At temperatures below about 1 K, a series of dc superconducting quantum interference devices (SQUID’s) exhibited an apparent flux noise with a spectral density scaling as 1/f α, where 0.58<α<0.80. Typically, the magnitude of the noise increased as the temperature was lowered below 1 K, tending to flatten out at low temperatures with a value of 7±3 μΦ0Hz−1/2 at 1 Hz that was nearly independent of the parameters and materials of the SQUID’s. Although a large number of hypothetical sources of the noise have been eliminated, the origin remains unidentified.

Low‐frequency noise in dc superconducting quantum interference devices below 1 K

The design, construction, and performance of miniature SQUID (superconducting quantum interference device) susceptometers is discussed. Spins (in units of mu /sub beta /) per square root Hz has been identified as an important figure of merit. Simple expressions for S/sub n/ (spin sensitivity) in miniature SQUID susceptometers are developed and the implications of dimensional scaling explored. The details of several existing and proposed designs are reviewed, including versions that utilize commercial SQUIDs. With thin-film DC SQUIDs, S/sub n/ values of a few thousand spins/ square root Hz have already been obtained, and it is projected that values of a few hundred will be achieved soon. >

Design, fabrication, and performance of integrated miniature SQUID susceptometers

Direct measurements of spin fluctuations are becoming the mainstream approach for studies of complex condensed matter, molecular, nuclear, and atomic systems. This review covers recent progress in the field of optical spin noise spectroscopy (SNS) with an additional goal to establish an introduction into its theoretical foundations. Various theoretical techniques that have been recently used to interpret results of SNS measurements are explained alongside examples of their applications.

https://iopscience.iop.org/article/10.1088/0034-4885/79/10/106501/pdf

The theory of spin noise spectroscopy: a review.

Semiconductor spin noise spectroscopy (SNS) has emerged as a unique experimental tool that utilizes spin fluctuations to provide profound insight into undisturbed spin dynamics in doped semiconductors and semiconductor nanostructures. The technique maps ever present stochastic spin polarization of free and localized carriers at thermal equilibrium via the Faraday effect onto the light polarization of an off-resonant probe laser and was transferred from atom optics to semiconductor physics in 2005. The inimitable advantage of spin noise spectroscopy to all other probes of semiconductor spin dynamics lies in the fact that in principle no energy has to be dissipated in the sample, i.e., SNS exclusively yields the intrinsic, undisturbed spin dynamics and promises optical non-demolition spin measurements for prospective solid state based optical spin quantum information devices. SNS is especially suitable for small electron ensembles as the relative noise increases with decreasing number of electrons. In this review, we first introduce the basic principles of SNS and the difference in spin noise of donor bound and of delocalized conduction band electrons. We continue the introduction by discussing the spectral shape of spin noise and prospects of spin noise as a quantum interface between light and matter. In the main part, we give a short overview about spin relaxation in semiconductors and summarize corresponding experiments employing SNS. Finally, we give in-depth insight into the experimental aspects and discuss possible applications of SNS.

Semiconductor Spin Noise Spectroscopy: Fundamentals, Accomplishments, and Challenges

This article gives an overview on the advance of spin noise spectroscopy (SNS) in semiconductors in the past 8 years from the first measurements in bulk n-GaAs [Oestreich et al., Phys. Rev. Lett. 95, 216603 (2005)] up to the recent achievement of optical detection of the intrinsic spin fluctuations of a single hole confined in an individual self-assembled quantum dot [Dahbashi et al., arXiv:1306.3183 (2013)]. We discuss the general technical implementation of optical SNS and the invaluable profit of the introduction of real-time fast Fourier transform analysis into the data acquisition. By now, the full spin dynamic from the milli- to picosecond timescales can be addressed by SNS and the technique quickly strides ahead to enable real quantum non-demolition measurements in semiconductors. 



Spin noise spectra recorded in 2005 in bulk n-GaAs with approximately 109 electron spins (Oestreich et al.) and 2013 (Dahbashi et al.) for a single hole spin. The integration time for the latter is more than a factor of 40 shorter due to the significant advances in the measurement technique.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pssb.201350291

The rise of spin noise spectroscopy in semiconductors: From acoustic to GHz frequencies

Mesure simultanee, par une nouvelle technique magnetometrique, du bruit lie aux fluctuations magnetiques et de la susceptibilite ac comme fonctions de la temperature; mise en evidence de la validite du theoreme de fluctuation-dissipation au-dessus et au-dessous de la temperature de gel; particularites des variations dans le domaine des basses frequences

Magnetic equilibrium noise in spin-glasses: Eu0.4Sr0.6S.

We analyze in detail a novel integrated SQUID technique for studying magnetic noise and susceptibility. In particular, we show how the susceptibility, π/2 rule and the fluctuation‐dissipation theorem (FDT) are affected by large demagnetizing factors and by the geometry of our pickup and field coils. We present data on the spin glass Eu0.4Sr0.6S as a function of temperature. We confirm the FDT and show evidence against the conventional power‐law dynamics in the spin‐glass phase.

A. P. Malozemoff

Papers

Magnetic equilibrium noise in spin-glasses: Eu0.4Sr0.6S.

Temperature dependence of the magnetic 1/f noise in Eu0.4Sr0.6S (invited)