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An important goal for nanoscale opto-electronics is the transfer of single electron spin states into single photon polarization states (and vice versa), thereby interfacing quantum transport and quantum optics. Such an interface enables new experiments in the field of quantum information processing. Single and entangled photon-pair generation can be used for quantum cryptography. Furthermore, photons can be used in the readout of a quantum computer based on electron spins. Semiconducting nanowires are a suitable electron (hole) channel, as they combine confinement of electrons (holes) in two dimensions with carrier transport in the third dimension. In addition, the small nanowire diameter allows for the combination of semiconductors with different lattice constants. Such heterostructures can be used to locally confine electrons and holes along the nanowire, creating an optically active quantum dot. Nanowire quantum dots are therefore a zero dimensional opto-electrical element embedded in a one dimensional electrical transport channel, which is ideal for quantum opto-electronics. In this thesis, we report a number of steps towards an electron spin to photon polarization interface based on nanowire quantum dots. First we develop single InAs0.25P0.75 quantum dots embedded in InP nanowires. We show that the nanowire quantum dots have optical emission linewidths as narrow as about 30 microeV. Due to the narrow emission lines, we are able to resolve individual spin states at magnetic fields of the order of 1 Tesla. We can prepare a given spin state by tuning the excitation polarization or excitation energy. To realize an electron-photon interface in a functional opto-electrical device, we contact the nanowires to obtain InP nanowire photodetectors with a single InAsP quantum dot as light absorbing element. For photon energies above the InP band gap, the nanowire photodetectors have a quantum efficiency of 4 %. Under resonant excitation of the quantum dot, the photocurrent amplitude depends on the polarization of the incident light. The photocurrent is enhanced (suppressed) for a linear polarization parallel (perpendicular) to the axis of the nanowire (contrast 0.83). The active detection volume under resonant excitation is 7 10^(-3) nm^(-3). These results show the promising features of quantum dots embedded in nanowire devices for electrical detection of light with a high spatial resolution. Next, we apply an electric field to induce single electron charging effects in the nanowire quantum dot. We perform optical experiments of a charge tunable, single nanowire quantum dot, in which the charge state is tuned with two independent voltages. First, we control tunneling events through an applied electric field along the nanowire growth direction. Second, we modify the electrochemical potential in the nanowire with a back-gate. We combine these two field-effects to isolate a single electron and independently tune the tunnel coupling of the quantum dot with the contacts. Such charge control is a requirement for opto-electrical experiments involving a single electron spin in a nanowire quantum dot. We successively develop lateral gates next to the optically active nanowire quantum dots. By applying a positive potential to both lateral gates, we observe energy modifications of the emission when one and two electrons are residing in the quantum dot. The energy shifts are explained by a reduction of the electron-electron Coulomb and s-p exchange interactions. In addition, we present large biexciton emission energy control when a lateral electric field is applied to the quantum dot. Here, the emission energy of the biexciton can be tuned to the same energy as the exciton emission energy, a key result for entangled photon pair generation. The coupling of the lateral gates to the negatively charged exciton is promising for future electron spin manipulation experiments in optically active nanowire quantum dots. To move towards on-chip excitation of the quantum dot, we present reproducible fabrication of InP-InAsP nanowire light emitting diodes in which electron-hole recombination is restricted to the quantum-dot-sized InAsP section. The nanowire geometry naturally self-aligns the InAsP section with the n-InP and p-InP ends of the wire, making these devices promising candidates for electrically-driven quantum optics experiments. We have investigated the operation of these nano-LEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for on-chip sources of single photons. Finally, we present the method to scale up the nanowire quantum dot synthesis in a regular array. We show single-photon and cascaded photon pair emission in the infrared, originating from a single InAsP quantum dot embedded in a standing InP nanowire. To perform electron spin manipulation and optical read-out, it is necessary to reduce the optical emission linewidth of the contacted nanowire quantum dots.

We experimentally investigate Quantum Dots, formed in Carbon Nanotubes. The first part of this thesis deals with charge sensing on such quantum dots. The charge sensor is a metallic Single-electron-transistor, sensitive to the charge of a single electron on the quantum dot. We use this technique for real-time charge readout and precise tuning of the tunnel barriers of the quantum dot. The second part of this thesis describes the realization of exceptionally clean Carbon Nanotube quantum dots. We create few-electon single, double and triple quantum dots. In a few electron double quantum dot, we observe an effect which is analogous to Klein tunneling in relativistic quantum mechanics.

