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arXiv:2008.01575 · SciPost Physics

Approaching the Tsirelson bound with a Sagnac source of polarization-entangled photons

Sandra Meraner, Robert J. Chapman, Stefan Frick, Robert Keil, Maximilian Prilmüller, Gregor Weihs

Published Jan 26, 2021• 11 citations
arXivHTMLPDF

Abstract

High-fidelity polarization-entangled photons are a powerful resource for quantum communication, distributing entanglement and quantum teleportation. The Bell-CHSH inequality S\leq2 S ≤ 2 is violated by bipartite entanglement and only maximally entangled states can achieve S=2\sqrt{2} S = 2 2 , the Tsirelson bound. Spontaneous parametric down-conversion sources can produce entangled photons with correlations close to the Tsirelson bound. Sagnac configurations offer intrinsic stability, compact footprint and high collection efficiency, however, there is often a trade off between source brightness and entanglement visibility. Here, we present a Sagnac polarization-entangled source with 2\sqrt{2}-S=(5.65\pm0.57\times10^{-3}) 2 2 − S = ( 5.65 ± 0.57 × 10 − 3 ) , on-par with the highest S S parameters recorded, while generating and detecting (4660\pm70)pairs/s/mW ( 4660 ± 70 ) p a i r s / s / m W , which is a substantially higher brightness than previously reported for Sagnac sources and around two orders of magnitude brighter than for traditional cone sources with the highest S S parameters. Our source records 0.9953\pm0.0003 0.9953 ± 0.0003 concurrence and 0.99743\pm0.00014 0.99743 ± 0.00014 fidelity to an ideal Bell state. By studying systematic errors in Sagnac sources, we identify that the precision of the collection focal point inside the crystal plays the largest role in reducing the S S parameter in our experiment. We provide a pathway that could enable the highest S S parameter recorded with a Sagnac source to-date while maintaining very high brightness.

