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Research in Optical Sciences

Spatially entangled 4-photons states from a periodically poled KTP crystal

Alexander J. H. van der Torren, S. Cigdem Yorulmaz, Jelmer J. Renema, Martin P. van Exter, M. J. A. de Dood

Published Jan 1, 2012• 1 citation
Publisher

Abstract

We explore four-photon spatial entanglement created by stimulated emission of photon pairs in a 2 mm long periodically poled KTP crystal. We vary the experimental conditions to explore and optimize the visiblity of stimulated pairs.
DOI
10.1364/qim.2012.qt4b.4

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Spatial quantum correlations are an important resource for quantum imaging and future quantum technologies based on high-dimensional entanglement. We experimentally explore spatial correlations in four-photon states generated by a highly efficient parametric down-conversion process in a 2 mm long periodically poled KTP crystal. The four-photon states produced in the crystal contain a contribution due to independent double pairs produced via spontaneous pair emission and a highly correlated four-photon state produced by stimulated emission of a second photon pair. We separate this contribution and introduce a joint spatial density for stimulated pair emission. We observe a maximum visibility $\ensuremath{\chi}=0.25$ of the four-photon state for a 1.5 nm bandpass filter.
We extend our earlier results of a bright source of non‐degenerate polarization entangled photon‐pairs by making a thorough theoretical and experimental investigation of its single‐mode emission and state‐decoherence. Using two perpendicular bulk crystals of periodically poled KTP, the source directly creates the polarization‐entangled state (|HH〉 + |VV〉)/√ with signal and idler wavelengths of 810 nm and 1550 nm, respectively. In order to optimize the coupling into single‐mode fiber we have developed a numerical algorithm which determines the emission mode and the fiber coupling efficiency for a given crystal length and focussing of the pump. For a 4.5 mm long crystal the optimal focussing of the pump is to a beam waist radius of 14 μm. With these focussing conditions the computer calculations give the lowest M2 value for the emission mode, M2 ≈ 3, and the best possible fiber coupling efficiency, γ ≈ 75%. Using the theoretical results to optimize our experimental setup we obtained photon count rates of 1.4 MHz singles and 12 kHz coincidences, and visibilities H, V: 99.6% and 45°: 94.2%.