Benchmarking the design, fabrication and shielding of superconducting qubits
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Nanotechnology (MPNAT), MSc
Publicerad
2024
Författare
Jakobsson, Halldór
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Superconducting qubits interact with their environment, which inherently limits
their coherence. In this thesis, we describe measurements and present experimental
results for characterising the qubits interaction with its environment. We performed
flux pulse distortion measurements which allowed us to correct the flux pulse with
0.2% error of the target pulse. A two-level-system (TLS) defect spectroscopy measurement showed a TLS density of 14 ± 2 TLS/GHz, with TLS-qubit coupling between 50 kHz and 650 kHz, and TLS lifetime less than 200 ns. Repeating the TLS
spectroscopy measurement every hour for twelve hours showed TLS drifting and
telegraphic switching between frequencies with a timescale on the order of hours.
Using a parity selective Ramsey sequence, we measured the qubit parity switching
rate due to quasiparticles (QPs) as 500 ± 100 Hz, which randomly varied between
200 Hz to 800 Hz over 24 hours. We designed and measured a device where the
qubit frequency could be tuned through the resonator. To this end, we measured
the qubit decay rate as a function of frequency and found the single mode Purcell
approximation to be negligible compared to TLS and QP losses in this device. In
addition, the TLS loss was measured as F δTLS = (3±1)· 10−6
, and the quasiparticle
density normalised to the density of Cooper pairs as xQP = (6±1)· 10−7
. In equilibrium, this quasiparticle density would corresponds to a qubit temperature of 147±2
mK, since the qubit is cooled down to 10 mK, we interpret this as the presence of
non-equilibrium quasiparticles.
Beskrivning
Ämne/nyckelord
Quantum, superconductor, transmon, TLS, quasiparticle, Purcell, defect, experiment, distortion, flux