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

TL;DR

This paper presents an FPGA-based platform enabling fast, high-fidelity active reset of a fluxonium qubit, significantly improving initialization speed and accuracy over passive thermalization methods.

Contribution

The authors developed a versatile FPGA control/readout platform that achieves measurement-based feedback with 428 ns latency, enabling rapid, high-fidelity qubit reset.

Findings

Achieved 99.4% reset fidelity for fluxonium qubit.

Reduced initialization time from 80 μs to approximately 1.5 μs.

Demonstrated effective active reset surpassing passive thermalization methods.

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 $1.5 \, \mathrm{\mu s}$ long. Compared to passive ground state initialization through thermalization, with the time constant given by $T_1 = ~ 80 \, \mathrm{\mu s}$, 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.