For small magnetic memories, the ultimate limit is a single magnetic particle, a single electron spin. A lot of research is put in developing devices that can utilize such a single spin memory. At the same time, there is also interest from a more fundamental point of view: on these small scales, Quantum Mechanics starts to play a role. Devices of this size have unique possibilities that have no parallel in the classical world around us. In this work we study a single electron spin that is captured in a small box, a quantum dot. This is all made inside a semiconducting nanowire. By measuring the current we can determine how the spin orients in a magnetic field. We can also see interactions of the spin with its environment. We study how the semiconductor crystal influences the spin, via spin-orbit interaction and via its nuclei. We also determine how this affects the interaction between two such spins.

This thesis presents different optical experiments performed on semiconductor quantum dots. These structures allow to confine a small number of electrons and holes to a tiny region of space, some nm across. The aim of this work was to study the basic properties of different types of quantum dots made of various materials and with different techniques. First we studied InAsP quantum dots in InP nanowires and demonstrated narrow optical transitions, with linewidths below 30 micro eV. It was also possible to produce electron-hole pairs in a given spin state and to show that, in the presence of a magnetic field, this state is preserved for a time comparable to the exciton lifetime. Measurements of the electron and hole g-factors in these dots are also presented. Other types of structures dealt in this thesis are GaAs quantum dots in AlGaAs and small InAs dots in GaAs. GaAs dots can be tuned to have optical transitions at the same energy as rubidium atoms. We studied InAs quantum rings and we observed energy oscillations that are compatible with the Aharonov-Bohm effect and that can be tuned by an electric field. The last chapter of this thesis deals with two-photon interference, a useful tool for different quantum information protocols. We demonstrated that a InAs quantum dot can emit pairs of indistiguishable photons with a delay of about 5~ns between them.

This thesis is about the development of a detector for single photons, particles of light. New techniques are being developed that require high performance single photon detection, such as quantum cryptography, single molecule detection, optical radar, ballistic imaging, circuit testing and fluorescence spectroscopy. Superconducting single photon detectors (SSPDs) are sensitive to single photons from the ultraviolet to the near infrared. In this thesis steps has been taken towards improving this type of detectors and implementing them in experiments. We have fabricated SSPDs in the Van Leeuwenhoek Laboratory at the TU Delft from NbTiN on an oxidized silicon substrate and we show world record system detection efficiencies at telecommunication wavelengths. In addtition, we have adjusted the geometry to get rid of the polarization dependence of the quantum efficiency. SSPDs fabricated from a new material show enhanced efficiency at longer wavelengths. Different read out schemes can scale a single pixel to an array of detectors. We have proven by implementing the SSPDs in a Hanbury-Brown Twiss setup that nanowire quantum dots emit single photons. We also have demonstrated that SSPDs are sensitive to single surface plasmon polaritons and single electrons.

This thesis describes experiments with surface plasmons: light confined to metal-dielectric interfaces. By using metals it is possible to make extremely small waveguides for light, even below the diffraction limit of dielectric structures. We study the quantum aspects of single plasmons in particular. To this end we create small circuits of gold waveguides with integrated detectors that are able to sense individual plasmons. In a beam splitter geometry we clearly observe quantum mechanical interaction between pairs of indistinguishable plasmons created using parametric down-conversion. We further describe simulations and experiments with optical antennas that allow to focus light to the nanoscale. Although the losses in metals present a significant challenge, these structures provide an interesting toolbox to bring light to the scale of typical electronic components.

