Broadband-laser-diode pumped periodically poled potassium titanyl phosphate-Sagnac polarization-entangled photon source

Abstract

We experimentally demonstrate a polarization-entangled photon source at 810 nm using a type-II phase-matched periodically poled potassium titanyl phosphate (PPKTP) crystal pumped by a low-cost, broadband laser diode with a central wavelength of 405 nm and a typical bandwidth of 0.53 nm. The PPKTP crystal is placed in a Sagnac loop to realize the compact size and high stability. The downconverted biphotons, the signal and the idler, have typical bandwidths of 5.57 and 7.32 nm. We prepare two Bell states <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo stretchy="false">|</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mi mathvariant="normal">Ψ</mml:mi> <mml:mo>+</mml:mo> </mml:msup> </mml:mrow> <mml:mo fence="false" stretchy="false">⟩</mml:mo> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo stretchy="false">|</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mi mathvariant="normal">Ψ</mml:mi> <mml:mo>−</mml:mo> </mml:msup> </mml:mrow> <mml:mo fence="false" stretchy="false">⟩</mml:mo> </mml:math> with the fidelities of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>0.948</mml:mn> </mml:mrow> <mml:mo>±</mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>0.004</mml:mn> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>0.963</mml:mn> </mml:mrow> <mml:mo>±</mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>0.002</mml:mn> </mml:mrow> </mml:math> . In the polarization correlation measurement, the visibilities are all higher than 96.2%, and in the Bell inequality test, the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi>S</mml:mi> </mml:mrow> </mml:math> value can achieve <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>2.78</mml:mn> </mml:mrow> <mml:mo>±</mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>0.01</mml:mn> </mml:mrow> </mml:math> . This high-quality and low-cost entangled photon source may have many practical applications in quantum information processing.