High-quality factor Ta2O5-on-insulator resonators with ultimate thermal stability.

Experiments in photonics, laser optics, and quantum technology require low-loss, thermal, and mechanical stability. While photonic integrated circuits on monolithic chips achieve interferometric stability, important nanophotonic material systems suffer from propagation loss, thermal drift, and noise that prevent, for example, precise frequency stabilization of resonators. Here we show that tantalum pentoxide (Ta2O5) on insulator micro-ring resonators combine quality factors beyond 1.8 Mio with vanishing temperature-dependent wavelength shift in a relevant 70 K to 90 K temperature range. Our Ta2O5-on-SiO2 devices will thus enable athermal operation at liquid nitrogen temperatures, paving the way for ultra-stable low-cost resonators, as desired for wavelength division multiplexing, on chip frequency stabilization and low-noise optical frequency comb generation.

Toward integrated tantalum pentoxide optical parametric oscillators.

We present a hybrid waveguide-fiber optical parametric oscillator (OPO) exploiting degenerate four-wave mixing in tantalum pentoxide. The OPO, pumped with ultrashort pulses at 1.55 µm wavelength, generated tunable idler pulses with up to 4.1 pJ energy tunable center wavelength between 1.63 µm and 1.68 µm. An upper bound for the total tolerable cavity loss of 32 dB was found, rendering a chip-integrated OPO feasible as a compact and robust light source.

Inverse design of nanophotonic devices using dynamic binarization.

The complexity of applications addressed with photonic integrated circuits is steadily rising and poses increasingly challenging demands on individual component functionality, performance and footprint. Inverse design methods have recently shown great promise to address these demands using fully automated design procedures that enable access to non-intuitive device layouts beyond conventional nanophotonic design concepts. Here we present a dynamic binarization method for the objective-first algorithm that lies at the core of the currently most successful inverse design algorithms. Our results demonstrate significant performance advantages over previous implementations of objective first algorithms, which we show for a fundamental TE00 to TE20 waveguide mode converter both in simulation and in experiments with fabricated devices.

Ultrafast quantum key distribution using fully parallelized quantum channels.

The field of quantum information processing offers secure communication protected by the laws of quantum mechanics and is on the verge of finding wider application for the information transfer of sensitive data. To improve cost-efficiency, extensive research is being carried out on the various components required for high data throughput using quantum key distribution (QKD). Aiming for an application-oriented solution, we report the realization of a multichannel QKD system for plug-and-play high-bandwidth secure communication at telecom wavelengths. We designed a rack-sized multichannel superconducting nanowire single photon detector (SNSPD) system, as well as a highly parallelized time-correlated single photon counting (TCSPC) unit. Our system is linked to an FPGA-controlled QKD evaluation setup for continuous operation, allowing us to achieve high secret key rates using a coherent-one-way protocol.

Scaling waveguide-integrated superconducting nanowire single-photon detector solutions to large numbers of independent optical channels.

Superconducting nanowire single-photon detectors are an enabling technology for modern quantum information science and are gaining attractiveness for the most demanding photon counting tasks in other fields. Embedding such detectors in photonic integrated circuits enables additional counting capabilities through nanophotonic functionalization. Here, we show how a scalable number of waveguide-integrated superconducting nanowire single-photon detectors can be interfaced with independent fiber optic channels on the same chip. Our plug-and-play detector package is hosted inside a compact and portable closed-cycle cryostat providing cryogenic signal amplification for up to 64 channels. We demonstrate state-of-the-art multi-channel photon counting performance with average system detection efficiency of (40.5 ± 9.4)% and dark count rate of (123 ± 34) Hz for 32 individually addressable detectors at minimal noise-equivalent power of (5.1 ± 1.2) · 10-18 W/Hz. Our detectors achieve timing jitter as low as 26 ps, which increases to (114 ± 17) ps for high-speed multi-channel operation using dedicated time-correlated single photon counting electronics. Our multi-channel single photon receiver offers exciting measurement capabilities for future quantum communication, remote sensing, and imaging applications.

