Fig. 4

Sensing experiments of intrinsic amplitude noise using a single ion. a Sensitivity calibration using k = 0 and ω _sid = ω _s , with ω _s  = 10 kHz. Measurements are taken for linearly sampled phases from 0 to 2π with varying modulation depth and interleaved noiseless pulses to identify the fidelity limits of the experiments (“Baseline”). Each measurement is repeated 200 times; the resulting projection noise is captured by the error bars. The overall signal is represented as the average over all phase realizations. b Frequency-dependent amplitude instability probed with band-shifted k = 0 DPSS-modulated pulses. The measurement is repeated twice by scanning the ω _s first from 2–100 kHz and then from 100–2 kHz. The bandwidth of the DPSS filter (FWHM) is ~150 Hz. (inset) A small offset pulse is added at the beginning of the main protocol and its area is varied over the range [−π, π] to demonstrate the frequency-dependent pulse offset behavior. High-contrast fringes indicate quantum coherence is maintained