1 

Quantum Dots in Vertical Nanowire Devices
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.

[PDF]
[Abstract]

2 

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.

[PDF]
[Abstract]

3 

Electron Spins in Semiconductor Quantum Dots
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: spinup
(parallel to the field) and spindown (antiparallel 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, singleshot readout 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 twodimensional electron gas (2DEG) of
a GaAs/AlGaAs heterostructure, by applying negative voltages to metallic gate
electrodes fabricated on top of the heterostructure.

[PDF]
[Abstract]

4 

Quantum transport in molecular devices and graphene
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 semimetal, 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 gateelectrode and hence the device behaves as a superconducting transistor. Our measurements provide new insights in the properties of this exotic material.

[PDF]
[Abstract]

5 

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.

[PDF]
[Abstract]

6 

Single photon emission and detection at the nanoscale utilizing semiconductor nanowires

[PDF]

7 

Photogeneration Diffusion and Decay of Charge Carriers in QuantumDot Solids
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 quantumdot (QD) solids, which are promising materials for many applications, such as photodetectors, fieldeffect transistors, solar cells, lightemitting 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 timeresolved microwave conductivity technique (TRMC) (Chapter 25), and Monte Carlo simulations (Chapter 4).

[PDF]
[Abstract]

8 

Electrical Control, Readout and Initialization of Single Electron Spins
An electron, in addition to its electric charge, possesses a small magnetic moment, called spin. The spin of an electron can point parallel (spinup) or antiparallel (spindown) to the magnetic field. These two states are analogous to zero and one of the logical bit in current digital electronic devices. However, according to the laws of quantum mechanics, the spin of an electron can be both up and down at the same time. Exploiting the spin degree of freedom has opened up a new era in the field of semiconductor electronics which may revolution current electronic devices. The electron spin could act as a quantum bit (qubit) in a futuristic quantum computer. With recent advances in nanotechnology, it is now feasible to create tiny electrostatic islands called quantum dots to controllably trap single electrons and explore their spin properties.
This thesis presents experiments aiming at combining the indispensable ingredients of a quantum computer: readout, control and initialization of single electron spins. It also seeks a deeper understanding of the properties of single electron spins in GaAs quantum dots.
The measurements are performed on a double quantum dot which is defined in a two dimensional electron gas (2DEG) of GaAs/AlGaAs heterostructure. Applying negative voltages to the metallic gates on top of the heterostructure depletes the electron gas beneath them and thereby creates the quantum dots. By applying more and more negative voltages on the gates, we remove the electrons from the quantum dots one by one, reaching the single electron regime. Applying a magnetic field creates an energy difference between the spin states and defines the two states of the qubit. The device used in the experiments is cooled down to about 100\,mK where quantum mechanical behaviour is observed.
In the first part of the thesis, we have realized independent singleshot readout of two electron spins in our double quantum dot. The capability to measure the quantum state of multiple qubits individually and in a singleshot manner is essential for efficient characterization of quantum information protocols. Additionally, in a quantum computer, the result of computation needs to be readout. The presented readout method is allelectrical and the cross talk between two measurements is negligible. The readout fidelities are about 86\% on average. This allows us to directly probe the anticorrelations between two spins prepared in a singlet state, an entangled twospin state.
The independent singleshot readout of two electron spins also enabled us to fully characterize the operation of the twoqubit exchange gate, an important operation in a spinbased quantum computer, on a complete set of basis states. We observe a deviation of the two qubit gate from the pure exchange which we later account for.
In the next step we combine the singleshot readout with electrical manipulation of single electrons. Manipulation of single electrons can be done using socalled electric dipole spin resonance (EDSR) where the electric field couples to the spin degree of freedom. In quantum dots, EDSR can be mediated in several ways such as spinorbit interaction, where the spin of an electron is coupled to its momentum, and the hyperfine interaction, where the electron spin is coupled to the nuclear spins of three isotopes of GaAs. We show that at high magnetic fields there is a clearly observable shift in the resonance condition between spinorbit mediated and hyperfinemediated EDSR. In these experiments, we introduce adiabatic rapid passage using fast frequency chirps as a robust technique to invert the electron spin in quantum dots. Furthermore, by modeling the EDSR response, we get a deeper understanding of the interplay between spinorbit and hyperfine mediated driving. These findings could be exploited for enhanced control of dynamic nuclear polarization processes, including selective control of the three nuclear spin species.
The focus of the penultimate part of the thesis is to combine singleshot readout with fast initialization of single electron spins. The capability of fast qubit initialization to a wellknown state is crucial for the implementation of the quantum computer for two reasons. First, at the start of the computation qubits need to be initialized. Second, for the error correction schemes, a continuous source of initialized qubits is required where the speed of initialization needs to be faster than the relevant gate operations. In the experiments described in the thesis, we demonstrate electrically controlled fast initialization of a singlespin qubit making use of ``hot spots'' where spin relaxation is enhanced by more than three orders of magnitude. These hot spots occur when the spin splitting matches the quantized orbital level spacing. Voltage pulses applied to the gates defining the double quantum dot allow us to rapidly move to one of the hot spots. There, spinorbit and hyperfine interactions efficiently mix the spin and orbital excited states and spinconserving orbital relaxation syphons the entire population to the ground state, thus achieving $\mu$sscale initialization.
In the last part of the thesis, allelectrical independent addressing of a singleelectron spins is presented. In those measurements we perform single electron manipulation using EDSR. Surprisingly, we observe wellseparated Zeeman splittings in neighbouring quantum dots. This finding provides a direct route to selective addressing of spins in quantum dot arrays without the need for microfabricated magnets. The observed splitting also accounts for the deviation of the twoqubit gate from pure exchange, as observed in the first part of the thesis.
All results presented in this thesis contribute to meeting the fundamental requirements for physical implementation of a spinbased quantum computer.

