Optical Chopping Enhanced Rydberg-Atom-Based Ultra-Low-Frequency Electric Field Measurement

Abstract

This study demonstrates a significant enhancement in ultra-low-frequency (ULF) electric field sensitivity using Rydberg atoms via an optical chopping amplification (OCA) technique. Conventional Rydberg-based ULF measurements are fundamentally limited by 1/f noise, which severely degrades sensitivity. Our approach modulates the coupling laser with an optical chopper before the vapor cell, inducing periodic Rydberg excitation at the chopping frequency. The photodetector (PD) output signal is demodulated by a lock-in amplifier (LIA) using the optical chopper's signal as the reference. This process effectively improves the signal-to-noise ratio (SNR) by shifting the 1/f noise to a higher frequency band where it can be filtered out. The OCA technique enhanced sensitivity by 19.1 dB for the frequency 7 Hz, which is down to 49.1 uV/cm/rt(Hz). For the frequency range from 10Hz to 1kHz, it also enhanced nearly 7dB. This OCA method for enhancing the sensitivity of Rydberg atoms in ULF electric field measurements enables the Rydberg sensor's detection range to span the entire spectrum from low frequency (LF) to ULF, thereby significantly broadening its application potential.

Figures (5)

• Figure 1: (a) Schematic of the experimental setup. An 852 nm probe laser propagates through a vapor cell. A pair of copper electrode plates is integrated into the cell in parallel with a spacing of 18 mm. A chopped 509 nm coupling laser counter-propagates and overlaps with the probe beam. The reference signal from the chopper and the output signal from the PD are fed into the LIA for demodulation. The resulting signal is measured by the SA. Abbreviations: DM, dichroic mirror; PD, photodetector; LIA, lock-in amplifier; SA, spectrum analyzer. (b) Energy-level diagram for the three-level Rydberg-EIT system. A probe laser resonantly couples the $\ket{6S_{1/2}}$ and $\ket{6P_{3/2}}$ states with Rabi frequency $\Omega_p$. A coupling laser drives the transition of $\ket{6P_{3/2}}$ and $\ket{57D_{5/2}}$ states with Rabi frequency $\Omega_c$. In the presence of an electric field, the Rydberg level exhibits $m_j = 1/2, 3/2, 5/2$ dependent Stark shifts and splitting. (c) The signal demodulation process within the LIA.
• Figure 2: (a) Power spectral density without optical chopping, showing the ULF signal obscured by LF noise. (b) Spectrum after optical chopping at $f_{\mathrm{chop}}$. The signal is upconverted to the sidebands at $f_{\mathrm{chop}} \pm f_s$. (c) Spectrum after LIA demodulation. The signal is downconverted back to the baseband, while the $1/f$ noise introduced by the PD and transmission line is upconverted to the chopping frequency region and filtered out. The overall process involves double modulation of the signal and single modulation of the noise.
• Figure 3: (a) to (g) represent the energy-level splitting of the EIT spectra when the direct DC electric field applied between the parallel electrodes is 0 mV, 600 mV, 650 mV, 700 mV, 750 mV, 800 mV, and 850 mV, respectively. (h) Calibration of the transmission factor when measuring the 66 Hz electric field in this work. The factor $F$ was determined by a linear fit of the electric field strength measured via the Rydberg atom response (using the $\ket{57D_{5/2}, m_j = 1/2}$ Stark shift) to the nominal field strength set by the signal generator, under strong-field conditions.
• Figure 4: Comparison of ULF electric field measurements with and without using OCA. (a)--(d) Electric field strength measured at 7 Hz, 33 Hz, 66 Hz, and 132 Hz.
• Figure 5: Electric field sensitivity versus frequency. A comparison of the theoretical limit for a 1 cm dipole antenna with experimentally reported sensitivities from this work (using and without using OCA) and other Rydberg-atom-based experiments.