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The gravitational field of a laser pulse of finite lifetime, is investigated in the framework of linearized gravity. Although the effects are very small, they may be of fundamental physical interest. It is shown that the gravitational field of a linearly polarized light pulse is modulated as the norm of the corresponding electric field strength, while no modulations arise for circular polarization. In general, the gravitational field is independent of the polarization direction. It is shown that all physical effects are confined to spherical shells expanding with the speed of light, and that these shells are imprints of the spacetime events representing emission and absorption of the pulse. Nearby test particles at rest are attracted towards the pulse trajectory by the gravitational field due to the emission of the pulse, and they are repelled from the pulse trajectory by the gravitational field due to its absorption. Examples are given for the size of the attractive effect. It is recovered that massless test particles do not experience any physical effect if they are co-propagating with the pulse, and that the acceleration of massless test particles counter-propagating with respect to the pulse is four times stronger than for massive particles at rest. The similarities between the gravitational effect of a laser pulse and Newtonian gravity in two dimensions are pointed out. The spacetime curvature close to the pulse is compared to that induced by gravitational waves from astronomical sources.
The differential cross-section for gravitational photon-photon scattering calculated in perturbative quantum gravity is shown to depend on the degree of polarization entanglement of the two photons. The interaction between photons in the symmetric Bell state is stronger than between not entangled photons. In contrast, the interaction between photons in the anti-symmetric Bell state is weaker than between not entangled photons. The results are interpreted in terms of quantum interference, and it is shown how they fit into the idea of distance-dependent forces. Copyright (C) EPLA, 2016
We present a simple setup that exploits the interference of entangled photon pairs. 'Signal' photons are sent through a Mach–Zehnder-like interferometer, while 'idlers' are detected in a variable polarization state. Two-photon interference (in coincidence detection) is observed with very high contrast and for significant time delays between signal and idler detection events. This is explained by quantum erasure of the polarization tag and a delayed choice protocol involving a non-local virtual polarizer. The phase of the two-photon fringes is scanned by varying the path length in the signal beam or by rotating a birefringent crystal in the idler beam. We exploit this to characterize one beam splitter of the signal photon interferometer (reflection and transmission amplitudes including losses), using only information about coincidences and control parameters in the idler path. This is possible because our bi-photon state saturates the Greenberger–Yelin–Englert inequality between contrast and predictability.
Quasi-monolithic ring resonator for efficient frequency doubling of an external cavity diode laser
(2010)
A quasi-monolithic second-harmonic-generation ring resonator assembled with miniaturized components is presented. The ring contains a 10-mm-long bulk periodically poled lithium niobate crystal for second-harmonic generation, four plane mirrors and two gradient-index lenses. All parts are mounted on a glass substrate with an overall size of 19.5 mmx8.5 mmx4 mm. As pump source a broad-area laser diode operated in an external resonator with Littrow arrangement is utilized. This external cavity diode laser provides near diffraction limited, narrow-bandwidth emission with an optical output power of 450 mW at a wavelength of 976 nm. Locking of the diode laser emission to the resonance frequency of the ring cavity was achieved by an optical self-injection locking technique. With this setup more than 126 mW of diffraction-limited blue light at 488 nm could be generated. The opto-optical conversion efficiency was 28% and a wall plug efficiency better than 5.5% could be achieved.
Stripe-array diode lasers naturally operate in an anti-phase supermode. This produces a sharp double lobe far field at angles ña depending on the period of the array. In this paper a 40 emitter gain guided stripe-array laterally coupled by off-axis filtered feedback is investigated experimentally and numerically. We predict theoretically and confirm experimentally that at doubled feedback angle 2a a stable higher order supermode exists with twice the number of emitters per array period. The theoretical model is based on time domain traveling wave equations for optical fields coupled to the carrier density equation taking into account diffusion of carriers. Feedback from the external reflector is modeled using Fresnel integration.