Improving Transmon Qudit Measurement on IBM Quantum Hardware
The Hilbert space of a physical qubit typically features more than two energy
levels. Using states outside the qubit subspace can provide advantages in
quantum computation. To benefit from these advantages, individual states of the
$d$-dimensional qudit Hilbert space have to be discriminated during readout. We
propose and analyze two measurement strategies that improve the
distinguishability of transmon qudit states. Based on a model describing the
readout of a transmon qudit coupled to a resonator, we identify the regime in
hardware parameter space where each strategy is optimal. We discuss these
strategies in the context of a practical implementation of the default
measurement of a ququart on IBM Quantum hardware whose states are prepared by
employing higher-order $X$ gates that make use of two-photon transitions.
Tobias Kehrer Tobias Nadolny Christoph Bruder