We demonstrate a new technique for detecting the amplitude of arbitrarily chosen components of radio-frequency waveforms based on stroboscopic backaction evading measurements. We combine quantum nondemolition measurements and stroboscopic probing to detect waveform components with magnetic sensitivity beyond the standard quantum limit. Using an ensemble of $1.5\times 10^6$ cold rubidium atoms, we demonstrate entanglement-enhanced sensing of sinusoidal and linearly chirped waveforms, with 1.0(2) and 0.8(3) dB metrologically relevant noise reduction, respectively. We achieve volume-adjusted sensitivity of $\delta B\sqrt{V}\approx 3.96\mathrm{fT}\sqrt{{\mathrm{cm}}^3/\mathrm{Hz}}$, comparable to the best rf magnetometers.

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• Atom optics
• Atomic & molecular processes in external fields
• Coherent control
• Cooling & trapping
• Light-matter interaction
• Quantum fluctuations & noise
• Quantum measurements
• Quantum optics