Nanosecond Spin Coherence Time of Nonradiative Excitons in GaAs/AlGaAs Quantum Wells.

We report on the experimental evidence for a nanosecond timescale spin memory based on nonradiative excitons with large in-plane wave vector. The effect manifests itself in magnetic-field-induced oscillations of the energy of the optically active (radiative) excitons. The oscillations detected by a spectrally resolved pump-probe technique applied to a GaAs/AlGaAs quantum well structure in a transverse magnetic field persist over a timescale, which is orders of magnitude longer than the characteristic decoherence time in the system. The effect is attributed to the spin-dependent electron-electron exchange interaction of the optically active and inactive excitons. The spin relaxation time of the electrons belonging to nonradiative excitons appears to be much longer than the hole spin relaxation time.

Nondestructive measurement of nuclear magnetization by off-resonant Faraday rotation.

We report on the nondestructive measurement of nuclear magnetization in n-GaAs via cavity enhanced Faraday rotation. In contrast with the existing optical methods, this detection scheme does not require the presence of detrimental out-of-equilibrium electrons. Specific mechanisms of the Faraday rotation are identified for (i) nuclear spins situated within the localized electron orbits, these spins are characterized by fast dynamics, (ii) all other nuclear spins in the sample characterized by much slower dynamics. Our results suggest that even in degenerate semiconductors nuclear spin relaxation is limited by the presence of localized electron states and spin diffusion, rather than by Korringa mechanism.

Proposal for a bosonic cascade laser.

We propose a concept of a quantum cascade laser based on transitions of bosonic quasiparticles (excitons) in a parabolic potential trap in a semiconductor microcavity. This laser would emit terahertz radiation due to bosonic stimulation of excitonic transitions. The dynamics of a bosonic cascade is strongly different from the dynamics of a conventional fermionic cascade laser. We show that populations of excitonic ladders are parity dependent and quantized if the laser operates without an external terahertz cavity.

Fast control of nuclear spin polarization in an optically pumped single quantum dot.

Highly polarized nuclear spins within a semiconductor quantum dot induce effective magnetic (Overhauser) fields of up to several Tesla acting on the electron spin, or up to a few hundred mT for the hole spin. Recently this has been recognized as a resource for intrinsic control of quantum-dot-based spin quantum bits. However, only static long-lived Overhauser fields could be used. Here we demonstrate fast redirection on the microsecond timescale of Overhauser fields on the order of 0.5 T experienced by a single electron spin in an optically pumped GaAs quantum dot. This has been achieved using coherent control of an ensemble of 10(5) optically polarized nuclear spins by sequences of short radiofrequency pulses. These results open the way to a new class of experiments using radiofrequency techniques to achieve highly correlated nuclear spins in quantum dots, such as adiabatic demagnetization in the rotating frame leading to sub-µK nuclear spin temperatures, rapid adiabatic passage, and spin squeezing.

Pumping of nuclear spins by optical excitation of spin-forbidden transitions in a quantum dot.

We demonstrate that efficient optical pumping of nuclear spins in semiconductor quantum dots (QDs) can be achieved by resonant pumping of optically forbidden transitions. This process corresponds to one-to-one conversion of a photon absorbed by the dot into a polarized nuclear spin, and also has potential for initialization of hole spin in QDs. We find that by employing this spin-forbidden process, nuclear polarization of 65% can be achieved, markedly higher than from pumping the allowed transition, which saturates due to the low probability of electron-nuclear spin flip-flop.

The puzzle of magnetic resonance effect on the magnetic compass of migratory birds.

Experiments on the effect of radio-frequency (RF) magnetic fields on the magnetic compass orientation of migratory birds are analyzed using the theory of magnetic resonance. The results of these experiments were earlier interpreted within the radical-pair model of magnetoreception. However, the consistent analysis shows that the amplitudes of the RF fields used are far too small to noticeably influence electron spins in organic radicals. Other possible agents that could mediate the birds' response to the RF fields are discussed, but apparently no known physical system can be responsible for this effect.

