State preparation of a fluxonium qubit with feedback from a custom FPGA-based platform

Abstract

We developed a versatile integrated control and readout instrument for experiments with superconducting quantum bits (qubits), based on a field-programmable gate array (FPGA) platform. Using this platform, we perform measurement-based, closed-loop feedback operations with $428\mathrm{ns}$ platform latency. The feedback capability is instrumental in realizing active reset initialization of the qubit into the ground state in a time much shorter than its energy relaxation time $T_1$. We show experimental results demonstrating reset of a fluxonium qubit with $99.4\%$ fidelity, using a readout-and-drive pulse sequence approximately $1.5\mu \mathrm{s}$ long. Compared to passive ground state initialization through thermalization, with the time constant given by $T_1=80\mu \mathrm{s}$, the use of the FPGA-based platform allows us to improve both the fidelity and the time of the qubit initialization by an order of magnitude.