Identify the origin of optical-power-dependent dephasing

Identify the physical mechanism responsible for the optical-power-dependent degradation of the niobium transmon’s Ramsey coherence time below the threshold for measurable excess relaxation.

Background

The niobium transmon exhibits a decrease in Ramsey coherence time as optical power increases, even at powers below the onset of measurable relaxation degradation and without a resolvable quasiparticle-induced qubit-frequency shift. This behavior indicates an error channel distinct from the protected relaxation process.

The paper does not establish whether the dephasing originates from quasiparticle fluctuations, optical or phononic effects, fluctuations in the device environment, or another mechanism. Determining its origin is necessary for improving protection against optical-field-induced errors.