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Optics Express

Polarization-entangled photon pairs from a periodically poled crystalline waveguide

Zachary H. Levine, Jingyun Fan, Jun Chen, Alan L. Migdall

Published Mar 24, 2011• 18 citations
Publisher

Abstract

A proposal is made for the generation of polarization-entangled photon pairs from a periodically poled crystal allowing for high collection efficiency, high entanglement, and stable operation. The theory is formulated for colinear propagation for application to waveguides. The key feature of the theory is the use of type II phase matching using both the +1 and -1 diffraction orders of the poling structure. Although these conditions are fairly restrictive in terms of operating parameters, practical operating conditions can be found. For example, we find that a HeNe pump laser may be used for a periodically poled rubidium-doped potassium titanyl phosphate (Rb:KTP) waveguide to yield single mode polarization-entangled pairs. Fidelities of 0.98 are possible under practical conditions.

DOI
10.1364/oe.19.006724

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A proposal is made for the generation of polarization-entangled photon pairs from a periodically poled crystal allowing for high collection efficiency, high entanglement, and stable operation. The theory is formulated for colinear propagation for application to waveguides. The key feature of the theory is the use of type II phase matching using both the +1 and -1 diffraction orders of the poling structure. Although these conditions are fairly restrictive in terms of operating parameters, practical operating conditions can be found. For example, we find that a HeNe pump laser may be used for a periodically poled rubidium-doped potassium titanyl phosphate (Rb:KTP) waveguide to yield single mode polarization-entangled pairs. Fidelities of 0.98 are possible under practical conditions.
The need for bright efficient sources of entangled photons has been a subject of tremendous research over the last decade. Researchers have been working to increase the brightness and purity to help overcome the spontaneous nature of the sources. Periodic poling has been implemented to allow for the use of crystals that would not normally satisfy the phase matching conditions. Utilizing periodic poling and single mode waveguide confinement of the pump field has yielded extremely large effective nonlinearities in sources easily producing millions of photon pairs. Here we will demonstrate these large nonlinearity effects in a periodically poled potassium titanyl phosphate (PPKTP) waveguide as well as characterizing the source purity.
In this paper, we propose a single integrated waveguide device to generate two different polarization-entangled Bell states involving nondegenerate photon pairs (with the signal photons being emitted in the telecom CCC band) by utilizing type-0, type-I, and type-II quasi-phase-matching (QPM). In this context, we have studied ppKTP (periodically-poled potassium titanyl phosphate) in detail. To obtain polarization entanglement of the emitted photons, we have used two different situations, two cascaded QPM gratings (of the same length) to obtain $|{\ensuremath{\psi}}^{+}\ensuremath{\rangle}$, and two separate QPM gratings (of different lengths) to obtain $|{\ensuremath{\phi}}^{+}\ensuremath{\rangle}$. In such a scenario, maximal entanglement can be quantified in terms of overlap bandwidths of the two phase-matched processes.
We investigated the high-purity entangled photon-pair generation in five kinds of “non-poled” potassium titanyl phosphate (KTP) isomorphs (i.e., KTiOPO4, RbTiOPO4, KTiOAsO4, RbTiOAsO4, and CsTiOAsO4). The technique is based on the spontaneous parametric down-conversion (SPDC) under Type II extended phase matching (EPM), where the phase matching and the group velocity matching are simultaneously achieved between the interacting photons in non-poled crystals rather than periodically poled (PP) KTPs that are widely used for quantum experiments. We discussed both theoretically and numerically all aspects required to generate photon pairs in non-poled KTP isomorphs, in terms of the range of the beam propagation direction (or the spectral range of photons) and the corresponding effective nonlinearities and beam walk-offs. We showed that the SPDC efficiency can be increased in non-poled KTP isomorphs by 29% to 77% compared to PPKTP cases. The joint spectral analyses showed that photon pairs can be generated with high purities of 0.995–0.997 with proper pump filtering. In contrast to the PPKTP case, where the EPM is achieved only at one specific wavelength, the spectral position of photon pairs in the non-poled KTP isomorphs can be chosen over the wide range of 1883.8–2068.1 nm.
We present two experiments geared toward the realization of a robust and intense source of polarization-entangled photons. First, we describe a novel source of polarization-entangled pairs that uses periodically-poled potassium titanyl phosphate (PPKTP) and an interferometer based on polarization beam displacers. The source emits an intense flux of high-quality single-mode entangled photons and is stable, robust, and easy to align. Second, we report on sources of correlated photons generated in PPKTP waveguides. Waveguide sources of correlated pairs have been shown to generate high fluxes of pairs: we theoretically and experimentally investigate spontaneous parametric down-conversion generation of photon pairs in waveguides at different wavelengths.
A scheme to generate multi-channel photon pairs is presented,based on a periodically poled(PP) Potassium Titanyl Phosphate(KTP) waveguide via spontaneous parametric downconversion(SPDC).Type-II and type-I phase matching cases with a pulsed and a cw pump laser are discussed respectively.It is analytically shown that several photon pairs could be generated in a single type-II PPKTP waveguide,and the wavelengths of all photon pairs are in the telecom band.In type-I case,several photon pairs can be prepared,but one photon of each pair is not in telecom band.And,a multi-channel photon pairs source could be realized.