This thesis describes a series of experiments on the control of the optical properties of the nitrogen-vacancy (NV) center in diamond, and on control of the electron and nuclear spin states associated with the NV center. The NV center is a fluorescing atomic defect center in diamond, consisting of a substitutional nitrogen atom adjacent to a vacancy in the diamond carbon lattice. The electron spin of the NV center can be initialized and read out optically, and coherently manipulated with high fidelity even at room temperature. This unique combination of properties has attracted much attention to the NV center in recent years for application in quantum information technologies. The experiments in this work focus on exploiting the intrinsic hybrid nature of the NV center. The NV center forms a hybrid spin register, as its electron spin is always coupled to the nuclear spin of its own nitrogen nucleus, and can furthermore be coupled to additional nuclear carbon-13 spins in the diamond. In addition, the electronic spin state can be mapped onto the state of a photon, in principle allowing long range transmission of quantum information and measurement-based entanglement of distant NV center registers. The long-term goal is to achieve a large scale quantum network where the different constituents each perform the task they are most well suited for: the stable nuclear spins store the quantum information, photons transfer quantum information over long distances, and the fast electron spin interfaces between spin and photon states. For measurement-based entanglement of distant NV centers it is of primary importance to maximize the emission and detection of coherent photons emitted by the NV center. Spontaneous emission can be controlled and enhanced by coupling an emitter to a photonic cavity. Chapters 3, 4, and 5 of this thesis describe experiments aimed at maximizing the coherent photon emission rate of an NV center by coupling the NV center to a photonic crystal cavity. One of the main challenges is to position a single NV center into the nanometer-sized cavity mode. Chapter 3 describes the development of a nanopositioning technique, which allows diamond nanocrystals ~50 nm in size containing single NV centers to be placed at arbitrary locations on a chip, with nanometer precision. By using a sharp etched tungsten tip, individual nanocrystals can be picked up, moved and placed under real-time imaging with an electron microscope. We explicitly demonstrate that the unique optical and spin properties of the NV center are conserved by the nanopositioning process. In chapter 4, we apply the nanopositioning technique to couple single NV centers contained in a diamond nanocrystals to gallium phosphide photonic crystal cavities. These cavities resonate in the visible with high quality factors, and are designed to have the lowest energy mode coincide with the zero-phonon line of the NV center. Efficient coupling is evidenced by a strong enhancement of NV center emission at the cavity wavelength. The high quality factor of a photonic crystal cavity is a result of careful design and fabrication. The introduction of a nanocrystal may therefore be expected to have a detrimental effect on the optical properties of a cavity. In chapter 5 we investigate the effect of a nanocrystal on the optical properties of photonic crystal cavities. Our simulations and measurements show that the effect of a nanocrystal is in fact only minor, a promising result for future work on cavity-QED systems in the solid state and with diamond defect centers in particular. Understanding and mitigating decoherence is a key challenge for quantum science and technology. The main source of decoherence for solid-state spin systems is the uncontrolled spin bath environment. Chapter 6 describes experiments that exploit quantum control of the NV center and recently developed dynamical decoupling techniques to study the properties of the surrounding bath of spins belonging to single substitutional nitrogen atoms. The coherence properties of a single NV center are affected by changes in the state of the bath spins, allowing a single NV center to be used as a probe to detect bath spin resonances and study the quantum dynamics of the spin bath. We use quantum control of the spin bath to extend the dephasing time of the NV center, important for the application of the NV center in DC magnetometry. In chapter 7 we describe the development and implementation of decoherence protected quantum gates for the hybrid spin register formed by the NV center electron spin that is coupled to the nuclear spin of its own nitrogen atom. Electron and nuclear spins evolve and decohere at vastly different rates, making it challenging to use such a hybrid system as a fully functional quantum register. We create a universal set of two-qubit quantum gates by integrating dynamical decoupling techniques into the gate operation. We demonstrate the power of our gate design by implementing for the first time Grover's quantum search algorithm on a solid-state spin register.

The field of quantum science and technology has generated many ideas for new revolutionary devices that exploit the quantum mechanical properties of small-scale systems. Isolated solid state spins play a large role in quantum technologies. They can be used as basic building blocks for a quantum computer or as ultra-sensitive magnetic-field probes which can detect the extremely weak magnetic field generated by a single proton. A major hurdle for realizing these applications is the loss of quantum coherence resulting from uncontrolled interactions with spins in the environment. In the experiments described in my thesis we studied spins associated with defect centers in diamond and used new strategies for mitigating decoherence involving advanced quantum control techniques and for fundamental studies of decoherence. We show that we can prolong the coherence time of a single spin associated with a Nitrogen-Vacancy (NV) defect center in diamond with dynamical decoupling techniques. Our experiments are accurately reproduced theoretically and from this theory we conclude that, with dynamically decoupling, the spin environment can in principle be made irrelevant for the decoherence of a single spin. This removes a major obstacle for using solid-state spins in quantum science and technology. Furthermore, the dynamics in the spin environment and its influence on the NV spin is thoroughly experimentally studied. By better understanding the mechanisms behind decoherence we may one day find the answer to unresolved fundamental issues in quantum physics such as the quantum measurement problem.

