Ultrafast Spin Noise Spectroscopy With Variable Probing Wavelength
ORAL
Abstract
Quantum fluctuations usually limit the precision in metrology according to the Heisenberg uncertainty principle, which states that a coherent state must exhibit finite fluctuations in conjugate
variables, defined as the standard quantum limit (SQL). In contrast, squeezed states have one quadrature with reduced noise below the SQL. Recent theory suggested that superradiant phase transition (SRPT) may support spin squeezing under finite temperatures and magnetic fields, and experiment indicated such a transition in orthoferrites. To directly probe spin fluctuations in complex oxides, we developed an ultrafast spin noise spectroscopy (SNS) platform with wavelength tunability and terahertz bandwidth, which is a passive, non-perturbative optical technique that detects intrinsic spin fluctuations through polarization correlations, enabling measurement of stochastic spin dynamics in equilibrium. We implemented a broadband pulse shaping scheme (730–1600 nm) to produce spectrally separable pairs of ultrashort probe pulses for sampling the spontaneous magnetization fluctuation at electronic resonance frequencies. The setup also simultaneously compensates large dispersion for both fiber-based configurations. The system achieves a noise-equivalent Faraday-rotation sensitivity of ∼ 10−12 rad2/√Hz, sufficient to resolve the spin fluctuation of orthoferrites. This tunable SNS technique establishes a versatile probe for magnetic phase transitions in complex magnets. Combining femtosecond temporal resolution, wavelength selectivity, and high sensitivity, it opens a new pathway to investigate non-classical magnetic phenomena such as SRPT and spin liquid.
variables, defined as the standard quantum limit (SQL). In contrast, squeezed states have one quadrature with reduced noise below the SQL. Recent theory suggested that superradiant phase transition (SRPT) may support spin squeezing under finite temperatures and magnetic fields, and experiment indicated such a transition in orthoferrites. To directly probe spin fluctuations in complex oxides, we developed an ultrafast spin noise spectroscopy (SNS) platform with wavelength tunability and terahertz bandwidth, which is a passive, non-perturbative optical technique that detects intrinsic spin fluctuations through polarization correlations, enabling measurement of stochastic spin dynamics in equilibrium. We implemented a broadband pulse shaping scheme (730–1600 nm) to produce spectrally separable pairs of ultrashort probe pulses for sampling the spontaneous magnetization fluctuation at electronic resonance frequencies. The setup also simultaneously compensates large dispersion for both fiber-based configurations. The system achieves a noise-equivalent Faraday-rotation sensitivity of ∼ 10−12 rad2/√Hz, sufficient to resolve the spin fluctuation of orthoferrites. This tunable SNS technique establishes a versatile probe for magnetic phase transitions in complex magnets. Combining femtosecond temporal resolution, wavelength selectivity, and high sensitivity, it opens a new pathway to investigate non-classical magnetic phenomena such as SRPT and spin liquid.
*This research is sponsored by Keck Foundation (995764) and National Science Foundation (DMR-2240106).
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Publication: [1] V. S. Zapasskii, "Spin-noise spectroscopy: from proof of principle to applications," Adv. Opt. Photon. 5, 131–168 (2013).
[2] I. Žutić, J. Fabian, and S. Das Sarma, "Spintronics: Fundamentals and applications," Rev. Mod. Phys. 76, 323–410 (2004).
[3] M. A. Weiss et al., "Discovery of ultrafast spontaneous spin switching in an antiferromagnet by femtosecond noise correlation
spectroscopy," Nat. Commun. 14, 7651 (2023).
[4] D. Kim et al., "Observation of the magnonic Dicke superradiant phase transition," Sci. Adv. 11, eadt1691 (2025).
Presenters
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Shung-An Koh
- Applied Physics Graduate Program, Smalley-Curl Institute, Rice University, Houston, TX 77005, USA