Integrated silicon nitride time-bin entanglement circuits.

Time-bin entangled photons allow robust entanglement distribution over quantum networks. Integrated photonic circuits positioned at the nodes of a quantum network can perform the important functions of generating highly entangled photons and precisely manipulating their quantum state. In this Letter, we demonstrate time-bin entangled photon generation, noise suppression, wavelength division, and entanglement analysis on a single photonic chip utilizing low-loss double-stripe silicon nitride waveguide structures. Quantum state tomography results show 91±0.7% fidelity compared with the ideal state, indicating that highly entangled photons are generated and analyzed. This work represents a crucial step toward practical quantum networks.

High repetition rate correlated photon pair generation in integrated silicon nanowires.

Integrated single-photon sources are a key component for photonic quantum technology but are generally limited to low single-photon rates. For sources based on photon pair generation by four-wave mixing, increasing the repetition rate of pump laser pulses is a straightforward way to enhance the single-photon rate, but the benefits and practical limitations have not yet been demonstrated and analyzed in a CMOS-compatible platform. In this work, we demonstrate correlated photon pair generation in integrated silicon nanowires and systematically analyze the count rate and coincidence to accidental ratio as the pump rate is varied between 156.25 MHz and 10 GHz. We show that the highest useful pump rate is limited by the timing resolution of the single-photon detection system, and that in this regime, the nonlinear loss of the silicon nanowire does not have a significant effect on the single-photon generation.

Uni-directional wavelength conversion in silicon using four-wave mixing driven by cross-polarized pumps.

We demonstrate optical frequency conversion between telecom wavelengths using four-wave mixing Bragg scattering powered by two pump pulses polarized on orthogonal axes of a silicon waveguide. This allows conversion in a single frequency direction while, with co-polarized pumps, the signal is redshifted or blueshifted with similar efficiency. Our approach exploits the birefringence of the waveguide and its effect on the phase matching of the four-wave mixing process. The blue or red direction can be selected by the input polarization of the signal, and 20 dB extinction ratios are observed with the unintended direction. This technique will allow efficient and controlled conversion between specified wavelength channels in integrated photonic devices.

Multiwavelength stabilization control of a thermo-optic system with adaptive reconfiguration.

Accurate temperature control is crucial for the reliable operation of photonic integrated circuits in the presence of internal thermal crosstalk or external thermal disturbance. We propose an adaptive multiple-input and multiple-output (MIMO) control scheme to stabilize the operation wavelength of on-chip wavelength demultiplexers that have many applications in photonic-chip-based optical signal processing. Using the MIMO control scheme, the wavelength drift is reduced from 0.5 to 0.1 nm when internal and external thermal disturbances occur.

Integrated sources of photon quantum states based on nonlinear optics.

The ability to generate complex optical photon states involving entanglement between multiple optical modes is not only critical to advancing our understanding of quantum mechanics but will play a key role in generating many applications in quantum technologies. These include quantum communications, computation, imaging, microscopy and many other novel technologies that are constantly being proposed. However, approaches to generating parallel multiple, customisable bi- and multi-entangled quantum bits (qubits) on a chip are still in the early stages of development. Here, we review recent advances in the realisation of integrated sources of photonic quantum states, focusing on approaches based on nonlinear optics that are compatible with contemporary optical fibre telecommunications and quantum memory platforms as well as with chip-scale semiconductor technology. These new and exciting platforms hold the promise of compact, low-cost, scalable and practical implementations of sources for the generation and manipulation of complex quantum optical states on a chip, which will play a major role in bringing quantum technologies out of the laboratory and into the real world.

Phase-sensitive tomography of the joint spectral amplitude of photon pair sources.

We present a novel measurement technique to perform full phase-sensitive tomography on the joint spectrum of photon pair sources, using stimulated four-wave mixing and phase-sensitive amplification. Applying this method to an integrated silicon nanowire source with a frequency chirped pump laser, we are able to observe a corresponding phase change in the spectral amplitude that would otherwise be hidden in standard intensity measurements. With a highly nonlinear fiber source, we show that phase-sensitive measurements have superior sensitivity to small spectral features when compared to intensity measurements. This technique enables more complete characterization of photon pair sources based on nonlinear photonics.

Frequency conversion in silicon in the single photon regime.

