Persistent control of a superconducting qubit by stroboscopic measurement feedback

Understand

Making a system state follow a prescribed trajectory despite fluctuations and errors commonly consists in monitoring an observable (temperature, blood-glucose level...) and reacting on its controllers (heater power, insulin amount ...).

• In the quantum domain, there is a change of paradigm in feedback since measurements modify the state of the system, most dramatically when the trajectory goes through superpositions of measurement eigenstates.
• Here, we demonstrate the stabilization of an arbitrary trajectory of a superconducting qubit by measurement based feedback.
• The protocol benefits from the long coherence time ($T_2>10 \mu$s) of the 3D transmon qubit, the high efficiency (82%) of the phase preserving Josephson amplifier, and fast electronics ensuring less than 500 ns delay.