Cryo-compatible opto-mechanical low-voltage phase-modulator integrated with superconducting single-photon detectors.

Photonic integrated circuits (PICs) have enabled novel functionality in quantum optics, quantum information processing and quantum communication. PICs based on Silicon Nitride (Si3N4) provide low-loss passive components and are compatible with efficient superconducting nanowire single-photon detectors (SNSPDs). For realizing functional quantum photonic systems, the integration with active phase-shifters is needed which is challenging at the cryogenic temperatures needed for operating SNSPDs. Here we demonstrate a cryo-compatible phase shifter using a low-voltage opto-mechanical modulator and show joint operation with SNSPDs at 1.3 K. We achieve a half-wave voltage of 4.6 V, single-photon detection with 88% on-chip detection efficiency (OCDE) and a low timing jitter of 12.2 ps. Our approach allows for operating reconfigurable quantum photonic circuits with low dissipation in a cryogenic setting.

Single-photon detection and cryogenic reconfigurability in lithium niobate nanophotonic circuits.

Lithium-Niobate-On-Insulator (LNOI) is emerging as a promising platform for integrated quantum photonic technologies because of its high second-order nonlinearity and compact waveguide footprint. Importantly, LNOI allows for creating electro-optically reconfigurable circuits, which can be efficiently operated at cryogenic temperature. Their integration with superconducting nanowire single-photon detectors (SNSPDs) paves the way for realizing scalable photonic devices for active manipulation and detection of quantum states of light. Here we demonstrate integration of these two key components in a low loss (0.2 dB/cm) LNOI waveguide network. As an experimental showcase of our technology, we demonstrate the combined operation of an electrically tunable Mach-Zehnder interferometer and two waveguide-integrated SNSPDs at its outputs. We show static reconfigurability of our system with a bias-drift-free operation over a time of 12 hours, as well as high-speed modulation at a frequency up to 1 GHz. Our results provide blueprints for implementing complex quantum photonic devices on the LNOI platform.

Optoelectromechanical phase shifter with low insertion loss and a 13π tuning range.

We present an on-chip optoectromechanical phase shifter with low insertion loss and low half-wave voltage using a silicon nitride platform. The device is based on a slot waveguide in which the electrostatic displacement of mechanical structures results in a change of the effective refractive index. We achieve insertion loss below 0.5 dB at a wavelength of 1550 nm in a Mach-Zehnder Interferometer with an extinction ratio of 31 dB. With a phase tuning length of 210 µm, we demonstrate a half-wave voltage of Vπ = 2.0 V and a 2π phase shift at V2π = 2.7 V. We measure phase shifts up to 13.3 π at 17 V. Our devices can be operated in the MHz range and allow for the generation of sub-µs pulses.

Integration of Diamond-Based Quantum Emitters with Nanophotonic Circuits.

Nanophotonics provides a promising approach to advance quantum technology by replicating fundamental building blocks of nanoscale quantum optic systems in large numbers with high reproducibility on monolithic chips. While photonic integrated circuit components and single-photon detectors offer attractive performance on silicon chips, the large-scale integration of individually accessible quantum emitters has remained a challenge. Here, we demonstrate simultaneous optical access to several integrated solid-state spin systems with Purcell-enhanced coupling of single photons with high modal purity from lithographically positioned nitrogen vacancy centers into photonic integrated circuits. Photonic crystal cavities embedded in networks of tantalum pentoxide-on-insulator waveguides provide efficient interfaces to quantum emitters that allow us to optically detect magnetic resonances (ODMR) as desired in quantum sensing. Nanophotonic networks that provide configurable optical interfaces to nanoscale quantum emitters via many independent channels will allow for novel functionality in photonic quantum information processors and quantum sensing schemes.

Superconducting nanowire single-photon detectors integrated with tantalum pentoxide waveguides.

