We experimentally demonstrate a source of nearly pure single photons in arbitrary temporal shapes heralded from a parametric down-conversion (PDC) source at telecom wavelengths. The technology is enabled by the tailored dispersion of in-house fabricated waveguides with shaped pump pulses to directly generate the PDC photons in on-demand temporal shapes. We generate PDC photons in Hermite-Gauss and frequency-binned modes and confirm a minimum purity of 0.81, even for complex temporal shapes.Preparing single photons in pure and controlled spectral-temporal modes is a key requirement for quantum photonic technologies. Diverse applications including quantum-enhanced metrology [1,2], quantum computation [3,4], and quantum encryption [5][6][7] rely on high-contrast interference through stable sources of pure single photons. In addition, widely customisable and precisely controllable temporal-mode shaping is necessary to ensure mode matching between individual sources [8], facilitate coupling between nodes in a quantum network [9], and enable temporal-mode based quantum communication [10] and source mupliplexing [11,12], among other applications. Furthermore, sources with high brightness are essential for scalable performance, and spatially single-mode behaviour is necessary for coupling to optical fibre networks and integrated waveguide devices.Sources based on parametric downconversion (PDC) have granted a simple solution to heralded single-photon generation for decades, but have not yet satisfied all of the above requirements simultaneously. Most PDC sources generate photons with strong spectral correlations which is undesirable for heralded single-photon sources. However, it is possible to minimise the spectral correlation in crystals offering specific dispersion properties along with an adapted pump bandwidth [8,[13][14][15][16][17][18][19][20][21]. This specific dispersion property is linked to the group velocities of the pump and the PDC photons and can be summarised in two categories: matching the group velocity of the pump photon with one of the PDC photons [8,20], or having the group velocity of the pump between the two PDC photons [15,[17][18][19].On the other hand, efficient temporal-mode shaping of the PDC photons is more challenging. Existing methods to create a broadband single photon in an arbitrary temporal mode rely on carving out the desired mode from the original wavepacket as depicted in Fig. 1(a), which can be accurately achieved by temporal or spectral modulation of the photon [22][23][24][25][26]. This method, however, necessarily introduces loss and leads to a reduced rate of pre- * vahid.ansari@uni-paderborn.de With an appropriately designed pump field and group-velocity engineered nonlinear medium, the PDC photons are emitted directly in a desired temporal shape. In both scenarios the purity of heralded single photon rely on the separability of the PDC state in terms of signal and herald fields.pared photons [27] and a low pair-symmetric heralding efficiencies [28]; this poses a practical lim...

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Photon pair sources in the visible to NIR wavelength region play a key role in quantum optics. The wavelength range around 800 nm provides an opportunity for using low cost detectors, which makes it highly interesting for practical, large scale quantum applications. Here, we report on the realization of single mode Rubidium (Rb) exchanged waveguides in periodically poled (PP) Potassium Titanyl Phosphate (Rb:KTiOPO4 or Rb:KTP) for frequency-non-degenerate type II parametric down-conversion pumped at 400 nm and generating pairs of photons at around 800 nm. The source exhibits a nonlinear conversion efficiency of 2.0%/(Wcm2), estimated from SHG measurements. Characterisation of the generated two-photon state confirms nonclassical photon-number correlations, characterized by g(1,1). The high nonlinear conversion efficiency and low temperature sensitivity make this source a promising candidate for operations in both classical and quantum integrated network applications.

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Integrated χ(2) devices are a widespread tool for the generation and manipulation of light fields, since they exhibit high efficiency, a small footprint and the ability to interface them with fibre networks. Surprisingly, some commonly used material substrates are not yet fully understood, in particular potassium titanyl phosphate (KTP). A thorough understanding of the fabrication process of waveguides in this material and analysis of their properties is crucial for the realization and the engineering of high efficiency devices for quantum applications. In this paper we present our studies on rubidium-exchanged waveguides fabricated in KTP. Employing energy dispersive X-ray spectroscopy (EDX), we analysed a set of waveguides fabricated with different production parameters in terms of time and temperature. We find that the waveguide depth is dependent on their widths by reconstructing the waveguide depth profiles. Narrower waveguides are deeper, contrary to the theoretical model usually employed. Moreover, we found that the variation of the penetration depth with the waveguide width is stronger at higher temperatures and times. We attribute this behaviour to stress-induced variation in the diffusion process.

Interference between single photons is key for many quantum optics experiments and applications in quantum technologies, such as quantum communication or computation. It is advantageous to operate the systems at telecommunication wavelengths and to integrate the setups for these applications in order to improve stability, compactness and scalability. A new promising material platform for integrated quantum optics is lithium niobate on insulator (LNOI). Here, we realise Hong-Ou-Mandel (HOM) interference between telecom photons from an engineered parametric down-conversion source in an LNOI directional coupler. The coupler has been designed and fabricated in house and provides close to perfect balanced beam splitting. We obtain a raw HOM visibility of (93.5 ± 0.7) %, limited mainly by the source performance and in good agreement with off-chip measurements. This lays the foundation for more sophisticated quantum experiments in LNOI.

We study the interaction of gray tracking and DC ionic conductivity in Potassium Titanyl Phosphate (KTiOPO4, KTP) and present a novel way to reduce conductivity via a potassium nitrate treatment improving the device quality.

We study the DC conductivity in potassium titanyl phosphate (KTiOPO4, KTP) and its isomorphs KTiOAsO4 (KTA) and Rb1%K99%TiOPO4 (RKTP) and introduce a method by which to reduce the overall ionic conductivity in KTP by a potassium nitrate treatment. Furthermore, we create so-called gray tracking in KTP and investigate the ionic conductivity in theses areas. A local unintended reduction of the ionic conductivity is observed in the gray-tracked regions, which also induce additional optical absorption in the material. We show that a thermal treatment in an oxygen-rich atmosphere removes the gray tracking and brings the ionic conductivity as well as the optical transmission back to the original level. These studies can help to choose the best material and treatment for specific applications.