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arXiv:2107.08070

An Optimal Polarization-Entanglement Source: Frequency-Converted SPDC with Degeneracy, Indistinguishability, and Ultra-High Purity Configurable Over a Large Spectral Range

Randy Lafler, R. Nicholas Lanning

Published Jul 16, 2021• 0 citations
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Abstract

Modeling and simulations of entanglement-based quantum-networking protocols commonly assume perfect entangled states. Some investigations have been performed which show how imperfections cause the efficiency of the protocols to rapidly deteriorate. For polarization-entangled states created by SPDC, the fundamentals of phase matching lead to a trade-off problem for the optimal properties of the state. We present a new SPDC method which circumvents the trade-off problem and allows one to obtain degeneracy, indistinguishability, and heralded-single-photon spectral purity greater than 99% for any target SPDC wavelength in the visible and NIR spectrum. Therefore, our method can in principle generate optimal polarization-entangled states for polarization-entanglement-based quantum-networking protocols.

DOI
10.48550/arxiv.2107.08070

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Modeling and simulations of entanglement-based quantum-networking protocols commonly assume perfect entangled states. Some investigations have been performed which show how imperfections cause the efficiency of the protocols to rapidly deteriorate. For polarization-entangled states created by SPDC, the fundamentals of phase matching lead to a trade-off problem for the optimal properties of the state. We present a new SPDC method which circumvents the trade-off problem and allows one to obtain degeneracy, indistinguishability, and heralded-single-photon spectral purity greater than 99% for any target SPDC wavelength in the visible and NIR spectrum. Therefore, our method can in principle generate optimal polarization-entangled states for polarization-entanglement-based quantum-networking protocols.
Modeling and simulations of entanglement-based quantum-networking protocols commonly assume perfect entangled states. Some investigations have been performed, which show how imperfections cause the efficiency of the protocols to rapidly deteriorate. For polarization-entangled states created by spontaneous-parametric down-conversion (SPDC), the fundamentals of phase matching lead to a trade-off problem for the optimal properties of the state. We present a SPDC method, which circumvents the trade-off problem and allows one to obtain degeneracy, indistinguishability, and heralded-single-photon spectral purity greater than $99\mathrm{%}$ for any target SPDC wavelength in the visible and near-infrared spectrum. Therefore, our method can, in principle, generate polarization-entangled states that are optimal for polarization-entanglement-based quantum-networking protocols.
The generation of high purity single photons and entangled photon pairs is a crucial first step in many quantum optics experiments, including those that require quantum teleportation [1], entanglement swapping [2], and the generation of multi-photon entanglement [3]. The most widely used and established approach for creating photon pairs is spontaneous parametric down conversion (SPDC) - where pump photons undergo a non-linear optical process to create a down-converted photon pair [4]. Traditionally, the generation of high-purity heralded single photons required severe spectral filtering, introducing additional loss. However, through careful engineering of spectral correlations of the down-converted photon pairs one can circumvent the need for filtering and avoid the accompanying reduction in heralding efficiency.
Polarization-entangled photon pairs can be efficiently prepared into pure Bell states with a high fidelity via type-II spontaneous parametric down-conversion (SPDC) of narrow-band pump light. However, the use of femtosecond pump pulses to generate multiphoton states with precise timing often requires spectral filtering to maintain a high quality of polarization entanglement. This typically reduces the efficiency of photon pair collection. We experimentally map the polarization correlations of photon pairs from such a source over a range of down-converted wavelengths with a high spectral resolution and find strong polarization correlations everywhere. A spectrally dependent imbalance between contributions from the two possible decay paths of SPDC is identified as the reason for a reduction in entanglement quality observed with femtosecond pump pulses. Our spectral measurements allow us to predict the polarization correlations for arbitrary filter profiles when the frequency degree of freedom of the photon pairs is ignored.
A biphoton polarization-frequency entanglement source was realized by shaping the phase-matching function of a periodically poled KTP crystal. It provides a simple method to achieve polarization (or spectral) entanglement in a simple collinear setup, based on single-pass SPDC and a dichroic (or polarizing) beam splitter. This is a robust and cost-effective configuration that can be easily implemented outside the laboratory environment. We characterized the source using two approaches: reconstructing the density matrix of the generated state with quantum state tomography, by recording the coincidences across 16 mutual polarization settings, in addition to a new method based on quantifying the symmetry of the joint spectral intensity by swapping between the signal and idler wavelengths. The polarization-entangled source violates the Clauser–Horne–Shimony–Holt inequality with a measured S =2.747±0.004. We also measured the polarization entanglement visibility in two mutually unbiased polarization bases and evaluated the squeezing level by characterizing the reduction in visibility at high pump power.
Entangled-photon sources with simultaneously near-unity heralding efficiency and indistinguishability are the fundamental elements for scalable photonic quantum technologies. We design and realize a degenerate telecommunication wavelength entangled-photon source from an ultrafast pulsed laser pumped spontaneous parametric down-conversion (SPDC), which shows simultaneously 97% heralding efficiency and 96% indistinguishability between independent single photons without narrow-band filtering. Such a beamlike and frequency-uncorrelated SPDC source allows generation of the first 12-photon genuine entanglement with a state fidelity of 0.572±0.024. We further demonstrate a blueprint of scalable scattershot boson sampling using 12 SPDC sources and a 12×12 mode interferometer for three-, four-, and five-boson sampling, which yields count rates more than 4 orders of magnitude higher than all previous SPDC experiments.