1 

Towards Fast Light at the Single Photon Level

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2 

Pulse optimization for multiqubit gates in transmon system
In transmon qubits, the leading source of errors for quantum gates is the existence of higher energy levels, besides the 0> and 1> states which form the computational subspace. Several methods have been developed to eliminate these errors systematically by clever use of the available experimental controls. In this bachelor thesis we focus on performing quantum gates on multiple qubits simultaneously, when transition frequencies of different qubits are close to each other. The existing solution is designed to perform a quantum gate on one qubit, while eliminating all the effects on the other one. We develop a procedure to create new analytic pulse shapes which produce lowerror gates for single qubits and we generalize this approach to multiqubit systems to apply multiple quantum gates at the same time. For both single and twoqubit gates these new pulses reduce errors by several orders of magnitude compared to simple driving pulses. In addition we combine these optimal analytical pulse shapes with multiparameter pulses. The parameters of this additional pulse are optimized numerically, to produce even lower gate errors.

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3 

Electrical actuation and frequency tuning of 2D mechanical resonators
The electrical actuation of suspended membrane nanomechanical resonators incorporated into a laser interferometer displacement setup was investigated. Graphene and MoS2 membranes have been used to show electrical actuation of both metals and semiconductors. The method to fabricate devices was optimized and the properties of the fabricated devices are documented in order to make future device farication easier when speciﬁc properties are required. The ﬁrst experiment performed with the electrical actuation was done in order to investigate the frequency tuning the of the 2D resonators. Resonance frequencies in the range of 1555 MHz are observed without frequency tuning, where variations are due to differences in diameter and thickness of the suspended drums and the builtin tension. Applying a DC voltage caused both a decrease and an increase in the resonance frequency, depending on the device and the magnitude of the voltage. A maximum (increasing) frequency tuning sensitivity of 7.62 MHz/V was achieved. Furthermore, the model of interactions in 2D circular membrane due to electric actuation and DC gate voltage tuning in the linear resonator regime is compared to the measurement results. This comparison gives insight in the interaction between electrostatic spring softening and mechanically induced strain in membranes with different properties. The investigated model was insufﬁcient to explain the measured frequency changes due to the applied DC voltages.

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4 

A boolean QPQ using single photons
I propose an implementation of the quantum private query protocol
as described in an article using a photon to encode a question and reflectionor transmission of the photon as answer options. Each question is represented by a photon in a transmission line with both ends returning to the user, and the answer is represented by reflection or transmission of this photon caused by the single photon transistor as described in another article. By solving the quantum Langevin equations for the 32 × 32dimensional operators describing the single photon transistor the system is analysed. This analysis shows that the user privacy is maintained when the returning transmission lines are under the user's control. The probabilities for reflection and transmission are calculated to verify the behaviour of the answering mechanism. By using pulse trains instead of numbered lines to represent questions, the scalability of the system could be improved.

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5 

Electrodynamics of strongly disordered superconductors
Thin films of superconducting materials with a high resistance in the normal state, such as TiN, NbTiN, NbN and InO, are intensively studied from both an application and a fundamental point of view. A spatially inhomogeneous superconducting state can arise in these materials as a result of the strong disorder. This Thesis provides an experimental study of the evolution of the electromagnetic response with increasing disorder of superconducting thin films.
We primarily investigate a series of disordered TiN films. The films are grown by means of plasmaenhanced atomiclayer deposition. We probe the electromagnetic response using superconducting microwave resonators. We find a gradual evolution of the electromagnetic response with disorder, deviating from the MattisBardeen equations.
This result might be attributed to changes in the quasiparticle density of states (DoS) induced by the disorder. We can describe the measured microwave response with a heuristic model which contains a disorderdependent effective pair breaking parameter that modifies the DoS.
Further, we compare the assumed DoS—used to describe the electrodynamics—to local tunnel spectra obtained using scanning tunneling spectroscopy. We find affirmative results for the lowest disordered film. However, we find a strong discrepancy for the most disordered film. Moreover, this film displays large variations in the local tunnel spectra. This result signals the breakdown of a model that is based on average properties, due to the emergence of a spatial inhomogeneous superconducting state.

