The silicon monovacancy in 4H-SiC is a promising candidate for solid-state quantum information processing. We perform high-resolution optical spectroscopy on single V2 defects at cryogenic temperatures. We find favorable low temperature optical properties that are essential for optical readout and coherent control of its spin and for the development of a spin-photon interface. The common features among individual defects include two narrow, nearly lifetime-limited optical transitions that correspond to ms= ± 3/2 and ms= ± 1/2 spin states with no discernable zero-field splitting fluctuations. Initialization and readout of the spin states is characterized by time-resolved optical spectroscopy under resonant excitation of these transitions, showing significant differences between the ±3/2 and ±1/2 spin states. These results are well-described by a theoretical model that strengthens our understanding of the quantum properties of this defect. arXiv:1811.01293v2 [cond-mat.mes-hall]

Endophilin, which participates in membrane vesiculation during receptor-mediated endocytosis, is a ∼40 kDa SH3 domain-containing protein that binds to the proline/arginine-rich domain of dynamin, a ∼100 kDa GTPase that is essential for endocytic membrane scission. It has been suggested that endophilin is monomeric in the cytoplasm and dimerizes only after it binds to membranes (or perhaps to dimers or tetramers of dynamin). To clarify this issue, we studied the oligomeric state of endophilin both in vitro using analytical ultracentrifugation and fluorescence anisotropy, and in living cells using two-photon fluorescence fluctuation spectroscopy. We analyzed the fluctuation data using the Q-analysis method, which allowed us to determine the intrinsic brightness of the labeled protein complexes and hence its aggregation state in the cytoplasmic regions of the cell. Although a relatively high K(d) (∼5-15 μM) was observed in vitro, the cell measurements indicate that endophilin is dimeric in the cytoplasm, even at submicromolar concentrations. We also demonstrate that endophilin significantly enhances the assembly of dynamin, and that this enhancement is proportional to the fraction of dimeric endophilin that is present. Moreover, there is correlation between the concentrations of endophilin that promote dynamin self-assembly and those that stimulate dynamin GTPase activity. These findings support the view that endophilin-dynamin interactions play an important role in endocytosis.

The direct measurement of Berry phases is still a great challenge in condensed-matter systems. The bottleneck has been the ability to adiabatically drive an electron coherently across a large portion of the Brillouin zone in a solid where the scattering is strong and complicated. We break through this bottleneck and show that high-order sideband generation (HSG) in semiconductors is intimately affected by Berry phases. Electron-hole recollisions and HSG occur when a near-band-gap laser beam excites a semiconductor that is driven by sufficiently strong terahertz-frequency electric fields. We carry out experimental and theoretical studies of HSG from three GaAs=AlGaAs quantum wells. The observed HSG spectra contain sidebands up to the 90th order, to our knowledge the highest-order optical nonlinearity reported in solids. The highest-order sidebands are associated with electron-hole pairs driven coherently across roughly 10% of the Brillouin zone around the Γ point. The principal experimental claim is a dynamical birefringence: the intensity and polarization of the sidebands depend on the relative polarization of the exciting near-infrared (NIR) and the THz electric fields, as well as on the relative orientation of the laser fields with the crystal. We explain dynamical birefringence by generalizing the three-step model for highorder harmonic generation. The hole accumulates Berry phases due to variation of its internal state as the quasimomentum changes under the THz field. Dynamical birefringence arises from quantum interference between time-reversed pairs of electron-hole recollision pathways. We propose a method to use dynamical birefringence to measure Berry curvature in solids.

Electron-hole recollisions are induced by resonantly injecting excitons with a near-IR laser at frequency fNIR into quantum wells driven by a 10 kV/cm field oscillating at fTHz = 0.57 THz. At T = 12 K, up to 18 sidebands are observed at frequencies f sideband = fNIR + 2nfTHz, with −8 ≤ 2n ≤ 28. Electrons and holes recollide with total kinetic energies up to 57 meV, well above the ELO = 36 meV threshold for longitudinal optical (LO) phonon emission. Sidebands with order up to 2n = 22 persist up to room temperature. A simple model shows that LO phonon scattering suppresses but does not eliminate sidebands associated with kinetic energies above ELO.The interaction of electrons and holes in semiconductors has long been a rich area of study in physics. In most cases, the electron and hole are treated as point particles within the effective mass approximation [1]. In this approximation, the effects of the lattice are parametrized by the dielectric constant of the semiconductor and the effective masses of the electrons and holes. Calculations based on the effective mass approximation successfully predict, for example, the binding energies and the frequencies of internal transitions of impurity-bound electrons and excitons in GaAs [2] and in GaAs/AlGaAs quantum wells [3][4][5]. The effective mass approximation, however, belies the rich complexity of the microscopic physics. In GaAs, for example, ground-state excitons are collective excitations of more than 10 5 atoms in the crystal. The advent of sources of intense terahertz electromagnetic radiation enables the study of semiconductors in a regime where time-dependent perturbation theory fails completely. Exciting new quantum coherent phenomena emerge, including the dynamical Franz-Keldysh effect [6,7], non-linear excitonic effects [8][9][10][11], and highorder sideband generation [12][13][14]. High-order sideband generation is a cousin of high-order harmonic generation from atoms in intense laser fields [15][16][17]. Excitons are created by a near-infrared laser in a semiconductor driven by an intense terahertz field. The terahertz field ionizes the exciton into an electron and a hole that it then pulls apart and smashes back together. Upon recollision, the electron and hole can recombine across the band gap with the acquired kinetic energy carried off by light with frequency above that of the near-IR laser.In this Letter, we report that the electron-hole recollision process-which, in its theoretical descriptions to date has been treated as a purely ballistic process-is surprisingly robust against scattering. We report electronhole recollisions both with kinetic energy well above the threshold for emission of a longitudinal optical (LO) phonon and at room temperature. We model our results by treating the electron-hole pair as a single particle with the appropriate reduced mass subject to dephasing and LO phonon scattering.Two quantum well (QW) samples with 20 QW repetitions were studied. Both samples were grown on semiinsulating GaAs substrates by molecular beam epitax...

Point defects in SiC are an attractive platform for quantum information and sensing applications because they provide relatively long spin coherence times, optical spin initialization, and spin-dependent fluorescence readout in a fabrication-friendly semiconductor. The ability to precisely place these defects at the optimal location in a host material with nano-scale accuracy is desirable for integration of these quantum systems with traditional electronic and photonic structures. Here, we demonstrate the precise spatial patterning of arrays of silicon vacancy ($${V}_{Si}$$ V Si ) emitters in an epitaxial 4H-SiC (0001) layer through mask-less focused ion beam implantation of Li+. We characterize these arrays with high-resolution scanning confocal fluorescence microscopy on the Si-face, observing sharp emission lines primarily coming from the $${V1}^{{\prime}}$$ V 1 ′ zero-phonon line (ZPL). The implantation dose is varied over 3 orders of magnitude, leading to $${V}_{Si}$$ V Si densities from a few per implantation spot to thousands per spot, with a linear dependence between ZPL emission and implantation dose. Optically-detected magnetic resonance (ODMR) is also performed, confirming the presence of V2 $${V}_{Si}$$ V Si . Our investigation reveals scalable and reproducible defect generation.