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We theoretically and experimentally investigate the spectral tunability and purity of photon pairs generated from spontaneous parametric down conversion in periodically poled KTiOPO(4) crystal with group-velocity matching condition. The numerical simulation predicts that the spectral purity can be kept higher than 0.81 when the wavelength is tuned from 1460 nm to 1675 nm, which covers the S-, C-, L-, and U-band in telecommunication wavelengths. We also experimentally measured the joint spectral intensity at 1565 nm, 1584 nm and 1565 nm, yielding Schmidt numbers of 1.01, 1.02 and 1.04, respectively. Such a photon source is useful for quantum information and communication systems.

Multi‐wavelength quantum light sources are extremely desired in establishing communication links among multiple users for realizing quantum networks. Despite recent impressive advances, developing such a quantum light source with high quality remains challenging. Here a multi‐wavelength quantum light source using a silicon nitride micro‐ring with a free spectral range of 200 GHz is demonstrated. The generation of eight‐wavelength‐paired photon pairs is ensured in a wavelength range of 25.6 nm. With device optimization and noise‐rejecting filters, this source enables the generation of heralded single‐photons at a rate of 62 kHz with gh(2)(0)=0.014±0.001$g^{(2)}_{\text{h}}(0)=0.014\pm 0.001$ and the generation of energy‐time entangled photons with a visibility of 99.39±0.45%$99.39\pm 0.45\%$ in the Franson interferometer. These results, at room temperature and telecom wavelength, in a CMOS‐compatible platform, represent an important step toward integrated quantum photonic devices, which pave the way for realizing a large‐scale quantum network. [ABSTRACT FROM AUTHOR]

Precise manipulation of the time–frequency modes of entangled photons is crucial for future quantum science and technologies. Recently, the frequency-domain-quantum-optical-synthesis (FD-QOS) method was demonstrated by creating a superposition of different joint temporal amplitudes: those temporal distributions can be controlled by manipulating the joint spectral amplitude in 2D frequency space via a Fourier optical relation. This FD-QOS method provides an efficient, flexible, and easy-to-control way to precisely modulate the temporal distributions of the entangled photon in an ultrafast region. However, manipulation of only the temporal modes is not sufficient for various applications in quantum information, since spectral modulations are also needed on many occasions. Here, we present a proof-of-concept experiment of two-photon spectral modulation via temporal manipulation of a biphoton wave packet. This protocol, called time-domain-quantum-optical-synthesis (TD-QOS), is achieved by adjusting the relative phases between two joint temporal distributions. In addition, the two-photon joint spectral distributions are characterized by measuring the joint spectral intensities and Hong–Ou–Mandel interferences. The combination of FD-QOS and TD-QOS enables complete control over the biphoton states. Our work would further develop quantum technologies that rely on the time–frequency modes of entangled photons. [ABSTRACT FROM AUTHOR]

Quantum light sources in the mid‐infrared (MIR) band play an important role in many applications, such as quantum sensing, quantum imaging, and quantum communication. However, there is still a lack of high‐quality quantum light sources in the MIR band, such as the spectrally pure single‐photon source. In this work, the generation of a spectrally‐pure state in an optimized poled lithium niobate crystal using a metaheuristic algorithm is presented. In particular, the particle swarm optimization algorithm is adopted to optimize the duty cycle of the poling period of the lithium niobate crystal. With this approach, the spectral purity can be improved from 0.820 to 0.998 under the third group‐velocity‐matched condition, and the wavelength‐tunable range from 3.0 to 4.0 µm for the degenerate case and 3.0 to 3.7 µm for the nondegenerate case. This work paves the way for developing quantum photonic technologies at the MIR wavelength band. [ABSTRACT FROM AUTHOR]

Summary: Spectrally intrinsically uncorrelated biphoton states generated from nonlinear crystals are very important but rare resources for quantum photonics and quantum information applications. Previously, such biphoton states were generated from several kinds of crystals, however, their wavelength ranges and nonlinear efficiencies were still limited for various applications. In order to explore new crystal for wider wavelength range and higher nonlinear efficiency, here we theoretically study the generation of spectrally uncorrelated biphoton states from 14 crystals in the "BBO family," including BBO, CLBO, KABO, KBBF, RBBF, CBBF, BABF, BiBO, LBO, CBO, LRB4, LCB, YCOB, and GdCOB. They satisfy three kinds of group‐velocity matching condition from near‐infrared to telecom wavelengths. Furthermore, heralded single photons can be generated with a purity as high as 0.98, which is achieved without any narrow filtering. The indistinguishability of photons from independent sources is examined by the Hong‐Ou‐Mandel interference, which results in a visibility of 98% also without any further filtering, that is, photons from different heralded single‐photon sources are highly indistinguishable. Our study may provide single‐photon sources with good performance for quantum information processing at near‐infrared and telecom wavelengths. [ABSTRACT FROM AUTHOR]