Frequency down-conversion of 637 nm light to the telecommunication band for non-classical light emitted from NV centers in diamond

Triply-resonant sum frequency conversion with gallium phosphide ring resonators

We demonstrate quasi-phase matched, triply-resonant sum frequency conversion in 10.6-µm-diameter integrated gallium phosphide ring resonators. A small-signal, waveguide-to-waveguide power conversion efficiency of 8 ± 1.1%/mW; is measured for conversion from telecom (1536 nm) and near infrared (1117 nm) to visible (647 nm) wavelengths with an absolute power conversion efficiency of 6.3 ± 0.6%; measured at saturation pump power. For the complementary difference frequency generation process, a single photon conversion efficiency of 7.2%/mW from visible to telecom is projected for resonators with optimized coupling. Efficient conversion from visible to telecom will facilitate long-distance transmission of spin-entangled photons from solid-state emitters such as the diamond NV center, allowing long-distance entanglement for quantum networks.

Frequency Conversion Interface towards Quantum Network: From Atomic Transition Line to Fiber Optical Communication Band

Quantum repeater is a key component of quantum network, and atomic memory is one of the important candidates for constructing quantum repeater. However, the atomic transition wavelength is not suitable for long-distance transmission in optical fiber. To bridge atomic memory and fiber communication, we demonstrate a frequency conversion interface from rubidium D1 line (795 nm) to the optical communication L-band (1621 nm) based on difference frequency generation. To reduce broadband noise of spontaneous Raman scattering caused by strong pumping light, we use a combination of two cascaded etalons and a Fabry-Perot cavity with low finesse to narrow the noise bandwidth to 11.7 MHz. The filtering system is built by common optical elements and is easy to use; it can be widely applied in frequency conversion process. We show that the signal-noise ratio of the converted field is good enough to reduce the input photon number below 1 under the condition of low external device conversion efficiency (0.51%) and large duration of input pulse (250 ns). The demonstrated frequency conversion interface has important potential application in quantum networks.

Low random duty-cycle errors in periodically poled KTP revealed by sum-frequency generation

Low-noise quantum frequency conversion in periodically poled nonlinear crystals has proved challenging when the pump wavelength is shorter than the target wavelength. This is-at least in large part-a consequence of the parasitic spontaneous parametric downconversion of pump photons, whose efficiency is increased by fabrication errors in the periodic poling. Here we characterize the poling quality of commercial periodically poled bulk potassium titanyl phosphate (ppKTP) by measuring the sum-frequency generation (SFG) efficiency over a large phase mismatch range from 0 to more than 400π. Over the probed range, the SFG efficiency behaves nearly ideally and drops to a normalized efficiency of 10^-6. Our results demonstrate that any background pedestal that would be formed by random duty-cycle errors in ppKTP is substantially reduced when compared to periodically poled lithium niobate. The standard deviation of the random duty-cycle errors can be estimated to be smaller than 2% of the domain length. From this, we expect a noise spectral density that is at least 1 order of magnitude smaller than that of current state-of-the-art single-step frequency converters.

Ultra-high-rate nonclassical light source with 50 GHz-repetition-rate mode-locked pump pulses and multiplexed single-photon detectors

Heralded single photons (HSPs) and entangled photon pairs (EPPs) via spontaneous parametric down-conversion are essential tools for the development of photonic quantum information technologies. In this paper, we report a novel ultra-high-rate nonclassical light source realized by developing 50 GHz-repetition-rate mode-locked pump pulses and multiplexed superconducting nanowire single-photon detectors. The presence of the single-photon state in the heralded photons with our setup was indicated by the second-order intensity correlation below 1/2 at the heralding rate over 20 Mcps. Even at the rate beyond 50 Mcps, the nonclassicality was still observed with the intensity correlation below unity. Moreover, our setup is also applicable to the polarization-EPP experiment, where we obtained the maximum coincidence rate of 1.6 Mcps with the fidelity of 0.881 ± (0.254 × 10^-3) to the maximally entangled state. Our versatile source could be a promising tool to explore various large-scale quantum-photonic experiments with low success probability and heavy attenuation.

