Quantum Key Distribution, or QKD, enables two remote parties, “Alice” and “Bob”, who are connected by a passive optical link to securely generate secret key material. Single-photon sources (SPSs) and single-photon detectors (SPDs) are key devices for enabling practical quantum key distributions (QKDs). Single photon generation is necessary for secure quantum transmission; otherwise, an eavesdropping party might intercept one of the transmitted photons and thus get a copy of the message. Moreover, Single-photon sources (SPSs) at Telecom-band are of special interest because the existing telecom backbone networks exhibit a minimal transmission loss around 1.55 μm.
Single photon light emitting diodes for on-chip integration
In 2016, Researchers from the Graphene Flagship reported to 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. Prof 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. Prof 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. Prof Kis and colleagues working at ETH Zurich and EPFL (Switzerland) also observed single photon emitters with narrow linewidths in WSe2. At the same time, Prof 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). Prof 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.”
Prof Andrea Ferrari (University of Cambridge, UK), Chair of the Graphene Flagship Management Panel, and the Flagship’s Science and Technology Officer, also co-authored the research. He adds “We are just scratching the surface of the many possible applications of devices prepared by combining graphene with other insulating, semiconducting, superconducting or metallic layered materials. In this case, not only have we demonstrated controllable photon sources, but we have also shown that the field of quantum technologies can greatly benefit from layered materials. We hope this will bring synergies between the Graphene Flagship and its researchers, and the recently announced Quantum Technologies Flagship, due to start in the next few years. Many more exciting results and applications will surely follow.”
Graphene single photon detectors
Considerable interest in new single-photon detector technologies has been scaling in this past decade. Nowadays, quantum optics and quantum information applications are, among others, one of the main precursors for the accelerated development of single-photon detectors. Capable of sensing an increase in temperature of an individual absorbed photon, they can be used to help us study and understand, for example, galaxy formation through the cosmic infrared background, observe entanglement of superconducting qubits or improve quantum key distribution methods for ultra-secure communications.
Current detectors are efficient at detecting incoming photons that have relatively high energies, but their sensitivity drastically decreases for low frequency, low energy photons. In recent years, graphene has shown to be an exceptionally efficient photo-detector for a wide range of the electromagnetic spectrum, enabling new types of applications for this field.
Thus, in a recent paper published in the journal Physical Review Applied, and highlighted in APS Physics, ICFO researcher and group leader Prof. Dmitri Efetov, in collaboration with researchers from Harvard University, MIT, Raytheon BBN Technologies and Pohang University of Science and Technology, have proposed the use of graphene-based Josephson junctions (GJJs) to detect single photons in a wide electromagnetic spectrum, ranging from the visible down to the low end of radio frequencies, in the gigahertz range.
In their study, the scientists envisioned a sheet of graphene that is placed in between two superconducting layers. The so created Josephson junction allows a supercurrent to flow across the graphene when it is cooled down to 25 mK. Under these conditions, the heat capacity of the graphene is so low, that when a single photon hits the graphene layer, it is capable of heating up the electron bath so significantly, that the supercurrent becomes resistive – overall giving rise to an easily detectable voltage spike across the device. In addition, they also found that this effect would occur almost instantaneously, thus enabling the ultrafast conversion of absorbed light into electrical signals, allowing for a rapid reset and readout.
The results of the study confirm that we can expect a rapid progress in integrating graphene and other 2-D materials with conventional electronics platforms, such as in CMOS-chips, and shows a promising path towards single-photon-resolving imaging arrays, quantum information processing applications of optical and microwave photons, and other applications that would benefit from the quantum-limited detection of low-energy photons.
Nu Quantum’s Graphene based single photon emitters and detectors
Launched in late 2018, Nu Quantum has now became an Associate Member of the EU-funded Graphene Flagship project. The start-up, a spin off from the Cavendish Laboratory at Graphene Flagship’s partner, the University of Cambridge, UK, specializes in single-photon quantum technology that is set to create unbreakable encryption keys. The start-up’s devices utilise the quantum properties of photons, single particles of light. Unlike other quantum technologies, which need to be cooled to around absolute zero to be stable, Nu Quantum’s components work at room temperature, enabling a wealth of commercial applications.
Nu Quantum is currently developing three technologies. Two are components; a single-photon emitter and a single photon detector. The third is a system, which combines the two components to create a quantum random number generator. Security systems rely on cryptographic keys, but these are traditionally produced by predictable algorithms. Quantum random number generation takes advantage of the behaviour of quantum particles, such as photons, to create completely random sequences that result in unbreakable encryption.
The start-up aims to achieve the highest efficiency photon emission and detection at room temperature, before creating more complex quantum photonic communications systems. Nu Quantum is currently undertaking pilot projects with telecommunications and space communications companies. Nu Quantum uses graphene and other layered-material technology in their devices, the atomically-thin nature of these materials crucial to accessing quantum light and matter control, as well as to improve efficiency, flexibility, size, weight and power requirements.
“Graphene is an ideal material for us to use as it helps improve the efficiency of our components. Graphene technology is also evolving very quickly so it aligns with our mission to be at the forefront of technological advancement,” commented Carmen Palacios-Berraquero, CEO of Graphene Flagship Associate Member Nu Quantum. Nu quantum has worked closely with Graphene Flagship partner the Cambridge Graphene Centre in Cambridge, UK, and recently joined the project as one of the project’s newest associate members.
“It can be difficult to certify whether new devices are truly quantum,” Palacios-Berraquero continued. “By using single quanta from the beginning, then using our algorithms to carefully process the data, we have created technologies that are unequivocally quantum. “We’re currently focusing on quantum cybersecurity, but our long-term vision is to branch out into other aspects of quantum technology, including sensors and computing.” Following a round of £650,000 pre-seed funding in 2018, Nu Quantum hasrecently obtained £3.6 million in government grants and is now raising a several-million seed round backed by different venture capital firms. Nu Quantum plans to launch its random number generator in 2021.
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