1 

Optical Properties of Semiconductor Quantum Dots
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 electronhole 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 gfactors 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 AharonovBohm effect and that can be tuned by an electric field.
The last chapter of this thesis deals with twophoton 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.

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2 

Superconducting Single Photon Detectors
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 HanburyBrown 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.

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3 

Single spins in semiconductor nanowires

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4 

Optoelectronics on Single Nanowire Quantum Dots
An important goal for nanoscale optoelectronics 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 photonpair 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 optoelectrical element embedded in a one dimensional electrical transport channel, which is ideal for quantum optoelectronics.
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 electronphoton interface in a functional optoelectrical 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 backgate. We combine these two fieldeffects 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 optoelectrical 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 electronelectron Coulomb and sp 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 onchip excitation of the quantum dot, we present reproducible fabrication of InPInAsP nanowire light emitting diodes in which electronhole recombination is restricted to the quantumdotsized InAsP section. The nanowire geometry naturally selfaligns the InAsP section with the nInP and pInP ends of the wire, making these devices promising candidates for electricallydriven quantum optics experiments. We have investigated the operation of these nanoLEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for onchip sources of single photons.
Finally, we present the method to scale up the nanowire quantum dot synthesis in a regular array. We show singlephoton 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 readout, it is necessary to reduce the optical emission linewidth of the contacted nanowire quantum dots.

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5 

Electron spins in nanowire quantum dots
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 spinorbit interaction and via its nuclei. We also determine how this affects the interaction between two such spins.

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6 

Single electronics with carbon nanotubes
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 Singleelectrontransistor, sensitive to the charge of a single electron on the quantum dot. We use this technique for realtime 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 fewelecton 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.

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7 

Quantum Plasmonics
This thesis describes experiments with surface plasmons: light confined to metaldielectric 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 downconversion. 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.

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8 

Josephson effects in carbon nanotube mechanical resonators and graphene

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9 

Feedback control of superconducting quantum circuits
Superconducting circuits have recently risen to the forefront of the solidstate prototypes for quantum computing. Reaching the stage of robust quantum computing requires closing the loop between measurement and control of quantum bits (qubits). This thesis presents the realization of feedback control of superconducting qubits based on projective measurement, with initial applications. The first application is a reset operation which initializes a qubit in the ground state fast and on demand. In a second experiment, we transform a cavity into a twoqubit parity meter, which is capable of generating entanglement starting from an unentangled superposition state. We then employ feedback to turn the generation of entanglement from probabilistic to deterministic, thus combining two key ingredients  parity measurement and feedback  of quantum error correction. As a third application, we transform a superconducting qubit into a chargeparity detector to investigate the limits of quasiparticle tunneling to qubit coherence. We find room for at least a tenfold increase in qubit coherence times before quasiparticles become the bottleneck. Finally, we present our latest results towards implementing the bitflip error correction code in a fivequbit processor and discuss the role of feedback control in the future development of superconducting quantum circuits.

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10 

Single spins in nanowire quantum dots with strong spinorbit coupling

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11 

Engineering the Optical Properties of Nanowire Quantum Dots
In the future world of quantum communication and computing, infomation cannot be hacked and computers solve difficult problems, such as factorizing large number or searching in a large databases, much faster than possible with ordinary computers. However, to realize such systems still much research is needed. One of the systems that has gained a lot of attention for applications in this field are optically active quantum dots, because of their ability to emit single and entangled photons. Also, they have the properties to form a bridge between stationary qubits and flying qubits. However, many challenges remain for their application in quantum information.
In this thesis, we study optically active quantum dots in nanowires to answer the questions: how we can improve extraction efficiencies and how can we gain more control over the optical properties.
The first part of this thesis discusses our experiments on InAsP quantumdots that are embedded in bottomup grown InP nanowires. We investigate the effect of the nanowire diameter on the extraction efficiency. We find that a nanowire waveguide on a gold mirror can improve extraction efficiencies up to at least 42%. The photons in such waveguides are emitted in the HE11 optical mode, that has a Gaussian profile. This allows the photons to be coupled to a fiber with an efficiency of 93%. We show that by growing a simple dielectric envelope by plasma enhanced chemical vapor deposition (PECVD), it is possible to shift the emission energy of the nanowire and quantum dot over 200 meV. We can tune the shift by changing the growth conditions, resulting in red and blue shifts. We explain that the observed shifts are induced by strain from the dielectric envelope and support this with tightbinding calculations. The quantum dots that are subject of the first part of this thesis are defined by a change in material (composition) and we refer to them as hetero quantum dots. One of the great challenges of such quantum dots is to control them in growth, allowing the control over the optical properties and the ability to design complex systems containing multiple quantum dots. In the second part of this thesis, we study a different type of quantum dots  crystal phase quantumdots  that are defined by a change in the crystallographic structure. Due to the high surface to volume ratio in nanowires, it is possible to grow crystal structures that are not found in bulk and a local change in crystallographic structure can form a quantum dot, such as a small zinc blende section in a wurtzite nanowire. This type of quantumdots would in principle allow for an unprecedented degree of control as the interface between the quantum dot and matrix is defined by the crystal structure and therefore within one monolayer.
In the second part, we optically study single crystal phase quantum dots in InP and GaP nanowires. We study InP nanowires containingmultiple crystal phase quantumdots andwe find that these are excellent single photon emitters, that can be source of cascaded single photon emission. We observe that the lines in our spectra our often split and we explain this with a comprehensive theory. In magnetic field, crystal phase quantumdots show a clear diamagnetic shift (1020 mueV/T2), but the observed behavior differs from what hasbeen observed for hetero quantum dots in magnetic field. In electric field, we find clear evidence of charging of the crystal phase quantum dots. However, the growth control of InP crystal phase quantum dots is not yet achieved. In GaP, the growth of crystal phase quantum dots can be well controlled, however, the understanding the optical properties is still subject of debate. By measuring the magnetic field, we try to identify the origin of some of the observed lines that could origin from crystal phase quantumdots.

