A new quantum device generates single photons and encodes information

“Our research shows that it is possible for a single layer of semiconductors to emit circularly polarized light without the aid of an external magnetic field,” said Han Hutun, a scientist at Los Alamos National Laboratory. “This effect has previously only been achieved through high magnetic fields generated by massive superconducting magnets, by coupling quantum emitters to highly complex nanoscale photonic structures, or by injecting spin-polarized carriers into quantum emitters. The effect approach is characterized by Our proximity to the advantage of low manufacturing cost and reliability.

The state of polarization is a way to encode a photon, so this realization is an important step towards quantum cryptography or quantum communication.

“With a source to generate a stream of single photons and a bias also introduced, we’ve basically combined two devices into one,” Hutton said.

Photoluminescence indentation key

As described in natural materialsThe research team at the Center for Integrated Nanotechnology worked on stacking a single-molecule layer of a semiconductor tungsten diselenide on a thicker layer of semiconductor nickel-phosphorus trisulfide. Xiangzi Li, a postdoctoral research associate, used atomic force microscopy to create a series of nanoscale indentations on a thin stack of material. The diameter of the prints is about 400 nanometers, so more than 200 of these prints can easily fit the width of a human hair.

The indentations created by the atomic microscopy instrument have proven useful for two effects when the laser is focused on the material stack. First, the indentation forms a trough, or depression, in the potential energy landscape. Electrons fall from the diselenide tungsten monolayer in the depression. This stimulates the emission of a stream of single photons from the well.

The nano-serration also disrupts the typical magnetic properties of a nickel-phosphorus trisulfide crystal, creating a local, upward-pointing magnetic moment in the material. This magnetic moment circularly polarizes the emitted photons. To provide experimental confirmation of this mechanism, the team first conducted high magnetic field optical spectroscopy experiments in collaboration with the Pulse Field Facility of the National High Magnetic Field Laboratory in Los Alamos. The team then measured the small magnetic field of local magnetic moments in collaboration with the University of Basel in Switzerland.

The experiments demonstrated that the team succeeded in demonstrating a new approach to controlling the polarization state of a single photon flux.

Quantum information coding

The team is currently exploring ways to modify the degree of circular polarization of single photons by applying electrical or microwave stimuli. This ability would provide a way to encode quantum information into a photon stream.

Coupling the photon flux into waveguides—microscopic light tubes—providing photonic circuits that allow photons to propagate in one direction. Circuits like these would be the building blocks of an ultra-secure quantum internet.