Storage of telecom-C-band heralded single photons with orbital-angular-momentum encoding in a crystal

A memory-based quantum repeater architecture provides a solution to distribute quantum information to an arbitrary long distance. Practical quantum repeaters are likely to be built in optical-fiber networks which take advantage of the low-loss transmission between quantum memory nodes. Most quantum memory platforms have characteristic atomic transitions away from the telecommunication band. A nondegenerate photon pair source is therefore useful for connection of a quantum memory to optical fibers. Here, we report a high-brightness narrowband photon-pair source which is compatible with a rare-earth-ion-doped crystal Pr³⁺:Y₂SiO₅. The photon-pair source is generated through a cavity-enhanced spontaneous parametric down-conversion process with the signal photon at 606 nm and the idler photon at 1540 nm. Moreover, using the telecom C-band idler photons for heralding, we demonstrate the reversible transfer of orbital-angular-momentum qubit between the signal photon and the quantum memory based on Pr³⁺:Y₂SiO₅.

Spectral noise in frequency conversion from the visible to the telecommunication C-band

We report a detailed study of the noise properties of a visible-to-telecom photon frequency converter based on difference frequency generation (DFG). The device converts 580 nm photons to 1541 nm using a strong pump laser at 930 nm, in a periodically poled lithium niobate ridge waveguide. The converter reaches a maximum device efficiency of 46 % (internal efficiency of 67%) at a pump power of 250 mW. The noise produced by the pump laser is investigated in detail by recording the noise spectra both in the telecom and visible regimes and measuring the power dependence of the noise rates. The noise spectrum in the telecom is very broadband, as expected from previous work on similar DFG converters. However, we also observe several narrow dips in the telecom spectrum, with corresponding peaks appearing in the 580 nm noise spectrum. These features are explained by sum frequency generation of the telecom noise at wavelengths given by the phase-matching condition of different spatial modes in the waveguide. The proposed noise model is in good agreement with all the measured data, including the power dependence of the noise rates, both in the visible and telecom regimes. These results are applicable to the class of DFG converters where the pump laser wavelength is in between the input and target wavelength.

Polarization insensitive frequency conversion for an atom-photon entanglement distribution via a telecom network

Long-lifetime quantum storages accessible to the telecom photonic infrastructure are essential to long-distance quantum communication. Atomic quantum storages have achieved subsecond storage time corresponding to 1000 km transmission time for a telecom photon through a quantum repeater algorithm. However, the telecom photon cannot be directly interfaced to typical atomic storages. Solid-state quantum frequency conversions fill this wavelength gap. Here we report on the experimental demonstration of a polarization-insensitive solid-state quantum frequency conversion to a telecom photon from a short-wavelength photon entangled with an atomic ensemble. Atom-photon entanglement has been generated with a Rb atomic ensemble and the photon has been translated to telecom range while retaining the entanglement by our nonlinear-crystal-based frequency converter in a Sagnac interferometer.

Using temperature to reduce noise in quantum frequency conversion

Quantum frequency conversion is important in quantum networks to interface nodes operating at different wavelengths and to enable long-distance quantum communication using telecommunications wavelengths. Unfortunately, frequency conversion in actual devices is not a noise-free process. One main source of noise is spontaneous Raman scattering, which can be reduced by lowering the device operating temperature. We explore frequency conversion of 1554 nm photons to 837 nm using a 1813 nm pump in a periodically poled lithium niobate waveguide device. By reducing the temperature from 85°C to 40°C, we show a three-fold reduction in dark count rates, which is in good agreement with theory.

Multimode fiber-coupled superconducting nanowire single-photon detector with 70% system efficiency at visible wavelength

We report the development of the multimode fiber-coupled superconducting nanowire single-photon detector with high system detection efficiency at visible wavelength. The detector consists of a 10.5-nm-thick and 150-nm-wide NbN nanowire meander fabricated on a Si substrate with a multilayer dielectric mirror and a quarter wavelength cavity for obtaining high optical absorptance. The meander area was 35 µm in diameter and coupled with the GRIN-lensed multimode optical fiber with a core diameter of 50 µm. The system reached detection efficiency of 70% with dark count rate of 100 Hz at the wavelength of 635 nm, 3 dB roll-off response counting rate of 8.5 Mcps, and timing jitter of 76 ps.