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Entangled Photon Pair Sources based on Periodically Poled Silica Fiber

Changjia Chen

Published Jun 1, 2022• 0 citations
OpenAlex

Abstract

Entangled photons are a fundamental resource in quantum applications. Broadband entangled photon sources have been used in applications such as high-resolution quantum sensing and wavelength-multiplexed quantum communication. In particular, in fiber-based quantum applications such as long-distance quantum clock synchronization and quantum key distribution in deployed fiber networks, a practical entangled source that is robust and compatible with silica fiber is highly sought after. Periodically poled silica fiber (PPSF) has been demonstrated to be a promising resource of entangled photon-pairs. PPSF uses the induced second-order nonlinearity in poled fiber to generate polarization-entangled photon pairs based on type-II spontaneous parametric down-conversion. The negligible group birefringence of the PPSF ensures high-quality and broadband polarization entanglement without compensation, making it particularly suitable for these quantum applications. The PPSF-based source has a lot of potentials to be tapped because of its high entanglement quality, broadband emission bandwidth, simple structure, and its compatibility with the existing fiber infrastructure. In this thesis, I focus on the development of the PPSF-based entangled photon sources. I first investigate the design and the manufacturing process of the PPSF where I try to produce PPSF with improved performance and innovative properties. Next, I study the compensation-free nature of the PPSF-based polarization entanglement source and discuss the interplay between dispersion, birefringence, emission bandwidth, and polarization entanglement. Based on the theory, a commercial PPSF-based photon-pair source prototype is developed and patented. It has to my knowledge the simplest design of all polarization-entangled sources reported so far in the literature. Furthermore, I broaden the biphoton bandwidth by building a PPSF photon-pair source that emits biphotons at telecom O-band. With more than 24THz bandwidth, the biphotons generated in this source are the broadest of all O-band polarization-entangled photon sources reported to my knowledge. I also propose and experimentally demonstrate a polarization-frequency hyperentangled photon source using PPSF, which provides higher dimensionality by making use of entanglement in multiple degrees of freedom. Overall, this thesis contributes to the development of PPSF technology for practical quantum applications. I anticipate that the PPSF-based photon-pair source will play an important role in the era of quantum technology.

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In this Letter, we report a polarization-entangled photon-pair source based on type-II spontaneous parametric downconversion at telecom O-band in periodically poled silica fiber (PPSF). The photon-pair source exhibits more than 130 nm ( ∼ 24 T H z ) emission bandwidth centered at 1306.6 nm. The broad emission spectrum results in a short biphoton correlation time, and we experimentally demonstrate a Hong–Ou–Mandel interference dip with a full width of 26.6 fs at half-maximum. Owing to the low birefringence of the PPSF, the biphotons generated from type-II SPDC are polarization-entangled over the entire emission bandwidth, with a measured fidelity to a maximally entangled state greater than 95.4%. The biphoton source provides the broadest bandwidth entangled biphotons at O-band to our knowledge.

