The current through a Josephson junction is governed by the current-phase relation (CPR) that depends on the phase difference between the electrodes. Notable applications are qubits, Josephson...Show moreThe current through a Josephson junction is governed by the current-phase relation (CPR) that depends on the phase difference between the electrodes. Notable applications are qubits, Josephson diodes and microscopic imaging techniques. This thesis presents a method to measure the CPR based on [1]. The junction under study is incorporated into a superconducting loop that is inductively coupled to a dc-SQUID magnetometer. The measured flux is proportional to the junction’s phase and by controlling the current through the junction’s loop it is possible to directly measure the CPR. This thesis outlines several important considerations and constraints of this method. Furthermore we provide CPR measurements of a superconductor-normal- superconductor (SNS) junction made from Nb and Cu. It shows a clear temperature dependence with a qualitative change in shape as well as a quantitative change in amplitude of the current-phase relation. These results are in agreement with theory. In the future a flux-locked loop can be used to further improve the measurements.Show less
Studying the position dependent absorption efficiency of superconducting single photon detectors (SSPD) is essential for understanding its detection process. We set out to assemble a scattering...Show moreStudying the position dependent absorption efficiency of superconducting single photon detectors (SSPD) is essential for understanding its detection process. We set out to assemble a scattering-scanning near field optical microscope (s-SNOM) capable of probing photons at sub 30 nm resolution. Tuning forks with glued tip are used in the aim of building a low temperature AFM. Temperature and pressure dependence of the tuning fork’s resonance frequency and quality factor have been studied. Temperature fluctuations during AFM scanning have been shown to cause height deviations of up to 650 mm/K at atmospheric pressure depending on the tip. AFM scanning has been successful at imaging a calibration sample with a height difference of 200 nm and locating 50 nm high meandering nanowire features of a MoGe SSPD.Show less
We measure the temperature dependent resistance of a niobium nitride (NbN) from room temperature to 4 K. The increasing resistance with decreasing temperature can be explained by tunneling of...Show moreWe measure the temperature dependent resistance of a niobium nitride (NbN) from room temperature to 4 K. The increasing resistance with decreasing temperature can be explained by tunneling of electrons between grain boundaries. Once the detector is in the superconducting regime single photon counts can be registered. We find an optimal setting of the trigger level of 0.2 V to register detection events while minimizing the influence of amplifier noise. From the measured voltage pulses we estimate a kinetic inductance of 200 nH for our devices. We explore the regime of high photon energies by plotting the count rate vs optical power on a double logarithmic scale. For photons with 500 nm wavelength the highest initial slope is equal to 2.6, indicating that detector tomography with 3 photon events realistic. Unfortunately, higher slopes are not observed and makes looking into detection events with more than 10 eV total energy difficult if not impossibleShow less
We study superconductivity and photon detection in a NbN nanobridge of 150 nm width and 4 nm thickness. Superconductivity is observed below Tc = 8.4K. The measured critical current as a function of...Show moreWe study superconductivity and photon detection in a NbN nanobridge of 150 nm width and 4 nm thickness. Superconductivity is observed below Tc = 8.4K. The measured critical current as a function of temperature shows a slow decrease up to 7.9 K and more rapid decrease between 7.9 K and 8.4K. Light counts are only observed below T = 8.4K and dark counts below 7.85K. We characterize the detector by measuring the count rate as a function of polarization and average power to explore a multi-photon polarization dependence. We find a large difference in the response curves of the detector that cannot be explained by a change in the absorption efficiency h. Unfortunately, at the moment of writing the origin of the effect is inconclusive and is at least partly caused by a shift in the position of the focused laser beam on the sample when rotating a l/2 plate to change the polarizationShow less
We investigate photon counting properties in an NbN superconducting nanodetector, and succesfully do quantum detector tomography. We find qualitative similarities to an earlier tomography result on...Show moreWe investigate photon counting properties in an NbN superconducting nanodetector, and succesfully do quantum detector tomography. We find qualitative similarities to an earlier tomography result on the same detector, although a larger power range may eliminate anomalies in the numerical fit. Furthermore, we investigate the resistive properties of a MoSi superconducting nanodetector, observing finite resistance below the critical temperature. Measuring current as a function of voltage, we find indications that indeed imply a finite resistance below Tc. Obtaining the critical current for different temperatures and applying a Ginzburg-Landau fit, we find a lower critical temperature than obtained from measurements of resistance as a function of temperature. Finally, we observe that photon counts for MoSi are not possible above T > 3.85 K. We calculate that electronic noise is responsible for this absence of photon counts.Show less
