Observation of magnetic amplification using dark spins

Abstract

Quantum amplification enables the enhancement of weak signals and is of great importance for precision measurements, such as biomedical science and tests of fundamental symmetries. Here, we observe a previously unexplored magnetic amplification using dark noble-gas nuclear spins in the absence of pump light. Such dark spins exhibit remarkable coherence lasting up to 6 min and the resilience against the perturbations caused by overlapping alkali-metal gas. We demonstrate that the observed phenomenon, referred to as "dark spin amplification," significantly magnifies magnetic field signals by at least three orders of magnitude. As an immediate application, we showcase an ultrasensitive magnetometer capable of measuring subfemtotesla fields in a single 500-s measurement. Our approach is generic and can be applied to a wide range of noble-gas isotopes, and we discuss promising optimizations that could further improve the current signal amplification up to [Formula: see text] with [Formula: see text]Ne, [Formula: see text] with [Formula: see text]Xe, and [Formula: see text] with [Formula: see text]He. This work unlocks opportunities in precision measurements, including searches for ultralight dark matter with sensitivity well beyond the supernova-observation constraints.

Keywords: magnetic amplification; noble gas; quantum sensing; spin magnetic resonance.

Fig. 1.

Schematic of noble-gas spin amplification in the dark. (A) A cubic vapor cell contains ¹²⁹Xe and ⁸⁷Rb atoms and buffer-gas N₂. A bias magnetic field is applied along $z$ to tune the ¹²⁹Xe Larmor frequency. (B) The measurement process can be divided into three stages: 1) Noble gas spin initialization. ¹²⁹Xe is polarized through spin-exchange collisions with optically polarized ⁸⁷Rb atoms (SI Appendix, Initialization) (44, 45); 2) Amplification of oscillating magnetic field. The pump light for ⁸⁷Rb is turned off. With the ¹²⁹Xe Larmor frequency tuned to match the frequency of the external field, ¹²⁹Xe spins are tilted away from $z$, and the induced nuclear magnetization generates an effective field on ⁸⁷Rb atoms with a large amplification factor; 3) Readout of the amplified field with pump light on. The embedded ⁸⁷Rb atoms act as a sensitive magnetometer to in situ read out the effective field.

Fig. 2.

Demonstration of noble-gas amplification in the dark. (A) Noble-gas spin response evolution under different frequency detunings. (B) Optimal amplification as a function of frequency detuning. The optimal amplification is defined as the maximum value of the amplification, as shown with the red dashed lines in A. The FWHM is measured to be $\approx 6.7$ mHz. (C) Optimal dark time as a function of frequency detuning.

Fig. 3.

Demonstration of sensitive magnetometry using dark spin amplification. (A) Sequence for bias magnetic fields and optical pumping. (B) Amplification as a function of resonance frequencies. Black points represent the experimental data. The red points denote the data after numerically correcting the bias-field gradient and yield the average value of amplification of $\Pi \approx$5,000. (C) Magnetic field measurement uncertainty as a function of resonance frequency. Black points represent the experimental data with error bars from nine repeated experiments.

Fig. 4.

Proposal of dark spin amplification using various noble gas. The projected maximum amplification of ³He-K system (red line), ¹²⁹Xe-⁸⁷Rb system (black line), and ²¹Ne-⁸⁷Rb system (blue line) could reach $8\times {10}^5$, $7.8\times {10}^4$, $2\times {10}^4$ at time $T_2$, respectively. The dark time denotes the evolution time of noble gas in the dark.