New 'dented' composite can 'twist' photons on demand

Los Alamos National Laboratory

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Researchers at Los Alamos National Laboratory have successfully developed a new way to produce a specific type of photon that could prove critical for quantum data exchange, notably encryption. The specific kind of photons, called "circularly polarized light," have thus far proved challenging to create and control, but this new technique makes the process easier and, importantly, cheaper. This was achieved, the team explains, by stacking two different, atomically thin materials to "twist" (polarize) photons in a predictable fashion.

“Our research shows that it is possible for a monolayer semiconductor to emit circularly polarized light without the help of an external magnetic field,” explained Han Htoon, a scientist at Los Alamos National Laboratory. “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," he added.

The polarization state effectively "encodes" generated photons, making this a crucial step for quantum cryptography and 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.

To achieve this, the research team at the Center for Integrated Nanotechnologies used atomic force microscopy to create a series of nanometer-scale indentations, or "dents," on the stack of materials. The stack consisted of a single-molecule-thick layer of tungsten diselenide semiconductor stacked onto a thicker layer of nickel phosphorus trisulfide magnetic semiconductor. Each of the approximately 400-nanometer diameter indentations made would fit over 200 across the width of a human hair.

The researchers then found that the "dents" caused the tungsten diselenide to emit individual light particles (photons). They also, it turned out, changed the magnetic properties of the bottom material in such a way that it gave the emitted photons a special twist ("circular polarization").

To confirm this mechanism, the team conducted optical spectroscopy experiments with the National High Magnetic Field Laboratory and measured the magnetic field of the local magnetic moments with the University of Basel. By doing so, the team successfully demonstrated a new method of controlling single-photon stream polarization in the experiments. Quite an impressive feat!

Moving forward, the team is exploring ways to modulate the degree of "circular polarization" of single photons using electrical or microwave stimuli that could, theoretically, encode quantum information into the photon stream. Microscopic conduits of light called waveguides could also allow the coupling of the photon stream, creating photonic circuits. If achievable, these "circuits" could form the foundations of an ultra-secure quantum internet.

You can view the study for yourself in the journal Nature Materials.

Study abstract:

Quantum light emitters capable of generating single photons with circular polarization and non-classical statistics could enable non-reciprocal single-photon devices and deterministic spin–photon interfaces for quantum networks. To date, the emission of such chiral quantum light relies on the application of intense external magnetic fields, electrical/optical injection of spin-polarized carriers/excitons or coupling with complex photonic metastructures. Here we report the creation of free-space chiral quantum light emitters via the nanoindentation of monolayer WSe2/NiPS3 heterostructures at zero external magnetic field. These quantum light emitters emit with a high degree of circular polarization (0.89) and single-photon purity (95%), independent of pump laser polarization. Scanning diamond nitrogen-vacancy microscopy and temperature-dependent magneto-photoluminescence studies reveal that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe2 monolayer and the out-of-plane magnetization of defects in the antiferromagnetic order of NiPS3, both of which are co-localized by strain fields associated with the nanoscale indentations.