Using the process of spontaneous parametric downconversion in a novel two-crystal geometry, we have generated a source of polarization-entangled photon pairs which is more than ten times brighter, per unit of pump power, than previous sources, with another factor of 30 to 75 expected to be readily achievable. We have measured a high level of entanglement between photons emitted over a relatively large collection angle, and over a 10-nm bandwidth. As a demonstration of the source capabilities, we obtained a 242-σ violation of Bell's inequalities in less than three minutes, and observed near-perfect photon correlations when the collection efficiency was reduced. In addition, both the degree of entanglement and the state purity should be readily tunable. [5], and quantum computation [6]. At present, the most accessible and controllable source of entanglement arises from the process of spontaneous parametric down-conversion in a nonlinear optical crystal. Here we describe a proposal for, and experimental realization of, an ultrabright source of polarization-entangled photon pairs, using two such nonlinear crystals. Because nearly every pair of photons produced is polarization-entangled, the total flux of emitted polarization-entangled pairs should be hundreds of times greater than is achievable with the best previous source, for comparable pump powers. The new technique has the added advantage that the degree of entanglement and the purity of the state may be readily tunable, heretofore impossible.It is now well known that the photons produced via the down-conversion process share nonclassical correlations [7]. In particular, when a pump photon splits into two daughter photons, conservation of energy and momentum lead to entanglements in these two continuous degrees of freedom [8]. Yet conceptually, the simplest examples of entangled states of two photons are the polarizationentangled "Bell states":where H and V denote horizontal and vertical polarization, respectively, and for convenience we omit the normalization factor (1/ √ 2). For instance, HV − V H is the direct analog of the spin-singlet considered by Bell [2]. To date there have been only two methods for producing such polarization-entangled photon pairs, and each has fairly substantial limitations. The first was an atomic cascade -a two-photon decay process from one state of zero angular momentum to another. The resulting photons do display nonclassical correlations (they were used in the first tests of Bell's inequalities [9,10]), but the correlations decrease if the photons are not emitted backto-back, as is allowed by recoil of the parent atom.This problem was circumvented with parametric downconversion, since the emission directions of the photons are well-correlated. In several earlier experiments downconversion photon pairs of definite polarization were incident on a beamsplitter, and nonclassical correlations observed for those post-selected events in which photons traveled to different output ports [11]. However, the photons were actually created i...

There are various intrinsic device aspects that allow a clean spin transport signal in Ic2, and that make it immune to fringe field-induced magnetoresistance and Hall effects: 1. The exponential spin selective mean free path dependence in the ferromagnetic films create very large spin polarizations. In principle this can approach 100%, allowing effective injection and detection at cryogenic and room temperatures 11 ; 3 2. Because the spin filtering is caused by bulk scattering in the ferromagnetic films, they are easy to reproduce, since there is no interface sensitivity to the spin filtering (as there is, e.g., in magnetic tunnel junctions); 3. This device, like a spin-valve transistor, is a high impedance current source. 11,12 Since Ic2 is driven by Ic1, and Ic1 by Ie, Ic2 is virtually independent of Vc1, the applied voltage across the Si drift region. This also means that any generated Hall voltage in the FZ-Si has no effect on Ic2. The underlying background to the insensitivity to resistance and voltage of the FZ-Si is that the

We use all-electrical methods to inject, transport, and detect spin-polarized electrons vertically through a 350-micron-thick undoped single-crystal silicon wafer. Spin precession measurements in a perpendicular magnetic field at different accelerating electric fields reveal high spin coherence with at least 13pi precession angles. The magnetic-field spacing of precession extrema are used to determine the injector-to-detector electron transit time. These transit time values are associated with output magnetocurrent changes (from in-plane spin-valve measurements), which are proportional to final spin polarization. Fitting the results to a simple exponential spin-decay model yields a conduction electron spin lifetime (T1) lower bound in silicon of over 500 ns at 60 K.

We present a symmetry analysis of electronic bandstructure including spin-orbit interaction close to the insulating gap edge in monolayer black phosphorus ('phosphorene'). Expressions for energy dispersion relation and spin-dependent eigenstates for electrons and holes are found via simplification of a perturbative expansion in wavevector $k$ away from the zone center using elementary group theory. Importantly, we expose the underlying symmetries giving rise to substantial anisotropy in optical absorption, charge and spin transport properties, and reveal the mechanism responsible for valence band distortion and possible lack of a true direct gap

A unique spin depolarization mechanism, induced by the presence of g-factor anisotropy and intervalley scattering, is revealed by spin-transport measurements on long-distance germanium devices in a magnetic field longitudinal to the initial spin orientation. The confluence of electron-phonon scattering (leading to Elliott-Yafet spin flips) and this previously unobserved physics enables the extraction of spin lifetime solely from spin-valve measurements, without spin precession, and in a regime of substantial electric-field-generated carrier heating. We find spin lifetimes in Ge up to several hundreds of nanoseconds at low temperature, far beyond any other available experimental results.