11 research outputs found

    Optical detection and storage of entanglement in plasmonically coupled quantum-dot qubits

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    Recent proposals and advances in quantum simulations, quantum cryptography, and quantum communications substantially rely on quantum entanglement formation. Contrary to the conventional wisdom that dissipation destroys quantum coherence, coupling with a dissipative environment can also generate entanglement. We consider a system composed of two quantum-dot qubits coupled with a common, damped surface plasmon mode; each quantum dot is also coupled to a separate photonic cavity mode. Cavity quantum electrodynamics calculations show that upon optical excitation by a femtosecond laser pulse, entanglement of the quantum-dot excitons occurs, and the time evolution of the g(2) pair correlation function of the cavity photons is an indicator of the entanglement. We also show that the degree of entanglement is conserved during the time evolution of the system. Furthermore, if coupling of the photonic cavity and quantum-dot modes is large enough, the quantum-dot entanglement can be transferred to the cavity modes to increase the overall entanglement lifetime. This latter phenomenon can be viewed as a signature of entangled, long-lived quantum-dot exciton-polariton formation. The preservation of total entanglement in the strong-coupling limit of the cavity–quantum-dot interactions suggests a novel means of entanglement storage and manipulation in high-quality optical cavities

    The control, manipulation and detection of surface plasmons and cold atoms.

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    Doctoral Degree. School of Chemistry and Physics, University of KwaZulu-Natal, Durban, 2017.Cold atoms and surface plasmons are now widely recognised as having a vast potential as sources for future quantum information technologies, including in quantum simulations, quantum computing and quantum-enhanced metrology. In the first part of this Thesis an experimental investigation of the decoherence of single surface plasmon polaritons in plasmonic waveguides is carried out. In the study, a Mach-Zehnder configuration previously considered for measuring decoherence in atomic, electronic and photonic systems, is used. By placing waveguides of di erent lengths in one arm measurements of the amplitude damping time, pure phase damping time and total phase damping time were achieved. Decoherence was found to be mainly due to amplitude damping and thus losses arising from inelastic electron and photon scattering play the most important role in the decoherence of plasmonic waveguides in the quantum regime. However, pure phase damping is not completely negligible. In the second part of the Thesis the properties of light in the fundamental mode of a subwavelengthdiameter plasmonic nanowire are also investigated. One of the applications of the light is the trapping of atoms by the optical force of the evanescent field and the subsequent guiding of the emitted light from the atoms. The quantum correlation functions of the emitted light from di erent numbers of atoms into the wave guided mode of the nanowire are investigated analytically. It is found that the nanowire provides an efficient method of generating quantum states of light - it gives a faster time scale for the dynamics and improved coupling e ciency compared to an equivalent dielectric nanofiber. The results of this Thesis will be useful for the design of plasmonic waveguide systems for carrying out phase-sensitive quantum applications, such as quantum sensing, and for the generation of novel quantum states of light for quantum computing and quantum communication. The probing techniques developed for the plasmonic waveguides may also be applied to other types of plasmonic nanostructures, such as those used as nanoantennas, as unit cells in metamaterials and as nanotraps for cold atoms

    Single emitter cryo-micro-spectroscopy of pyramidal quantum dots, fluorescent proteins, and light-harvesting complexes

