  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.
