For quantum computing applications, it will be necessary to control emissions from quantum emitters, or quantum dots (QDs), and to produce quantum entanglement of the emission from pairs of QDs. A new photon-emitting device developed at Trinity College Dublin enables controllable, directional emission of single photons from QDs and can produce entangled states of pairs of QDs. To create the device, the researchers focused a plasmonic waveguide within a few nm of a surface containing the QDs. When they excited the tip of the waveguide, it generated enough power over a sufficiently small region to strongly drive quantum emitters. This enabled ultrafast single-photon emission, and also created entangled states between two emitters when performing a controlled-NOT operation. “The device works by placing a metal tip within a few nanometers of a surface containing the quantum dots,” professor John Donegan said. “The tip is excited by light and produces an electric field of such enormous intensity that it can greatly increase the number of single photons emitted by the dots. This strong field can also couple emission from pairs of quantum dots, entangling their states in a way that is unique to quantum emitters of light.” By using a movable plasmonic waveguide instead of stationary nanostructures, the system enables high-speed rasterization between sets of qubits and enables spatially flexible data storage and quantum information processing. The researchers demonstrated cavity-free dynamic entanglement through subdiffraction focusing of the waveguide. “By scanning the metal tip over the surface containing the quantum dots, we can generate the single-photon emission as required,” professor Ortwin Hess said. “Such a device is much simpler than current systems that attempt to fix a metal tip, or a cavity, in close proximity to a quantum dot.” The Trinity team believes that its device could reduce fabrication constraints and increase the speed and scalability of nanophotonic quantum devices and advance research in emitters for quantum technologies. The team plans to fabricate devices that will demonstrate controlled single-photon emission and contribute strongly to the research effort in quantum technologies in Ireland. The research was published in Nano Letters (www.doi.org/10.1021/acs.nanolett.0c01705).