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6 

Quantum transport in graphene
After the experimental discovery of graphene a single atomic layer of graphite a scientific rush started to explore graphene’s electronic behaviour. Graphene is a fascinating twodimensional electronic system, because its electrons behave as relativistic particles. Moreover, it is a promising material for future highspeed nanoelectronic applications. In this thesis, several experiments are described to reveal graphene’s electronic transport properties. We have shown that we can control the bandstructure of bilayer and trilayer graphene. Simply by applying a perpendicular electric field in a graphene device, we could tune the bandgap in the bilayer and the bandoverlap in the trilayer. Furthermore, we have described transport measurements on graphene devices (length = 0.11 micrometer) showing that electronic transport in graphene is phase coherent at cryogenic temperatures (4 K or less). We have observed weak localization, bipolar supercurrents and the AharonovBohm effect. We have also shown that in narrow graphene nanoribbons (width less than 100 nm) a transport gap appears, which can be well explained by strong localization of electronic states. Our experimental results provide a better understanding of electronic transport in graphene, and are also a first step towards the realization of graphene nanoelectronic devices.

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7 

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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8 

Quantum Dots and Andreev Reflections in Graphene
Graphene is an exceptionally thin semiconductor that consists of only one atomic layer of carbon atoms. The electrons in graphene live in a strictly twodimensional (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 nanodevices. Overall, two topics are explored in this thesis. First we engineer an energy gap in graphene, which is naturally a zerogap 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.

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9 

Two dimensional photonic crystal devices

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10 

In situ Electrical measurements in Transmission Electron Microscopy
In the present thesis the combination of realtime electricalmeasurements on nanosampleswith simultaneous examination by transmission electron microscope (TEM) is discussed. Application of an electrical current may lead to changes in the samples thus the possibility to correlate such changes with the corresponding IV measurements is very important. Using the TEMalong with inhouse built sample holder and measurement setup, some important results were obtained. Firstly, current induced grain growth in polycrystalline Pt nanobridges (14 nm thick, 200 nm wide and 300 nm long) was investigated. Direct correlation was found between the evolution of the grain size and the change in the resistance. Secondly, the electromigration in Pt and Pd nanobridges was studied by in situ TEM technique. The material transfer during direct and reverse EM process in Pd bridges with different geometry was followed insitu using scanning TEM.
Further, the results of application of the Helium Ion Microscope (HIM) as a sculpting tool for nanoscaled samples are presented. We discuss the possibility to combine modification of the sample by the focused heliumion beam with local heating of the specimens. Heating is facilitated by using MEMS based heaters developed inhouse. The detailed analysis of the modified samples was carried out with FEI Titan transmission electron microscope (TEM) operated at 300 kV. With the proposed method it is also possible to carry out the electrical measurements on a wide range of materials such as metallic and semiconductor nanowires, nanobridges, nanopatricles and novel materials such as graphene.

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11 

The triggering probability of radioloud AGN: A comparison of high and low excitation radio galaxies in hosts of different colors

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12 

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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13 

