Characterizing Thermal Populations in a Coupled Superconducting Aluminum 3Dcavity and a Transmon Qubit
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Physics (MPPHS), MSc
Publicerad
2023
Författare
Hagström, Ivan
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Circuit quantum electrodynamics (cQED), which combines superconducting qubits
with microwave resonators, has become one of the leading platforms for quantum
computation [1]. Continuous variable quantum computation (CVQC), which uses
the bosonic modes of a quantum microwave resonator as a quantum memory, has
some advantages over discrete quantum computing, including long lifetimes and
simple schemes for error correction [2]. The presence of thermal excitations in a
continuous variable system can increase error rates and reduce the fidelity of quantum
operations. This provides the motivation to characterize the residual thermal
population of our systems. In this thesis we demonstrate methods for characterizing
the residual thermal populations in a coupled superconducting 3D-cavity and transmon
qubit. First, the qubit population was measured following the scheme from
Geerlings et al. (2013) [3]. The measured population was 0.25 ± 0.06% corresponding
to an effective temperature of 50.2 ± 1.9 mK. Second, two different methods
were used to measure the thermal population of the 3D-cavity. One based on Rabi
driving and one based on Ramsey interferometry. Of the two, the method using
Rabi driving seemed to perform better at small populations in terms of standard
deviation. With this method we measuring a residual population of 0.34 ± 0.03%
corresponding to an effective temperature of 37.6 ± 0.3 mK. Finally, the thermal
population of an on-chip readout resonator coupled to the qubit was estimated using
the photon induced dephasing of the qubit. This gave a population of < 0.18%
corresponding to an effective temperature of < 56.1 mK. These results compare favorable
to previous studies such as J. M. Gertler et al. (2021) who measured a cavity
population of 1-2% and a transmon population of 5% in a similar system [4], and R.
W. Heeres et al. (2017) who measured a population of 2% in their cavity and 5% in
their transmon [5]. An additional sub-goal of this thesis was measuring the quality
factor of a niobium 3D-cavity. Using a circle fit [6], a quality factor of approximately
4.3 million was measured, which is an expected result for an unpolished cavity.
Beskrivning
Ämne/nyckelord
3D-cavity , Circuit quantum electrodynamics , Continuous variable quantum computing , qubit , thermal excitation , thermal population