The physical realisation of a photonic quantum computer requires a reliable, fast, ondemand source of single indistinguishable photons. A promising candidate for one of these sources is a single dye molecule in a solid state matrix placed within the evanescent field of a photonic waveguide. This thesis explores the possibility of coupling of single dibenzoterrylene (DBT) molecules in an anthracene matrix to a silicon nitride waveguide at room temperature. I first discuss the theory by which photons from a DBT molecule can be evanescently coupled to a ridge waveguide. I present a novel growth method to form DBT-doped anthracene crystals which is very promising for applications to photonic devices. I discuss the methodology and the theory of such growth. I describe the confocal microscope I developed and used to image and characterise the emission of the single DBT molecules embedded in these crystals. My measurements show that the molecules are extremely stable single quantum emitters with a well-de ned polarization relative to the crystal axes. Measurements of the saturation intensity at room temperature allow me to estimate that a single DBT molecule could deliver at least 1012 photons before bleaching. This method of growth was used to deposit DBT molecules on top of a silicon nitride ridge waveguide. The results of the coupling experiment are shown. These include confocal images, saturation and lifetime measurements. The coupling efficiency is calculated and compared to what was simulated. The challenges of such structures are then presented. To tackle these challenges I deposited the molecules on top of lithium niobate and in silicon nitride slots. I conclude with a proposal for constructing the best photonic structure which would guarantee an easy deposition of the molecules and high coupling efficiencies.
The physical realisation of a photonic quantum computer requires a reliable, fast, ondemand source of single indistinguishable photons. A promising candidate for one of these sources is a single dye molecule in a solid state matrix placed within the evanescent field of a photonic waveguide. This thesis explores the possibility of coupling of single dibenzoterrylene (DBT) molecules in an anthracene matrix to a silicon nitride waveguide at room temperature. I first discuss the theory by which photons from a DBT molecule can be evanescently coupled to a ridge waveguide. I present a novel growth method to form DBT-doped anthracene crystals which is very promising for applications to photonic devices. I discuss the methodology and the theory of such growth. I describe the confocal microscope I developed and used to image and characterise the emission of the single DBT molecules embedded in these crystals. My measurements show that the molecules are extremely stable single quantum emitters with a well-de ned polarization relative to the crystal axes. Measurements of the saturation intensity at room temperature allow me to estimate that a single DBT molecule could deliver at least 1012 photons before bleaching. This method of growth was used to deposit DBT molecules on top of a silicon nitride ridge waveguide. The results of the coupling experiment are shown. These include confocal images, saturation and lifetime measurements. The coupling efficiency is calculated and compared to what was simulated. The challenges of such structures are then presented. To tackle these challenges I deposited the molecules on top of lithium niobate and in silicon nitride slots. I conclude with a proposal for constructing the best photonic structure which would guarantee an easy deposition of the molecules and high coupling efficiencies.
Single photons are very attractive for quantum information processing, given their long coherence time and their ability to carry information in many degrees of freedom. A current challenge is the generation of single photons in a photonic chip in order to scale up the complexity of quantum operations. Solid state emitters could offer this possibility. Quantum dots are one promising approach, but photons produced by different dots are generally distinguishable. Nitrogen-vacancy (N-V) centres in diamond are also promising, however it is challenging to place these in photonic circuits and the emitted photons have many frequencies. By contrast, dibenzoterrylene (DBT) molecules embedded in a thin film of anthracene (AC) at cryogenic temperatures produce a high yield of indistinguishable photons [1, 2]. DBT acts as a two-level system with a narrow 30 MHz near-infrared (785 nm) zero-phonon line [1]. We have proposed that a DBT molecule, deposited in the vicinity of a nanoscale waveguide, could emit these photons efficiently into the waveguide [3]. Conventional spin-coating methods [1] do not offer sufficient control over the surface coverage and the morphology of the AC crystal for deposition on photonic structures, while crystals grown with sublimation are too thick. Another method is needed.
Single photons are highly desirable for encoding, sending and processing quantum information. Suitable engineering of single quantum emitters and their environment can lead to a reliable single photon source and to mediated photon-photon interactions, essential assets for quantum communication and computation purposes. Dye molecules, our chosen single quantum emitter, have attracted much attention in recent years due to their high photostability and narrow emission lines. In this thesis I present a detailed analysis of the optical properties of single dibenzoterrylene molecules in an anthracene matrix. Single photon emission is demonstrated at room and cryogenic temperatures. In the latter case, an analysis of the second-order correlation function of the emission of a single molecule is studied at various temperatures. The data agree with a full solution of the optical Bloch equations over a wide range of temperature, and provide some insight into the dephasing process of the molecules. I then present a technique for integrating single dye molecules with nano-photonic devices. These are hybrid plasmonic waveguides, designed to guide light and to enhance the interaction with a single emitter by a tight confinement of the waveguide mode. Single molecules are deposited over the waveguides and a first evidence of coupling is presented. I conclude with proposed improvements to the optical setup and to the deposition methods. Finally, I present another type of integrated device, where the light matter interaction is enhanced by inserting single quantum emitters into coupled optical resonators. The theoretical description for the case of resonant two-photon absorption from single atoms in coupled photonic crystal cavities is developed. I conclude by proposing a possible realisation of this system, and describing the first steps towards the fabrication of such a device.
