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Photonics for Quantum (PfQ)

Fibre-optic quantum entanglement sources

Li Qian

Published Aug 27, 2021• 0 citations
Publisher

Abstract

Fibre optics is a mature technology that revolutionized telecommunication. Quantum photonics is an emerging technology still under development. The fusion of the two can bring technologically-robust solutions for quantum applications. Here we give one concrete example of a fiber-optic entangled photon source. Free of alignment, fiber coupling or any birefringence compensating schemes, it is arguably world’s simplest polarization-entangled photon source, and perfectly compatible with the vast fibre-optic network infrastructure. Furthermore, using it to form a fibre-based nonlinear interferometer results in the coupling of the spectral domain with the polarization domain, and tunability in both domains. By manipulating the dispersion and birefringence properties of the linear medium in the interferometer, we experimentally demonstrate altering the biphoton spectrum through biphoton interference, as well as tuning the degree of polarization entanglement. Due to the low-birefringence nature of both the nonlinear and linear media in the interferometer, we can preserve the high-quality polarization entanglement while shaping the biphoton spectrum. Additionally, the coupling of the polarization degree of freedom with the spectral degree of freedom, allowing us to tune the degree of entanglement by simply tuning wavelength. Our fibre-based quantum entanglement sources demonstrate a great deal of flexibility in entanglement engineering for future fibre-optic quantum networks.

DOI
10.1117/12.2611214

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•Jun 1, 2022•0 citations
Fibre optics is a mature technology that revolutionized telecommunication. Quantum photonics is an emerging technology still under development. The fusion of the two can bring technologically-robust solutions for quantum applications. Here we give one concrete example of a fiber-optic entangled photon source. Free of alignment, fiber coupling or any birefringence compensating schemes, it is arguably world’s simplest polarization-entangled photon source, and perfectly compatible with the vast fibre-optic network infrastructure. Furthermore, using it to form a fibre-based nonlinear interferometer results in the coupling of the spectral domain with the polarization domain, and tunability in both domains. By manipulating the dispersion and birefringence properties of the linear medium in the interferometer, we experimentally demonstrate altering the biphoton spectrum through biphoton interference, as well as tuning the degree of polarization entanglement. Due to the low-birefringence nature of both the nonlinear and linear media in the interferometer, we can preserve the high-quality polarization entanglement while shaping the biphoton spectrum. Additionally, the coupling of the polarization degree of freedom with the spectral degree of freedom, allowing us to tune the degree of entanglement by simply tuning wavelength. Our fibre-based quantum entanglement sources demonstrate a great deal of flexibility in entanglement engineering for future fibre-optic quantum networks.
Abstract Quantum entanglement is an integral part of quantum optics and has been exploited in areas such as computation, cryptography and metrology. The entanglement between photons can be present in various degrees of freedom (DOFs), and even the simplest bi-partite systems can occupy a large Hilbert space. Therefore, it is desirable to exploit this multi-dimensional space for various quantum applications by fully controlling the properties of the entangled photons in multiple DOFs. While current entangled-photon sources are capable of generating entanglement in one or more DOFs, there is currently a lack of practical techniques that can shape and control the entanglement properties in multiple DOFs. Here we show that cascading two or more entangled-photon sources with tunable linear media in between allows us to generate photon-pairs whose entanglement properties can be tailored and shaped in the frequency and polarisation domains. We first develop a quantum mechanical model to study the quantum state generated from the cascade structure with special considerations paid to the effects of pump temporal coherence, linear dispersion, and in-structure polarisation transformation applied between the entangled-photon sources. We then experimentally generate photon-pairs with tunable entanglement properties by manipulating the dispersion and birefringence properties of the linear medium placed in between two entangled-photon sources. This is done in an all-fibre, phase stable, and alignment-free configuration. Our results show that the cascade structure offers a great deal of flexibility in tuning the properties of entangled photons in multiple DOFs, opening up a new avenue in engineering quantum light sources.
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.
We present a fiber-based source of polarization-entangled photon pairs that is well suited for quantum communication applications in the 1.5μμμm band of standard telecommunication fiber. Quantum-correlated signal and idler photon pairs are produced when a nonlinear-fiber Sagnac interferometer is pumped in the anomalous-dispersion region of the fiber. Recently, we have demonstrated nonclassical properties of such photon pairs by using Geiger-mode InGaAs/InP avalanche photodiodes. Polarization entanglement in the photon pairs can be created by pumping the Sagnac interferometer with two orthogonally polarized pulses. In this case the parametrically scattered signal-idler photons yield biphoton interference with $>$90% visibility in coincidence detection, while no interference is observed in direct detection of either the signal or the idler photons.
Fiber-based entangled photon sources are a key resource in emerging quantum technologies due to compactness, long-term stability and alignment-free operation. Fast control of the quantum state properties is also necessary in practical scenarios, where a universal source can be employed with different temporal and spectral configurations. Here, we report on a flexible source design that produces high-quality entanglement from continuous-wave up to GHz-pulsed operation modes. Our approach uses off-the-shelf optical components in a Sagnac configuration to generate polarization-entangled photon pairs at telecom wavelength. This design further allows phase modulation above MHz speeds with high fidelity. Together with its 60 nm spectral bandwidth, the proposed system is entirely suitable for wavelength-multiplexed and reconfigurable quantum networks, where state modulation or active bandwidth allocation is required.
We present a fiber-based source of polarization-entangled photons that is well suited for quantum communication applications in the 1550 nm band of standard fiber-optic telecommunications. Polarization entanglement is created by pumping a nonlinear-fiber Sagnac interferometer with two time-delayed orthogonally polarized pump pulses and subsequently removing the time distinguishability by passing the parametrically scattered signal and idler photon pairs through a piece of birefringent fiber. Coincidence detection of the signal and idler photons yields biphoton interference with visibility greater than 90%, while no interference is observed in direct detection of either signal or idler photons. All four Bell states can be prepared with our setup and we demonstrate violations of the Clauser-Horne-Shimony-Holt form of Bell's inequality by up to 10 standard deviations of measurement uncertainty.
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.