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Optics Express

Spectrally pure photons generated in a quasi-phase matched xenon-filled hollow-core photonic crystal fiber

Walker D. Larson, Trevor L. Courtney, Christian K. Keyser

Published Jan 17, 2022• 4 citations
Publisher

Abstract

Spectrally pure photons heralded from unentangled photon pair sources are crucial for any quantum optical system reliant on the multiplexing of heralded photons from independent sources. Generation of unentangled photon pairs in gas-filled hollow-core photonic crystal fibers specifically remains an attractive architecture for integration into quantum-optical fiber networks. The dispersion design offered by selection of fiber microstructures and gas pressure allows considerable control over the group-velocity profile which dictates the wavelengths of photon pairs that can be generated without spectral entanglement. Here, we expand on this design flexibility, which has previously been implemented for four-wave mixing, by modeling the use of a static, periodically poled electric field to achieve an effective quasi-phase-matched three-wave mixing nonlinearity that creates spontaneous parametric downconversion. Electric-field-induced quasi-phase-matched spontaneous parametric downconversion enables control of phase matching conditions that is independent of the group velocity, allowing phase matching at arbitrary wavelengths without affecting the entanglement of photons at those wavelengths. This decoupling of entanglement engineering and phase matching facilitates spectrally pure photon pair generation with efficiency and wavelength-tunability that is otherwise unprecedented.

