This thesis discusses the digital signal processing involved in building a Vector Network Analyser for qubit readout. Existing VNAs are aggregated and used to construct a programme of requirements for this application. An architecture is constructed and explained, and the stages IQ decomposition and data reduction are analysed mathematically. The Discrete Fourier Transform is used to extract DC signals for this application and its properties are compared to different filters. Common digital logic functions such as AXI, Direct Digital Synthesis, and Direct Memory Access are explained, as well as the implementations of custom blocks for this application such as accumulators and a sequencer. These IP blocks are demonstrated individually and integrated to be implemented on a Red Pitaya STEMLab 125-14 board containing the Zynq 7010 SoC. The implementation is tested using simulated input signals and the resulting measurements are analysed. The implementation is found to have good absolute accuracy of within 2% of expected absolute amplitude, 1% of the expected relative amplitude and 8 mrad of expected relative phase. Modulation of the input signals is tested to work as expected and no major cross-modulation is found. Future improvements are identified and the limitations of the used data reduction are discussed in relation to a Vector Signal Analyser mode.

One way to tackle the rise in antibiotic resistance, is to develop new techniques for testing the susceptibility of cells to antibiotics. In this paper, a comparison is made between a novel bacterial susceptibility testing method and a modification on this method. Both methods rely on bacterial samples deposited on graphene cavities, where the bacteria will stick to the graphene by the addition of APTES. By making use of a 632.8 nm He-Ne laser, the sample is probed, and the cavities then serve as ultrasenstive sensors for determining bacterial nanomotion. The existing method (single spot readout) is based on focusing a laser on graphene drums, where the drums are read out one at a time by the use of a photodiode. The modified method (parallel readout) makes, by the addition of one lens, use of an expanded laser, and a CMOS camera. At 100 frames per second, four drums are read out simultaneously. This technique hypothetically makes very high throughput possible for antimicrobial testing.

Both methods rely on converting a signal based on the intensity of the incoming light to the membrane deflection in nanometer, and the bacterial motility is found by taking the variance. The comparison of the two methods is done by performing multiple experiments, in order to relate the quality of the signal by finding the standard deviation (noted as S) of the variance of the deflection σ_z² . 

From an analysis of S, statistical quantities describing the distances between probability distributions have been conceived, and a criterion is proposed to differentiate between the noise levels of the two techniques. One such quantity is the normalized distance between the signals of two types of experiments, the one being an experiment without bacteria as a reference and the other being an experiment with living bacteria.
In the case of hypermotile bacteria, parallel readout has an average variance of deflection of 5.95 nm², it is substantially higher than the method of single spot readout, having average 2.92 nm². The unitless metric D for the distance between two signals however shows that both methods score similarly in probing nonmotile (∆-MotAB) E. Coli, as for the parallel readout the measure has for ∆-MotAB a value 0.34 and for single spot readout it has the value 0.31. In the case of hypermotile (7740) E. Coli, parallel readout scores again better with a value of 2.35 versus 0.39, which is the value obtained for single spot readout.

Finally an outlook is given where interesting findings have been summarized. With the use of power spectral densities and heatmaps of either average intensity or variance of the signal, interesting phenomena are noted and are topics for future research.

One of the main components used in superconducting quantum chips is the Josephson junction. Currently when fabricating Josephson junctions, the resistance uniformity at wafer-scale is not optimal. It is also known that an annealing process can alter the junction resistance and with it the qubit frequency. However, laser annealing has only shown to be able to increase the junction resistance. An equally effective and minimally invasive technique to decrease junction resistance is necessary to have full control on frequency
targeting. Thermal annealing in a reducing environment is known to result in a global decrease in junction resistance. To get better control on frequency targeting the techniques could be combined. A die containing not-capped Manhattan type Josephson junctions has been annealed using forming gas at 200°C for 2 minutes, based on a non-linear least squares reciprocal function fit to the data an asymptotic lower bound of 467 μSμm^-2 for the change in conductance per unit area has been found. Smaller junctions with an area
of approximately 0.03 μm² undergo a bigger change in conductance per unit area of around 800 μSμm^-2. Annealing at higher temperatures such as 300 and 400°C results in a decrease of the conductance. There is no substantial change in yield of usable SQUIDs when using a rapid thermal annealing process.

