Programmable frequency-bin quantum states in a nano-engineered silicon device.

Photonic qubits should be controllable on-chip and noise-tolerant when transmitted over optical networks for practical applications. Furthermore, qubit sources should be programmable and have high brightness to be useful for quantum algorithms and grant resilience to losses. However, widespread encoding schemes only combine at most two of these properties. Here, we overcome this hurdle by demonstrating a programmable silicon nano-photonic chip generating frequency-bin entangled photons, an encoding scheme compatible with long-range transmission over optical links. The emitted quantum states can be manipulated using existing telecommunication components, including active devices that can be integrated in silicon photonics. As a demonstration, we show our chip can be programmed to generate the four computational basis states, and the four maximally-entangled Bell states, of a two-qubits system. Our device combines all the key properties of on-chip state reconfigurability and dense integration, while ensuring high brightness, fidelity, and purity.

Mitigating indistinguishability issues in photon pair sources by delayed-pump intermodal four wave mixing.

Large arrays of independent, pure and identical heralded single photon sources are an important resource for linear optical quantum computing protocols. In the race towards the development of increasingly ideal sources, delayed-pump intermodal four wave mixing (IFWM) in multimode waveguides has recently emerged as one of the most promising approaches. Despite this, fabrication imperfections still spoil the spectral indistinguishability of photon pairs from independent sources. Here we show that by tapering the width of the waveguide and by controlling the delay between the pump pulses, we add additional spectral tunability to the source while still inheriting all the distinctive metrics of the IFWM scheme. This feature is used to recover spectral indistinuishability in presence of fabrication errors. Under realistic tolerances on the waveguide dimensions, we predict >99.5% indistinguishability between independent sources on the same chip, and a maximum degradation of the heralded Hong-Ou-Mandel visibility <0.35%.

Reservoir computing based on a silicon microring and time multiplexing for binary and analog operations.

Photonic implementations of reservoir computing (RC) promise to reach ultra-high bandwidth of operation with moderate training efforts. Several optoelectronic demonstrations reported state of the art performances for hard tasks as speech recognition, object classification and time series prediction. Scaling these systems in space and time faces challenges in control complexity, size and power demand, which can be relieved by integrated optical solutions. Silicon photonics can be the disruptive technology to achieve this goal. However, the experimental demonstrations have been so far focused on spatially distributed reservoirs, where the massive use of splitters/combiners and the interconnection loss limits the number of nodes. Here, we propose and validate an all optical RC scheme based on a silicon microring (MR) and time multiplexing. The input layer is encoded in the intensity of a pump beam, which is nonlinearly transferred to the free carrier concentration in the MR and imprinted on a secondary probe. We harness the free carrier dynamics to create a chain-like reservoir topology with 50 virtual nodes. We give proof of concept demonstrations of RC by solving two nontrivial tasks: the delayed XOR and the classification of Iris flowers. This forms the basic building block from which larger hybrid spatio-temporal reservoirs with thousands of nodes can be realized with a limited set of resources.

On the modeling of thermal and free carrier nonlinearities in silicon-on-insulator microring resonators.

The temporal dynamics of integrated silicon resonators has been modeled using a set of equations coupling the internal energy, the temperature and the free carrier population. Owing to its simplicity, Newton's law of cooling is the traditional choice for describing the thermal evolution of such systems. In this work, we theoretically and experimentally prove that this can be inadequate in monolithic planar devices, leading to inaccurate predictions. A new equation that we train to reproduce the correct temperature behaviour is introduced to fix the discrepancies with the experimental results. We discuss the limitations and the range of validity of our refined model, identifying those cases where Netwon's law provides, nevertheless, accurate solutions. Our modeling describes the phenomena underlying thermal and free carrier instabilities and is a valuable tool for the engineering of photonic systems which rely on resonator dynamical states, such as all optical spiking neural networks or reservoirs for neuromorphic computing.

Phase-resolved joint spectra tomography of a ring resonator photon pair source using a silicon photonic chip.

The exponential growth of photonic quantum technologies is driving the demand for tools to measure the quality of their information carriers. One of the most prominent is stimulated emission tomography (SET), which uses classical coherent fields to measure the joint spectral amplitude (JSA) of photon pairs with high speed and resolution. While the modulus of the JSA can be directly addressed from a single intensity measurement, the retrieval of the joint spectral phase (JSP) is far more challenging and received minor attention. However, a wide class of spontaneous sources of technological relevance, as chip integrated micro-resonators, have a JSP with a rich structure that carries correlations hidden in the intensity domain. Here, using a compact and reconfigurable silicon photonic chip, the complex JSA of a micro-ring resonator photon pair source is measured for the first time. The photonic circuit coherently excites the ring and a reference waveguide, and the interferogram formed by their stimulated fields is used to map the ring JSP through a novel phase reconstruction technique. This tool complements the traditionally bulky and sophisticated methods implemented so far, simultaneously minimizing the set of required resources.

Four Wave Mixing control in a photonic molecule made by silicon microring resonators.

Four Wave Mixing (FWM) is the main nonlinear interaction in integrated silicon devices, which finds diffuse use in all-optical signal processing and wavelength conversion. Despite the numerous works on coupled resonator devices, which showed record conversion efficiencies and broadband operation, the possibility to coherently control the strength of the stimulated FWM interaction on a chip has received very limited attention. Here, we demonstrate both theoretically and experimentally, the manipulation of FWM in a photonic molecule based on two side coupled silicon microring resonators. The active tuning of the inter-resonator phase and of their eigenfrequencies allows setting the molecule in a sub-radiant state, where FWM is enhanced with respect to the isolated resonators. On the other hand, we can reconfigure the state of the photonic molecule to have energy equipartition among the resonators, and suppress FWM by making the two Signal waves to interfere destructively in the side coupled waveguides. This work constitutes an experimental demonstration of the control of a nonlinear parametric interaction via coherent oscillation phenomena in an integrated optical device.

Tuning the strain-induced resonance shift in silicon racetrack resonators by their orientation.

In this work, we analyze the role of strain on a set of silicon racetrack resonators presenting different orientations with respect to the applied strain. The strain induces a variation of the resonance wavelength, caused by the photoelastic variation of the material refractive index as well as by the mechanical deformation of the device. In particular, the mechanical deformation alters both the resonator perimeter and the waveguide cross-section. Finite element simulations taking into account all these effects are presented, providing good agreement with experimental results. By studying the role of the resonator orientation we identify interesting features, such as the tuning of the resonance shift from negative to positive values and the possibility of realizing strain insensitive devices.