Single Molecules on Optical Nanofibres

Photons are ideal carriers of quantum information. They can be precisely manipulated, are well decoupled from the environment and can be transported over long distances using optical fibers. To implement optical quantum networks, efficient interfacing of quantum emitters with light fields is required and scalability of these systems is sought. The here proposed project will interface single organic molecules with the evanescent light field guided by an optical nanofiber. A nanofiber is the waist of a tapered commercial optical fiber which has a diameter that is smaller than the wavelength of light it is guiding. The molecules are embedded in a nanocrystal host that ensures stability and allows spectrally addressing of single molecules by a narrowband laser. This is possible as every molecule experiences a slightly different environment and therefore has a slightly different resonance transition frequency. The single molecule doped nanocrystals are deposited on the surface of the optical nanofiber and the nanofibers strong transverse confinement of the light field ensures that the interaction with a single molecule can be significant. We will use this new platform to study fundamental questions in quantum optics and to implement important components of a quantum network. We will study the nonlinear effect that a single molecule induces on the light field, which means that a single photon or a distinct number of photons interacting with the molecule will have a measureable effect. We will exploit this to realise a photon sorter, which is a number resolving detector for photons. This is not only a desired component in itself but a key constituent of more complex devices for quantum networks. The brightness of single molecules in solids together with their favorable internal level structure makes them a natural choice for single photon sources at a wide range of wavelengths. We will show the implementation of a stable, triggered single photon source by coupling a single molecule to a fiber Bragg grating-resonator which will enhance emission into the optical nanofiber. A triggered single photon source is a prerequisite for linear optical quantum computation and can be used to perform sub-shot noise spectroscopy. As the cavity increases the coupling of the molecules to the light field, this experimental platform allows the study of long range interactions between a distinct number of single molecules. The transition frequency of these molecules can be changed by an applied electric field and this is exploited to tune single molecules into resonance with each other. The interactions between different single molecules induced by the light field open up a wealth of experiments in nano-optics and for the implementation and control of entanglement in such solid state systems, an effect which lies at the core of quantum technologies.

Photons are ideal carriers of quantum information. They can be precisely manipulated, are well decoupled from the environment and can be transported over long distances using optical fibers. To implement optical quantum networks, efficient interfacing of photons with quantum emitters, such as single molecules, is required and scalability of these systems is sought. In our work, we show how single molecules in solids can be interfaced with the guided mode of an optical nanofiber. A nanofiber is the waist of a tapered commercial optical fiber which has a sub-wavelength diameter. Organic terrylene molecules are embedded in a para-terphenyl nanocrystal that ensures stability and allows one to spectrally address single molecules by a narrowband laser. The molecule-doped nanocrystals are deposited on the surface of the optical nanofiber and the strong transverse confinement of the guided light field ensures a significant interaction with even a single molecule. In our set-up, single molecules which are efficient quantum emitters are fully fiber-integrated. The excitation and detection of the molecules can be carried out solely via the optical nanofiber interface. By comparing the saturation intensities of different molecules, we show that we can probe molecules within a distance of 160 nm from the nanofiber surface. In order to enhance the light-matter interaction further, we have also implemented a fiber-based cavity that consists of a nanofiber section and two fiber Bragg grating mirrors. We have successfully shown that such a cavity can reach the strong coupling regime even at cryogenic temperatures, an environment necessary for most solid-state quantum emitters. In order to reduce the scattering from a host crystal and achieve stronger coupling efficiencies, other emitters have also been investigated. The quantum emitters that have shown the most promising results are colour centers in hexagonal Boron nitride. We have shown that these quantum emitters can be coupled to the guided modes of an optical nanofiber and have characterized the non-classical light emission of these type of quantum emitters. The new experimental platform based on optical nanofibers and solid-state quantum emitters is not only useful for studying fundamental questions in quantum optics, but it also lends itself for the implementation of key components of an optical fiber-based quantum network.