Nonlinear beam mechanics
In this Thesis, nonlinear dynamics and nonlinear interactions are studied from a micromechanical point of view. Single and doubly clamped beams are used as model systems where nonlinearity plays an important role. The nonlinearity also gives rise to rich dynamic behavior with phenomena like bifurcations, stochastic switching and amplitudedependent resonance frequencies.
The theoretical background of micromechanical systems involving the relevant nonlinearities for beams clamped on one (cantilever) or two sides (clampedclamped beam) are discussed in chapter 2. First, the linear response of a mechanical resonator is discussed. Then, the linear equations are extended with nonlinear terms accounting for geometric and inertial effects. Specifically, the origin of the Duffing nonlinearity in the equation of motion of a clampedclamped beam is shown. It is shown that the nonlinearity couples the flexural vibration modes of a beam.
Microcantilevers are widely used in mass sensing and force microscopy. At small resonance amplitudes, cantilever motion is described by a harmonic oscillator model, while at high amplitudes, the motions is limited by nonlinearities. In chapter 3, the intrinsic mechanical nonlinearity in microcantilevers is studied. It is shown that although the origin is different, the nonlinearity resembles a Duffing nonlinearity resulting in hysteresis and bistable amplitudes. This bistability is then used to implement a mechanical memory.
The bistability of microcantilevers can also be used to study the switching characteristics when noise is applied. Chapter 4 shows the experimental implementation of this stochastic switching of microcantilever motion. It is shown that upon increasing the noise intensity, the switching rate rises exponentially as expected from Kramer's law. However, at higher noise intensities, the switching rate saturates and eventually even decreases, which suggests that the noise influences the dynamical parameters of the system.
In chapter 5, we investigate in detail the coupling between the flexural vibration modes of a clampedclamped beam. The coupling arises from the displacementinduced tension. A theoretical model based on the nonlinearity is developed, which is experimentally verified by driving two modes of the beam at high amplitudes and reading out their motion at the two frequencies. The experiments show that the resonance frequency of one flexural mode depends on the amplitude of another flexural mode and the theory is in excellent agreement with the experiments.
The nonlinearity not only couples the flexural modes in a clampedclamped beam, but we show in chapter 6 that also the cantilever modes are coupled. Here, the mechanism causing the nonlinearity is different, as there is no displacementinduced tension. The microcantilever is driven using a piezo actuator and its motion is read out using an optical setup. At high vibration amplitudes, the resonance frequency of one mode depends on the amplitude of the other modes. The torsional modes also show nonlinear behavior as evidenced by a frequency stiffening of the response.
The modal interactions in a microcantilever can also be used in a allmechanical analogue of a cavityoptomechanics, where one mode is used as a cavity mode, which influences the probe mode. In chapter 7, we show that by exciting at the sum and difference frequencies of the two modes, the $Q$ factor of the probe mode could be suppressed over a wide range.
In chapter 8, the interaction between a directly and parametericallydriven resonance mode is studied. Parametric driving means that the spring constant of the beams is modulated at twice the resonance frequency. Clampedclamped beams with an integrated piezoactuator on top, designed for applications as gas sensors, are used in the experiments. First, the parametric amplification and oscillation of the beam is studied, then the motion of a parametricallydriven mode is detected by a change in resonance frequency of the directlydriven mode. There is a linear dependence of the oscillation frequency of the parametricallydriven mode and the resonance frequency of the directlydriven mode. A potential application as a linear frequency converter is suggested.

