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Spectroscopy of spontaneous spin noise as a probe of spin dynamics and magnetic resonance


Not all noise in experimental measurements is unwelcome. Certain fundamental noise sources contain valuable information about the system itself—a notable example being the inherent voltage fluctuations (Johnson noise) that exist across any resistor, which allow the temperature to be determined1,2. In magnetic systems, fundamental noise can exist in the form of random spin fluctuations3,4. For example, statistical fluctuations of N paramagnetic spins should generate measurable noise of order √(N) spins, even in zero magnetic field5,6. Here we exploit this effect to perform perturbation-free magnetic resonance. We use off-resonant Faraday rotation to passively7,8 detect the magnetization noise in an equilibrium ensemble of paramagnetic alkali atoms; the random fluctuations generate spontaneous spin coherences that precess and decay with the same characteristic energy and timescales as the macroscopic magnetization of an intentionally polarized or driven ensemble. Correlation spectra of the measured spin noise reveal g-factors, nuclear spin, isotope abundance ratios, hyperfine splittings, nuclear moments and spin coherence lifetimes—without having to excite, optically pump or otherwise drive the system away from thermal equilibrium. These noise signatures scale inversely with interaction volume, suggesting a possible route towards non-perturbative, sourceless magnetic resonance of small systems.

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Figure 1: Spontaneous spin noise in Rb or K vapour, probed via Faraday rotation.
Figure 2: Total integrated spin noise from 85Rb as a function of laser detuning from the D1 (black points) and D2 (blue points) transitions.
Figure 3: Spin noise from 85Rb versus spin density and beam cross-section.
Figure 4: The ground-state Zeeman and hyperfine structure of 87Rb and 39K as revealed by stochastic spin coherences.


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We thank P. Littlewood, S. Gider, P. Crowell and P. Crooker for discussions. This work was supported by the Los Alamos LDRD programme.

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Correspondence to S. A. Crooker.

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Supplementary information

Supplementary Figure

An additional figure of spin noise data, this time from atoms having nuclear spin 5/2. (PDF 169 kb)

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Crooker, S., Rickel, D., Balatsky, A. et al. Spectroscopy of spontaneous spin noise as a probe of spin dynamics and magnetic resonance. Nature 431, 49–52 (2004).

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