Quantum communication networks require single photon frequency converters, whether to shift photons between wavelength channels, to shift photons to the operating wavelength of a quantum memory, or to shift photons of different wavelengths to be of the same wavelength, to enable a quantum interference. Here, we demonstrate frequency conversion of laser pulses attenuated to the single photon regime in an integrated silicon-on-insulator device using four-wave mixing Bragg scattering, with conversion efficiencies of up to 12%, or 32% after correcting for nonlinear loss created by the pump lasers. The frequency shift can be conveniently chosen by tuning of the pump frequencies. We demonstrate that such frequency conversion enables interference between photons at different frequencies.

Bi-photon spectral correlation measurements from a silicon nanowire in the quantum and classical regimes.

The growing requirement for photon pairs with specific spectral correlations in quantum optics experiments has created a demand for fast, high resolution and accurate source characterisation. A promising tool for such characterisation uses classical stimulated processes, in which an additional seed laser stimulates photon generation yielding much higher count rates, as recently demonstrated for a χ(2) integrated source in A. Eckstein et al. Laser Photon. Rev. 8, L76 (2014). In this work we extend these results to χ(3) integrated sources, directly measuring for the first time the relation between spectral correlation measurements via stimulated and spontaneous four wave mixing in an integrated optical waveguide, a silicon nanowire. We directly confirm the speed-up due to higher count rates and demonstrate that this allows additional resolution to be gained when compared to traditional coincidence measurements without any increase in measurement time. As the pump pulse duration can influence the degree of spectral correlation, all of our measurements are taken for two different pump pulse widths. This allows us to confirm that the classical stimulated process correctly captures the degree of spectral correlation regardless of pump pulse duration, and cements its place as an essential characterisation method for the development of future quantum integrated devices.

Degenerate photon-pair generation in an ultracompact silicon photonic crystal waveguide.

We demonstrate degenerate, correlated photon-pair generation via slow-light-enhanced spontaneous four-wave mixing in a 96 µm long silicon photonic crystal waveguide. Our device represents a more than 50 times smaller footprint than silicon nanowires. We have achieved a coincidence-to-accidental ratio as high as 47 at a photon generation rate of 0.001 pairs per pulse and 14 at a photon generation rate of 0.023 pairs per pulse, which are both higher than the useful level of 10. This demonstration provides a path to generate indistinguishable photons in an ultracompact platform for future quantum photonic technologies.

Integrated optical auto-correlator based on third-harmonic generation in a silicon photonic crystal waveguide.

The ability to use coherent light for material science and applications is linked to our ability to measure short optical pulses. While free-space optical methods are well established, achieving this on a chip would offer the greatest benefit in footprint, performance and cost, and allow the integration with complementary signal-processing devices. A key goal is to achieve operation at sub-watt peak power levels and on sub-picosecond timescales. Previous integrated demonstrations require either a temporally synchronized reference pulse, an off-chip spectrometer or long tunable delay lines. Here we report a device capable of achieving single-shot time-domain measurements of near-infrared picosecond pulses based on an ultra-compact integrated CMOS-compatible device, which could operate without any external instrumentation. It relies on optical third-harmonic generation in a slow-light silicon waveguide. Our method can also serve as an in situ diagnostic tool to map, at visible wavelengths, the propagation dynamics of near-infrared pulses in photonic crystals.

Chalcogenide fiber-based distributed temperature sensor with sub-centimeter spatial resolution and enhanced accuracy.

We demonstrate a sub-centimeter spatial resolution fiber-based distributed temperature sensor with enhanced measurement accuracy and reduced acquisition time. Our approach employs time domain analysis of backscattered Stokes and anti-Stokes photons generated via spontaneous Raman scattering in a chalcogenide (ChG) As2S3 fiber for temperature monitoring. The sensor performance is significantly improved by exploiting the high Raman coefficient and increased refractive index of the ChG fiber. We achieve a temperature uncertainty of ± 0.65 °C for a short measurement time of only 5 seconds; whilst the detection uncertainty is less than ± 0.2 °C for a longer integration time of 2 minutes. We also investigate the optimum Stokes and anti-Stokes bands for optimal sensing performance. Our theoretical analysis shows that a small detuning frequency regime from a pump is more suitable for rapid measurements while a large detuning regime provides higher temperature resolution.

Multi-photon absorption limits to heralded single photon sources.

