Dissertation Defense: Katrina Sliwa, Yale University, “Minimizing Effects Detrimental to the Heisenberg Back-Action of Qubit Measurements with Parametric Amplifiers”

Event time: 
Friday, January 29, 2016 - 9:00am to 10:00am
Becton Engineering and Applied Science Center (BCT), Becton Seminar Room See map
15 Prospect St.
New Haven, CT 06511
(Location is wheelchair accessible)
Event description: 

The quantum back-action of the measurement apparatus arising from the Heisenberg uncertainty principle is both a fascinating phenomenon and a powerful manipulation tool. Unfortunately, there are other effects such as dephasing and reduced measurement efficiency which may overwhelm the Heisenberg back-action. This thesis focuses on minimizing these two effects in dispersive measurements of superconducting qubits made with two commonly used ultra-low-noise parametric amplifiers, the Josephson bifurcation amplifier (JBA) and the Josephson parametric converter (JPC). Qubit dephasing is primarily a problem in measurements made with the JBA, where a strong, resonant, pump tone is traditionally used to provide the energy for amplification. Replacing this pump tone with two detuned pump tones minimized this dephasing to the point where the Heisenberg back-action of measurements made with the JBA could be observed.
The second effect, reduced measurement efficiency, arises primarily from losses between the qubit and the parametric amplifier. Both the JBA and the JPC operate in reflection, requiring additional lossy, magnetic elements known and circulators both to separate input from output, and to protect the qubits from dephasing due to the amplified reflected signal. This work presents two alternative directional elements, the Josephson circulator, which is both theoretically loss-less and does not rely upon the strong magnetic fields needed for traditional circulators, and the Josephson directional amplifier which does not send any amplified signal back toward the qubit. Both of these elements achieve directionality by interfering multiple parametric processes inside a single JPC, allowing for in-situ switching between the two. This brings incredible experimental flexibility, and also makes these devices strong candidates for `on-chip’ integration, which would in turn eliminate loss between the qubit and parametric amplifier as a dominant source of reduced measurement efficiency.