Superconducting qubits require a low-noise, high-gain parametric amplifier to overcome the added noise during their readout. Josephson junction arrays are suitable candidates, as they have quantum-limited added noise, large saturation power and a tunable resonance frequency. Dimer Josephson Junction Amplifier Arrays (DJJAAs) have a mode structure, which is engineered in such a way that it exploits the wave mixing processes in JPAs when amplifying incoming signals. The resonant modes in this device form pairs, referred to as dimers. The behaviour of two resonant modes in a DJJAA when a drive tone is applied at an intermediate frequency, is analyzed. A maximum gainGmax = 43.6 dB and saturation power P1dB = 105.9 dBm is measured. It is demonstrated that the mode resonance frequency shifts if the mode photon population changes. If the drive power was increased above a threshold, self-oscillations were observed in this device. A fabrication process for DJJAAs using overlap Josephson junctions was carried out iteratively. Several issues in the fabrication process were resolved. In the fabricated DJJAAs it was possible to identify and drive resonant modes for gain G > 20 dB. The resonant modes in a fabricated device did not form dimers due to offset in the Josephson inductance from its Josephson junctions compared to the design. Future efforts can improve the quality of fabricated overlap Josephson junctions and on studying nonlinear effects that were observed in the device.

Spin qubit in semiconductor quantum dot arrays offers a promising platform for future scalable quantum computing with its small size and compatibility with modern semiconductor industry. To scale up the quantum dot arrays, one of the major challenges is the wiring bottleneck, as a high density of control lines might need to be integrated into a small chip. A proposal to solve this problem is using the shared control protocol, in which multiple qubits could be controlled by a shared line. For this, the most critical requirement is to realize uniformity across the quantum dot array, such that a single control signal could lead to an identical response in all dots involved. However, such uniformity is hard to achieve due to the variation of the device fabrication, and tackling this problem via materials and fabrication optimization only appears to be a daunting challenge.

In this thesis, we propose a potential solution to achieve uniformity of threshold voltage in such share-controlled systems. This solution is based on the hysteresis behavior of turn-on voltage in the heterostructure field-effect transistor (HFET) devices hosting the quantum dot array. In the Ge/SiGe HFET devices with hole as carrier, we found that the drift of turn-on voltage can be caused by population of 2DHG under negative gate voltage and reversed by applying positive gate voltage. We attribute this effect to trapping and detrapping processes on the dielectric surface of the device. Following this discovery, an automatic feedback control program was designed, in which gate voltage pulses are applied to control the trap filling level such that potential landscape in the device corresponds to the desired turn-on voltage. Using this program, we performed deeper investigations of the turn-on voltage shift including its relaxation and history-dependent stability. A hypothetical physical model for observations in these experiments is followed. For practical application of this effect, the feasibility to locally define and control the turn-on voltage is also demonstrated. Based on these results, we present a proposal for addressable manipulation of potential landscape in share-controlled quantum dot array, which might potentially realize the threshold voltage uniformity for scalable quantum dot array in the future.

Generation of high fidelity entanglement between quantum nodes is a key component of a future quantum internet. Heralded entanglement generation of two spatially separated qubit nodes can be established by interference and measurement of two photons, each entangled with one qubit state. The two-node entanglement fidelity is limited by the degree of indistinguishability of the photons, which can be measured in a Two-Photon Quantum Interference (TPQI) experiment. In this thesis, a TPQI experiment has been performed with photons emitted by a single Nitrogen-Vacancy (NV) center. This self-interference experiment shows a visibility of V=0.91±0.02 and a photon indistinguishability of J=0.945 in the Monte-Carlo method obtained 1σ-confidence interval of [0.920, 0.966] after correction for system imperfections, demonstrating near-perfect indistinguishability of zero-phonon line photons emitted by a single NV center. Furthermore, an extensive TPQI model was developed that includes possible arrival time- and frequency differences of the photons. This model predicts a dark- and noise count limited V=0.79±0.06 for a future two-node NV TPQI experiment with quantum frequency-converted photons, at a distinguishable photon coincidence rate of 1.2mHz, allowing for an experimentally feasible double-click two-node entanglement fidelity of 0.89±0.03.