· · · Institutional Repository

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.

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.

As a result of progress in nanotechnology, smaller and smaller electronic circuits can be made. The stage of electrically contacting even a single molecule has now been reached. This stimulates both fundamental and applied research alike. Molecular electronics is hence a booming new field that draws a lot of attention. In this research project we have studied fundamental electrical transport properties of single molecules at low temperatures. In collaboration with chemists, a special kind of molecules has been synthesized for this purpose: molecular magnets. These molecules individually behave as tiny magnets. In this thesis, we describe the effect of the magnetic properties on the conductance of the molecule. Quantum mechanical effects play an important role in this respect. Furthermore, we looked at the conductance of a novel material system: graphene an atomic layer of graphite. Graphene is a semi-metal, in which electrons behave as relativistic, massless particles. By coupling graphene to superconducting electrodes, we were able to induce a supercurrent in graphene. The supercurrent in graphene can be tuned by a gate-electrode and hence the device behaves as a superconducting transistor. Our measurements provide new insights in the properties of this exotic material.

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.

Semiconductor nanocrystals (NCs), which can have a variety of sizes, shapes and chemical compositions, will be a large and important family of future advanced materials.This thesis focuses on colloidal semiconductor NC solids, also called quantum-dot (QD) solids, which are promising materials for many applications, such as photo-detectors, field-effect transistors, solar cells, light-emitting diodes, and lasers. The thesis presents studies on the charge carrier properties of PbSe QD solids, going through the charge carrier photogeneration, thermalization, diffusion and decay, which together are the ``life and fate'' of the charge carriers. Diverse tools have been utilized to reveal the whole picture of the ``life and fate''. The most important ones are: femtosecond transient absorption spectroscopy (TA) (Chapter 2, 3, 4), picosecond Terahertz spectroscopy (THz) (Chapter 2), the nanosecond time-resolved microwave conductivity technique (TRMC) (Chapter 2-5), and Monte Carlo simulations (Chapter 4).

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

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 is about random fluctuations over time (or noise) of electric currents and voltages occuring in small (mesoscopic) electronic devices with typical sizes of micro- to nanometre. Even though the theory presented is of a more general nature, research into such systems has been greatly pushed forward by the prospect of building a quantum computer. There are two important aspects to noise which are addressed in this work. The first can be summarised as detection and refers to the idea, that the fluctuations carry information about the microscopic details (geometric design, scattering, temperature) of transport which causes them. The theoretical problem we investigate is then to relate detector signals to fundamental properties of the sample. Secondly, fluctuations can trigger a variety of processes in the environment. Depending on the system one may wish to enhance or diminish such effects. To achieve this goal we study noise-induced effects and the coupling between noise sources and their environment. The precise way in which these fluctuations occur can be found from the theory of Full Counting Statistics (FCS) which provides a cornerstone for this thesis. In chapter 3 the effect of a weak electromagnetic environment on the Full Counting Statistics of a coherent conductor is investigated. We obtain explicit expressions for the correction to the FCS which are further studied by analytical and numerical means. We also present a reinterpretation of the correction in terms of elementary physical events. The major result in that chapter is a universal relation for Full Counting Statistics which holds at arbitrary voltage, temperature and with no regard to the concrete realization of the contact. For FCS this relation takes the form of detailed balance. In chapter 4 we investigate the detection of finite frequency noise using a quantum tunnelling detector. We focus on a concrete experimental setup consisting of a coherent conductor taking the role of the noise source and a tunnel junction (the detector) which is capacitively coupled to it. We show that the detector rate in a certain parameter range is dominated by a two-photon process and a process involving two interacting electrons in the coherent conductor. We find an explicit analytical expression for the detector signal in terms of system parameters: tunnel coupling, transmissions, environment, voltage over the conductor and coupling parameter. Our results facilitate the detection of many-particle events in the context of quantum transport, particularly electron-electron interactions. The non-Gaussian higher moments of the distribution of current fluctuations in a mesoscopic conductor contain more information than is present in average current and noise. However they are inherently difficult to measure. In order to facilitate such experiments, we propose a completely new way for measuring the Full Counting Statistics in chapter 5. We study threshold detection with a Josephson junction coupled to a mesoscopic conductor. We show that the detailed dependence of the junction's escape rate is sensitive to the distinct FCS of specific conductors (tunnel junction, diffusive, ballistic). We also address issues related to the measurement procedure notably feedback and dispersiveness of the detector. Our theoretical results facilitate a new type of electric noise measurement: direct measurement of the full distribution of transferred charge.

Colloidal semiconductor nanoparticles, also called quantum dots, have unique opto-electronic properties that make them promising candidates for many applications such as solar cells, light–emitting diodes, lasers, or biological imaging. One of the most interesting features is that the bandgap energy can be tuned by changing the particle size. This allows the design of solar cells with optimized absorption of the solar spectrum, resulting in improved power conversion efficiency. Quantum-dot-based opto-electronic devices require photoconductive nanocrystal assemblies, i.e. assemblies in which charge carrier photogeneration and transport are efficient. However, in assemblies of colloidal quantum dots, both the yield for charge carrier photogeneration and the charge mobility are initially low. This is due to the presence of long (1-2 nanometer), insulating molecules that are present at the surface of the dots for particle stabilization and surface passivation. The goal of this thesis was to produce photoconductive films of quantum dots and to understand the mechanisms governing charge generation, transport and decay in those films. Three options to increase the film photoconductivity were investigated in this thesis: capping removal (Chapter 2), capping exchange with short, organic molecules (Chapter 3, 5 and 6), and capping exchange with short, inorganic molecules (Chapter 4). Mobilities greater than 1 cm2/Vs were achieved in films of CdSe and PbSe quantum dots (Chapter 4 and 5), and in some cases, unity quantum yields of charge carrier photogeneration were attained (Chapter 5). Furthermore, charge extraction from quantum dot layers has been demonstrated with encouraging efficiency (Chapter 2). Those results show that quantum dots fulfill the requirements for use as the active material in solar cells.