Structural analysis of strained quantum dots using nuclear magnetic resonance.

Strained semiconductor nanostructures can be used to make single-photon sources, detectors and photovoltaic devices, and could potentially be used to create quantum logic devices. The development of such applications requires techniques capable of nanoscale structural analysis, but the microscopy methods typically used to analyse these materials are destructive. NMR techniques can provide non-invasive structural analysis, but have been restricted to strain-free semiconductor nanostructures because of the significant strain-induced quadrupole broadening of the NMR spectra. Here, we show that optically detected NMR spectroscopy can be used to analyse individual strained quantum dots. Our approach uses continuous-wave broadband radiofrequency excitation with a specially designed spectral pattern and can probe individual strained nanostructures containing only 1 × 10(5) quadrupole nuclear spins. With this technique, we are able to measure the strain distribution and chemical composition of quantum dots in the volume occupied by the single confined electron. The approach could also be used to address problems in quantum information processing such as the precise control of nuclear spins in the presence of strong quadrupole effects.

Suppression of electron spin relaxation in Mn-doped GaAs.

We report a surprisingly long spin relaxation time of electrons in Mn-doped p-GaAs. The spin relaxation time scales with the optical pumping and increases from 12 ns in the dark to 160 ns upon saturation. This behavior is associated with the difference in spin relaxation rates of electrons precessing in the fluctuating fields of ionized or neutral Mn acceptors, respectively. For the latter, the antiferromagnetic exchange interaction between a Mn ion and a bound hole results in a partial compensation of these fluctuating fields, leading to the enhanced spin memory.

Exciton spin decay modified by strong electron-hole exchange interaction.

We study exciton spin decay in the regime of strong electron-hole exchange interaction, which occurs in a wide variety of semiconductor nanostructures. In this regime the electron spin precession is restricted within a sector formed by the external magnetic field and the effective exchange fields triggered by random spin flips of the hole. Using Hanle effect measurements, we demonstrate that this mechanism dominates our experiments in CdTe/(Cd,Mg)Te quantum wells. We present calculations that provide a consistent description of the experimental results, which is supported by independent measurements of the parameters entering the model.

Direct observation of the electron spin relaxation induced by nuclei in quantum dots.

We have studied the electron spin relaxation in semiconductor InAs/GaAs quantum dots by time-resolved optical spectroscopy. The average spin polarization of the electrons in an ensemble of p-doped quantum dots decays down to 1/3 of its initial value with a characteristic time T(Delta) approximately 500 ps, which is attributed to the hyperfine interaction with randomly oriented nuclear spins. We show that this efficient electron spin relaxation mechanism can be suppressed by an external magnetic field as small as 100 mT.

Quantum theory of spin dynamics of exciton-polaritons in microcavities.

We present the quantum theory of momentum and spin relaxation of exciton-polaritons in microcavities. We show that giant longitudinal-transverse splitting of the polaritons mixes their spin states, which results in beats between right- and left-circularly polarized photoluminescence of microcavities, as was recently experimentally observed [Phys. Rev. Lett. 89, 077402 (2002)]]. This effect is strongly sensitive to the bosonic stimulation of polariton scattering.

Suppression of Dyakonov-Perel spin relaxation in high-mobility n-GaAs.

We report a large and unexpected suppression of the free electron spin-relaxation in lightly doped n-GaAs bulk crystals. The spin-relaxation rate shows a weak mobility dependence and saturates at a level 30 times less than that predicted by the Dyakonov-Perel theory. The dynamics of the spin-orbit field differs substantially from the usual scheme: although all the experimental data can be self-consistently interpreted as a precessional spin-relaxation induced by a random spin-orbit field, the correlation time of this random field, surprisingly, is much shorter than, and is independent of, the momentum relaxation time determined from transport measurements.