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arXiv:0810.0998 · Physical Review A

Experimental verification of high spectral entanglement for pulsed waveguided spontaneous parametric down-conversion

Malte Avenhaus, Maria V. Chekhova, Leonid A. Krivitsky, Gerd Leuchs, Christine Silberhorn

Published Apr 30, 2009• 49 citations
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Abstract

We study the spectral properties of spontaneous parametric down-conversion (SPDC) in a periodically poled waveguided structure of potassium-titanyl-phosphate (KTP) crystal pumped by ultrashort pulses. Our theoretical analysis reveals a strongly entangled and asymmetric structure of the two-photon spectral amplitude for type-II SPDC. We confirm these predictions experimentally by measuring single-photon spectra, on one hand, and the dependence of Hong-Ou-Mandel interference visibility on the width of spectral filtering, on the other hand.

DOI
10.1103/physreva.79.043836

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We study the spectral properties of spontaneous parametric down-conversion (SPDC) in a periodically poled waveguided structure of potassium-titanyl-phosphate (KTP) crystal pumped by ultrashort pulses. Our theoretical analysis reveals a strongly entangled and asymmetric structure of the two-photon spectral amplitude for type-II SPDC. We confirm these predictions experimentally by measuring single-photon spectra, on one hand, and the dependence of Hong-Ou-Mandel interference visibility on the width of spectral filtering, on the other hand.
Spontaneous parametric down-conversion (SPDC) is a reliable and robust source of photons for quantum information applications. For applications that involve operations such as entanglement swapping or single-photon heralding, two-photon states are required to be factorable (uncorrelated) in their spectral and spatial degrees of freedom. We report the design and experimental characterization of an SPDC source that has been optimized for high spectral and spatial purity. The source is pumped by the 776 nm output of a mode-locked Ti:Sapphire laser and consists of a periodically-poled Potassium Titanyl Phosphate (PPKTP) crystal phase-matched for collinear type-II SPDC. The dispersive properties of PPKTP at these wavelengths is such that it is possible to minimize the spectral entanglement by matching the widths of the pump to the spectral phase-matching function. The spatial entanglement is minimized through careful control of the pump focus, yielding nearly single-mode emission. An advantage of this approach is that the emission rate into the collection modes is very high, resulting in a very bright SPDC source. We also report a scheme that employs the output of collinear sources such as these to produce polarization-entangled photon pairs. The scheme, which requires only simple polarization elements, can be scaled to N-photon GHZ states.
The properties of the backward-wave spontaneous parametric down-conversion (SPDC) in a periodically poled potassium titanyl phosphate (KTP) waveguide are studied in the context of creating narrowband heralded sources of single-photon states. The effective index of refraction and spatial profile of different waveguide modes, efficiency of different SPDC processes and purity of heralded photons are calculated numerically for a given waveguide. Compared to the usual co-propagating SPDC, spectral narrowing of the backward-wave SPDC was observed as should be expected. Generation biphoton states in backward-wave regime is experimentally observed in two-photon detection scheme.
The frequency entangled biphoton source generated via spontaneous parametric down-conversion process (SPDC) has found numerous applications in quantum information processing and relevant fields. We report on an experimental generation of coincident-frequency entanglement from periodically poled potassium titanyl phosphate, pumped by an ultra-short pulsed optical source with duration less than 20 fs. Based on the Hong-Ou-Mandel interferometric coincidence measurement setup, a visibility of about 42% is demonstrated, which indicates degraded frequency indistinguishibility of the down-converted biphotons. Through theoretical investigation, such a degradation can be perfectly explained by the nonnegligible second-order dispersion terms in the Taylor-expanded phase mismatching function for the case of ultra-broadband spectrum of the pulsed pump. The fitting to the experimental results is further used and perfect agreement is achieved. The results imply that the spectral bandwidth of the pump can affect the generated coincident-frequency entanglement.
The properties of the backward-wave spontaneous parametric down-conversion (SPDC) in a periodically poled potassium titanyl phosphate (KTP) waveguide are studied in the context of creating narrowband heralded sources of single-photon states. The effective index of refraction and spatial profile of different waveguide modes, efficiency of different SPDC processes and purity of heralded photons are calculated numerically for a given waveguide. Compared to the usual co-propagating SPDC, spectral narrowing of the backward- wave SPDC was observed as should be expected.
Over the past decade, counter-propagating spontaneous parametric down-conversion (CPSPDC) has attracted interest due to its ability to generate narrowband and spectrally pure photon pairs without the need for strong spectral filtering. While prior work has established the spectral characteristics of CPSPDC, its performance as a heralded single-photon source (HSPS) remains limited. Here, we present a detailed HSPS characterization of a first-order quasi-phase-matched CPSPDC source based on a sub-μm periodically poled potassium titanyl phosphate crystal. We measure an internal pair-generation rate of 1.3×10 3 pairs/s/mW under CW pumping, in addition to a high coincidence-to-accidental ratio exceeding 1.4×10 5 , and furthermore obtain the lowest heralded second-order correlation value of (8.57±4.95)×10 -6 ever reported. We also show a Hong–Ou–Mandel interference experiment that yields a raw visibility of 70±1%, confirming photon indistinguishability. These results demonstrate CPSPDC as a strong candidate for generating high-quality single photons for quantum information processing and long-distance quantum communication applications.