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arXiv:2208.10140

Construction and characterization of a Sagnac-based entanged-photon source

Vasile-Laurenţiu Dosan, Andrei Nazîru, Mona Mihăilescu, Radu Ionicioiu

Published Aug 22, 2022• 1 citation
arXivHTMLPDF

Abstract

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 $|S|\le 2$ by $n=22$ standard deviations. With visibilities up to ${\cal V}= 98.9\%$ and fidelity ${\cal F}= 97\%$, 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.

DOI
10.48550/arxiv.2208.10140

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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.
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 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.
In the field of quantum optics and its wide variety of applications it is essential to have high brightness sources of entangled photons. In this Bachelor project, the aim has been to produce such a source based on recent experimental setups [1] [2] with a so-called Sagnac interferometer to generate polarization-entangled photons. The source built shows high quality entanglement. High visibility polarisation coincidence fringes (99.5% visibility) were observed. Moreover a Bell’s inequality [6] in a CHSH-form[36] was violated by more than 44 standard deviations. Also the wavelength tuning of the source with respect to temperature and pump wavelength was analysed, revealing shifts in the degeneracy temperature of 0.020 nm/K and a shift of the signal and idler wavelengths of 0.046 nm/K. This thesis begins with an introduction which reviews the fundamental ideas of entanglement and it’s role in quantum mechanics in section 1. This section is continued by a general discussion of photonic entanglement, different types of photonic entanglement and a short discussion of how to measure this entanglement is presented. This is followed by a review of different tecniques of generating entangled photons in sections 2 and 3. In section 4, the experimental process of assembling the Sagnac interferometer is presented. This is followed by a review of experimental results, which includes a characterisation of the source in relation to temperature and pump wavelength tunability in section 5. Then, in section 6, a presentation and an experimental test of Bell’s inequalities are presented. Thereafter, concluding remarks are made regarding the outcome of this project and the potential of further investigations utilizing the source in sections 7 and 8.
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.
A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate (PPLN) waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. High normalized brightness (10.3 kpairs/s/nm/mW2^22) is achieved on a standard 100 GHz ITU channel pair, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96 % over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.
Polarization entangled photon pair source is widely used in many quantum information processing applications such as teleportation, quantum communications, quantum computation and high precision quantum metrology. We report on the generation of a continuous-wave pumped 1550 nm polarization entangled photon pair source at telecom wavelength using a type-II periodically poled KTiOPO(4) (PPKTP) crystal in a Sagnac interferometer. Hong-Ou-Mandel (HOM) interference measurement yields signal and idler photon bandwidth of 2.4 nm. High quality of entanglement is verified by various kinds of measurements, for example two-photon interference fringes, Bell inequality and quantum states tomography. The source can be tuned over a broad range against temperature or pump power without loss of visibilities. This source will be used in our future experiments such as generation of orbital angular momentum entangled source at telecom wavelength for quantum frequency up-conversion, entanglement based quantum key distributions and many other quantum optics experiments at telecom wavelengths.