Photonic integrated circuits hold great potential for realizing quantum technology. Efficient single-photon detectors are an essential constituent of any such quantum photonic implementation. In this regard waveguide-integrated superconducting nanowire single-photon detectors are an ideal match for achieving advanced photon counting capabilities in photonic integrated circuits. However, currently considered material systems do not readily satisfy the demands of next generation nanophotonic quantum technology platforms with integrated single-photon detectors, in terms of refractive-index contrast, band gap, optical nonlinearity, thermo-optic stability and fast single-photon counting with high signal-to-noise ratio. Here we show that such comprehensive functionality can be realized by integrating niobium titanium nitride superconducting nanowire single-photon detectors with tantalum pentoxide waveguides. We demonstrate state-of-the-art detector performance in this novel material system, including devices showing 75% on-chip detection efficiency at tens of dark counts per second, detector decay times below 1 ns and sub-30 ps timing accuracy for telecommunication wavelengths photons at 1550 nm. Notably, we realize saturation of the internal detection efficiency over a previously unattained bias current range for waveguide-integrated niobium titanium nitride superconducting nanowire single-photon detectors. Our work enables the full set of high-performance single-photon detection capabilities on the emerging tantalum pentoxide-on-insulator platform for future applications in integrated quantum photonics.

Tantalum pentoxide nanophotonic circuits for integrated quantum technology.

Nanophotonics holds great promise for integrated quantum technologies, but realizing all functionalities for processing quantum states of light in optical waveguides poses an outstanding challenge. Here we show that tantalum pentoxide-on-insulator offers significant advantages for such purpose and experimentally demonstrate crucial photonic integrated circuit components. Exploiting advanced nanophotonic design and state-of-the-art nanofabrication processes, we realize low-loss waveguiding with 1 dB/cm propagation loss, efficient optical fiber-chip interfaces with more than 100 nm bandwidth, micro-ring resonators with quality factors of 357,200 and tunable directional couplers. We further achieve active functionality with nano-electromechanical phase-shifters. Our work enables reconfigurable photonic circuit configurations in the Ta2O5 material system with highly favorable optical properties for integrated quantum photonics.

Broadband out-of-plane coupling at visible wavelengths.

Photonic integrated circuits require efficient interfaces to fiber optic components for applications in optical communication, computing, and sensing. Current optical interconnects rely on edge or grating couplers with limitations in terms of alignment tolerances, efficiency, and bandwidth, respectively. Here, we present a scalable coupling concept that allows fiber-to-chip coupling to the fundamental transverse electric and transverse magnetic modes based on three-dimensional nanostructures exploiting total internal reflection. We demonstrate close to octave-spanning highly efficient coupling to nanophotonic waveguides in the visible wavelength range. Our coupling scheme can be adjusted for other wavelength regimes and fiber mode-field diameters.

Parametric down-conversion photon-pair source on a nanophotonic chip.

Quantum-photonic chips, which integrate quantum light sources alongside active and passive optical elements, as well as single-photon detectors, show great potential for photonic quantum information processing and quantum technology. Mature semiconductor nanofabrication processes allow for scaling such photonic integrated circuits to on-chip networks of increasing complexity. Second-order nonlinear materials are the method of choice for generating photonic quantum states in the overwhelming majority of linear optic experiments using bulk components, but integration with waveguide circuitry on a nanophotonic chip proved to be challenging. Here, we demonstrate such an on-chip parametric down-conversion source of photon pairs based on second-order nonlinearity in an aluminum-nitride microring resonator. We show the potential of our source for quantum information processing by measuring the high visibility anti-bunching of heralded single photons with nearly ideal state purity. Our down-conversion source yields measured coincidence rates of 80 Hz, which implies MHz generation rates of correlated photon pairs. Low noise performance is demonstrated by measuring high coincidence-to-accidental ratios. The generated photon pairs are spectrally far separated from the pump field, providing great potential for realizing sufficient on-chip filtering and monolithic integration of quantum light sources, waveguide circuits and single-photon detectors.

Design and characterization of integrated components for SiN photonic quantum circuits.