Superconducting quantum circuits came out as promising candidates for the exploration of topological phenomena that are currently inaccessible in condensed
matter systems. One such circuit is a Cooper pair transistor which has already
been widely studied in different regimes of operation due to its importance in
quantum computation. However, it has only recently been appreciated that
a Cooper pair transistor hosts a non-trivial Chern number and topologically
protected current switching behavior. We provide here a more detailed analysis
of Cooper pair transistor operation for different parameter regimes and explore
the quantized ac current.

The defining challenge of connecting NISQ quantum computers over large distances is efficient microwave to optical transduction. In this work, we demonstrate a new platform for piezo-optomechanical conversion by combining a crystalline silicon nanobeam photonic crystal cavity and a suspended thin-film lithium niobate acoustic resonator. The goal is to combine the excellent optomechanical properties of the silicon nanobeam cavity with coherent excitations in a piezoelectric resonator in order to achieve large effective coupling between the microwave excitation and the optical cavity. In order to fabricate this hybrid silicon-on-lithium niobate device, we explore a technique termed ‘slapping’ where a loosely connected suspended nanostructure is patterned such that a tapered optical fiber can be used to rip it away and place it in an arbitrary location on another chip. We report a single photon optomechanical coupling rate g_0 = 5.1 kHz and a single photon microwave to optical efficiency of η_{μ→o} = 6.1 × 10−6 at 25μW of input optical power in a device limited by a suboptimal optical interface.

The quantum internet will allow for communication via qubits, enabling for improved clock synchronization, blind quantum computing and quantum key distribution. Key components of such a quantum network are quantum repeaters, which help to overcome the exponential loss of photons in optical fibers. In this thesis we focus on one type of quantum repeater based on atomic ensembles. In particular, we build a versatile and elaborate simulation model based on a discrete event simulator for quantum networks, capable of simulating quantum repeater chains of arbitrary length with a vast range of tunable simulation parameters. We validate this model by comparing it to an analytical one and investigate the effects of additional sources of noise that cannot be taken into account in the analytical model. Furthermore, we give a detailed theoretical overview and performance comparison of two types of quantum memories based on atomic ensembles: atomic frequency comb and electromagnetically induced transparency. Finally, we present two integer linear programming formulations for determining the optimal positioning of quantum repeaters in two spatial dimensions and demonstrate their use on a European-scale network.

Microwave Kinetic Inductance Detectors (MKIDs) are extremely sensitive radiation detectors based on superconducting resonators that can be combined in large arrays on a single readout line within a limited frequency bandwidth. This makes MKIDs ideal detectors for the ultimate far-infrared observatory: a future space-based actively cooled telescope with its performance solely limited by the low universe background radiation. However, to reach these detector requirements, state-of-the-art MKIDs still need a order of magnitude improvement in device sensitivity. In this work, the MKID sensitivity is improved by reducing the aluminium volume that absorbs pair-breaking radiation into quasiparticle excitations, while making sure all radiation is still absorbed. Furthermore, a key requirement is sufficient reduction of excess noise as to keep the device intrinsically limited by thermally driven random fluctuations in the number of quasiparticles in absence of radiation, or Generation-Recombination (G-R) noise. To this end, a model is developed that describes the noise contributions as function of device geometry, readout power, material properties and radiation power. Subsequently, a realistic MKID design is presented and tested that reduces excess noise and maximises the sensitivity, expressed as Noise Equivalent Power (NEP). At high temperatures, good overall agreement is found between the measured noise spectra and the model. At low temperature T = 120 mK, the measurement results give an optical NEP similar to current state-of-the-art MKIDs. The NEP is not as low as expected due to short quasiparticle lifetimes, an unexpected decrease in the G-R noise level and a very high excess noise attributed to Two-Level Systems (TLS) noise that starts to dominate the already low G-R noise spectrum at low temperatures. Possibly, the quick quasiparticle lifetime saturation and noise level drop are caused by a strong readout power effect, as the readout power is known to create excess quasiparticles and to cause a strongly non-thermal electron energy distribution in the aluminium strip of the MKID. However, the exact microscopic details of these effects are unknown and not studied in this project. Based on the current chip design, a straightforward way to improve device performance and study the readout power effect in more detail is a reduction of the high TLS noise levels, which is possibly fabrication related. This would allow an unobstructed view of the G-R noise spectrum at low temperatures, thereby allowing both a study of the readout power effect on the quasiparticle system, and ultimately achieving the improvement in NEP needed reach the detector requirements for the ultimate space-based far-infrared observatory.