DOI
10.21468/scipostphys.10.1.017

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arXiv:quant-ph/9810003•Physical Review A•Aug 1, 1999•1,142 citations
High-fidelity polarization-entangled photons are a powerful resource for quantum communication, distributing entanglement and quantum teleportation. The Bell-CHSH inequality S\leq2 S ≤ 2 is violated by bipartite entanglement and only maximally entangled states can achieve S=2\sqrt{2} S = 2 2 , the Tsirelson bound. Spontaneous parametric down-conversion sources can produce entangled photons with correlations close to the Tsirelson bound. Sagnac configurations offer intrinsic stability, compact footprint and high collection efficiency, however, there is often a trade off between source brightness and entanglement visibility. Here, we present a Sagnac polarization-entangled source with 2\sqrt{2}-S=(5.65\pm0.57\times10^{-3}) 2 2 − S = ( 5.65 ± 0.57 × 10 − 3 ) , on-par with the highest S S parameters recorded, while generating and detecting (4660\pm70)pairs/s/mW ( 4660 ± 70 ) p a i r s / s / m W , which is a substantially higher brightness than previously reported for Sagnac sources and around two orders of magnitude brighter than for traditional cone sources with the highest S S parameters. Our source records 0.9953\pm0.0003 0.9953 ± 0.0003 concurrence and 0.99743\pm0.00014 0.99743 ± 0.00014 fidelity to an ideal Bell state. By studying systematic errors in Sagnac sources, we identify that the precision of the collection focal point inside the crystal plays the largest role in reducing the S S parameter in our experiment. We provide a pathway that could enable the highest S S parameter recorded with a Sagnac source to-date while maintaining very high brightness.
High-fidelity polarization-entangled photons are a powerful resource for quantum communication, distributing entanglement and quantum teleportation. Sagnac sources offer intrinsic phase stability, compact footprint and high collection efficiency as no generated photons are discarded. To-date, however, Sagnac sources have failed to reach the Bell-CHSH S parameter values measured with traditional cone sources. Here, we present a Sagnac polarization-entangled source with 2 2 − S = ( 5.65 ± 0.57 ) × 10 − 3 , over an order of magnitude closer to the Tsirelson bound than previously reported. Our source records 0.995 ± 0.001 concurrence, 0.9974 ± 0.0007 fidelity to an ideal Bell state and is around two orders of magnitude brighter than the highest S parameter cone source. By studying errors in Sagnac sources, we identify that multi-pair generation and the nonlinear crystal position have the greatest impact on the S parameter in our experiment. We provide a clear pathway of improving Sagnac sources that could enable the highest S parameters measured to-date.
Entangled photon source is a basic building block of various quantum technologies such as quantum communication, QKD, quantum metrology, etc. Spontaneous parametric down conversion (SPDC) process is widely reported method for generation of entanglement due to its ease of implementation and stable performance. In this paper, we report design and development of type-II Sagnac loop based entangled photon source with a high brightness of 8.42×1048.42\times 10^{4}8.42×104 pairs/s/mW with 98.18% visibility, a CHSH inequality violation of S=2.777±0.006 and state fidelity of 97.8%. An automated test setup has also been developed for carrying out the measurements. The essential formulated design procedure to achieve optimal performance has been discussed and presented. The measured performance of developed source is also compared to other reported sources having the same configuration. In this work, higher brightness is achieved with comparable fidelity to previously reported works. This methodology can be extended to any collinear Sagnac loop based entangled photon sources.
We develop a Sagnac-type source of ultrabright, non-degenerate, polarization-entangled photon pair that is highly stable and compact simultaneously. We use a 20 mm20~\text{mm}20 mm long PPLN bulk crystal which, upon pumping continuously with 532 nm532~\text{nm}532 nm wavelength, produces polarization-entangled photon-pairs of 785 nm785~\text{nm}785 nm signal and 1651 nm1651~\text{nm}1651 nm idler wavelengths via the type-0 spontaneous parametric down-conversion (SPDC) process. All optical components used in the setup are off-the-shelf components, readily available commercially; hence, no custom-designed or costly multi-wavelength polarization optics are required. At the same time, long-term phase stability is achieved without any additional active stabilization; due to the geometry of our Sagnac-type design. We also report one of the highest brightness of non-degenerate polarization-entangled photon pairs available in the literature. Even with a very low pump power of 0.034 mW0.034~\text{mW}0.034 mW, we detect a coincidence rate of (6.96±0.03)×104(6.96\pm 0.03)\times10^{4}(6.96±0.03)×104 entangled pairs/sec/mW (averaged over three polarization-basis measurements: H/V, D/A, and R/L basis). The source's brightness is calculated to be (6.17±0.04)×106(6.17\pm 0.04)\times10^{6}(6.17±0.04)×106 entangled pairs/sec/mW for the signal line width of 0.4 nm0.4~\text{nm}0.4 nm. From the raw coincidence counts (without any background coincidence correction), the fidelity of the entangled state is measured to be (96.72±0.01)%(96.72\pm 0.01)\%(96.72±0.01)% with a concurrence of (94.68±0.20)%(94.68\pm 0.20)\%(94.68±0.20)%. Bell-CHSH inequality violation is reported as S=2.71±0.06S=2.71\pm 0.06S=2.71±0.06.
We present an ultrabright polarization-entangled photon source that is optimally coupled into single-mode fibers (SMFs). This study theoretically and experimentally examines the characteristics of spontaneous parametric down-conversion (SPDC) photons, including their spectrum, bandwidth, emission angle, and intensity, as functions of crystal length, temperature and beam waist condition. Notably, we measure the collinear spatial modes of photon-pairs and collection optics under various beam waist conditions and analyze them using a collinear Gaussian approximation model. By employing a simple mode-matching optical setup, we optimize the SMF coupling and heralding efficiencies of the photon-pairs. Consequently, we achieve a spectral brightness exceeding 2.0 MHz/mW/nm from a fiber-coupled entangled photon source, utilizing a 30-mm ppKTP crystal inside a polarization Sagnac interferometer. This represents the highest spectral brightness of SPDC photons generated using a CW laser pumped bulk crystal to date. Polarization entanglement was verified by a quantum state tomography and a polarization-correlation measurement. The fidelity of the entangled state is measured to be 97.8 % and the Bell-CHSH value S = 2.782 +- 0.04. The results obtained here provide practical insights for designing high-performance SPDC sources for satellite-based communication and long-distance optical links with extremely high-photon loss.
Entangled photons are crucial for the development of quantum technologies and especially important in quantum communications. Hence it is paramount to have a reliable, high-fidelity source of entangled photons. Here we describe the construction and characterization of a polarization-entangled photon source. We generate maximally-entangled Bell states using a type-II PPKTP nonlinear crystal inside a Sagnac interferometer. We characterize the source in terms of brightness, visibility, Bell-CHSH test and we perform quantum-state tomography of the density matrix. Our source violates Bell-CHSH inequality ∣S∣≤2|S|\le 2∣S∣≤2 by n=22n=22n=22 standard deviations. With visibilities up to V=98.9%{\cal V}= 98.9\%V=98.9% and fidelity F=97%{\cal F}= 97\%F=97%, our source is highly competitive, with state-of-the-art performance. To the best of our knowledge, this is the first source of entangled-photons designed and build in Romania and as such, represents an important step for the development of quantum technologies in our country. We envisage that our results will stimulate the progress of quantum technologies in Romania and will educate the future generation of quantum engineers.
Using the process of spontaneous parametric down-conversion in a two-crystal geometry, we have generated a source of polarization-entangled photon pairs that is more than ten times brighter, per unit of pump power, than previous sources, with another factor of 30 to 75 expected to be readily achievable. We have measured a high level of entanglement between photons emitted over a relatively large collection angle, and over a 10-nm bandwidth. As a demonstration of the source capabilities, we obtained a 242-\ensuremath{\sigma} violation of Bell's inequalities in less than three minutes, and observed near-perfect photon correlations when the collection efficiency was reduced. In addition, both the degree of entanglement and the state purity should be readily tunable.