Title: Harnessing Unconventional and Multiple Parametric Couplings for Superconducting Quantum Information
Abstract: Josephson-junction based parametric amplifiers have become a ubiquitous component in superconducting quantum machines. Although parametric amplifiers regularly achieve near-quantum limited performance, they have many limitations, including low saturation power, lack of directionality, and narrow bandwidth. The primary aim of this thesis is to attack these problems by using multiparametric drives to enhance the good features of the amplifier and mitigate the bad through Hamiltonian engineering. We will discuss an assortment of multiparametric pumping schemes, and in particular, present an amplifying scheme that is created by applying imbalanced, simultaneous, strong gain and conversion pump tones in a Josephson Parametric Converter. This mode of amplification features broadband gain in transmission that is 10X the bandwidth of a typical, single-pumped JPC amplifier and is matched in reflection up to -10 dB, making it only one scattering parameter away from the ideal quantum-limited amplifier. Surprisingly, this mode of amplification also has the same saturation power as the single-pumped mode, which doesn't match any presently understood theories. Since this amplifier is matched in reflection, we also explore the possibility of being able to chain these amplifiers in series to enhance the saturation power and work towards a fully directional amplifier.
In addition, we will discuss a project focused on using superconducting, thin-film Van der Waals materials for use in superconducting circuits. We began by studying the nature of the contact made between 3D bulk aluminum and these 2D flakes, and discovered we could create truly superconducting contact at their interface. Next, we created a superconducting SQUID from two of these atypical junctions, and studied its quantum interference pattern. We were able to measure critical currents
of 10-100's of uA, and also found that the effective area of both the junctions and the superconducting loop itself is much larger than their physical size due to the geometry of the flake. In addition, we studied the kinetic inductance of these flakes by placing them in NbTiN superconducting resonators, and found that the kinetic inductance we were measuring is around 2 orders of magnitude higher than what was predicted by theory.
The thesis paves the way for using nontraditional elements and pumping schemes in superconducting circuits to generate a new generation of devices for use in superconducting quantum information processes. In the future, hopefully, these fabrication techniques and experimental discoveries will bring the field closer to a fault tolerant, universal quantum computer.