Browsing by Author "Peltonen, Joonas, Dr., Aalto University, Finland"
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- Applications of hybrid single-electron turnstiles: To current standards and beyond
School of Science | Doctoral dissertation (article-based)(2023) Marín Suárez, MarcoHybrid single-electron transistors formed of a normal-metal island and superconducting leads (SINIS) have been used for generating single-electron currents. These single-electron turnstiles (ST) are voltage biased while a periodical voltage modulation is applied to its gate electrode. Accuracy in current generation in this device is limited, however, investigation of the physics behind its operation has allowed applications beyond metrological purposes. For example, they serve as a probe of the concentration of superconducting excitations in its leads. In this thesis, these applications are used first by probing extraction of superconducting excitations generated in the leads of the device by its bare operation. These quasiparticles (QPs) are unpaired electrons in the superconducting condensate of Cooper-pairs. The extraction is done by voltage biased Josephson junctions which share one lead with the SINIS ST and have other with higher energy gap. A reduction of one order of magnitude in QP density is observed by using the deviation of the generated current as an accurate probe. Then, an extension of the SINIS ST applications is presented. By driving this device with a signal of frequency f, two QPs are created in the leads close to the energy gap Δ so that a power 2Δf is generated in total. This enables to envision the development of the SINIS ST into a standard for the unit of power. It is also shown that such power generation is possible even in the absence of net particle current at zero bias. Furthermore, an analysis about the ultimate possible accuracy of power generation in a simplified version of this device is presented in this thesis. It is seen that errors increase with increasing operation frequency, tunnel resistance, temperature and presence of sub-gap states. Additionally, it is shown that detection efficiency of QP energy can be >99% at typical cryogenic temperatures. Following this, it is shown that by injecting an extra modulation to the source electrode of the transistor, different driving trajectories can be drawn in its stability diagram. With this, a new driving method with twice the frequency applied to the drain-source bias compared to the one applied to the gate is proposed. By doing so, tunneling events occurring against the biased direction are suppressed. These tunneling events lower the current below the expected outcome. Accuracy of current generated by SINIS ST is increased by one order of magnitude using the new driving method. Furthermore, it is shown that a similar driving method can be used for generating single-electron currents at zero-average bias, which had not been investigated until now in SINIS STs. - Fast thermometry and energy relaxation measurements on metallic calorimeters
School of Science | Doctoral dissertation (article-based)(2020) Viisanen, KlaaraThe development of single photon detectors (SPD's) in the microwave regime is a fundamental challenge for obtaining a better understanding of the phenomena and components employed in quantum information technologies. Particularly, the field of quantum thermodynamics has until now been mainly theory oriented due to the lack of suitable detectors. In this thesis I present the first steps in developing a high fidelity SPD suitable for detecting itinerant microwave photons in a transmission line. The device is of mesoscopic size and directly integrable into superconducting quantum circuits. Out of the two main approaches in microwave SPD's, atom-like systems and thermal detectors, we pursue the latter. In a thermal detector, the energy of the photon is transferred into the absorber of the detector, causing an abrupt increase in its temperature. This leads to the destruction of the photon, which can be avoided with an atom-like detector. However, an advantage of the thermal approach is that a broadband microwave detector, operated in a continuous manner, can be realized. This is essential for several experiments for example in quantum thermodynamics. The absorber of our micro calorimeter is a normal metal nanowire. The electron gas in the wire is used for the photon absorption, whereas the phonons work as a heat sink. The absorber is connected to a superconducting electrode via a thin tunnel barrier and grounded through a direct normal metal-superconductor contact, which acts as a heat mirror. The temperature of the electron gas is probed via the differential conductance of a tunnel-junction. For realising a sensitive SPD, it is essential to minimize the heat capacity of the absorber. Thermal properties of small metal structures can deviate significantly from the properties of bulk materials due to the large surface-to-volume ratio. We have measured thermal relaxation in Cu and Ag thin-film nanowires at sub-kelvin temperatures and observed an anomalously long relaxation time in the Cu wires. In addition to a large specific heat, these results may also originate from the slow thermalisation in the granular structure observed in the evaporated Cu wires.