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We report measurements of the longitudinal and transverse spin relaxation times of holes in an ensemble of self-assembled InAs/InAlGaAs quantum dots (QDs), emitting in the telecom spectral range. The spin coherence of a single carrier is determined using spin mode-locking in the inhomogeneous ensemble of QDs. Modeling the signal allows us to extract the hole spin coherence time to be in the range of T$_2 = 0.02-0.4$ $\mu$s. The longitudinal spin relaxation time T$_1 = 0.5$ $\mu$s is measured using the spin inertia method.

Quantum measurement back action is fundamentally unavoidable when manipulating electron spins. Here we demonstrate that this back action can be efficiently exploited to tune the spin relaxation of localized electrons induced by the hyperfine interaction. In optical pump-probe experiments, powerful probe pulses suppress the spin relaxation of electrons on Si donors in an InGaAs epilayer due to the quantum Zeno effect. By contrast, an increase of the probe power leads to a speed up of the spin relaxation for electrons in InGaAs quantum dots due to the quantum anti-Zeno effect. The microscopic description shows that the transition between the two regimes occurs when the spin dephasing time is comparable to the probe pulse repetition period., Comment: 6+5 pages, 2+8 figures

We demonstrate the realization of the resonant spin amplification (RSA) effect in Faraday geometry where a magnetic field is applied parallel to the optically induced spin polarization so that no RSA is expected. However, model considerations predict that it can be realized for a central spin interacting with a fluctuating spin environment. As a demonstrator, we choose an ensemble of singly-charged (In,Ga)As/GaAs quantum dots, where the resident electron spins interact with the surrounding nuclear spins. The observation of RSA in Faraday geometry requires intense pump pulses with a high repetition rate and can be enhanced by means of the spin-inertia effect. Potentially, it provides the most direct and reliable tool to measure the longitudinal $g$ factor of the charge carriers., Comment: 7 pages including 4 figures; 8 pages supplement including 4 figures

The dynamics of the coupled electron-nuclear spin system is studied in an ensemble of singly-charged (In,Ga)As/GaAs quantum dots (QDs) using periodic optical excitation at 1 GHz repetition rate. In combination with the electron-nuclei interaction, the highly repetitive excitation allows us to lock the electron spins into magnetic resonance in a transverse external magnetic field. Sweeping the field to higher values, the locking leads to an effective "diamagnetic" response of significant strength due to dynamic nuclear polarization, which shields the QD electrons at least partly from the external field and can even keep the internal magnetic field constant up to 1.3 T field variation. We model the effect through a magnetic field-dependent polarization rate of the nuclei, from which we suggest a strategy for adjusting the nuclear polarization through the detuning between optical excitation and electronic transition, in addition to tuning the magnetic field., Comment: 12 pages, 9 figures

The coherent electron spin dynamics of an ensemble of singly charged (In,Ga)As/GaAs quantum dots in a transverse magnetic field is driven by periodic optical excitation at 1 GHz repetition frequency. Despite the strong inhomogeneity of the electron $g$ factor, the spectral spread of optical transitions, and the broad distribution of nuclear spin fluctuations, we are able to push the whole ensemble of excited spins into a single Larmor precession mode that is commensurate with the laser repetition frequency. Furthermore, we demonstrate that an optical detuning of the pump pulses from the probed optical transitions induces a directed dynamic nuclear polarization and leads to a discretization of the total magnetic field acting on the electron ensemble. Finally, we show that the highly periodic optical excitation can be used as universal tool for strongly reducing the nuclear spin fluctuations and preparation of a robust nuclear environment for subsequent manipulation of the electron spins, also at varying operation frequencies., Comment: 14 pages, 7 figures

Nuclear spin coherence and relaxation dynamics of all constituent isotopes of an n-doped CdTe/(Cd,Mg)Te quantum well structure are studied employing optically detected nuclear magnetic resonance. Using time-resolved pump-probe Faraday ellipticity, we generate and detect the coherent spin dynamics of the resident electrons. The photogenerated electron spin polarization is transferred into the nuclear spin system, which becomes polarized and acts back on the electron spins as the Overhauser field. Under the influence of resonant radio frequency pulses, we trace the coherent spin dynamics of the nuclear isotopes $^{111}$Cd, $^{113}$Cd, and $^{125}$Te. We measure nuclear Rabi oscillations, the inhomogeneous dephasing time $T_2^*$, the spin coherence time $T_2$, and the longitudinal relaxation time $T_1$. Furthermore, we investigate the influence of the laser excitation and the corresponding electron spin polarization on the nuclear spin relaxation time and find a weak extension of this time induced by interaction with the electron spins., Comment: 5 pages, 2 figures

