Spin noise spectroscopy -- equilibrium and out-of-equilibrium spin dynamics

A conceptual experiment wherein spin interactions in a multi-component spin system are revealed via their intrinsic spin fluctuations while in thermal equilibrium. Different probes detect spin fluctuations in the different spin species, A and B. Interactions are revealed via cross-correlations of the two spin noise signals. Adapted from Scientific Reports 5, 9573 (2015)

The spin of electrons and holes is an essential ingredient in modern quantum electronics, spintronics and information technologies as a carrier of quantum information. There has been a substantial amount of recent activity to probe dynamical properties of spins in various atomic gases, semiconductors, and correlated materials both in equilibrium and out-of-equilibrium settings driven by a variety of perturbations. To use spin as a quantum bit (qubit) in quantum information technologies, we need to probe the quantum dynamics of spins nondestructively. The spin noise spectroscopy (SNS) is one such promising measurement tool to monitor spontaneous spin fluctuations, or “spin noise” of a material with an off-resonant linearly polarized laser beam. The SNS was developed at Los Alamos to measure dynamical spin properties of dilute atomic gases, bulk semiconductors and nanostructures (e.g. quantum dots) in equilibrium. In this measurement approach, the spin noise power spectra are obtained by measuring the fluctuations of the optical Faraday rotation of a linearly polarized laser beam passing through a region of the sample with spins. During my stay at Los Alamos, we extended the applications of SNS to measure correlations between different spin resonances and cross-correlations among the various spin species in a mixture.

We are interested in applying the SNS to detect a transition from diffusive to many-body localized (MBL) phase in disordered correlated spin systems, such as the Heisenberg spin chain, when the strength of disorder (i.e., random local magnetic field) is increased. The many-body localization has been experimentally investigated in ultracold atoms in optical lattices, polar molecules, and isolated spin impurities in solids by destructive one-time measurements. We wish to use the advantage of the SNS to measure two-time quantum correlations within the so-called weak measurement protocol to detect MBL transition. We have numerically calculated two-time quantum correlators locally and between remote regions of a random-field XXZ spin chain which is believed to exhibit an MBL phase. We identify the different behavior of two-time correlators between remote regions in the localized and delocalized phases. Currently, we are developing the SNS set-up at the LAMP group of RRI for measuring spin dynamics in ultracold atoms.

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M. Swar, D. Roy, S. Bhar, S. Roy and S. Chaudhuri, Phys. Rev. Res. 3, 043171 (2021)