Superconductor–semiconductor hybrid devices are interesting not only for their known and potential applications but also for the associated novel physical processes. One such example is the proposal for the realization of Majorana zero-modes, which are robust against noise and have applications in quantum information processing. Although the Josephson effect is known for decades, the recent advances in the experimental technologies made it possible only recently to make highly tunable hybrid devices. In this thesis, we study the superconductor–normal-metal–superconductor Josephson junctions and propose new effects or analyze experimental findings. In a Josephson junction, it is difficult to determine whether the flowof supercurrent is ballistic or diffusive. We propose an hourglass-shaped Josephson junction geometry to probe the nature of transport. In this device, the measurement of a critical current as a function of an external magnetic field produces a clear signature of the ballistic supercurrent. In metal-based Josephson junctions, the supercurrent flows uniformly through the scattering region. In contrast, semiconductor-based Josephson junctions allow tunable supercurrent due to the tunable carrier density of the semiconductors. We model a bilayer graphene Josephson junction with a split-top and back gate in the presence of an applied magnetic field to analyze the experimental measurements. The opening of bandgap in bilayer graphene in the gated area by applying tunable electrostatic potential allows spatial manipulation of supercurrent. The magnetic field is then used to probe the supercurrent flow in the device. In general, an applied magnetic field strongly suppresses supercurrent in Josephson junctions because it randomizes the contribution of the individual states. However, we show that graphene Josephson junctions are special and avoid the suppression of critical current under an applied in-plane magnetic field. The critical current as a function of the Zeeman field has a plateau whose size depends on the junction detail. Finally, we study a Josephson junction coupled with a microwave transmission line resonator in collaboration with an experimental group. We model this system to analyze and explain an unexpected experimental result of the system. We show that the unexpected outcome of the experiment is due to the coupling of the higher modes of the transmission line resonator.@en

We study the resonant tunneling of ultraslow atoms through a system of high quality microwave cavities. We find that the phase tunneling time across the two coupled cavities exhibits more frequent resonances as compared to the single cavity interaction. The increased resonances are instrumental in the display of an alternate sub and superclassical character of the tunneling time along the momentum axis with increasing energies of the incident slow atoms. Here, the intercavity separation appears as an additional controlling parameter of the system that provides an efficient control of the superclassical behavior of the phase tunneling time. Further, we find that the phase time characteristics through two cavity system has the combined features of the tunneling through a double barrier and a double well arrangements.

@en

The Josephson effect is one of the most studied macroscopic quantum phenomena in condensed matter physics and has been an essential part of the quantum technologies development over the last decades. It is already used in many applications such as magnetometry, metrology, quantum computing, detectors or electronic refrigeration. However, developing devices in which the induced superconductivity can be monitored, both spatially and in its magnitude, remains a serious challenge. In this work, we have used local gates to control confinement, amplitude and density profile of the supercurrent induced in one-dimensional nanoscale constrictions, defined in bilayer graphene-hexagonal boron nitride van der Waals heterostructures. The combination of resistance gate maps, out-of-equilibrium transport, magnetic interferometry measurements, analytical and numerical modelling enables us to explore highly tunable superconducting weak links. Our study opens the path way to design more complex superconducting circuits based on this principle, such as electronic interferometers or transition-edge sensors.

@en

Superconducting electronic devices have reemerged as contenders for both classical and quantum computing due to their fast operation speeds, low dissipation, and long coherence times. An ultimate demonstration of coherence is lasing. We use one of the fundamental aspects of superconductivity, the ac Josephson effect, to demonstrate a laser made from a Josephson junction strongly coupled to a multimode superconducting cavity. A dc voltage bias applied across the junction provides a source of microwave photons, and the circuit's nonlinearity allows for efficient down-conversion of higher-order Josephson frequencies to the cavity's fundamental mode. The simple fabrication and operation allows for easy integration with a range of quantum devices, allowing for efficient on-chip generation of coherent microwave photons at low temperatures.

@en

We consider the resonant interaction of an ultracold two-level atom with an electromagneticfield insidea high-Qmicromaser cavity. In particular, we study the tunneling and traversal of ultracold atomsthrough vacuum-induced potentials for secant hyperbolic square and sinusoidal cavity mode functions.The phase time which may be considered as an appropriate measure of the time required for the atomsto cross the cavity, significantly modifies with the change of cavity mode profile. For example, switchingbetween the sub and superclassical behaviors in phase time can occur due to the mode function. Si-milarly, negative phase time appears for the transmission of the two-level atoms in both excited andground states for secant hyperbolic square mode function which is in contrast to the mesa mode case@en