Advanced Seminar on Condensed Matter Physics
Wintersemester 2019/2020URL to ICS calendar of this seminar
KIP SR 01.404
The nuclear first excited state in Th-229 (called Th-229m) has the lowest excitation energy of all known nuclear states.
The energy of only 7.8(5) eV and its lifetime (in the range of minutes to hours) make it a promising candidate for a nuclear optical clock. The large uncertainty in the excitation energy, however, currently impedes further progress. Therefore the objective of our experiment is a precise determination of the Th-229m excitation energy via internal conversion electron spectroscopy.
The concept and possible applications of a nuclear optical clock as well as a first direct energy measurement will be presented.
The axion is an hypothetical beyond the Standard Model particle, first introduced in the seventies as a consequence of the strong CP problem of QCD. Axions can be the main constituents of the galactic Dark Matter halos. Their experimental search can be carried out with Earth-based instruments immersed in the Milky Way's halo, which are therefore called ``haloscopes''. Nowadays haloscopes rely on the inverse Primakoff effect to detect axion-induced excesses photons in a microwave cavity under a static magnetic field.
A ferromagnetic axion haloscope does not exploit the axion-to-photon conversion but its interaction with the electron spin. It consists in an axionic-to-electromagnetic signal transducer, which is then measured by a suitable rf detector. The transducer is an hybrid system formed by a magnetic material coupled to a microwave cavity through a static magnetic field, while the detector is a quantum-limited Josephson parametric amplifier. As it measures variation in the magnetization of a sample, the ferromagnetic haloscope is configured as a spin-magnetometer.
We report on the design and operation of such haloscope. Our prototype reached the sensitivity limit imposed by quantum mechanics, the Standard Quantum Limit, and can be improved only by quantum technologies like single photon counters.
Superconducting quantum information processing machines are predominantly based on microwave circuits with relatively low characteristic impedance, about 100 Ohm, and small anharmonicity, which can limit their coherence and logic gate fidelity. A promising alternative are circuits based on so-called superinductors, with characteristic impedances exceeding the resistance quantum R_Q = 6.4 kOhm. However, previous implementations of superinductors, consisting of mesoscopic Josephson junction arrays, can introduce unintended nonlinearity or parasitic resonant modes in the qubit vicinity, degrading its coherence.
I will present a fluxonium qubit design based on a granular aluminum (grAl) superinductor strip . I will argue that granular aluminium forms a compact effective junction array with high kinetic inductance and low nonlinearity , and it can be in-situ integrated with standard aluminum circuit processing. The measured qubit coherence time T_2^* = 30 µs illustrates the potential of grAl for applications ranging from protected qubit designs to quantum limited amplifiers and detectors, even though quasiparticle poisoning is a limiting factor  and should be addressed in future works.
 Grunhaupt, Spiecker et al. Nature Materials 18, 816-819 (2019)
 Maleeva et al. Nature Comm. 9, 3889 (2018)
 Grunhaupt et al., Phys. Rev. Lett. 121, 117001 (2018)
The goal of the Karlsruhe Tritium Neutrino (KATRIN) experiment is to measure the effective electron anti-neutrino mass with a sensitivity of 200 meV/c2 (90 % C.L.), by investigating the energy spectrum of tritium beta-decay electrons close to the endpoint. The first neutrino mass measurement campaign took place in spring 2019. With this first dataset an upper limit on the neutrino mass of 1.1 eV (90 % C.L.) was obtained, improving upon previous limits by nearly a factor of two. In this talk, the operating principle and the first result of the KATRIN experiment are presented. A special focus will be on background investigations and their relevance in view of this first measurement.
Lithographically fabricated nanowires are a promising alternative to Josephson tunnel junctions as a nonlinear element in superconducting quantum circuits. They potentially offer low intrinsic loss, a high impedance, and simple fabrication. In the superconducting regime, a key requirement for these wires is a high kinetic inductance stemming from a low normal state conductance. We study single high ohmic nanowires made from disordered oxidised aluminium with a new technique, which allows us to alter the the wire resistance in situ by two orders of magnitude. We present low temperature coherence measurements as well as transport properties. Here we observe the insulating, metallic and superconducting regime.
Superconducting circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the coherence of superconducting quantum bits is severely reduced by atomic-scale material defects which provide a bath of parasitic two-state tunnelling systems, so-called TLS.
We review experiments where a qubit is operated as a sensitive detector to probe the quantum properties of individual TLS defects while they are tuned via applied mechanical strain and electric fields. This provides expressive spectroscopic data revealing coherent TLS interactions and the fluctuation dynamics of thermally activated TLS at low energies.
A new method is presented to determine whether a given defects resides within the tunnel barrier of a qubit’s Josephson junctions or on an interface of the qubit electrodes. Using spatially tailored electric fields, we can also distinguish on which thin-film circuit interface a defect is located. These techniques yield valuable information to guide improvements in sample fabrication that are urgently needed to obtain higher coherence in qubits and other micro-fabricated quantum devices by learning how to avoid the formation of material defects.