Quantum bits (qubits) are the fundamental building blocks of quantum information processors, such as quantum computers. A qubit comprises a pair of well characterized quantum states that can in principle be manipulated quickly compared to the time it takes them to decohere by coupling to their environment. Much remains to be understood about the manipulation and decoherence of semiconductor qubits. Here we show that hydrogen-atom-like motional states of electrons bound to donor impurities in currently available semiconductors can serve as model qubits. We use intense pulses of terahertz radiation to induce coherent, damped Rabi oscillations in the population of two low-lying states of donor impurities in GaAs. Our observations demonstrate that a quantum-confined extrinsic electron in a semiconductor can be coherently manipulated like an atomic electron, even while sharing space with approximately 10(5) atoms in its semiconductor host. We anticipate that this model system will be useful for measuring intrinsic decoherence processes, and for testing both simple and complex manipulations of semiconductor qubits.

We isolate the two-step mechanism involving a real intermediate photon from the one-step mechanism involving a virtual photon for the trident process in a constant crossed field. The two-step process is shown to agree with an integration over polarised sub-processes. At low to moderate quantum non-linearity parameter, the one-step process is found to be suppressed. When the parameter is large, the two decay channels are comparable if the field dimensions are not much greater than the formation length.

When exposed to intense electromagnetic fields, the quantum vacuum is expected to exhibit properties of a polarizable medium akin to a weakly nonlinear dielectric material. Various schemes have been proposed to measure such vacuum polarization effects using a combination of high-power lasers. Motivated by several planned experiments, we provide an overview of experimental signatures that have been suggested to confirm this prediction of quantum electrodynamics of real photon-photon scattering.Keywords: Heisenberg-Euler; photon-photon scattering; vacuum birefringence; vacuum polarization MotivationThe increasing availability of multi-hundred TW and PW lasers [1] brings the confirmation of long-predicted phenomena of strong-field quantum electrodynamics (QED) [2,3] closer. A multitude of effects on the polarization, wavevector and frequency of photons that probe the polarization of the charged virtual pairs of the vacuum have been theoretically investigated. All of these effects can be understood in terms of the single process of 'photon-photon scattering'. The current best experimental limit on the predicted crosssection for photon-photon scattering using just high-power laser pulses lies eighteen orders of magnitude above QED [4] , but recent laser-cavity experiments such as BMV [5] and PVLAS [6] have reduced this to six and three orders of magnitude, respectively (or three orders of magnitude and a factor 50, respectively, at the level of the refractive index). Moreover, coinciding with the completion of the XFEL laser at DESY, an experiment at the HIBEF facility [7] plans to measure one manifestation of photon-photon scattering, namely the birefringence of the vacuum, using the XFEL beam and a 1 PW optical laser. This has generated much interest in vacuum polarization effects.The aims of this work are two-fold. First, the main analytical approaches used to study photon-photon scattering will be shown to be essentially equivalent for predictions of planned laser experiments. Second, an overview of the predicted signatures of real photon-photon scattering in various experimental scenarios will be provided, which is also hoped to be useful for the nonspecialist and, in particular, promote discussions between theorists and experimentalists. Introduction: vacuum polarizationVacuum polarization, depicted in the Feynman diagram of Figure 1, is a basic radiative correction that modifies the propagation of photons in vacuum through the appearance of virtual pairs in a 'fermion loop'.There are two complementary interpretations of this effect. The first is based on what is called 'old-fashioned' perturbation theory which emphasizes energy considerations at the price of manifest covariance [8] . In this interpretation, Heisenberg's uncertainty relation is invoked to show how quantum mechanics predicts energy and momentum conservation may be violated. The amount of this violation is inversely proportional to the space-time scale over which it occurs. This effect is represented by short-lived 'virtual' particles. T...

We test current numerical implementations of laser-matter interactions by comparison with exact analytical results. Focusing on photon emission processes, it is found that the numerics accurately reproduce analytical emission spectra in all considered regimes, except for the harmonic structures often singled out as the most significant high-intensity (multiphoton) effects. We find that this discrepancy originates in the use of the locally constant field approximation.

Double-slits provide incoming photons with a choice. Those that survive the passage have chosen from two possible paths which interfere to distribute them in a wave-like manner. Such wave-particle duality continues to be challenged and investigated in a broad range of disciplines with electrons, neutrons, helium atoms, C60 fullerenes, Bose-Einstein condensates and biological molecules. All variants have hitherto involved material constituents. We present a matterless double-slit scenario in which photons generated from virtual electron-positron pair annihilation in head-on collisions of a probe laser field with two ultra-intense laser beams form a double-slit interference pattern. Such electromagnetic fields are predicted to induce material-like behaviour in the vacuum, supporting elastic scattering between photons. Our double-slit scenario presents on the one hand a realisable method to observe photon-photon scattering, and demonstrates on the other, the possibility of both controlling light with light and non-locally investigating features of the quantum vacuum's structure.Comment: Originally submitted 1st July 2009, published 10th January 201