A new approach to quantum light emitters generates a stream of circularly polarized single photons, that may be useful for a range of quantum information and communication appli­cations. A Los Alamos National Laboratory team stacked two different, atomically thin materials to realize this chiral quantum light source. “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. “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 is a means of encoding the photon, so this achievement is an important step in the direction of quantum crypto­graphy or 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. The research team worked at the Center for Integrated Nanotechnologies to stack a single-molecule-thick layer of tungsten diselenide semiconductor onto a thicker layer of nickel phosphorus trisulfide magnetic semiconductor. Postdoc Xiangzhi Li used atomic force microscopy to create a series of nanometer-scale indenta­tions on the thin stack of materials. The indenta­tions are approxi­mately 400 nanometers in diameter.

The indentations created by the atomic microscopy tool proved useful for two effects when a laser was focused on the stack of materials. First, the indentation forms a well, or depression, in the potential energy landscape. Electrons of the tungsten diselenide monolayer fall into the depression. That stimulates the emission of a stream of single photons from the 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 materials. That magnetic moment circu­larly polarizes the photons being emitted.

To provide experimental confirmation of this mechanism, the team first performed high magnetic field optical spectroscopy experiments in colla­boration with National High Magnetic Field Laboratory’s Pulsed Field Facility at Los Alamos. The team then measured the minute magnetic field of the local magnetic moments in collaboration with the university of Basel in Switzerland. The experiments proved that the team had success­fully demonstrated a novel approach to control the polari­zation state of a single photon stream. 

The team is currently exploring ways to modulate the degree of circular polarization of the single photons with the appli­cation of electrical or microwave stimuli. That capabi­lity would offer a way to encode quantum information into the photon stream. Further coupling of the photon stream into waveguides would provide the photonic circuits that allow the propa­gation of photons in one direction. Such circuits would be the fundamental building blocks of an ultra-secure quantum internet. (Source: LANL)

Reference: X. Li et al.: Proximity-induced chiral quantum light generation in strain-engineered WSe₂/NiPS₃ heterostructures, Nat. Mat., online 17 August 2023; DOI: 10.1038/s41563-023-01645-7

Link: Center for Integrated Nanotechnologies, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, USA