DOI
10.1364/oe.446488

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Spectrally pure photons heralded from unentangled photon pair sources are crucial for any quantum optical system reliant on the multiplexing of heralded photons from independent sources. Generation of unentangled photon pairs in gas-filled hollow-core photonic crystal fibers specifically remains an attractive architecture for integration into quantum-optical fiber networks. The dispersion design offered by selection of fiber microstructures and gas pressure allows considerable control over the group-velocity profile which dictates the wavelengths of photon pairs that can be generated without spectral entanglement. Here, we expand on this design flexibility, which has previously been implemented for four-wave mixing, by modeling the use of a static, periodically poled electric field to achieve an effective quasi-phase-matched three-wave mixing nonlinearity that creates spontaneous parametric downconversion. Electric-field-induced quasi-phase-matched spontaneous parametric downconversion enables control of phase matching conditions that is independent of the group velocity, allowing phase matching at arbitrary wavelengths without affecting the entanglement of photons at those wavelengths. This decoupling of entanglement engineering and phase matching facilitates spectrally pure photon pair generation with efficiency and wavelength-tunability that is otherwise unprecedented.
We present a comprehensive numerical investigation of entangled photon pair generation via spontaneous four-wave mixing (SFWM) in hollow-core photonic crystal fibers (HC-PCFs). By analyzing various fiber geometries, dispersion profiles, and anti-resonant characteristics, we identify optimal configurations that enhance photon-pair generation efficiency. Our study highlights the crucial roles of group velocity dispersion engineering, pump parameters, and phase-matching conditions in shaping the joint spectral amplitude (JSA) of the emitted photons. Utilizing gas-filled or evacuated HC-PCFs significantly reduces Raman noise by minimizing field overlap with the glass, while maintaining strong nonlinear interactions. The results reveal practical regimes for improving spectral purity and heralding efficiency. These findings provide practical design guidelines for scalable, low-noise quantum light sources suitable for quantum communication technologies. We bridge fiber design and nonlinear optics to create a predictive model and practical design charts for application-specific quantum light sources.
We present a comprehensive numerical investigation of entangled photon pair generation via spontaneous four-wave mixing (SFWM) in hollow-core photonic crystal fibers (HC-PCFs). By analyzing various fiber geometries, dispersion profiles, and anti-resonant characteristics, we identify optimal configurations that enhance photon-pair generation efficiency. Our study highlights the crucial roles of group velocity dispersion engineering, pump parameters, and phase-matching conditions in shaping the joint spectral amplitude (JSA) of the emitted photons. Utilizing gas-filled or evacuated HC-PCFs significantly reduces Raman noise by minimizing field overlap with the glass, while maintaining strong nonlinear interactions. The results reveal practical regimes for improving spectral purity and heralding efficiency. These findings provide practical design guidelines for scalable, low-noise quantum light sources suitable for quantum communication technologies.
We present a comprehensive numerical investigation of entangled photon pair generation via spontaneous four-wave mixing (SFWM) in hollow-core photonic crystal fibers (HC-PCFs). By analyzing various fiber geometries, dispersion profiles, and anti-resonant characteristics, we identify optimal configurations that enhance photon-pair generation efficiency. Our study highlights the crucial roles of group velocity dispersion engineering, pump parameters, and phase-matching conditions in shaping the joint spectral amplitude (JSA) of the emitted photons. Utilizing gas-filled or evacuated HC-PCFs significantly reduces Raman noise by minimizing field overlap with the glass, while maintaining strong nonlinear interactions. The results reveal practical regimes for improving spectral purity and heralding efficiency. These findings provide practical design guidelines for scalable, low-noise quantum light sources suitable for quantum communication technologies.
We demonstrate theoretically and experimentally a high level of control of the four-wave mixing process in an inert gas-filled inhibited-coupling guiding hollow-core photonic crystal fiber. The specific multiple-branch dispersion profile in such fibers allows both correlated and separable bi-photon states to be produced. By controlling the choice of gas and its pressure and the fiber length, we experimentally generate various joint spectral intensity profiles in a stimulated regime that is transferable to the spontaneous regime. The generated profiles may cover both spectrally separable and correlated bi-photon states and feature frequency tuning over tens of THz, demonstrating a large dynamic control that will be very useful when implemented in the spontaneous regime as a photon pair source.
Summary form only given. Parametric downconversion (PDC) allows the generation of highly coherent, spectrally pure single photons for quantum photonics. In a typical heralded PDC source, the signal photon emerges in a spectrally mixed state due to strong (anti)correlation in the joint spectrum of PDC photon pairs.One method for reducing spectral correlations is spectral filtering, but this compromises the heralding efficiency and the brightness of the source. A more sophisticated method for increasing the heralded photon purity is group-velocity matching, which works by erasing part of the timing information between the pump and PDC photons. What group-velocity matching cannot address, however, are the remaining correlations in the joint spectrum that come from the sinc-shaped phase-matching function (PMF) typical to standard PDC crystals. Further source engineering is therefore required to remove these correlations. Several studies have introduced domain-engineering techniques for shaping the phase matching function (PMF) in periodically poled crystals to improve single photon purities [1-4]. We present a new technique for tailoring crystal nonlinearities which combines the advantages of a deterministic method for domain engineering [4] and an annealing method developed for classical applications in higher-harmonic generation [5]. In our method, we first choose each domain orientation such that the generated field amplitude along the crystal matches the target field amplitude obtained by integrating the desired nonlinearity profile. Using a one-block structure instead of a two-blocks structure (as in [4]) results in a higher flexibility of the algorithm in tracking the target function and yields to more accurate results. We then use an annealing algorithm to slightly vary the width of each domain, further improving the effective PMF. Our technique outperforms its predecessors, particularly for short crystals, leading to high single photon purities, as shown in Fig.1. We experimentally characterise our method through a high-precision measurement of multiphoton interference between two heralded photons generated by two independent sources.We also notice that by considering a complex target field amplitude function it is possible to generalise the algorithm in [5] to any constant domain width (not necessarily equal to the coherence length), leading to a better approximation of the desired phase matching function, especially in the very short crystal regime. Moreover, tracking both the real and imaginary components of the target function allows us to approximate more exotic cases such as chirped nonlinearity profiles. Since scalable quantum photonics will require integrated architectures in which nonlinear processes are typically employed for single-photon generation, it will be interesting to explore new techniques similar to in-fibre Bragg grating structures which might yield quasi-periodic four-wave mixing and apply our algorithm to these new platforms.
For advanced quantum information technology, sources of photon pairs in quantum mechanically factorable states are of great importance for realizing high-fidelity photon-photon quantum gate operations. Here we experimentally demonstrate a technique to produce spectrally factorable photon pairs utilizing multi-order quasi-phase-matching (QPM) conditions in spontaneous parametric downconversion (SPDC). In our scheme, a spatial nonlinearity profile of a nonlinear optical crystal is shaped with current standard poling techniques, and the associated phase-matching function can be approximated to a Gaussian form. By the measurement of a phase-matching function and the second-order autocorrelation function, we demonstrate that telecom-band photon pairs produced by our custom-poled crystal are highly factorable with > 95% single-photon purity.