1,720,978 research outputs found
Confinement of charge carriers in bilayer graphene
No full textIn this thesis we investigate the fundamental properties of electronic transport in bilayer graphene. We do this by confining electrons to narrow constrictions and small islands. Our key result is the fabrication and measurement of nanoscale devices that permit confinement with electric fields in bilayer graphene for the first time on a substrate. We observed the quantum mechanical nature of electrical transport in a narrow constriction. Moreover we measured single electrons tunnelling onto a small island in the bilayer graphene. The platform we developed paves the way for further investigations into the fundamentals of electronic transport in (bilayer) graphene.Quantum NanoscienceApplied Science
Coherent Coupling of Qubits in Small Quantum Dot Arrays
No full textThe coherent coupling of electron charges and spins is investigated in small quantum dot arrays.TNWApplied Science
Quantum Dots and Andreev Reflections in Graphene
No full textGraphene is an exceptionally thin semiconductor that consists of only one atomic layer of carbon atoms. The electrons in graphene live in a strictly two-dimensional (2D) world. In addition to this remarkable 2Dness, it is also peculiar that the behavior of the electrons in graphene is governed by the Dirac equation rather than the well known Schrödinger’s equation, leading to the discovery of several new physics phenomena. Such unusual properties of graphene have stirred up great excitements since it was first isolated in the lab about five years ago. In this thesis, we investigate the low temperature transport properties of the electrons and holes in several graphene based nano-devices. Overall, two topics are explored in this thesis. First we engineer an energy gap in graphene, which is naturally a zero-gap semiconductor, and further form quantum dot devices on the gapped graphene. The low temperature electronic transport properties of the confined electrons are then studied experimentally in such graphene dots. In a second project,we fabricated Josephson junction devices on graphene using a high critical field superconductor as leads. Here the goal is to research on the interactions between the electrons from graphene and the Cooper pairs from the superconductor in the quantum Hall regime.Kavli Institute of Nanoscience DelftApplied Science
Quantum Computation and Simulation - Spins Inside
No full textQuantum computation has captivated the minds of many for almost two decades. For much of that time, it was seen mostly as an extremely interesting scientific problem. In the last few years, we have entered a new phase as the belief has grown that a large-scale quantum computer can actually be built. Quantum bits encoded in the spin state of individual electrons in silicon quantum dot arrays, have emerged as a highly promising direction [1]. In this talk, I will present our vision of a large-scale spin-based quantum processor, and ongoing work to realize this vision.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.QN/Vandersypen La
Electrical Control, Read-out and Initialization of Single Electron Spins
No full textAn electron, in addition to its electric charge, possesses a small magnetic moment, called spin. The spin of an electron can point parallel (spin-up) or antiparallel (spin-down) 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: read-out, 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 single-shot read-out of two electron spins in our double quantum dot. The capability to measure the quantum state of multiple qubits individually and in a single-shot manner is essential for efficient characterization of quantum information protocols. Additionally, in a quantum computer, the result of computation needs to be read-out. The presented read-out method is all-electrical and the cross talk between two measurements is negligible. The read-out fidelities are about 86\% on average. This allows us to directly probe the anticorrelations between two spins prepared in a singlet state, an entangled two-spin state. The independent single-shot read-out of two electron spins also enabled us to fully characterize the operation of the two-qubit exchange gate, an important operation in a spin-based 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 single-shot read-out with electrical manipulation of single electrons. Manipulation of single electrons can be done using so-called 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 spin-orbit 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 spin-orbit mediated and hyperfine-mediated 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 spin-orbit 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 single-shot read-out with fast initialization of single electron spins. The capability of fast qubit initialization to a well-known 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 single-spin 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, spin-orbit and hyperfine interactions efficiently mix the spin and orbital excited states and spin-conserving orbital relaxation syphons the entire population to the ground state, thus achieving s-scale initialization. In the last part of the thesis, all-electrical independent addressing of a single-electron spins is presented. In those measurements we perform single electron manipulation using EDSR. Surprisingly, we observe well-separated 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 micro-fabricated magnets. The observed splitting also accounts for the deviation of the two-qubit 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 spin-based quantum computer.Kavli Institute of NanoscienceApplied Science
Graphene Nanodevices
No full textThis thesis describes a divergent set of experiments on graphene, a one-atom thin sheet of carbon. We employ graphene’s unique properties to explore fundamental physics and novel applications. This is done by nano fabricating graphene to nanodevices, which are subject to experiments. Here we first developed a water-based transfer method. The electronic quality of wet-transferred graphene does not significantly degrade, despite the presence of wrinkling in graphene. This transfer method allowed us to realise the first graphene nanopore device. With that device we were able detect single DNA molecules passing the nanopore. This is the first step towards a proto-type DNA sequencing device. Electronic transport from graphene in a superconductor is carried by Andreev reflection, notable by a doubling of the conductance. In disordered graphene/NbTiN junctions we have observed more than a doubling of the conductance, which can be understood by an enhancement of Andreev reflection mediated by disorder. We have used the tip of an atomic force micrscope to mechanically clean a graphene surface. The electronic quality improved after this cleaning treatments. This new method is particularly relevant when other methods are not effective or undesirable. For the first time we have observed ballistic transport in chemical vapour deposited (CVD) graphene on micron length scales. To realise this we have adapted a dry and clean transfer method to transfer CVD graphene onto hBN flakes. CVD graphene has the advantage that it is scalable and controllable, hence applicable for industrial purposes. While exfoliated graphene is widely used as a platform for fundamental research, CVD graphene may soon become an attractive alternative.Quantum NanoscienceApplied Science
Electrical manipulation and detection of single electron spins in quantum dots
No full textKavli Institute of Nanoscience DelftApplied Science
Manipulation and Read-out of Spins in Quantum Dots
No full textBesides an electric charge, electrons also have a tiny magnetic moment, called spin. In a magnetic field, the spin has two possible orientations: 'spin-up' (parallel to the field) and 'spin-down' (anti-parallel to the field) and can therefore be used as a quantum bit, the computational unit of a quantum computer. For quantum computations, quantum bits must have long relaxation and coherence times. Furthermore, one needs to be able to manipulate and read out the quantum bits. The research in this thesis aims at developing a solid-state quantum bit using electron spins, confined in quantum dots. We perform single-shot read-out of electron spin states and observe long relaxation times. These measurements also reveal that lattice vibrations (phonons) play a dominant role in the spin relaxation process. To increase the spin read-out fidelity we introduce a novel approach to ultrafast charge detection: a high electron mobility transistor operated as a cryogenic pre-amplification stage. Another essential requirement is the ability to coherently manipulate the electron spin, which we achieve by generating an oscillating magnetic field close to the dot. We also use this electron spin resonance technique to indirectly control the surrounding nuclear spins. This electron-nuclear interaction might be used to prepare a nuclear spin environment where fluctuations are reduced.Applied Science
Over cowboys, uitvinders en kwantum verstrengeling
No full textIntreerede uitgesproken 20 maart 2009 ter gelegenheid van het ambt van Antoni van Leeuwenhoek hoogleraar aan de Technische Universiteit Delft, Faculteit Technische NatuurwetenschappenApplied Science
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