Hybrid quantum networks rely on efficient interfacing of dissimilar quantum nodes, as elements based on parametric downconversion sources, quantum dots, colour centres or atoms are fundamentally different in their frequencies and bandwidths. Although pulse manipulation has been demonstrated in very different systems, to date no interface exists that provides both an efficient bandwidth compression and a substantial frequency translation at the same time. Here we demonstrate an engineered sum-frequency-conversion process in lithium niobate that achieves both goals. We convert pure photons at telecom wavelengths to the visible range while compressing the bandwidth by a factor of 7.47 under preservation of non-classical photon-number statistics. We achieve internal conversion efficiencies of 61.5%, significantly outperforming spectral filtering for bandwidth compression. Our system thus makes the connection between previously incompatible quantum systems as a step towards usable quantum networks.

An integrated chip with quantum state generation, active polarization manipulation, and precise time control is demonstrated.

Entangled photon pair sources based on bulk optics are approaching optimal design and implementation, with high state fidelities, spectral purities and heralding efficiencies, but generally low brightness. Integrated entanglement sources, while providing higher brightness and low-power operation, often sacrifice performance in output state quality and coupling efficiency. Here we present a polarization-entangled pair source based on a hybrid approach of waveguiding and bulk optics, addressing every metric simultaneously. We show 96 % fidelity to the singlet state, 82 % Hong-Ou-Mandel interference visibility, 43 % average Klyshko efficiency, and a high brightness of 2.9 × 10 6 pairs/(mode·s·mW), while requiring only microwatts of pump power.Our source combines for the first time high performance in all parameters simultaneously. Integrated single-mode photon pair sourcesOver the last two decades, efforts in improving entangled photon-pair sources based on bulk crystals and bulk optics have resulted in impressive performance in many measures (see Table 1 in the appendix for comparison). Entanglement fidelities above 99 % are readily achieved [1,2,17,30], and even above 99.9 % is possible [4]. Klyshko (heralding) efficiency [31], defined as the ratio of coincidence to singles counts, can reach 75 % [1,2,30,32,33]. The spectral purity, required to interfere photons from separate sources for multi-photon experiments, has been shown above 99 % [34].Unfortunately, bulk sources suffer from an intrinsic tradeoff between the brightness, or emitted photon rate per pump power (taken in the source before losses, but considering only modes which will reach the detectors), and the Klyshko efficiency [11]; for example setting the pump focus to enable coupling photon pairs to single mode fiber with 95 % efficiency necessarily reduces the brightness by a factor of ten from the maximum [35]. This tradeoff arises due to conflicting requirements on the focusing conditions: high brightness requires a tight pump focus which concentrates the down-converted light into the spatial modes collected by the fibers [36]. High Klyshko efficiency, however, requires a weak focus which more strongly correlates the spatial modes of signal and idler photons such that if one photon is coupled into fiber, the other is likely to be coupled too [37]. This tradeoff means the fundamental performance limits of bulk sources have largely been saturated. Furthermore, sources at telecommunications wavelengths are much less bright than those with visible-range photons, due to the wavelength dependence of the down-conversion efficiency [38].Integrated photon sources can surpass these limits, as the waveguide, rather than spatial phasematching, defines the allowed modes into which photons are emitted [39]. Singlespatial-mode waveguides in particular completely decouple the brightness from the focusing conditions [40], and can be produced with appropriate choice of the waveguide width and height. Then the the maximum coupling efficiency depends only on the mode ov...

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...

We present the generation of continuous-wave optical squeezing from a titanium-indiffused lithium niobate waveguide resonator. We directly measure 2.9 ± 0.1 dB of single-mode squeezing, which equates to a produced level of 4.9 ± 0.1 dB after accounting for detection losses. This device showcases the current capabilities of this waveguide architecture and precipitates more complicated integrated continuous-wave quantum devices in the continuous-variable regime.