Coherent control of a multi-qubit dark state in waveguide quantum electrodynamics

Superconducting qubits in a waveguide have long-range interactions mediated by photons that promote the emergence of collective states. Destructive interference between the qubits can collectively decouple them from the waveguide environment, creating dark states. Their inability to emit photons into the waveguide makes dark states a valuable resource for preparing long-lived many-body states and realizing quantum information protocols in open quantum systems. However, they also decouple from fields that drive the waveguide, making manipulation a challenge. Here we show coherent control of a collective dark state that is realized by controlling the interactions between four superconducting transmon qubits and local drives. The dark state's protection against decoherence results in decay times that exceed those of the waveguide-limited single qubits by more than two orders of magnitude. We perform phase-sensitive spectroscopy of the two-excitation manifold and reveal bosonic many-body statistics in the transmon array. Our dark state qubit provides a starting point for implementing quantum information protocols with collective states.