Researchers from the Graphene Flagship use layered materials to create an all-electrical quantum light emitting diodes (LED) with single-photon emission. These LEDs have potential as on-chip photon sources in quantum information applications.
Atomically thin LEDs emitting one photon at a time have been developed by researchers from the Graphene Flagship. Constructed of layers of atomically thin materials, including transition metal dichalcogenides (TMDs), graphene, and boron nitride, the ultra-thin LEDs showing all-electrical single photon generation could be excellent on-chip quantum light sources for a wide range of photonics applications for quantum communications and networks. The research, reported in Nature Communications, was led by the University of Cambridge, UK.
The ultra-thin devices reported in the paper are constructed of thin layers of different layered materials, stacked together to form a heterostructure. Electrical current is injected into the device, tunnelling from single-layer graphene, through few-layer boron nitride acting as a tunnel barrier, and into the mono- or bi-layer TMD material, such as tungsten diselenide (WSe2), where electrons recombine with holes to emit single photons. At high currents, this recombination occurs across the whole surface of the device, while at low currents, the quantum behaviour is apparent and the recombination is concentrated in highly localised quantum emitters.
All-electrical single photon emission is a key priority for integrated quantum optoelectronics. Typically, single photon generation relies on optical excitation and requires large-scale optical set-ups with lasers and precise alignment of optical components. This research brings on-chip single photon emission for quantum communication a step closer. Professor Mete Atatüre (Cavendish Laboratory, University of Cambridge, UK), co-author of the research, explains “Ultimately, in a scalable circuit, we need fully integrated devices that we can control by electrical impulses, instead of a laser that focuses on different segments of an integrated circuit. For quantum communication with single photons, and quantum networks between different nodes – for example, to couple qubits – we want to be able to just drive current, and get light out. There are many emitters that are optically excitable, but only a handful are electrically driven” In their devices, a modest current of less than 1 µA ensures that the single-photon behaviour dominates the emission characteristics.
The layered structure of TMDs makes them ideal for use in ultra-thin heterostructures for use on chips, and also adds the benefit of atomically precise layer interfacing. The quantum emitters are highly localised in the TMD layer and have spectrally sharp emission spectra. The layered nature also offers an advantage over some other single-photon emitters for feasible and effective integration into nanophotonic circuits. Professor Frank Koppens (ICFO, Spain), leader of Work Package 8 – Optoelectronics and Photonics, adds “Electrically driven single photon sources are essential for many applications, and this first realisation with layered materials is a real milestone. This ultra-thin and flexible platform offers high levels of tunability, design freedom, and integration capabilities with nano-electronic platforms including silicon CMOS.”
This research is a fantastic example of the possibilities that can be opened up with new discoveries about materials. Quantum dots were discovered to exist in layered TMDs only very recently, with research published simultaneously in early 2015 by several different research groups including groups currently working within the Graphene Flagship. Dr Marek Potemski and co-workers working at CNRS (France) in collaboration with researchers at the University of Warsaw (Poland) discovered stable quantum emitters at the edges of WSe2 monolayers, displaying highly localised photoluminescence with single-photon emission characteristics. Professor Kis and colleagues working at ETH Zurich and EPFL (Switzerland) also observed single photon emitters with narrow linewidths in WSe2. At the same time, Professor van der Zant and colleagues from Delft University of Technology (Netherlands), working with researchers at the University of Münster (Germany) observed that the localised emitters in WSe2 are due to trapped excitons, and suggested that they originate from structural defects. These quantum emitters have the potential to supplant research into the more traditional quantum dot counterparts because of their numerous benefits of the ultrathin devices of the layered structures.
With this research, quantum emitters are now seen in another TMD material, namely tungsten disulphide (WS2). Professor Atatüre says “We chose WS2 because it has higher bandgap, and we wanted to see if different materials offered different parts of the spectra for single photon emission. With this, we have shown that the quantum emission is not a unique feature of WSe2, which suggests that many other layered materials might be able to host quantum dot-like features as well.”
Scientists Overcome Limiting Factor on the Way to an Optical Quantum Computer
Whether for use in safe data encryption, ultrafast calculation of huge data volumes or so-called quantum simulation of highly complex systems: Optical quantum computers are a source of hope for tomorrow’s computer technology. For the first time, scientists now have succeeded in placing a complete quantum optical structure on a chip, as outlined in the “Nature Photonics” journal. This fulfills one condition for the use of photonic circuits in optical quantum computers. (DOI: 10.1038/nphoton.2016.178)
“Experiments investigating the applicability of optical quantum technology so far have often claimed whole laboratory spaces,” explains Professor Ralph Krupke of the KIT. “However, if this technology is to be employed meaningfully, it must be accommodated on a minimum of space.” Participants in the study were scientists from Germany, Poland, and Russia under the leadership of Professors Wolfram Pernice of the Westphalian Wilhelm University of Münster (WWU) and Ralph Krupke, Manfred Kappes, and Carsten Rockstuhl of the Karlsruhe Institute of Technology (KIT).
The light source for the quantum photonic circuit used by the scientists for the first time were special nanotubes made of carbon. They have a diameter 100,000 times smaller than a human hair, and they emit single light particles when excited by laser light. Light particles (photons) are also referred to as light quanta. Hence the term “quantum photonics.”
That carbon tubes emit single photons makes them attractive as ultracompact light sources for optical quantum computers. “However, it is not easily possible to accommodate the laser technology on a scalable chip,” admits physicist Wolfram Pernice. The scalability of a system, i.e. the possibility to miniaturize components so as to be able to increase their number, is a precondition for this technology to be used in powerful computers up to an optical quantum computer.
As all elements on the chip now developed are triggered electrically, no additional laser systems are required any more, which is a marked simplification over the optical excitation normally used. “The development of a scalable chip on which a single-photon source, detector, and waveguide are combined, is an important step for research,” emphasizes Ralph Krupke, who conducts research at the KIT Institute for Nanotechnology and the Institute of Materials Science of the Darmstadt Technical University. “As we were able to show that single photons can be emitted also by electric excitation of the carbon nanotubes, we have overcome a limiting factor so far preventing potential applicability.”
About the methodology: The scientists studied whether the flow of electricity through carbon nanotubes caused single light quanta to be emitted. For this purpose, they used carbon nanotubes as single-photon sources, superconducting nanowires as detectors, and nanophotonic waveguides. One single-photon source and two detectors each were connected with one waveguide. The structure was cooled with liquid helium to allow single light quanta to be counted. The chips were produced in an electron beam scribing device.
The scientists’ work is fundamental research. It is not yet clear whether and when it will lead to practical applications. Wolfram Pernice and the first author, Svetlana Khasminskaya, were supported by the Deutsche Forschungsgemeinschaft and the Helmholtz-Gemeinschaft, Ralph Krupke was funded by the Volkswagen Foundation.
The University of Münster (German: Westfälische Wilhelms-Universität Münster, WWU) is a public university located in the city of Münster, North Rhine-Westphalia in Germany.
The WWU is part of the Deutsche Forschungsgemeinschaft, a society of Germany’s leading research universities. The WWU has also been successful in the German government’s Excellence Initiative.
With almost 40,000 students and over 130 fields of study in 15 departments, it is Germany’s third largest university and one of the foremost centers of German intellectual life. The university offers a wide range of subjects across the sciences, social sciences and the humanities. Several courses are also taught in English, including PhD programmes as well as postgraduate courses in geoinformatics, geospational technologies or information systems).
Professors and former students have won nine Leibniz Prizes, the most prestigious as well as the best-funded prize in Europe, and one Fields Medal.