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We present results of angle-dependent magnetoresistance measurements and direct optical images of underdoped $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ which reveal partial detwinning by action of a 14 T magnetic field. The relative change in the twin domain population for the given field is modest, of order $5--15\text{ }\mathrm{%}$. The associated field-induced hysteretic changes in the resistivity are up to 5% for intermediate Co concentrations, implying a large in-plane resistivity anisotropy in the broken-symmetry state. Based on the generic anisotropic susceptibility of collinear antiferromagnets, we infer a smaller resistivity along the antiferromagnetic ordering direction. The observation of field-induced motion of twin boundaries indicates a substantial magnetoelastic coupling in this material.

The phase coherence of a physical qubit is essential for quantum information processing, motivating fast control methods to preserve that phase. Ultrafast optical techniques allow complete spin control to be performed on a much faster timescale than microwave or electrical control (ps vs. ns at best). Using our ultrafast control techniques, we demonstrate our experimental approach towards a spin echo sequence on the spin of a single electron confined in a semiconductor quantum dot (QD), increasing the observed decoherence time of a single QD electron spin from nanoseconds to several microseconds. The ratio of the observed decoherence time to the demonstrated single-qubit gate time exceeds 10 5 , suggesting strong promise for future quantum information processors.

We report the observation of a feedback process between the nuclear spins in a single charged quantum dot and its trion transition, driven by a periodic sequence of optical pulses. The pulse sequence intersperses off-resonant ultrafast pulses for coherent electron-spin rotation and resonant narrow-band optical pumping. The feedback manifests as a hysteretic triangle-like pattern in the free-induction-decay of the single spin. We present a simple, quasi-analytic numerical model to describe this observation, indicating that the feedback process results from the countering effects of optical nuclear pumping and nuclear spin-diffusion inside the quantum dot. This effect allows dynamic tuning of the electron Larmor frequency to a value determined by the pulse timing, potentially allowing more complex coherent control operations., 4 pages, 3 figures

We report an ultrafast downconversion quantum interface, where 910-nm single photons from a quantum dot are downconverted to the 1.5-μm telecom band with sub-10 picosecond pulses at 2.2-μm, enabling the demonstration of quantum-dot spin-photon entanglement.