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IEEE Microwaves, Antennas, and Propagation Conference (MAPCON)

Strategies to achieve high brightness in collinear Sagnac polarization entangled photon source

Jay B. Bhatt, Parthkumar V. Sakhiya, Adarsh Jain, M. Senthilkumar, Dinesh Kumar Singh

Published Dec 11, 2023• 0 citations
Publisher

Abstract

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 $8.42\times 10^{4}$ 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.

DOI
10.1109/mapcon58678.2023.10464076

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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.
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.
We have experimentally created a robust, ultrabright and phase-stable polarization-entangled state close to maximally entangled Bell-state with %98-fidelity using the type-II spontaneous parametric down-conversion (SPDC) process in periodically-poled KTiOPO 4 (PPKTP) collinear crystal inside a Sagnac interferometer (SI). Bell inequality measurement, Freedman's test, as the different versions of CHSH inequality, and also visibility test which all can be seen as the nonlocal realism tests, imply that our created entangled state shows a strong violation from the classical physics or any hidden-variable theory. We have obtained very reliable and very strong Bell violation as S = 2.78 ± 0.01 with high brightness V HV = % 99.969±0.003 99.969 ± 0.003 99.969±0.003 and V DA = % 96.751±0.002 96.751 ± 0.002 96.751±0.002 and very strong violation due to Freedman test as δ F = 0.01715 ± 0.00001 . Furthermore, using the tomographic reconstruction of quantum states together a maximum-likelihood-technique (MLT) as the numerical optimization, we obtain the physical non-negative definite density operator which shows the nonseparability and entanglement of our prepared state. By having the maximum likelihood density operator, we calculate some important entanglement-measures and entanglement entropies. The Sagnac configuration provides bidirectional crystal pumping yields to high-rate entanglement source which is very applicable in quantum communication, sensing and metrology as well as quantum information protocols, and has potential to be used in quantum illumination-based LIDAR and free-space quantum key distribution (QKD).
Entangled photon sources with high spectral brightness are important resources for photonic quantum information processing, particularly in quantum communication and quantum networking where usable photon flux of entangled photons is often constrained by channel loss and source inefficiency. Here, we demonstrate a spectrally engineered type-0 spontaneous parametric down-conversion (SPDC) source with enhanced spectral brightness for entanglement distribution. By pumping a 30-mm ppKTP crystal with an ultra-narrowband laser slightly detuned from degeneracy, photon-pair generation is concentrated into a narrow spectral bandwidth while retaining the strong nonlinear interaction of type-0 phase matching. The source produces a coincidence rate of 44.6 kHz corresponding to a detected spectral brightness of 0.507 MHz/mW/nm. We further integrate the source into a Sagnac interferometer to generate polarization-entangled photon pairs and demonstrate entanglement distribution through a 2.56 km free-space round-trip channel. Our results show that spectral engineering provides a practical route to compact, spectrally bright entangled-photon sources for quantum communication applications.
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.