Efficient quantum light sources and non-linear optical elements at the few photon level are the basic ingredients for most applications in nano and quantum technologies. On the other hand, a scalable platform for quantum ICT typically requires reliable light matter interfaces and on-chip integration. In this work we demonstrate the potential of a novel hybrid technology which combines single organic molecules as quantum emitters and dielectric chips [1]. Dibenzoterrylene molecules in anthracene crystals (DBT:Ac) are particularly suitable quantum systems for this task, since they exhibit long-term photostability in thin samples [2], easy fabrication methods and life-time limited emission at cryogenic temperatures [3]. We demonstrate at room temperature the emission of single photons from DBT molecules into ridge waveguides with a branching ratio up to 40%. The overall single-photon source efficiency, including emission into the guided mode, propagation losses, and emission into a quasi-gaussian mode in free space, is estimated around 16%. These results are competitive with state-of-the-art single photon emission into propagating guided modes from solid state systems [4], while offering a novel platform with unprecedented versatility. References [1] P. Lombardi et al., Arxiv: 1701.00459v1 (2017). [2] C. Toninelli et al., Opt. Express 18, 6577 (2010). [3] A. A. L. Nicolet et al., ChemPhysChem 8, 1929 (2007). [4] I. Zadeh et al., Nano Lett. 16, 2289 (2016); R. S. Daveau et al., Arxiv: 1610.08670v1 (2016). [5] J. Hwang et. al., New J. Phys. 13, 085009 (2011); H.-W. Lee et al., Phys. Rev. A 63, 012305 (2000).
Single organic molecules hold great promise for generating, manipulating, and storing single photons. Such processes form the basis for new quantum-enhanced technologies such as sensing, communications, and computation. In this talk I will focus on a particularly promising molecule – dibenzoterrylene (DBT). When DBT is introduced into a solid crystal of anthracene it is photostable and emits light between 780 and 795 nm. When cooled to cryogenic temperature, DBT is isolated from phonon-induced dephasing meaning that the photons have a lifetime-limited. I will present recent results in growing DBT-doped anthracene crystals, introducing these to nanophotonic interfaces for enhancing the collection of photons, tuning the DBT emission wavelength to coincide with rubidium atomic absorption, and finally pump-probe spectroscopy experiments to find long-lived triplet states in DBT which will be useful for building a single-molecule quantum memory.
The on-chip integration of quantum light sources and nonlinear elements constitutes a major step toward scalable photon-based quantum information processing and communication. In this work we demonstrate the potential of a hybrid technology that combines organic-molecule-based quantum emitters and dielectric chips consisting of ridge waveguides and grating far-field couplers. In particular, dibenzoterrylene molecules in thin anthracene crystals are used as single-photon sources, exhibiting long-term photostability, easy fabrication methods, almost unitary quantum yield, and lifetime-limited emission at cryogenic temperatures. We couple such single emitters to silicon nitride ridge waveguides, showing a coupling efficiency of up to 42 ± 2% over both propagation directions. Our results open a novel path toward a fully integrated and scalable photon-processing platform.
Placing single emitters inside optical cavities provides a way of drastically modifying their interaction with the electric field. Such systems can be used to test our fundamental understanding of quantum electrodynamics and to develop devices that exploit this new found knowledge. This thesis describes work towards coupling single dye molecules to optical fibre microcavities at cryogenic temperatures. I present the design of an oven growth chamber and develop a method to grow cosublimated crystals of anthracene doped with dibenzoterrylene. I then describe the characterisation of these samples using a confocal microscope setup both at ambient and cryogenic temperatures. The samples grown show bright, stable, single emitters in a defined orientation in the anthracene crystals. By varying the growth parameters of the homebuilt crystal growing chamber, we can control the density of dibenzoterrylene molecules and grow at an optimum density to use these crystals in conjunction with optical fibre microcavities. I show how optical fibre microcavities have been developed that can be cooled to the liquid helium temperatures required to take advantage of the lifetime-limited emission from single dibenzoterrylene molecules, that only occurs below 4K. I also present cavity quantum electrodynamic simulations of the cavities, showing what we expect to see when we take reflection spectra of single molecules coupled to these cavities. In addition, I describe the optical setup that has been developed to take these measurements. I conclude with proposed improvements to the cavity setup that will enable these reflection spectra to be more easily taken. This will allow these molecule-cavity systems to be used as infrared single photon sources for quantum optics experiments.