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Entangled photons are a fundamental resource in quantum applications. Broadband entangled photon sources have been used in applications such as high-resolution quantum sensing and wavelength-multiplexed quantum communication. In particular, in fiber-based quantum applications such as long-distance quantum clock synchronization and quantum key distribution in deployed fiber networks, a practical entangled source that is robust and compatible with silica fiber is highly sought after. Periodically poled silica fiber (PPSF) has been demonstrated to be a promising resource of entangled photon-pairs. PPSF uses the induced second-order nonlinearity in poled fiber to generate polarization-entangled photon pairs based on type-II spontaneous parametric down-conversion. The negligible group birefringence of the PPSF ensures high-quality and broadband polarization entanglement without compensation, making it particularly suitable for these quantum applications. The PPSF-based source has a lot of potentials to be tapped because of its high entanglement quality, broadband emission bandwidth, simple structure, and its compatibility with the existing fiber infrastructure. In this thesis, I focus on the development of the PPSF-based entangled photon sources. I first investigate the design and the manufacturing process of the PPSF where I try to produce PPSF with improved performance and innovative properties. Next, I study the compensation-free nature of the PPSF-based polarization entanglement source and discuss the interplay between dispersion, birefringence, emission bandwidth, and polarization entanglement. Based on the theory, a commercial PPSF-based photon-pair source prototype is developed and patented. It has to my knowledge the simplest design of all polarization-entangled sources reported so far in the literature. Furthermore, I broaden the biphoton bandwidth by building a PPSF photon-pair source that emits biphotons at telecom O-band. With more than 24THz bandwidth, the biphotons generated in this source are the broadest of all O-band polarization-entangled photon sources reported to my knowledge. I also propose and experimentally demonstrate a polarization-frequency hyperentangled photon source using PPSF, which provides higher dimensionality by making use of entanglement in multiple degrees of freedom. Overall, this thesis contributes to the development of PPSF technology for practical quantum applications. I anticipate that the PPSF-based photon-pair source will play an important role in the era of quantum technology.
In this Letter, we report a polarization-entangled photon-pair source based on type-II spontaneous parametric downconversion at telecom O-band in periodically poled silica fiber (PPSF). The photon-pair source exhibits more than 130 nm ( ∼ 24 T H z ) emission bandwidth centered at 1306.6 nm. The broad emission spectrum results in a short biphoton correlation time, and we experimentally demonstrate a Hong–Ou–Mandel interference dip with a full width of 26.6 fs at half-maximum. Owing to the low birefringence of the PPSF, the biphotons generated from type-II SPDC are polarization-entangled over the entire emission bandwidth, with a measured fidelity to a maximally entangled state greater than 95.4%. The biphoton source provides the broadest bandwidth entangled biphotons at O-band to our knowledge.
Quantum sources that provide broadband biphotons entangled in both polarization and time-energy degrees of freedom are a rich quantum resource that finds many applications in quantum communication, sensing, and metrology. Creating such a source while maintaining high entanglement quality over a broad spectral range is a challenge, which conventionally requires various compensation steps to erase temporal, spectral, or spatial distinguishabilities. Here, we point out that in fact compensation is not always necessary. The key to generate broadband polarization-entangled biphotons via type-II spontaneous parametric downcoversion (SPDC) without compensation is to use nonlinear materials with sufficiently low group birefringence that the biphoton bandwidth becomes dispersion-limited. Most nonlinear crystals or waveguides cannot meet this condition, but it is easily met in fiber-based systems. We reveal the interplay of group birefringence and dispersion on SPDC bandwidth and polarization entanglement quality. We show that periodically poled silica fiber (PPSF) is an ideal medium to generate high-concurrence (>0.977) polarization-entangled photons over a broad spectral range (>77nm), directly and without compensation. This is the highest polarization-entanglement concurrence reported that is maintained over a broad spectral range from a compensation-free source.
This thesis explores a method to enhance the practical utility of hyper-entangled photons generatedin periodically poled silica fiber for quantum communication applications. Hyper-entanglement, which involves quantum correlations in multiple degrees of freedom simultaneously, offers increased information capacity per particle compared to single-degree entanglement. Periodically poled sil- ica fiber has shown promise in generating high-quality, broadband polarization-frequency hyper- entangled photon pairs collinearly through type-II spontaneous parametric down-conversion. How- ever, the collinear nature of these photon pairs poses a challenge for applications requiring hyper- entanglement, as they occupy the same spatial mode. This research develops a technique for sep- arating these photon pairs while preserving their information capacity, measures the resulting en- tanglement quality degradation, and discusses potential improvements. This work contributes to the development of efficient fiber-based hyper-entanglement sources, advancing the field of practical quantum technologies and long-distance quantum communication.
Hyperentangled photons are particles of light that exhibit quantum correlations in multiple degrees of freedom simultaneously. The simultaneous entanglement in these degrees of freedom allows hyperentanglement to provide a distinct advantage over entanglement by increasing the information capacity per particle. Therefore, there is much demand for a high quality hyperentangled source, especially in fiber-based applications such as long distance quantum communication. Periodically poled silica fiber is a strong candidate for directly generating photon pairs in fiber, producing broadband collinear polarization-frequency hyperentangled photon pairs. However, collinear photon pairs cannot be simply accessed for practical applications without reducing their available information capacity. In this thesis, I investigate a method for separating these photon pairs while maintaining their original information capacity. I demonstrate the separation of the photon pairs, measure the degradation of the entanglement quality, and discuss the sources of this degradation as well as possible improvements to avoid degradation.