Quantum Amplified Sensing of Arbitrary Frequency Electric Fields below the Standard Quantum Limit*

Trapped ions serve as sensitive probes of electromagnetic fields but have previously been limited to sensing a narrow frequency band around the secular frequency of the trapped ion oscillator or around optical transitions within the ion. We demonstrate a new technique which utilizes motional Raman transitions to sense the amplitude, frequency, and phase of an electric field (arXiv:2311.12263). This technique extends the ion’s sensitivity to arbitrary frequency fields across the radio and microwave frequency range, and we demonstrate sensitivity to a frequency range 800 times larger than previous sensing schemes.

Furthermore, quantum amplification via squeezing can improve sensitivity to weak electromagnetic fields (Science 364, 1163). We demonstrate that motional Raman transitions for sensing of arbitrary frequency electric fields are compatible with quantum amplification via squeezing by performing two different squeezing protocols: extrinsic and intrinsic squeezing. For extrinsic squeezing we first prepare a squeezed state and then sense via motional Raman transitions to achieve a sensitivity 3.4(20) dB below the standard quantum limit. Intrinsic squeezing is a continuous time version of this protocol where the probing fields themselves squeeze while performing motional Raman transitions.