Polarization-preserving confocal microscope for optical experiments in a dilution refrigerator with high magnetic field

We present the design and operation of a fiber-based cryogenic confocal microscope. It is designed as a compact cold-finger that fits inside the bore of a superconducting magnet, and which is a modular unit that can be easily swapped between use in a dilution refrigerator and other cryostats. We aimed at application in quantum optical experiments with electron spins in semiconductors and the design has been optimized for driving with, and detection of optical fields with well-defined polarizations. This was implemented with optical access via a polarization maintaining fiber together with Voigt geometry at the cold finger, which circumvents Faraday rotations in the optical components in high magnetic fields. Our unit is versatile for use in experiments that measure photoluminescence, reflection, or transmission, as we demonstrate with a quantum optical experiment with an ensemble of donor-bound electrons in a thin GaAs film.Comment: 9 pages, 7 figure

Electromagnetically Induced Transparency with an Ensemble of Donor-Bound Electron Spins in a Semiconductor

We present measurements of electromagnetically induced transparency with an ensemble of donor- bound electrons in low-doped n-GaAs. We used optical transitions from the Zeeman-split electron spin states to a bound trion state in samples with optical densities of 0.3 and 1.0. The electron spin dephasing time T* \approx 2 ns was limited by hyperfine coupling to fluctuating nuclear spins. We also observe signatures of dynamical nuclear polarization, but find these effects to be much weaker than in experiments that use electron spin resonance and related experiments with quantum dots.Comment: 4 pages, 4 figures; Improved analysis of data in Fig. 3, corrected factors of 2 and p

Towards quantum optics and entanglement with electron spin ensembles in semiconductors

We discuss a technique and a material system that enable the controlled realization of quantum entanglement between spin-wave modes of electron ensembles in two spatially separated pieces of semiconductor material. The approach uses electron ensembles in GaAs quantum wells that are located inside optical waveguides. Bringing the electron ensembles in a quantum Hall state gives selection rules for optical transitions across the gap that can selectively address the two electron spin states. Long-lived superpositions of these electron spin states can then be controlled with a pair of optical fields that form a resonant Raman system. Entangled states of spin-wave modes are prepared by applying quantum-optical measurement techniques to optical signal pulses that result from Raman transitions in the electron ensembles.Comment: Proceedings E-MRS 2007, session on solid-state quantum information, 7 pages, 3 figure

Quantum Optical Control of Donor-bound Electron Spins in GaAs

This thesis presents experimental research on coherent manipulation by laser light of the quantum state of an ensemble of electron spins in a solid. The medium is formed by donor-bound electron spins in GaAs. The coherent manipulation is achieved with a technique that was till now mostly explored with atomic vapors. Applying this to a donor-bound electron ensemble is possible at low temperature and low donor concentration, where neighboring electrons do not interact. Under these conditions, the electron ensembles and atomic vapors have similar properties. However, the solid-state medium has the advantage of easy integration with existing semiconductor technologies, and is thereby a more suitable system for quantum information technologies. Moreover, the rich interactions between the spins and their environment in GaAs make it an interesting system for exploring new physics. In order to control the electron spins with lasers at low temperature we built a dedicated experimental setup, which allows mechanical positioning of the sample in a helium cryostat with sub-micrometer precision. The laser light is delivered to the sample by an optical fiber. In a high magnetic field, we observed and controlled quantum phenomena that result in optical transparency for transitions that are addressed with a resonant laser. This gives access to a robust technique for controlling strong interactions between quantum optical signal fields and quantum states of spins. Our results thereby provide all the experimental techniques that are needed for future experiments on demonstrating the preparation of nonlocal quantum entanglement between spin excitations in two different ensembles.