Tailoring light on three-dimensional photonic chips: a platform for versatile OAM mode optical interconnects

Explosive growth in demand for data traffic has prompted exploration of the spatial dimension of lightwaves, which provides a degree of freedom to expand data transmission capacity. Various techniques basedon bulky optical devices have been proposed to tailor light waves in the spatial dimension. However, theirinherent large size, extra loss, and precise alignment requirements make these techniques relativelydifficult to implement in a compact and flexible way. In contrast, three-dimensional (3D) photonic chips withcompact size and low loss provide a promising miniaturized candidate for tailoring light in the spatialdimension. Significantly, they are attractive for chip-assisted short-distance spatial mode optical interconnectsthat are challenging to bulky optics. Here, we propose and fabricate femtosecond laser-inscribed 3D photonicchips to tailor orbital angular momentum (OAM) modes in the spatial dimension. Various functions on theplatform of 3D photonic chips are experimentally demonstrated, including the generation, (de)multiplexing,and exchange of OAM modes. Moreover, chip-chip and chip–fiber–chip short-distance optical interconnectsusing OAM modes are demonstrated in the experiment with favorable performance. This work paves the wayto flexibly tailor light waves on 3D photonic chips and offers a compact solution for versatile opticalinterconnects and other emerging applications with spatial modes.

Quantum entanglement on photonic chips:a review

Entanglement is one of the most vital properties of quantum mechanical systems,and it forms the backbone of quantum information technologies.Taking advantage of nano/microfabrication and particularly complementary metal-oxide-semiconductor manufacturing technologies,photonic integrated circuits(PICs)have emerged as a versatile platform for the generation,manipulation,and measurement of entangled photonic states.We summarize the recent progress of quantum entanglement on PICs,starting from the generation of nonentangled and entangled biphoton states,to the generation of entangled states of multiple photons,multiple dimensions,and multiple degrees of freedom,as well as their applications for quantum information processing.

On-chip quantum NOON state sensing for temperature and humidity

A maximal photon number entangled state,namely NOON state,can be adopted for sensing with a quantum enhancedprecision.In this work,we designed silicon quantum photonic chips containing two types of Mach-Zehnder interferometerswherein the two-photon NOON state,sensing element for temperature or humidity,is generated.Compared with classicallight or single photon case,two-photon NOON state sensing shows a solid enhancement in the sensing resolution andprecision.As the first demonstration of on-chip quantum photonic sensing,it reveals the advantages of photonic chips forhigh integration density,small-size,stability for multiple-parameter sensing serviceability.A higher sensing precision isexpected to beat the standard quantum limit with a higher photon number NOON state.

Quantum photonic network on chip

We provide an overview of quantum photonic network on chip. We begin from the discussion of the pros and cons of several material platforms for engineering quantum photonic chips. Then we introduce and analyze the basic building blocks and functional units of quantum photonic integrated circuits. In the main part of this review, we focus on the generation and manipulation of quantum states of light on chip and are particularly interested in some applications of advanced integrated circuits with different functionalities for quantum information processing, including quantum communication, quantum computing, and quantum simulation. We emphasize that developing fully integrated quantum photonic chip which contains sources of quantum light, integrate circuits, modulators, quantum storage, and detectors are promising approaches for future quantum photonic technologies. Recent achievements in the large scale photonic chips for linear optical computing are also included. Finally, we illustrate the challenges toward high performance quantum information processing devices and conclude with promising perspectives in this field.

Experimental demonstration of a fast calibration method for integrated photonic circuits with cascaded phase shifters

With the development of research on integrated photonic quantum information processing,the integration level of the integrated quantum photonic circuits has been increasing continuously,which makes the calibration of the phase shifters on the chip increasingly difficult.For the calibration of multiple cascaded phase shifters that is not easy to be decoupled,the resources consumed by conventional brute force methods increase exponentially with the number of phase shifters,making it impossible to calibrate a relatively large number of cascaded phase shifters.In this work,we experimentally validate an efficient method for calibrating cascaded phase shifters that achieves an exponential increase in calibration efficiency compared to the conventional method,thus solving the calibration problem for multiple cascaded phase shifters.Specifically,we experimentally calibrate an integrated quantum photonic circuit with nine cascaded phase shifters and achieve a high-precision calibration with an average fidelity of 99.26%.

Hybrid-integrated chalcogenide photonics

High-quality photonic materials are critical for promoting integrated photonic devices with broad bandwidths,high efficiencies,and flexibilities for high-volume chip-scale fabrication.Recently,we designed a home-developed chalcogenide glass(ChG)-Ge_(25)Sb_(10)S_(65)(GeSbS)for optical information processing chips and systems,which featured an ultrabroad transmission window,a high Kerr nonlinearity and photoelastic coefficient,and compatibility with the photonic hybrid integration technology of silicon photonics.Chip-integrated GeSbS microresonators and microresonator arrays with high quality factors and lithographically controlled fine structures were fabricated using a modified nanofabrication process.Moreover,considering the high Kerr nonlinearity and photoelastic effect of ChGs,we realised a novel ChG hybrid integrated chip,inspired by recent advances in integrated soliton microcombs and acousto-optic(AO)modulators.

Topologically Protected Polarization Quantum Entanglement on a Photonic Chip

Quantum entanglement,as the strictly non-classical phenomenon,is the kernel of quantum computing and quantum simulation,and has been widely applied ranging from fundamental tests of quantum physics to quantum information processing.Meanwhile,the topolog-ical phase is found inherently capable of protecting physical fields from unavoidable fabrication-induced disorder,which inspires the po-tential application of topological protection to quantum states.Here,we present the experimental demonstration of topologically protected quantum entangled states on a photonic chip.The process tomogra-phy shows that quantum entanglement can be well preserved by the topological states even when the chip material introduces disorder and relative polarization rotation in phase space.Our work links the fields of materials,topological science and quantum physics,opening the door to wide applications of topological enhancement in quantum regime.

