The real-time spin dynamics and the spin noise spectra are calculated for p and n-charged quantum dots within an anisotropic central spin model extended by additional nuclear electric quadrupolar interactions (QC) and augmented by experimental data studied using identical excitation conditions. Using realistic estimates for the distribution of coupling constants including an anisotropy parameter, we show that the characteristic long time scale is of the same order for electron and hole spins strongly determined by the QC even though the analytical form of the spin decay differs significantly consistent with our measurements. The low frequency part of the electron spin noise spectrum is approximately 1/3 smaller than those for hole spins as a consequence of the spectral sum rule and the different spectral shapes. This is confirmed by our experimental spectra measured on both types of quantum dot ensembles in the low power limit of the probe laser.PACS numbers: 78.67. Hc, Introduction: The promising perspective of combining traditional electronics with novel spintronics devices lead to intensive studies of the spin dynamics of a single electron (n) or hole (p) confined in a semiconductor quantum dot (QD) [1][2][3][4]. In contrast to defects in diamonds [5,6], such QDs may be integrated into conventional semiconductor devices. While the strong confinement of the electronic wave function in QDs reduces the interaction with the environment and suppresses electronic decoherence mechanisms, it simultaneously enhances the hyperfine interaction between the confined electronic spin and the nuclear spin bath formed by the underlying lattice.Generally it is believed [3,4,7,8] that the hyperfine interaction dominates the spin relaxation in QDs. The s-wave character of the electron-wave function at the nuclei leads to an isotropic central spin model (CSM) [9] for describing the electron-nuclear hyperfine coupling, while for p-charged QDs, the couplings to the nuclear spins can be mapped onto an anisotropic CSM [4,10]. Since the coupling constants for p-charged QDs are reduced compared to the n-charged QDs [4,10], and additionally a large anisotropy factor Λ > 1 suppresses the spin decay of the S z component [4,10], p-charged QDs have been considered as prime candidates for long lived spin excitations in spintronics applications.Experimentally, however, there is evidence for comparable spin-decay times of the S z components [11][12][13][14][15] in p-and n-charged QDs: hence the anisotropic CSM provides only an incomplete description of the relevant spin-relaxation processes in such systems.In this paper, we resolve this puzzle by investigating the effect of an additional realistic nuclear electric quadrupolar interaction term (QC) [16] onto the spin decoherence. Most of the Ga and As isotopes have a nuclear spin I = 3/2 which is subject to a quadrupolar split-

The spin fluctuations of electron and hole doped self-assembled quantum dot ensembles are measured optically in the low-intensity limit of a probe laser in absence and presence of longitudinal or transverse static magnetic fields. The experimental results are modeled by two complementary approaches based either on semiclassical or quantum mechanical descriptions. This allows us to characterize the hyperfine interaction of electron and hole spins with the surrounding bath of nuclei on time scales covering several orders of magnitude. Our results demonstrate (i) the intrinsic precession of the electron spin fluctuations around the effective nuclear Overhauser field caused by the host lattice nuclear spins, (ii) the comparably long time scales for electron and hole spin decoherence, as well as (iii) the dramatic enhancement of the spin lifetimes induced by a longitudinal magnetic field due to the decoupling of nuclear and charge carrier spins.

'Spin noise spectroscopy' is an optical technique for probing electron and hole spin dynamics that is based on detecting their intrinsic fluctuations while in thermal equilibrium. Here we show that fluctuation correlations can be further exploited in multi-probe noise studies to reveal information that in general cannot be accessed by conventional linear optical spectroscopy, such as the underlying homogeneous linewidths of individual constituents within inhomogeneously broadened systems. This is demonstrated in singly charged (In,Ga)As quantum-dot ensembles using two weak probe lasers: When the lasers have similar wavelengths, they probe the same quantum dots in the ensemble and show correlated spin fluctuations. In contrast, mutually detuned probe lasers measure different subsets of quantum dots, giving uncorrelated fluctuations. The noise correlation versus laser detuning directly reveals the quantum dot homogeneous linewidth even in the presence of a strong inhomogeneous broadening. Such noise-based correlation techniques are not limited to semiconductor spin systems, but are applicable to any system with measurable intrinsic fluctuations.

Per the fluctuation-dissipation theorem, the information obtained from spin fluctuation studies in thermal equilibrium is necessarily constrained by the system's linear response functions. However, by including weak radio frequency magnetic fields, we demonstrate that intrinsic and random spin fluctuations even in strictly unpolarized ensembles can reveal underlying patterns of correlation and coupling beyond linear response, and can be used to study nonequilibrium and even multiphoton coherent spin phenomena. We demonstrate this capability in a classical vapor of (41)K alkali atoms, where spin fluctuations alone directly reveal Rabi splittings, the formation of Mollow triplets and Autler-Townes doublets, ac Zeeman shifts, and even nonlinear multiphoton coherences.

We attract attention to the fact that the ultimate (shot-noise-limited) polarimetric sensitivity can be enhanced by orders of magnitude leaving the photon flux incident onto the photodetector on the same low level. This opportunity is of crucial importance for present-day spin noise spectroscopy, where a direct increase of sensitivity by increasing the probe beam power is strongly restricted by the admissible input power of the broadband photodetectors. The gain in sensitivity is achieved by replacing the 45-deg polarization geometry commonly used in conventional schemes with balanced detectors by geometries with stronger polarization extinction. The efficiency of these high-extinction polarization geometries with enhancement of the detected signal by more than an order of magnitude is demonstrated by measurements of the spin noise spectra of bulk n:GaAs in the spectral range 835-918 nm. It is shown that the inevitable growth of the probe beam power with the sensitivity gain makes spin noise spectroscopy much more perturbative, but, at the same time, opens up fresh opportunities for studying nonlinear interactions of strong light fields with spin ensembles.Comment: 8 pages, 9 figure