It is an advance towards the use of single photons in quantum communication and information processing.
In quantum computing, photons play a fundamental role as carriers of quantum information and as photonic qubits, which are the basic unit of information, just as bits are the unit of information in electronic computers, which, as the name suggests, use electrons.
Why photons? Photons are particles of light that possess quantum properties such as superposition and entanglement, making them ideal candidates for implementing qubits in quantum systems. The problem, however, is generating photons with the correct polarization.
Using a new approach to quantum light emitters, scientists at Los Alamos National Laboratory were able to generate a stream of circularly polarized single photons. The team stacked two materials a few atoms thick to create this chiral quantum light source, meaning it exhibits a unique, entangled direction and polarization that could be useful for quantum information and communications applications.
“Our research shows that it is possible for a single-layer semiconductor to emit circularly polarized light without the help of an external magnetic field,” says Han Htoon, a scientist at Los Alamos National Laboratory. “This effect has so far only been achieved with high magnetic fields generated by bulky superconducting magnets, the coupling of quantum emitters with very complex nanoscale photonic structures, or the injection of spin-polarized charge carriers into quantum emitters.” Our proximity effect approach has the advantage of low manufacturing costs and Reliability.”
The polarization of light is the plane in which the light oscillates. Due to the superposition of two polarized crystals that filter a single direction of vibration perpendicular to each other, no light can pass through. The polarization state is an intermediate way of encoding the photon, so this achievement is an important step towards quantum cryptography or quantum communication.
The cracks, the key to photoluminescence
As described in the Nature Materials paper, the research team at the Center for Integrated Nanotechnologies worked to stack a single-molecule-thick layer of tungsten diselenide semiconductor on top of a thicker layer of magnetic phosphorus trisulfide semiconductor and nickel.
Xiangzhi Li, a postdoc, used atomic microscopy to create a series of nanometer-sized pits in the thin stack of material. The slits are about 400 nanometers in diameter, so more than 200 of them can easily fit the width of a human hair.
The indentations created by the atomic microscopy tool were useful for two purposes, as a laser was focused on the pile of material. First, the fissure forms a depression or depression in the potential energy landscape. Electrons in the tungsten diselenide monolayer fall into the cavity. This stimulates the emission of a stream of single photons from the borehole.
The nano-indentation also alters the typical magnetic properties of the underlying phosphorus-nickel trisulfide crystal, creating a local magnetic moment pointing up and away from the materials. This magnetic moment circularly polarizes the emitted photons. To experimentally confirm this mechanism, the team first performed experiments using high-magnetic-field optical spectroscopy in collaboration with the Pulsed Field Facility at the Los Alamos National High Magnetic Field Laboratory. Next, in collaboration with the University of Basel, Switzerland, the team measured the tiny magnetic field of the local magnetic moments.
The experiments showed that the team had developed a novel way to control the polarization state of a single stream of photons.
Encoding of quantum information
The team is currently exploring ways to modulate the degree of circular polarization of individual photons by applying electrical or microwave stimuli. This would allow quantum information to be encoded in the photon stream.
The subsequent coupling of the flow of photons in waveguides — microscopic channels of light — would create the photonic circuits that allow photons to propagate in one direction. These circuits would be the basic components of a highly secure quantum internet.
Proximity-induced chiral quantum light generation in strain-engineered WSe2/NiPS3 heterostructures