Entwicklung magnetischer Mikrokalorimeter für die hochauflösende Spektroskopie des beta-Emitters 36Cl

Im Rahmen der vorliegenden Arbeit wurde ein Detektorprototyp für ener- giedispersive Messungen von -Spektren mit magnetischen Kalorimetern entwickelt und an einer 36Cl-Quelle getestet. Beim Zerfall des Isotops ent- stehen hochenergetische Elektronen mit einer Energie bis zu 709,6 keV, die im Experiment in einem 4pi-Gold-Absorber gestoppt wurden. Dieser Ener- gieeintrag kann in Form einer Temperaturänderung und der damit ver- bundenen Magnetisierungsänderung des paramagnetischen Temperatur- sensors durch ein sensitives DC-SQUID-Magnetometer nachgewiesen wer- den. Für den 36Cl-Detektorprototyp wurde zur Messung der Magnetisierung eine flache, mäanderförmige Spulengeometrie gewählt, deren Abmessun- gen für das spezielle Experiment optimiert wurden. Die dafür nötigen su- praleitenden Strukturen konnten im Reinraum des Institutes durch den ei- gens dafür entwickelten 4-Lagen-Prozess in Dünnschichttechnik hergestellt werden. In der Arbeit wird der Detektor beschrieben, sowie dessen Quan- tenausbeute, Rauschquellen und die erreichbare Energieauflösung disku- tiert. Das damit gewonnene 36 Cl-Spektrum mit einer Energieauflösung von EFWHM = 750 eV wird vorgestellt, und mit existierenden experimentellen und theoretischen Daten verglichen

Transmon Qubit in a Magnetic Field: Evolution of Coherence and Transition Frequency

We report on spectroscopic and time-domain measurements on a fixed-frequency concentric transmon qubit in an applied in-plane magnetic field to explore its limits of magnetic field compatibility. We demonstrate quantum coherence of the qubit up to field values of $B={40}\,\mathrm{mT}$, even without an optimized chip design or material combination of the qubit. The dephasing rate $\Gamma_\varphi$ is shown to be not affected by the magnetic field in a broad range of the qubit transition frequency. For the evolution of the qubit transition frequency, we find the unintended second junction created in the shadow angle evaporation process to be non-negligible and deduce an analytic formula for the field-dependent qubit energies. We discuss the relevant field-dependent loss channels, which can not be distinguished by our measurements, inviting further theoretical and experimental investigation. Using well-known and well-studied standard components of the superconducting quantum architecture, we are able to reach a field regime relevant for quantum sensing and hybrid applications of magnetic spins and spin systems.Comment: 9 pages, 8 figure

Analog quantum simulation of the Rabi model in the ultra-strong coupling regime

The quantum Rabi model describes the fundamental mechanism of light-matter interaction. It consists of a two-level atom or qubit coupled to a quantized harmonic mode via a transversal interaction. In the weak coupling regime, it reduces to the well-known Jaynes-Cummings model by applying a rotating wave approximation (RWA). The RWA breaks down in the ultra-strong coupling (USC) regime, where the effective coupling strength $g$ is comparable to the energy $\omega$ of the bosonic mode, and remarkable features in the system dynamics are revealed. We demonstrate an analog quantum simulation of an effective quantum Rabi model in the USC regime, achieving a relative coupling ratio of $g/\omega \sim 0.6$. The quantum hardware of the simulator is a superconducting circuit embedded in a cQED setup. We observe fast and periodic quantum state collapses and revivals of the initial qubit state, being the most distinct signature of the synthesized model.Comment: 20 pages, 13 figure

Random telegraph fluctuations in granular microwave resonators

Microwave circuit electrodynamics of disordered superconductors is a very active research topic spawning a wide range of experiments and applications. For compact superconducting circuit elements, the transition to an insulating state poses a limit to the maximum attainable kinetic inductance. It is therefore vital to study the fundamental noise properties of thin films close to this transition, particularly in situations where a good coherence and temporal stability is required. In this paper, we present measurements on superconducting granular aluminum microwave resonators with high normal state resistances, where the influence of the superconductor to insulator phase transition is visible. We trace fluctuations of the fundamental resonance frequency and observe, in addition to a 1/f noise pattern, a distinct excess noise, reminiscent of a random telegraph signal. The excess noise shows a strong dependency on the resistivity of the films as well as the sample temperature, but not on the applied microwave power.Comment: 6 pages, 4 figure

Observation of giant two-level systems in a granular superconductor

Disordered thin films are a common choice of material for superconducting, high impedance circuits used in quantum information or particle detector physics. A wide selection of materials with different levels of granularity are available, but, despite low microwave losses being reported for some, the high degree of disorder always implies the presence of intrinsic defects. Prominently, quantum circuits are prone to interact with two-level systems (TLS), typically originating from solid state defects in the dielectric parts of the circuit, like surface oxides or tunneling barriers. We present an experimental investigation of TLS in granular aluminum thin films under applied mechanical strain and electric fields. The analysis reveals a class of strongly coupled TLS having electric dipole moments up to 30 eA, an order of magnitude larger than dipole moments commonly reported for solid state defects. Notably, these large dipole moments appear more often in films with a higher resistivity. Our observations shed new light on granular superconductors and may have implications for their usage as a quantum circuit material.Comment: 12 pages, 8 figure