In this Master thesis I report results on a route to find Majorana fermions in indium antimonide nanowires in contact with a superconductor. Theoretically Majorana fermions appear in one-dimensional nanowires with strong spin-orbit coupling, in proximity with a superconductor and an external magnetic field applied parallel to the nanowire. The nanowires are deposited by a deterministic method, in this way the external magnetic field is perfect aligned with the nanowires up to a few degrees. Results we observed are a possible magnetic field tunable pi-junction, measurements of an induced gap in the nanowire and a robust zero-bias peak that persist in both gate and magnetic field scans. This zero-bias peak can be split and recombine with varying the applied magnetic field and the local gate potential.

This thesis describes a series of experiments aimed at understanding the low-temperature electrical transport properties of semiconductor nanowires. The semiconductor nanowires (1-100 nm in diameter) are grown from nanoscale gold particles via a chemical process called vapor-liquid-solid (VLS) growth. The huge versatility of this material system (e.g. in size and materials) results in a wide range of potential applications in (opto-)electronics. During the last few years many important proofs of concept have already been provided like lasers, field-effect transistors, light emitting diodes, and biochemical sensors. Simultaneously, the versatility of semiconductor nanowires creates new opportunities for the study of quantum transport phenomena. The quantum mechanical properties of semiconductor nanowires become visible at low temperatures (below a few Kelvin) and can be very different from room-temperature transport properties. For instance, the confinement of electrons in a small nanowire segment results in a discrete electronic energy spectrum forming a quantum dot, or artificial atom.

Electronic transport through nanostructures can be very different from trans- port in macroscopic conductors, especially at low temperatures. Carbon na- notubes are tiny cylinders made of carbon atoms. Their remarkable electronic and mechanical properties, together with their small size (a few nm in diameter), make them very attractive for scientific research, both from the basic as well as from the technological point of view. This thesis describes experimental research aimed at understanding electronic transport through carbon nanotubes (CNTs) at low temperatures.

An electron does not only have an electric charge, but also a small magnetic moment, called spin. In a magnetic field, the spin can point in the same direction as the field (spin-up) or in the opposite direction (spin-down). However, the laws of quantum mechanics also allow the spin to exist in both states at the same time (so-called superposition state). The experiments described in this thesis aim at controlling the quantum state of a single electron spin which is confined in a quantum dot. Using the level of control achieved in these experiments, we investigated the properties of one and two-electron spin states, for example by measuring how the environment affects the superposition states. In addition to unraveling these fundamental properties, this research also aims at the development of a so-called quantum bit. This is an important building block for the future (much more powerful) quantum computer.

This thesis describes a series of experiments aimed at understanding and controlling the behavior of the spin degree of freedom of single electrons, confined in semiconductor quantum dots. This research work is motivated by the prospects of using the electron spin as a quantum bit (qubit), the basic building block of a quantum computer. Here, the envisioned basis states (logical 0 and 1) of the qubit are the two possible orientations of the spin in a magnetic field: spin-up (parallel to the field) and spin-down (anti-parallel to the field). In this thesis, a number of important steps towards the use of electron spins as qubits are reported: the isolation of a single electron in a quantum dot, energy spectroscopy of the electron spin states, development of a new technique to probe a nearlyisolated quantum dot, single-shot read-out of the electron spin orientation, and increased understanding of the interaction of the electron spin with its environment. A quantum dot can be thought of as a small box filled with a controllable number of electrons. This box is coupled via tunnel barriers to reservoirs, with which electrons can be exchanged, and is coupled capacitively to one or more gate electrodes that allow the number of electrons on the dot to be varied. Due to the small dot size (typically ? 50 nm), comparable to the Fermi wavelength of the electrons, it exhibits a discrete energy spectrum. The quantum dot devices studied in this work are defined in a two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure, by applying negative voltages to metallic gate electrodes fabricated on top of the heterostructure.