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14 

Silicon Quantum Electronics

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15 

Superterahertz heterodyne spectrometer using a quantum cascade laser
Highresolution spectroscopy at superterahertz frequencies (26 THz) can play a vital role in astronomical observation and atmospheric remote sensing. It provides unique and important information on the history of our universe and its evolution, by getting into the insight of the physical and chemical conditions. Moreover, it can help to address questions about our own atmosphere such as ozone layer depletion and climate change problems. However, up to now this frequency region has rarely been accessible for highresolution spectroscopy due to the lack of suitable local oscillator technology. The recently developed terahertz quantum cascade lasers become the most promising candidate as a novel solidstate terahertz source.
Terahertz quantum cascade lasers, after one decade development from the first demonstration in 2002, are now capable of delivering milliwatts or more of continuouswave coherent radiation over the terahertz frequency range. For the local oscillator application, a heterodyne sensitivity measurement has been performed, which proved a prominent power stability with no additional inher ent noise for a terahertz quantum cascade laser. However, a finial and crucial step to demonstrate a heterodyne receiver system is a direct highresolution heterodyne spectroscopic experiment, which is also an important approach to characterize the performance of the entire system.
In this thesis, we have realized a highresolution heterodyne spectrometer by introducing a terahertz quantum cascade laser as a local oscillator, a super conducting hot electron bolometer as a mixer and a Fast Fourier Transform Spectrometer as a backend spectrometer. The first molecular spectrum by using a terahertz quantum cascade laser as local oscillator was obtained at a frequency of 2.9 THz. We push further this heterodyne spectrometer up to 3.5 THz with a more advanced terahertz quantum cascade laser, where the first 3.5 THz methanol (CH3OH) spectra were obtained with ∼1 GHz tuning range from the local oscillator. Excellent agreement between the measured spectra to the theoretical calculation was achieved with respect to both line intensity and frequency.
Furthermore, we have explored the frequency locking capability of such terahertz quantum cascade lasers by using a terahertz molecular absorption line as a reference frequency. Based on a compact gas cell and a power detector, the frequency stabilization is achieved with a minimal linewidth of 18 kHz and a Gaussianlike shape. Such kHz linewidth with a compact locking scheme is favorable for any space or ballonborne instrument.
For actual observation applications, the effective integration time is a cru cial issue that determines the efficiency of the observation. A robust exper imental scheme has been demonstrated to simultaneously stabilize the fre quency and amplitude of a terahertz quantum cascade laser. The frequency stabilization has been realized using a methanol absorption line, a power de tector and a proportionalintegralderivative loop. The amplitude stabilization of the incident power has been achieved using a swingarm voice coil actuator as a fast optical attenuator, and using the direct detection output of a super conducting mixer in combination with a 2nd feedback loop. As a result, a fully stabilized heterodyne spectrometer at superterahertz freqeuencies was demon strated, with improved Allan Variance times, and also supported by measured heterodyne molecular spectra.
Based on all this work, terahertz quantum cascade lasers become techno logically much more mature and convincing to be used as local oscillator. A di rect outcome is a new NASA mission: Galactic/Xgalactic Ultra long duration balloon Spectroscopic Stratospheric THz Observatory (GUSSTO), in which terahertz quantum cascade lasers have been proposed as local oscillators for the 4.7 THz receiver channel. Within the PhaseAConcept study period, in collaboration with Q. Hu’s group at MIT and C. Walker’s group at University of Arizona, we demonstrated a heterodyne receiver using an advanced third order distributed feedback quantum cascade laser as a local oscillator, whose emission frequency is only a few GHz away from the OI line at 4.7448 THz. Excellent receiver sensitivity together with a heterodyne spectrum have been demonstrated. All these efforts should lead soon to the first realization of a terahertz quantum cascade laser for astronomical application in a telescope. Also the local oscillator technology described in this thesis, offers the technique for other instruments such as Oxygen Heterodyne Camera (OCAM) proposed on SOFIA and also creates new mission opportunities in the future.

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16 

Stimulated terahertz stokes emission of silicon crystals doped with antimony donors
Article / letter to the editor 
Applied Sciences
2006

Author: 
Pavlov, S.G.
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Hubers, H.W.
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Hovenier, J.N.
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Klaassen, T.O.
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Carder, D.A.
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Phillips, P.J.
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Redlich, B.
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Riemann, H.
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Zhukavin, R.Kh.
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Shastin, V.N.


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17 

Using a quantum dot as a highfrequency shot noise detector

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18 

Antenna model for wire lasers
Article / letter to the editor 
Applied Sciences
2006

Author: 
Orlova, E.E.
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Hovenier, J.N.
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Klaassen, T.O.
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Kasalynas, I
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Adam, A.J.L.
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Gao, J.R.
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Klapwijk, T.M.
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Williams, B.S.
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Kumar, S.
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Hu, Q.
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Reno, J.L.


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19 

Shotnoise detection in a carbon nanotube quantum dot

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20 

Quantum Pumping and Adiabatic Transport in Nanostructures
This thesis consists of a theoretical exploration of quantum transport phenomena and quantum dynamics in nanostructures. Specifically, we investigate adiabatic quantum pumping of charge in several novel types of nanostructures involving open quantum dots or graphene. For a bilayer of graphene we find that at the Dirac point and for a wide bilayer the pumped current scales linearly with the sample length when this length is much smaller than the interlayer coupling length, exhibits a maximum when both of these length scales are comparable, and crosses over to a logarithmic dependence if the sample length is much larger than the interlayer coupling length. This behavior is markedly different from the behavior of the conductance in a graphene bilayer.
Futher we study possibilities for adiabatic evolution and computing in an extended version of the quantum Ising model, which includes beyondnearest neighbour interactions and an additional sitedependent longitudinal magnetic field. We calculate the energy spectrum of this model, treating the interactions exactly and using perturbation theory in the longitudinal field and find that the presence of nextnearestneighbour interactions enhances the minimal energy gap between the ground state and the first excited state.

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