{Wideband electric field sensing via a single trapped ion beyond the Fourier and Standard Quantum Limits}

Ultrasensitive measurement of the frequency, phase, and amplitude of radio-frequency (RF) and microwave electric fields underpins modern communications, high-fidelity qubit control, cosmology, and dark-matter searches. Quantum harmonic-oscillator platforms—especially trapped ions—offer nanometer-scale spatial resolution and exceptional field sensitivity, but are typically confined to narrow detection bands near either the ion’s motional frequency or an optical transition.

In the first part, I introduce a quantum vector signal analyzer (QVSA) based on motional Raman transitions in a single trapped ion, enabling state-of-the-art sensitivity to frequency, phase, and amplitude across a bandwidth more than 800× larger than previous approaches. The method is compatible with quantum amplification via squeezing and Fock-basis readout, achieving performance 3.4(2.0) dB below the standard quantum limit[1].

In the second part, I show how combining QVSA with dynamical decoupling resolves two RF tones within the system’s spectral linewidth. We discriminate two randomly chosen ~100-MHz electric fields separated by 5 Hz, measuring a 5.0(1.6) Hz difference with only 1 ms acquisition per run—an improvement of 200× beyond the traditional spectral-resolution limit[2].

In the last part, I leverage QVSA’s phase sensitivity with a correlation protocol to surpass the conventional limit set by the sensor’s coherence time, reaching mHz-level precision on GHz signals. In this regime, the ultimate precision is limited by the signal-generator stability, not the coherence time of quantum sensor.