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12 

Topology in twodimensional systems
This thesis includes two research directions both aim to discover a building block for topological quantum computing. First, in Chapter 3, a novel setup is designed, built, and tested, that can electrostatically gate a material without endangering the materials pristine quality. The setup was designed with the goal to reveal nonAbelian quasiparticles in the v=5/2 fractional quantum Hall effect. To stabilize these particles, material quality should be as high as possible. Second, and the larger part of this thesis, is devoted to InAs/GaSb double quantum wells. They are investigated for their possible use in topological quantum bits. Unprecedented material growth allowed us to investigate this material beyond existing (gate) bounds. By interfacing this material with superconducting leads we set the first step towards topological quantum computing in this material. However, we also found topologicallike signatures arising from trivial mechanisms.

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13 

Quantum transport in onedimensional nanostructures

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14 

Nanowirebased Quantum Photonics
In this thesis work, I studied individual quantum dots embedded in onedimensional nanostructures called nanowires. Amongst the effects given by the nanometric dimensions, quantum dots enable the generation of single light particles: photons. Single photon emitters and detectors are central building blocks of future communication technologies. As the miniaturization in electronics is driving towards the quantum limit, we envision future telecommunication as based on single photons. Single photons enable the use of quantum properties of light to encode information and share it with unbreakable security provided by quantum cryptography. Based on the principle that a single photon cannot be cloned and that its quantum state cannot be observed without altering it the information that the photon is carrying, a single particle of light represents the ideal mean of transportation for future secure telecommunication. During my work, nanowires have been tailored into waveguides providing directional propagation along the nanostructure that outcouples to the macroscopic world with negligible losses. The optical properties of the quantum dots have been explored through photoluminescence spectroscopy and improved in order to obtain a pure flux of single photons. Simultaneously, the material composition and shape of the nanowire have been designed to achieve efficient collection of these photons. Nanowires are tailored into waveguides providing directional propagation along the nanostructure that outcouples to the macroscopic world with negligible losses. This system we developed can be seen as an analogue of an nano optical fiber where photons are propagating one at a time. Several optical properties of the emitted single photons have been analyzed and subsequently optimized such as the temporal emission statistics, the shape and dispersion of the energy spectrum and the spatial emission profile. In addition, we demonstrated the possibility of creating an excitation in the quantum dot and translate this excitation into an electrical signal transmitted along the nanowire, thereby detecting the generation of individual excitations in the nanostructure.

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15 

Photocurrent in Carbon Nanotubes

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16 

Quantum control and coherence of interacting spins in diamond
The field of quantum science and technology has generated many ideas for new revolutionary devices that exploit the quantum mechanical properties of smallscale 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 ultrasensitive magneticfield 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 NitrogenVacancy (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 solidstate 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.

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17 

Quantum control of single spins and single photons in diamond
This thesis describes a series of experiments on the control of the optical properties of the nitrogenvacancy (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 carbon13 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 measurementbased entanglement of distant NV center registers. The longterm 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 measurementbased 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 nanometersized 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 realtime 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 zerophonon 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 cavityQED 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 solidstate 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 twoqubit 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 solidstate spin register.

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18 

Quantum measurement and entanglement of spin quantum bits in diamond
This thesis presents a set of experiments that explore the possible realisation of a macroscopic quantum network based on solidstate quantum bits. Such a quantum network would allow for studying quantum mechanics on large scales (meters, or even kilometers), and can open new possibilities for applications in quantum information processing.
A promising candidate platform for such a network is the nitrogenvacancy (NV) center in diamond. The NV center is a lattice defect in diamond that has excellent quantum properties and allows optical access to a quantum register of individual electronic and nuclear spins. We use optical spectroscopy and magnetic resonance techniques at liquid helium temperature to manipulate individual spins, and perform projective quantum measurements on them.
We first show an experiment in which we create entanglement between two noninteracting nuclear spins by a projective quantum measurement. Second, we demonstrate the creation of an entangled state between two NV electronic spins located in different setups at a distance of three meters. Finally, we demonstrate the deterministic teleportation of the quantum state of a nuclear spin onto an electronic spin over three meters. Our results demonstrate the great potential of the NV center for the realization of a largescale quantum network.

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19 

Quantum dot spin engineering for quantum optics

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20 

Majorana fermions in well aligned InSbnanowires with superconducting and normal contacts
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 onedimensional nanowires with strong spinorbit 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 pijunction, measurements of an induced gap in the nanowire and a robust zerobias peak that persist in both gate and magnetic field scans. This zerobias peak can be split and recombine with varying the applied magnetic field and the local gate potential.

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