Reducing Decoherence in dc SQUID Phase Qubits
Przybysz, Anthony Joseph
Wellstood, Frederick C.
This thesis examines sources of dissipation and dephasing in a dc SQUID phase qubit. Coupling of the qubit to the bias lines and lossy dielectrics causes the qubit to lose quantum information through a process known generally as decoherence. Using knowledge of the possible sources of decoherence, a dc SQUID phase qubit is designed with parameters that should have made it resistant to dissipation and dephasing from those sources. Device PB9 was a dc SQUID with one small area 0.23 (μm)2 Josephson junction with a critical current of 130 nA, which was meant to be the qubit junction, and a larger area 5 (μm)2 junction with a critical current of 8.6 μA, which acted as part of an inductive isolation network. The qubit junction was shunted by a 1.5 pF low-loss interdigitated capacitor. The dc current bias line had an on-chip LC filter with a cutoff frequency of 180 MHz. The other control lines were also designed to minimize coupling of dissipative elements to the qubit. According to a theoretical model of the dissipation and dephasing, the qubit was expected to have an energy relaxation T<sub>1</sub> ≤ 8.4 μs and dephasing time T<sub>phi</sub> ~ 1 μs. Because of the relatively high Josephson inductance of the qubit junction, the device did not act perform like a conventional isolated single-junction phase qubit. Instead, the resonant modes that I observed were the normal modes of the entire SQUID. At 20 mK and a frequency of 4.047 GHz, the maximum energy relaxation time of the device was found to be 350 ± 70 ns, despite the optimized design. Through a study of T<sub>1</sub> versus applied flux, T<sub>1</sub> was found to depend on the strength of the coupling of the microwave drive line to the qubit. When the line was more coupled, T<sub>1</sub> was shorter. This was evidence that the microwave line was overcoupled to the qubit, and was limiting the lifetime of the excited state T<sub>1</sub>. Through a study of the spectroscopic coherence time T<sub>2</sub><super>*</super>, which measured the effects of low-frequency inhomogeneous broadening and higher frequency dephasing from noise, I discovered that device PB9 has several sweet spots. In particular, the presence of a sweet spot with respect to critical current fluctuations allowed me to identify critical current noise as a major source of broadening and dephasing in the qubit. From the spectroscopy I estimated the 1/f critical current noise power density at 1 Hz was and the 1/f flux noise power spectral density at 1 Hz was . Both of these values were quite high, possibly due to switching of the device between measurements.