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    In this thesis, I present experiments involving low-temperature single emitter optical spectroscopy of pyramidal quantum dots, light harvesting complexes, and fluorescent proteins. Additionally, ensemble optical spectroscopy studies on fluorescent proteins are included. I investigate indium gallium arsenide pyramidal quantum dots with a double quantum dot system, having two different interdot separations. Micro-photoluminescence spectroscopy at cryogenic temperature is employed to study the spontaneous emission lineshape, characterised by the spectrally sharp zero-phonon line emission superimposed on a broad emission due to phonon-assisted transitions. Excitation power dependence and polarisation resolved photoluminescence measurements are conducted to identify the exciton, biexciton and trions emission lines. Further, using two-dimensional four-wave mixing measurements, coherent coupling of the excitons between two distant quantum dots is investigated. For the double quantum dot system with 10 nm interdot separation, coherent coupling between exciton due to static dipole-dipole interactions is revealed with a coupling strength of 150 micro-eV. I develop a sample design and preparation protocol for plasmonically enhanced low-temperature single emitter micro-photoluminescence measurements. A slow evaporation drop-casting method is developed to form monolayers of a polymer film containing the emitter around plasmonic nanoparticles to have efficient coupling. The choice of shape and size of plasmonic nanoparticles is discussed, and the optical scattering cross section of silica-coated plasmonic gold nanorods is calculated using the boundary element method. The Purcell factor and fluorescence enhancement of an emitter near plasmonic gold nanorods are estimated. I study plasmonically enhanced emission from individual light harvesting complexes LH2 at a temperature of 5 K. Plasmonically enhanced emitter positions show spectrally sharp zero-phonon emission lines that undergo spectral diffusion, contrasting with the broader emission observed from the unenhanced emitters. From the statistics of the different measured positions, an emission linewidth of 0.5 milli-eV is observed. I also analysed experimental data for LH2 emission at low temperatures without plasmonic enhancement from the literature to extract intrinsic lineshapes using a novel method separating the jitter, yielding a linewidth, and providing an alternative interpretation of the data. I study low-temperature photoluminescence and surface enhanced Raman scattering on mRhubarb720 fluorescent protein complex using plasmonic gold nanorods using a variant of the sample design used for LH2. The plasmonic properties of the gold nanorods are estimated using the boundary element method. The emission spectrum is characterised by spectrally sharp resonant Raman emission lines from individual fluorescent proteins. I measured power dependence, and polarisation resolved time traces of emission lines. From a fitting procedure of emission lines, I calculated the correlation between different emission lines. I also used non-negative matrix factorisation to explain observed time traces of emission lines by a combination of different spectral components. Finally, I study the ultrafast spectroscopy of three different fluorescent proteins at room temperature in an ensemble. Pump-probe spectroscopy measurements are conducted on the thin film of fluorescent proteins. From a Fourier transform analysis of the transient absorption spectrum, low-frequency vibrational beating at 250 per cm is observed for different fluorescent proteins. Two dimensional electronic spectroscopy measurements are also conducted to reveal the ultrafast relaxation within the molecules. I conducted an ensemble study of different fluorescent proteins using fluorescence line narrowing emission at low temperatures. These fluorescent proteins, variants of the same type but featuring distinct functional groups attached to them, exhibit different absorption and emission lineshapes. From the excitation power dependence of the emission spectra, I analyse the change in the emission lineshapes for different variants

    Light-Matter Interaction in Hybrid Quantum Plasmonic Systems

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    Attempting to implement quantum information related applications utilizing atoms and photons, as they naturally form quantum systems supporting superposition states, hybrid quantum plasmonic systems emerged in the past as a platform to study and engineer light-matter interaction. This platform combines the unrivaled electromagnetic field localization of surface plasmon polaritons, boosting the light-matter coupling rate, with the tremendous integration potential of truly nanoscale structures, and both the significant emission rates of nanoantennas and photonic transmission velocities. In this work, a classical description of surface plasmon polaritons is combined with a light-matter interaction model based on a cavity quantum electrodynamical formalism. The resulting composite semi-classical method, introduced and described in this thesis, provides efficient and versatile means to simulate the dynamical behavior of radiative atomic transitions coupled to plasmonic cavity modes in the weak incoherent coupling regime. Both the emission into the far field and various dissipation mechanisms are included by expanding the model to an open quantum system. The variety of light-matter interaction applications that can be modeled with the outlined method is indicated by the four different exemplary scenarios detailed in the application chapter of this thesis. The classical description of localized surface plasmon polaritons is benchmarked by reproducing the experimental measurements of the molecular fluorescence manipulation through optical nanoantennas in a collaborative effort with experimental partners. Furthermore, in the weak light-matter coupling regime, the potential of achieving a higher nanoantenna functionality and simultaneously realizing more elaborate quantum dynamics is revealed by the three remaining applications. Each pivotally involving a bimodal nanoantenna and demonstrating different quantum optical phenomena, the implementation of cavity radiation mode conversion, non-classical cavity emission statistics, and non-classical cavity emission properties is shown and described in the application chapter

    Nanofiber quantum photonics

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    Recent advances in the coherent control of single quanta of light, photons, is a topic of prime interest, and is discussed under the banner of quantum photonics. In the last decade, the subwavelength diameter waist of a tapered optical fiber, referred to as an optical nanofiber, has opened promising new avenues in the field of quantum optics, paving the way toward a versatile platform for quantum photonics applications. The key feature of the technique is that the optical field can be tightly confined in the transverse direction while propagating over long distances as a guided mode and enabling strong interaction with the surrounding medium in the evanescent region. This feature has led to surprising possibilities to manipulate single atoms and fiber-guided photons, e.g. the efficient channeling of emission from single atoms and solid-state quantum emitters into the fiber-guided modes, high optical depth with a few atoms around the nanofiber, trapping atoms around a nanofiber, and atomic memories for fiber-guided photons. Furthermore, implementing a moderate longitudinal confinement in nanofiber cavities has enabled the strong coupling regime of cavity quantum electrodynamics to be reached, and the long-range dipole–dipole interaction between quantum emitters mediated by the nanofiber offers a platform for quantum nonlinear optics with an ensemble of atoms. In addition, the presence of a longitudinal component of the guided field has led to unique capabilities for chiral light–matter interactions on nanofibers. In this article, we review the key developments of the nanofiber technology toward a vision for quantum photonics on an all-fiber interface
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