Single photons are of paramount importance to future quantum technologies, including quantum communication and computation. Nonlinear photonic devices using parametric processes offer a straightforward route to generating photons, however additional nonlinear processes may come into play and interfere with these sources. Here we analyse spontaneous four-wave mixing (SFWM) sources in the presence of multi-photon processes. We conduct experiments in silicon and gallium indium phosphide photonic crystal waveguides which display inherently different nonlinear absorption processes, namely two-photon (TPA) and three-photon absorption (ThPA), respectively. We develop a novel model capturing these diverse effects which is in excellent quantitative agreement with measurements of brightness, coincidence-to-accidental ratio (CAR) and second-order correlation function g((2))(0), showing that TPA imposes an intrinsic limit on heralded single photon sources. We build on these observations to devise a new metric, the quantum utility (QMU), enabling further optimisation of single photon sources.

Heralded single-photon source in a III-V photonic crystal.

In this Letter we demonstrate heralded single-photon generation in a III-V semiconductor photonic crystal platform through spontaneous four-wave mixing. We achieve a high brightness of 3.4×10(7) pairs·s(-1) nm(-1) W(-1) facilitated through dispersion engineering and the suppression of two-photon absorption in the gallium indium phosphide material. Photon pairs are generated with a coincidence-to-accidental ratio over 60 and a low g(2) (0) of 0.06 proving nonclassical operation in the single photon regime.

High-efficiency frequency conversion in the single-photon regime.

In this Letter we demonstrate frequency conversion in the single-photon regime through Bragg-scattering four-wave mixing with near-unit efficiency in a 750 m long commercially available dispersion-engineered highly nonlinear fiber, where all photons and pump laser frequencies are in the low-loss telecommunications band. We achieve 99.1%±4.9% downconversion and 98.0%±5.0% upconversion of photons by 12 nm using a weak coherent state with an average input of 0.27 photons per detection gate window.

Mode multiplexed single-photon and classical channels in a few-mode fiber.

We classically measure the entire propagation matrix of a few-mode fiber and use a spatial light modulator to undo modal mixing and recover single-photons launched onto each of the eigenmodes of the fiber at one end, but arriving as mixed modal superpositions at the other. We exploit the orthogonality of these modal channels to improve the isolation between a quantum and classical channel launched onto different spatial and polarization modes at different wavelengths. The spatial diversity of the channels provides an additional 35dB of isolation in addition to that provided by polarization and wavelength.

Low Raman-noise correlated photon-pair generation in a dispersion-engineered chalcogenide As2S3 planar waveguide.

We demonstrate low Raman-noise correlated photon-pair generation in a dispersion-engineered 10 mm As2S3 chalcogenide waveguide at room temperature. We show a coincidence-to-accidental ratio (CAR) of 16.8, a 250 times increase compared with previously published results in a chalcogenide waveguide, with a corresponding brightness of 3×10(5) pairs·s(-1)·nm(-1) generated at the chip. Dispersion engineering of our waveguide enables photon passbands to be placed in the low spontaneous Raman scattering (SpRS) window at 7.4 THz detuning from the pump. This Letter shows the potential for As2S3 chalcogenide to be used for nonlinear quantum photonic devices.

Continuous wave, dual-wavelength-pumped supercontinuum generation in an all-fiber device.

We propose a continuous wave dual-wavelength-pumped scheme for visible supercontinuum (SC) generation. The scheme is numerically studied in this paper. In the scheme, the dual-wavelength pump source is produced through a four-wave mixing process in a photonic crystal fiber. SC generation is numerically investigated by solving the generalized nonlinear Schrödinger equation. The results verify that the visible SC can be generated by the scheme, which implies that the scheme is promising for generating visible SC with high spectral power densities.

Ultrahigh-bandwidth, on-chip all-optical pulse erasure using the χ(3) process in a nonlinear chalcogenide waveguide.

We demonstrate on-chip all-optical pulse erasure based on four-wave mixing and cross-phase modulation in a dispersion engineered chalcogenide (As(2)S(3)) rib waveguide. We achieve an erasure efficiency of ~15 dB for picosecond pulses in good agreement with numerical simulations using the nonlinear Schrödinger equation. The combined effect of the high instantaneous optical nonlinearity (γ = 9900 (W km)(-1)) and small group-velocity dispersion (D = 29 ps/nm km), which reduces pulse walk-off, will enable all-optical pulse erasure for ultrafast signal processing.