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 singlephoton 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.

Biofunctionalized all-polymer photonic lab on a chip with integrated solid-state light emitter

A photonic lab on a chip(PhLOC),comprising a solid-state light emitter(SSLE)aligned with a biofunctionalized optofluidic multiple internal reflection(MIR)system,is presented.The SSLE is obtained by filling a microfluidic structure with a phenyltrimethoxysilane(PhTMOS)aqueous sol solution containing a fluorophore organic dye.After curing,the resulting xerogel solid structure retains the emitting properties of the fluorophore,which is evenly distributed in the xerogel matrix.Photostability studies demonstrate that after a total dose(at λ5365 nm)greater than 24 J cm^(-2),the xerogel emission decay is only 4.1%.To re-direct the emitted light,the SSLE includes two sets of air mirrors that surround the xerogel.Emission mapping of the SSLE demonstrates that alignment variations of 150 mm(between the SSLE and the external pumping light source)provide fluctuations in emitted light smaller than 5%.After this verification,the SSLE is monolithically implemented with a MIR,forming the PhLOC.Its performance is assessed by measuring quinolone yellow,obtaining a limit of detection(LOD)of(0.6060.01)mM.Finally,the MIR is selectively biofunctionalized with horseradish peroxidase(HRP)for the detection of hydrogen peroxide(H_(2)O_(2))target analyte,obtaining a LOD of(0.760.1)μM for H_(2)O_(2),confirming,for the first time,that solid-state xerogel-based emitters can be massively implemented in biofunctionalized PhLOCs.

On-chip topological nanophotonic devices

On-chip topological nanophotonic devices,which take photons as in-formation carriers with topological protection during light propaga-tion,have great application potential in the next generation photonic chips.The topological photonic states enable the nanophotonic de-vices to be robust and stable,immune to scattering even with imper-fect structures.The development,opportunities and challenges of the on-chip topological nanophotonic devices have attracted great atten-tion of scholars,and desired to be known.In this review,topologi-cal devices were introduced in the order of functionalities on an in-tegrated photonic chip,i.e.topological light source,topological light waveguiding,topological light division and selection,topological light manipulation and topological light detecting.Finally,we gave out-looks for predicting and promoting the performances of on-chip topo-logical nanophotonic devices from the angles of non-Hermitian sys-tems,non-Abelian topology,metasurfaces,intelligent algorithms and multiple functional topological nanophotonic integration.This review provides rich knowledge about on-chip topological nanophotonic de-vices.The insights in this paper will spark inspiration and inspire new thinking for the realization of topological photonic chips.

On-chip single-photon chirality encircling exceptional points

Exceptional points(EPs),which are typically defined as the degener-acy points of a non-Hermitian Hamiltonian,have been investigated in various physical systems such as photonic systems.In particular,the intriguing topological structures around EPs have given rise to novel strategies for manipulating photons and the underlying mechanism is especially useful for on-chip photonic applications.Although some on-chip experiments with the adoption of lasers have been reported,EP-based photonic chips working in the quantum regime largely re-main elusive.In the current work,a single-photon experiment was proposed to dynamically encircle an EP in on-chip photonic waveg-uides possessing passive anti-parity-time symmetry.Photon coinci-dences measurement reveals a chiral feature of transporting single photons,which can act as a building block for on-chip quantum de-vices that require asymmetric transmissions.The findings in the cur-rent work pave the way for on-chip experimental study on the physics of EPs as well as inspiring applications for on-chip non-Hermitian quantum devices.

Femtosecond laser-inscribed optical waveguides in dielectric crystals:a concise review and recent advances

Femtosecond laser inscription or writing has been recognized as a powerful technique to engineer various materials toward a number of applications.By efficient modification of refractive indices of dielectric crystals,optical waveguides with diverse configurations have been produced by femtosecond laser writing.The waveguiding properties depend not only on the parameters of the laser writing but also on the nature of the crystals.The mode profile tailoring and polarization engineering are realizable by selecting appropriate fabrication conditions.In addition,regardless of the complexity of crystal refractive index changes induced by ultrafast pulses,several three-dimensional geometries have been designed and implemented that are useful for the fabrication of laser-written photonic chips.Some intriguing devices,e.g.,waveguide lasers,wavelength converters,and quantum memories,have been made,exhibiting potential for applications in various areas.Our work gives a concise review of the femtosecond laser-inscribed waveguides in dielectric crystals and focuses on the recent advances of this research area,including the fundamentals,fabrication,and selected photonic applications.

Low-depth optical neural networks

Optical neural network(ONNs)are emerging as attractive propos-als for machine-learning applications.However,the stability of ONNs decreases with the circuit depth,limiting the scalability of ONNs for practical uses.Here we demonstrate how to compress the circuit depth to scale only logarithmically in terms of the dimension of the data,leading to an exponential gain in terms of noise robustness.Our low-depth(LD)-ONN is based on an architecture,called Optical Com-puTing Of dot-Product UnitS(OCTOPUS),which can also be applied individually as a linear perceptron for solving classification problems.We present both numerical and theoretical evidence showing that LD-ONN can exhibit a significant improvement on robustness,compared with previous ONN proposals based on singular-value decomposition.