[PDF]
[Abstract]

9 

Electron spins in fewelectron lateral quantum dots
This thesis describes a series of experiments aimed at understanding and controlling single electron spins confined in semiconductor lateral quantum dots, with the longterm goal of creating of a smallscale 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. Excitedstate spectroscopy enables us to identify the ground state spin configuration of a quantum dot containing 15 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 dotlead 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 singleelectron tunnel events in real time. Then, we demonstrate one of the key ingredients for a quantum computer: singleshot readout of the spin states. To convert the spin information to charge information, we have exploited the spindependent energy, and spindependent tunnel rates, achieving a measurement visibility of more than 80%. Both for a single spin and for the twoelectron 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 spinorbit interaction as the dominant

[PDF]
[Abstract]

10 

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.

[PDF]
[Abstract]

11 

Effects and detection of quantum noise
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 noiseinduced 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 twophoton 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 manyparticle events in the context of quantum transport, particularly electronelectron interactions.
The nonGaussian 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.

[PDF]
[Abstract]

12 

Charge Injection, Charge Trapping and Charge Transfer in QuantumDot Solids
This study reports on fundamental processes in QuantumDot Solids, after light absorption. Transient Absorption and Timeresolved Photoluminescence spectrocopy reveal the dynamics of charge transfer and charge trapping processes. Typically, both occur on a picosecond time scale and compete with each other. We find that the efficiency of these processes depends on the Fermi level in the QuantumDot Solid. The latter can be controlled electrochemically, via charge injection into the QuantumDot Solid, using a potentiostat. The presented findings aid the rational design of optoelectronic devices based on Quantum Dots, such as solar cells or LEDs.

[PDF]
[Abstract]

13 

Photoconductivity of Quantum Dot Films Towards ThirdGeneration Solar Cells
Colloidal semiconductor nanoparticles, also called quantum dots, have unique optoelectronic properties that make them promising candidates for many applications such as solar cells, lightâ€“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. Quantumdotbased optoelectronic 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 (12 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.

[PDF]
[Abstract]