The design, fabrication, and detailed calibration of essential building blocks towards fully integrated linear-optics quantum computation are discussed. Photonic devices are made from silicon nitride rib waveguides, where measurements on ring resonators show small propagation losses. Directional couplers are designed to be insensitive to fabrication variations. Their offset and coupling lengths are measured, as well as the phase difference between the transmitted and reflected light. With careful calibrations, the insertion loss of the directional couplers is found to be small. Finally, an integrated controlled-NOT circuit is characterized by measuring the transmission through different combinations of inputs and outputs. The gate fidelity for the CNOT operation with this circuit is estimated to be 99.81% after post selection. This high fidelity is due to our robust design, good fabrication reproducibility, and extensive characterizations.

Waveguide integrated low noise NbTiN nanowire single-photon detectors with milli-Hz dark count rate.

Superconducting nanowire single-photon detectors are an ideal match for integrated quantum photonic circuits due to their high detection efficiency for telecom wavelength photons. Quantum optical technology also requires single-photon detection with low dark count rate and high timing accuracy. Here we present very low noise superconducting nanowire single-photon detectors based on NbTiN thin films patterned directly on top of Si3N4 waveguides. We systematically investigate a large variety of detector designs and characterize their detection noise performance. Milli-Hz dark count rates are demonstrated over the entire operating range of the nanowire detectors which also feature low timing jitter. The ultra-low dark count rate, in combination with the high detection efficiency inherent to our travelling wave detector geometry, gives rise to a measured noise equivalent power at the 10(-20) W/Hz(1/2) level.

Nonlinear optical effects of ultrahigh-Q silicon photonic nanocavities immersed in superfluid helium.

Photonic nanocavities are a key component in many applications because of their capability of trapping and storing photons and enhancing interactions of light with various functional materials and structures. The maximal number of photons that can be stored in silicon photonic cavities is limited by the free-carrier and thermo-optic effects at room temperature. To reduce such effects, we performed the first experimental study of optical nonlinearities in ultrahigh-Q silicon disk nanocavities at cryogenic temperatures in a superfluid helium environment. At elevated input power, the cavity transmission spectra exhibit distinct blue-shifted bistability behavior when temperature crosses the liquid helium lambda point. At even lower temperatures, the spectra restore to symmetric Lorentzian shapes. Under this condition, we obtain a large intracavity photon number of about 40,000, which is limited ultimately by the local helium phase transition. These new discoveries are explained by theoretical calculations and numerical simulations.

Integrated GaN photonic circuits on silicon (100) for second harmonic generation.

We demonstrate second order optical nonlinearity in a silicon architecture through heterogeneous integration of single-crystalline gallium nitride (GaN) on silicon (100) substrates. By engineering GaN microrings for dual resonance around 1560 nm and 780 nm, we achieve efficient, tunable second harmonic generation at 780 nm. The χ2 nonlinear susceptibility is measured to be as high as 16 ± 7 pm/V. Because GaN has a wideband transparency window covering ultraviolet, visible and infrared wavelengths, our platform provides a viable route for the on-chip generation of optical wavelengths in both the far infrared and near-UV through a combination of χ2 enabled sum-/difference-frequency processes.

Carrier and thermal dynamics of silicon photonic resonators at cryogenic temperatures.

We describe measurement results of silicon photonic circuits at cryogenic temperatures. The interplay between optically induced heating and free carrier dynamics in nano-photonic ring resonators is investigated at temperatures down to 1.8 K. We find that the life-time of free carriers generated by two-photon absorption in silicon waveguides is reduced from 1.9 ns at room temperature to less than 100 ps below 10K. At the same time the thermal relaxation time is significantly elongated. Our work provides the first cryogenic measurement of ultra-short free-carrier lifetimes in silicon waveguides. The results further indicate that integrated optical chips can be easily thermo-optically stabilized at low temperatures.

Complete deterministic linear optics Bell state analysis.

We show how hyperentanglement allows us to deterministically distinguish between all four polarization Bell states of two photons. In this proof-of-principle experiment, we employ the intrinsic time-energy correlation of photon pairs generated with high temporal definition in addition to the polarization entanglement obtained from parametric down-conversion. For the identification, no nonlinear optical elements or auxiliary photons are needed. The new possibilities this complete Bell measurement offers are demonstrated by realizing an optimal dense coding protocol.