A single photon interacting with a single atom is the most fundamental form of light interacting with matter and has been extensively studied in the field of Cavity Quantum Electrodynamics (cavity QED). Here, a non-linearity like an atom is coupled to a single mode of the electromagnetic field in a cavity. Another field which explores the quantum mechanical nature of photons is Circuit Quantum Electrodynamics (cQED) where photons are the quantized excitations of a superconducting microwave resonator and non-linearity is introduced by the Josephson junction. Like this, setups analog to that of in cavity QED can be copied to cQED, with a number of differences. For example, the photons propagating in a transmission line are more confined and the circuits are made with conventional lithography techniques, allowing for more freedom in engineering the system parameters.
In the first part of the thesis, we build a numerical model in order to examine the feasibility of quenching the ground state of a coplanar waveguide (CPW) interrupted by a tunable coupling element. Next, by means of experiments and simulations we considered the feasibility of observing experimentally a synchronization effect in a driven CPW with its central conductor interrupted by equally spaced capacitively shunted Josephson junctions (Josephson crystal) based on a recent proposal. Finally, we made a first step in understanding the
synchronization from a classical perspective by modelling a Josephson crystal of two junctions as two degenerate non-linearly coupled Duffng oscillators.
Concerning the quenching experiment, we found that the plasma frequency must be tuned faster than 1/f with f the resonance frequency of the CPW, which is in the sub-nanosecond regime and therefore unfeasible with current state the art electronics. We also found that, in contrast to what was claimed in the proposal, the synchronization effect cannot be observed for the parameters common in cQED. One way would be to push the limits of the capacitances to several picofarads. Finally, we found that the non-linear coupling causes the two degenerate non-linearly coupled Duffng oscillator to synchronize, which is a first step in understanding the proposed synchronization effect in a fully classical way.

Superconducting resonators used in mm/sub-mm (MMW) astronomy would greatly benefit from deposited dielectrics with low dielectric loss. The excess loss in deposited dielectrics is mainly due to two-level systems (TLS), and there is no consensus on their microscopic origin. To study the relation between hydrogenated amorphous silicon’s (a-Si:H) microwave (MW) loss at 120 mK and its void volume fraction, hydrogen content, microstructure parameter, bond-angle disorder, and infrared (IR) refractive index, we deposited films at substrate temperatures of 100°C, 250°C and 350°C using plasma-enhanced chemical vapor deposition (PECVD). We measured the room temperature properties of the films using Fourier-transform infrared spectroscopy, Raman spectroscopy and ellipsometry. All room temperature properties except the IR refractive index decrease monotonically with increasing substrate temperature. The IR refractive index approaches the refractive index of crystalline silicon (c-Si) when increasing the substrate temperature to 350 °C. We measured the dielectric losses using superconducting coplanar waveguide resonators. Interestingly, we do not see a correlation of the room temperature results with the MW losses. All films have an excellent 120 mK MW loss tangent below 1e−5 at −50 dBm internal resonator power. More research on the loss tangents is recommended, for example using microstrip lines or lumped element parallel plate capacitors. The low dielectric losses make these films promising for application in MW kinetic inductance detectors and on-chip filters. These promising results could lead to the application of the dielectrics in the integrated superconducting spectrometer DESHIMA 2.0.