Slow propagation line-based superconducting devices for quantum technology

Doctoral thesis, 2016

In the field of circuit quantum electrodynamics (c-QED), the coherent interaction of two-level systems (TLSs) with photons, confined in a superconducting microwave resonator, opens up new possibilities for quantum computing experiments. This thesis contributes to the expansion of c-QED tool-box with slow propagation line-based solutions for ubiquitous techniques, such as cryogenic Near Field Scanning Microwave Microscopy (NSMM), Electron Spin Resonance (ESR) spectroscopy and Traveling Wave Parametric Amplifier (TWPA). For NSMM, novel compact superconducting fractal resonators have been developed to be directly integrated on a scanning probe. We report NSMM operation based on a microwave high-Q resonator populated with less than 10^3 photons and demonstrate a capacitive sensitivity of 0.38 aF/rtHz. The unique properties and the design flexibility of fractal resonators also boost their resiliency to strong magnetic fields for ESR studies. The reported high Q-factors above 10^5 in a magnetic field up to 0.4 T translate into ESR sensitivity of 5 10^5 spins/rtHz. Furthermore, we demonstrate the operation of a practical TWPA based on a slow propagation fractal line. We achieve per unit length gain of > 0.5 dB/cm and total gain of ~6 dB for a 10 cm long line. Due to a radically shortened line, the amplifier is less vulnerable to fabrication defects. Moreover, due to a successful impedance matching between the amplifier line and in/out terminals, the obtained gain vs frequency characteristic has only moderate ripples. To mitigate a common TWPA problem of coupling to parasitic ground plane resonances, we deploy a novel multilayer fabrication technology, which combines high and low kinetic inductance (KI) elements. Finally, we present an alternative implementation of a slow propagation line: a microstrip line with a thin film Atomic Layer Deposition (ALD) Al2O3 oxide. The resonator, based on a segment of a microstrip line, has a Q-factor on the order of 10^4 at single photon powers, reaching up to 10^5 at higher powers. As an additional functionality, we incorporate dc current control over KI so that the resonance frequency is tuned by 62 MHz range, which corresponds to a KI-related nonlinearity of 3%.