Chiral quantum light, made up of a stream of single, circularly polarized photons, is a crucial enabler of quantum information networks and other exciting emerging applications. In a paper published in Nature Materials, researchers from the Los Alamos National Laboratory led by S. Crooker and H. Htoon, in collaboration with the Quantum Sensing Lab (University of Basel) led by P. Maletinsky recently showed a new chiral quantum light source that is smaller, easier to produce, and more effective than existing ones.

The group created a photon source using a sandwich of van der Waals materials – an atomically thin monolayer of tungsten di-selenide (WSe₂) with unique optical properties that was coupled to an antiferromagnetic nickel-phosphorus tri-sulfide (NiPS₃) monolayer. By using atomic force microscopy, a series of nanoindentations was made on the substrate. Such indentations are well-known to produce quantum light sources in WSe₂, albeit not chiral ones, as of now. By directing a continuous-wave laser at the substrate at cryogenic temperatures, the authors then realized that their novel combination of the two materials (WSe₂ and NiPS₃) led to a stream of circularly polarized photons – the sought-after chiral quantum light source.

Detecting the magnetic field

Typically, chiral light sources depend on the application of high magnetic fields, which ensure circularly polarized quantum light emission. How, then, could an antiferromagnetic substrate produce such a pure stream of circularly polarized photons? The Crooker’s and Htoon’s teams hypothesized that the nanoindentations deformed the NiPS₃ monolayer such that a magnetic field would emerge and act on the WSe₂ quantum light sources. The resulting, local “exchange field”, was then, locally, strong enough to ensure circularly polarized photon emission.

To verify their hypothesis, the authors needed a detector sensitive to measure the magnetic field emerging from the NiPS₃ with highest resolution and sensitivity. Here entered Scanning NV Magnetometry.  While the magnetic signal from single NiPS₃ was at the edge of detectability, Maletinsky’s team using their cryogenic SNVM system was able to provide the direct evidence for the magnetic field that the researchers were looking for.