New quantum devices will aid single-photon communications!

A new approach to quantum light emitters generates a stream of circularly polarized single photons, or light particles, that could be useful for a range of quantum information and communication applications. Now, a team of researchers at Los Alamos National Laboratory (LNL) has realized this chiral quantum light source by stacking two different atomically thin materials together.

"Proximity-induced chiral quantum light generation in strain-engineered WSe2/NiPS3 heterostructures."

Han Htoon, a scientist at Los Alamos National Laboratory, explains, "Our study shows that it is possible for a single-layer semiconductor to emit circularly polarized light without the help of an external magnetic field. Previously, this was only possible with high magnetic fields generated by large superconducting magnets, coupling quantum emitters to very complex nanoscale photonics structures, or injecting spin-polarized carriers into quantum emitters. Our proximity effect approach has the advantage of low-cost fabrication and high reliability."

"With a light source that produces a single photon stream and also introduces polarization, we have essentially combined two devices into one."

Polarization states are a means of encoding photons, so this result is an important step toward quantum cryptography or quantum communication.

Working at the Center for Integrated Nanotechnology, the research team superimposed a single molecule-thick layer of tungsten diselenide semiconductor onto a thicker layer of nickel phosphorus trisulfide magnetic semiconductor. Postdoctoral assistant researcher Xiangzhi Li used an atomic force microscope to create a series of nanoscale indentations (indentations) in this thin layer: these indentations are about 400 nanometers in diameter, so more than 200 of these indentations could easily pass through the width of a single hair.

It turns out that when the laser is focused on this pile of material, the indentations produced by the atomic microscopy tool have two effects. First, the indentation creates a well or depression in the potential energy diagram. Electrons from the tungsten diselenide monolayer fall into the depression. This stimulates the emission of a single photon stream from the well.

The nanoindentation also disrupts the typical magnetism of the underlying nickel trisulfide diphosphate crystals, creating a localized magnetic moment that points upward from the material. This magnetic moment circularly polarizes the emitted photons. To confirm this mechanism experimentally, the team first performed high-field optical spectroscopy experiments in collaboration with the National High Magnetic Field Laboratory's Pulsed Magnetic Field Facility in Los Alamos. Then, the team measured the tiny magnetic fields of localized magnetic moments in collaboration with the University of Basel, Switzerland.

As evidenced by the experiments, the team successfully demonstrated a new method for controlling the polarization state of a single photon stream.

The WSe2 /NiPS3 heterostructure has a strong degree of spontaneous circular polarization.

Quantum photoemission demonstration of WSe2 /NiPS3 heterostructure.

Correlation of chiral quantum photoemission with local magnetization intensity.

The team is now exploring how to modulate the degree of circular polarization of single photons by applying electrical or microwave stimulation. This ability would provide a way to encode quantum information in a stream of photons.

Further, coupling photon streams to waveguides (microscopic conduits of light) will provide photonic circuits that allow photons to propagate in one direction: such circuits will be the basic building blocks of an ultra-secure quantum internet.