Quantum Photonics Spawns Optical Age

Today quantum information is encoded onto the state of a particle with mass—an electron or atomic nuclei. However, a brighter future would result from processing quantum information encoded on massless photons, according to a recent paper written by researchers at the Technical University of Munich (TUM, Germany) et al and published in the journal Nature.

The first applications will likely be optical quantum sensors, optical quantum transistors, optically uncrackable quantum encryption, and eventually room-temperature optical quantum computers.

"This work could form the basis of photonic quantum computers at room temperature and marks an important milestone toward artificially producing quantum light sources for quantum sensors, quantum transistors and quantum encryption," said crystal defect expert Alexander Weber-Bargioni, who was not a contributor to the research, but who is director of the Characterization Facility at Lawrence Berkeley National Labs.

To date, quantum-encoded light sources have been found in nature but their origins were a mystery, since the lattice defects enabling them could not be accurately created by design. In contrast, the team of researchers authoring this paper accurately created by design new quantum light sources by introducing atomic-scale defects into a two-dimensional (2D) monolayer of molybdenum disulfide (MoS₂). The defects here are intentionally created atom-sized vacancies in a MoS₂ crystalline lattice. The vacancies—empty spots created when a particle beam knocks an atom from the MoS₂ lattice—can trap excitons (energized electron-hole pairs) which when recombined emit quantum light to release their energy.

"We have already managed to place 2D semiconductors onto prototype photonic circuits and have shown that localized excitons can release and absorb radiation," said Jonathan Finley, a physicist at the Technical University of Munich's Walter Schottky Institute, and corresponding author of the paper. "The next step will be to characterize the quality of the quantum light that is generated by these defects and after that to see how we can control the quantum polarization of the photons via the quantum spin of the excitons. If that works out, then one could perform, for example, quantum cryptography with such defects."

The team has achieved nanoscale accuracy in placing vacancy defects in its atomically thin molybdenum disulfide—a key capability previously not possible. Other materials, such as silicon and diamond, have had similar vacancy defects utilized in their 3D crystalline lattices. Unfortunately, the 3D vacancies could not be created but had to be found according to Weber-Bargioni, which prevented them from accurately aligning with other photonic components, such as waveguides, that are essential to optical quantum circuitry.

How it works

The international team of researchers included scientists from TUM, the Nanosystems Initiative (Munich), the Universität Bremen (Germany), the University at Buffalo (New York), The State University of New York (Buffalo), the National Institute for Materials Science (Tsukuba, Japan) and the Max-Planck-Institut für Quantenoptik (Garching, Germany). Each performed different stages of the work, and together they sandwiched a monolayer of MoS₂ crystals between hexagonal boron nitride (hBN) layers, after which they encapsulated MoS₂ crystals with hBN and placed the structure atop a 290-nanometer-thick insulating silicon dioxide (SiO₂) wafer. The MoS₂ and encapsulating hBN layers all were exfoliated from natural crystals.

"The hBN acts as an atomically flat substrate and has been previously shown to improve the quality of the MoS₂ when used to encapsulate it," said Finley.

The MoS₂ atoms were ejected with nanoscale accuracy, enabling the researchers to insert excitons into the vacancy. The excitons, in turn, became quantum light sources by virtue of recombination of the electron and hole, which release their energy as photons.

According to the paper, these very accurately placed quantum light sources will be "highly beneficial for integration into photonic circuits," enabling the creation of nanoscale sensors, as well as aiding in investigating nuclear spin physics in general. Through the tightly controlled use of highly accurate helium-ion beams to knock out MoS₂ atoms from its lattice, the group will create precisely spaced vacancy defects, a technique that is expected to enable the formation of periodic two-dimensional patterns of optically active defects with nanoscale accuracy. Such a quantum light array could reveal the details of exotic many-body physics structures in atomic lattice systems, as well as be used to entangle photons for the interconnection (by Einsteinian action at a distance) of quantum transistors in quantum computers, quantum encryption hardware, and quantum sensors, according to quantum emitter expert professor Mete Atature, a professor of physics at Cambridge Univerity.

"In contrast to the past where we had to find randomly placed natural defects, this work's deterministic creation of quantum emitters in an ordered array opens the door to investigating the collective effects of many-body physics," said Atature. "The authors achieved both a high level of spatial control through the helium ion irradiation used to create the vacancies, as well as demonstrate superior optical properties for the generated emitters. These results will fuel a new line of research in the field of atomically thin layered materials."

The resulting optical circuitry will use the photons emitted by vacancy-trapped excitons to entangle groups of photons and transmit the resultant quantum information encoded on the photon's polarization for computations and for sending information over optical data transmission lines. Instead of wires, the conductors in this photonic circuity will be free-space waveguides and optical fibers, as well as the identical encodings on entangled photons even when separated by great distances. The end of a photonic transmission line will use special optical detectors to decode and/or pass along the quantum information encoded on the photon's polarization.

Because the vacancy defects created with helium ion beams are virtually identical, quantum encoded photons can easily become entangled, which will provide alternative methods for quantum encoding, transmission, and information processing.

"The emitted photons could encode quantum information not only on their polarization but also their energy," said Finley. "In the exciton, the quantum information can also be stored in several different ways, including its spin direction and energy level."

"This work is so important, because in the past quantum light-emitting defects were found in materials for which we did not know the precise origin of the defect. In contrast, this work shows exactly from where the defect comes, plus explains how to create regularly space arrays of them. And as far as quantum light emitters go, the more the merrier, for everything from making more-sensitive quantum sensors, to entangling adjacent qubits, to making connections among quantum transistors in circuits, to accurately controlling the polarization of photons for optical quantum computers," said Weber-Bargioni.

"We are interested in the way that different excitons will interact with their neighbors when the defects are placed on a regular 2D lattice," said Finley. "Also 'quantum simulators' of the real world can be built using our systems which are capable of realizing variants of the Bose-Hubbard model [namely, the solid-state physics of atoms in a crystal]. Until now, most of the studies of Bose-Hubbard physics have been done with ultra-cold atoms in optical lattices, but here we have a method to realize similar systems at elevated temperatures. Someday we may even be able to perform quantum simulations at room temperature on a chip."

R. Colin Johnson is a Kyoto Prize Fellow who has worked as a technology journalist for two decades.