In this thesis, the spin and charge degree of freedom of electrons in semiconductor lateral and vertical quantum dots are experimentally investigated. The lateral quantum dot devices are defined in a two-dimensional electron gas (2DEG) below the surface of a GaAs/AlGaAs heterostructure, by metallic surface gates. The vertical quantum dots are sub-micron pillars fabricated in an In/Al/GaAs double-barrier heterostructure, and surrounded by a metal gate electrode. Both kinds of quantum dots behave in many ways as artificial atoms. In the first part of this thesis, we describe experiments aimed at using a single electron in a lateral quantum dot as a spin qubit, building block of a quantum computer. We first develop the spin qubit hardware: a device consisting of two coupled quantum dots that can be filled with one electron spin each, with a controllable inter-dot tunnel coupling. We then use a nearby quantum point contact (QPC) as an electrometer to characterize the quantum dot in the regime of very weak coupling to the reservoirs. In particular, we measure the Zeeman splitting between the proposed qubit states, i.e. the spin-up and spin-down state of a single electron spin in a large magnetic field. Finally, we develop a fully electrical technique to perform single-shot measurement of the spin orientation of an individual electron in a quantum dot. We find a very long spin relaxation time of 0.85 ms at a magnetic field of 8 T, indicating that the electron spin degree of freedom is only weakly disturbed by the environment. We conclude part one of this thesis with an overview of the progress made towards creating a spin qubit. Part two focusses on quantum dots that are strongly coupled to the reservoirs. We observe a strong Kondo effect in a lateral quantum dot, with the conductance reaching the unitary limit. In a vertical quantum dot containing six electrons, we observe an unexpected Kondo effect at the transition between a spin singlet and a spin triplet ground state. Finally, we conclude with an investigation of elastic and inelastic cotunneling in a vertical quantum dot containing two to six electrons.

The research in this thesis is motivated by an interest in quantum physics and by the prospect of new applications based on the spin of electrons or holes. This work focuses on confining single spins in quantum dots, which can serve as building blocks of a future quantum computer. Such a computer exploits the unique features of quantum mechanics to perform computations that are not possible classically. Long spin lifetimes are crucial to carry out operations on spin quantum bits. We report a number of important steps towards the creation of spin quantum bits in a material with an expected long spin lifetime: the demonstration of single quantum dots in Si nanowires, the isolation of a single hole in a Si quantum dot, energy and magnetic field spectroscopy of the first four spin states, and the use of a scanning probe microscope to locate quantum dots inside InAs nanowires. Additionally we try to make novel spintronic devices that exceed modern-day silicon IC-technology in terms of data processing speed, power consumption, non-volatility and integration densities. The demonstration of electric field control of the magnetoresistance in InP nanowires shows our ability to combine the functionalities of semiconductors and magnetic materials.

This thesis describes a series of experiments aimed at understanding and controlling single electron spins confined in semiconductor lateral quantum dots, with the long-term goal of creating of a small-scale quantum computer. The confinement of these electrons results in a quantized energy spectrum, and therefore, the quantum dots can be regarded as artificial atoms. At first, the quantum dots are analyzed by conventional transport experiments. Here, we can energetically resolve the Zeeman splitting of a single electron when a strong magnetic field is applied. Excited-state spectroscopy enables us to identify the ground state spin configuration of a quantum dot containing 1-5 electrons. Furthermore, by using fast voltage pulses, we find a lower bound on the spin doublet relaxation time of 50 microseconds. Second, a novel method was developed for finding the relevant dot parameters in the regime of very weak dot-lead coupling. Here a quantum point contact electrostatically coupled to the quantum dot, is used as a fast and sensitive charge detector allowing us to resolve single-electron tunnel events in real time. Then, we demonstrate one of the key ingredients for a quantum computer: single-shot read-out of the spin states. To convert the spin information to charge information, we have exploited the spin-dependent energy, and spin-dependent tunnel rates, achieving a measurement visibility of more than 80%. Both for a single spin and for the two-electron spin states, we find that the relaxation can be very slow (relaxation times up to milliseconds). We find a strong magnetic field dependence that hints at spin-orbit interaction as the dominant

The research described in this thesis is aimed at constructing a quantum interface between a single electron spin and a photon, using a nanowire quantum dot. Such a quantum interface enables information transfer from a local electron spin to the polarization of a photon for long distance readout. Important aspects for the realization of such a device are the optical quality of the quantum dot and access to the optical polarization. In this thesis, we demonstrate high quality nanowire quantum dots and observe spin memory. By embedding them in a vertical nanowire device, we are able to isolate a single electron on the dot, while maintaining access to the intrinsic polarization properties of the nanowire quantum dot. These results demonstrate that quantum dots in vertical nanowire device are promising as an optical interface for single electron spins.