratory.A new quantum light emitter can generate a stream of circularly polarized single photons, or particles of light, that may be useful for a range of quantum information and communication applications. To create this chiral quantum light source, a team of researchers from Los Alamos National Laboratory simply stacked together two different, atomically thin materials.
“Our research shows that it is possible for a monolayer semiconductor to emit circularly polarized light without the help of an external magnetic field,” said Han Htoon, a scientist at Los Alamos. “This effect has only been achieved before with high magnetic fields created by bulky superconducting magnets, by coupling quantum emitters to very complex nanoscale photonics structures or by injecting spin-polarized carriers into quantum emitters. Our proximity-effect approach has the advantage of low-cost fabrication and reliability.”
The polarization state provides a means for encoding a photon, so this achievement represents an important step in the direction of quantum cryptography and quantum communication. “With a source to generate a stream of single photons and also introduce polarization, we have essentially combined two devices in one,” Htoon said.
As they report in a paper in Nature Materials, the researchers stacked a single-molecule-thick layer of a tungsten diselenide semiconductor onto a thicker layer of a nickel phosphorus trisulfide magnetic semiconductor. Xiangzhi Li, a postdoctoral research associate at Los Alamos, then used an atomic force microscope (AFM) to create a series of nanometer-scale indentations in this thin stack of materials. The indentations are approximately 400nm in diameter, meaning that over 200 could easily be fitted across the width of a human hair.
The indentations created by the AFM induce two useful effects when a laser is focused on the stack of materials. First, they form wells, or depressions, in the potential energy landscape. Electrons in the tungsten diselenide monolayer fall into these depressions, stimulating the emission of a stream of single photons from each well.
The nanoindentation also disrupts the typical magnetic properties of the underlying nickel phosphorus trisulfide crystal, creating a local magnetic moment pointing up out of the material. This magnetic moment circularly polarizes the photons being emitted.
To provide experimental confirmation of this mechanism, the researchers first performed high magnetic field optical spectroscopy experiments, in collaboration with the National High Magnetic Field Laboratory’s Pulsed Field Facility at Los Alamos. They then measured the minute magnetic field of the local magnetic moments, in collaboration with the University of Basel in Switzerland.
These experiments provided a successfully demonstration of this novel approach to controlling the polarization state of a single photon stream.
The team is currently working on modulating the degree of circular polarization of the single photons by applying electrical or microwave stimuli. This capability would offer a way to encode quantum information into the photon stream.
Further coupling of the photon stream into waveguides – microscopic conduits of light – would produce photonic circuits that allow the propagation of photons in one direction. Such circuits could be fundamental building blocks of an ultra-secure quantum internet.
This story is adapted from material from Los Alamos National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.