Abstract
A periodically poled lithium niobate (PPLN) crystal has been used as an efficient source of non-collinearly generated polarization-entangled photon pairs at 810 and 1550 nm. The PPLN crystal was endowed with a specially designed poling pattern and the entangled photons were generated via the nonlinear optical process of spontaneous parametric down conversion (SPDC). A novel design based on overlapping two concurrent type-I quasi-phase-matching structures in a single PPLN crystals produced correlated pairs of alternatively polarized photons in largely separated spectral regions. The phase of the resulting two-photon state is directly linked to parameters of the nonlinear grating. Continuous tunability of the generated Bell state, from Phi(+) to Phi(-), has been demonstrated by translating a slightly wedged crystal perpendicular to the pump beam.
Highlights
Spontaneous parametric downconversion (SPDC) [1] has been widely used as a source of correlated and entangled photon pairs in many quantum optics experiments
Practical quantum cryptography is expected to benefit from the availability of a compact and efficient source of polarization entanglement between photons whose frequencies are well separated, when one of the wavelengths falls in the telecommunication window near 1550 nm and the other lies in the visible region [16]
State to another, requires the insertion of auxiliary birefringent elements in the outgoing beams. Is it possible to create a compact single-crystal source, pumped by a single beam, that generates polarization-entangled photon pairs, with well separated frequencies in a non-collinear geometry, that does not requires additional optics for tuning the output state? We demonstrate, both theoretically and experimentally, that a specially engineered periodically poled crystal (PPC) can serve as just such a source when suitably prepared
Summary
Spontaneous parametric downconversion (SPDC) [1] has been widely used as a source of correlated and entangled photon pairs in many quantum optics experiments. A polarization-entangled state can be engineered by making use of fully overlapping correlated-photon outcomes from separate nonlinear interactions in two thin type-I nonlinear crystals placed immediately adjacent to each other; downconversion with orthogonal polarizations emerges from the crystal pair when pumped by a single beam polarized at 45◦ [19]. An additional birefringent plate must be placed in the path of the pump beam to tune the phase of a Bell-state An extension of this approach that makes use of two orthogonally oriented PPKTP crystals has led to the collinear generation of nondegenerate polarization-entangled pairs [20]. State to another, requires the insertion of auxiliary birefringent elements in the outgoing beams Is it possible to create a compact single-crystal source, pumped by a single beam, that generates polarization-entangled photon pairs, with well separated frequencies in a non-collinear geometry, that does not requires additional optics for tuning the output state? We demonstrate the tuning the state from Φ+ to Φ− by a simple translation of the crystal
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