The periodic optical orientation of electron spins in (In,Ga)As/GaAs quantum dots leads to the formation of electron spin precession modes about an external magnetic field which are resonant with the pumping periodicity. As the electron spin is localized within a nuclear spin bath, its polarization imprints onto the spin polarization of the bath. The latter acts back on the electron spin polarization. We implement a pulse protocol where a train of laser pulses is followed by a long, dark gap. It allows us to obtain a high-resolution precession mode spectrum from the free evolution of the electron spin polarization. Additionally, we vary the number of pump pulses in a train to investigate the build-up of the precession modes. To separate out nuclear effects, we suppress the nuclear polarization by using a radio-frequency field. We find that a long-living nuclear spin polarization imprinted by the periodic excitation significantly speeds up the buildup of the electron spin polarization and induces the formation of additional electron spin precession modes. To interpret these findings, we extend an established dynamical nuclear polarization model to take into account optically detuned quantum dots for which nuclear spins activate additional electron spin precession modes., Comment: 9 pages, 10 figures

The spin relaxation dynamics of Ce$^{3+}$ ions in heavily cerium-doped YAG crystals is studied using pulse-electron paramagnetic resonance and time-resolved pump-probe Faraday rotation. Both techniques address the 4$f$ ground state, while pump-probe Faraday rotation provides also access to the excited 5$d$ state. We measure a millisecond spin-lattice relaxation time $T_1$, a microsecond spin coherence time $T_2$ and a $\sim 10$ ns inhomogeneous spin dephasing time $T_2^*$ for the Ce$^{3+}$ ground state at low temperatures. The spin-lattice relaxation of Ce$^{3+}$ ions is due to modified Raman processes involving the optical phonon mode at 125 cm$^{-1}$. The relaxation at higher temperature goes through a first excited level of the $^{2}$F$_{5/2}$ term at about $\hbar \omega \approx 228$ cm$^{-1}$. Effects provided by the hyperfine interaction of the Ce$^{3+}$ with the $^{27}$Al nuclei are observed., Comment: 10 pages, 13 figures

Electron spin dephasing in a singly charged semiconductor quantum dot can partially be suppressed by periodic laser pulsing. We propose a semi-classical approach describing the decoherence of the electron spin polarization governed by the hyperfine interaction with the nuclear spins as well as the probabilistic nature of the photon absorption. We use the steady-state Floquet condition to analytically derive two subclasses of resonance conditions excellently predicting the peak locations in the part of the Overhauser field distribution which is projected in the direction of the external magnetic field. As a consequence of the periodic pulsing, a non-equilibrium distribution develops as a function of time. The numerical simulation of the coupled dynamics reveals the influence of the hyperfine coupling constant distribution onto the evolution of the electron spin polarisation before the next laser pulse. Experimental indications are provided for both subclasses of resonance conditions., Comment: 21 pages, 21 figures

We develop an extended pump-probe Faraday rotation technique to study the submicrosecond electron spin dynamics with picosecond time resolution in a wide range of magnetic fields. The electron spin dephasing time $T_2^*$ and the longitudinal spin relaxation time $T_1$, both approaching $250$ ns in weak fields, are measured thereby in $n$-type bulk GaAs. By tailoring the pump pulse train through increasing the contained number of pulses, the buildup of resonant spin amplification is demonstrated for the electron spin polarization. The spin precession amplitude in high magnetic fields applied in the Voigt geometry shows a non-monotonic dynamics deviating strongly from a mono-exponential decay and revealing slow beatings. The beatings indicate a two spin component behavior with a $g$-factor difference of $\Delta g \sim 4\times10^{-4}$, much smaller than the $\Delta g$ expected for free and donor-bound electrons. This $g$-factor variation indicates efficient, but incomplete spin exchange averaging., Comment: 5 pages, 3 figures

We demonstrate the potential of a periodic laser-pulse protocol for dynamic decoupling of hole spins in (In,Ga)As quantum dots from surrounding baths. When doubling the repetition rate of inversionlaser pulses between two reference pulses for orienting the spins, we find that the spin coherence time is increased by a factor of two., Comment: A comment on the different versions of this article can be found at the end of this version