Transporting Cold Atoms towards a GaN-on-Sapphire Chip via an Optical Conveyor Belt

Trapped atoms on photonic structures inspire many novel quantum devices for quantum information processing and quantum sensing.Here,we demonstrate a hybrid photonic-atom chip platform based on a Ga N-onsapphire chip and the transport of an ensemble of atoms from free space towards the chip with an optical conveyor belts.Due to our platform’s complete optical accessibility and careful control of atomic motion near the chip with a conveyor belt,successful atomic transport towards the chip is made possible.The maximum transport efficiency of atoms is about 50%with a transport distance of 500μm.Our results open up a new route toward the efficient loading of cold atoms into the evanescent-field trap formed by the photonic integrated circuits,which promises strong and controllable interactions between single atoms and single photons.

Tunable Optical Bandpass Filter via a Microtip-Touched Tapered Optical Fiber

We demonstrate a tunable bandpass optical filter based on a tapered optical fiber(TOF)touched by a hemispherical microfiber tip(MFT).Other than the interference and selective material absorption effects,the filter relies on the controllable and wavelength-dependent mode–mode interactions in TOF.Experimentally,a large range of tunability is realized by controlling the position of the MFT in contact with the TOF for various TOF radii,and two distinct bandpass filter mechanisms are demonstrated.The center wavelength of the bandpass filter can be tuned from 890 nm to 1000 nm,while the FWHM bandwidth can be tuned from 110 nm to 240 nm when the MFT touches the TOF in the radius range from 160 nm to 390 nm.The distinction ratio can reach 28±3 dB experimentally.The combined TOF-MFT is an in-line tunable bandpass optical filter that has great application potential in optical networks and spectroscopy,and the principle could also be generalized to other integrated photonic devices.

Breaking the efficiency limitations of dissipative Kerr solitons using nonlinear couplers

Dissipative Kerr solitons(DKS) have long been suffering from poor power conversion efficiency when driven by continuous-wave lasers. By deriving the critical coupling condition of a multimode nonlinear optics system in a generalized theoretical framework,two efficiency limitations of the conventional pump method of DKS are revealed: the effective coupling rate is too small and is also power-dependent. A general approach is provided to resolve this challenge by introducing two types of nonlinear couplers to couple the soliton cavity and CW input through nonlinear processes. The collective coupler opens multiple coupling channels and the self-adaptive coupler builds a power-independent effective external coupling rate to the DKS for approaching the generalized critical coupling condition, which promises near-unity power conversion efficiencies. For instance, a conversion efficiency exceeding 90% is predicted for aluminum nitride microrings with a nonlinear coupler utilizing second-harmonic generation. The mechanism applies to various nonlinear processes, including Raman and Brillouin scattering, and thus paves the way for micro-solitons toward practical applications.

Atom-referenced and stabilized soliton microcomb

For the applications of the frequency comb in microresonators,it is essential to obtain a fully frequency-stabilized microcomb laser source.In this study,we present a system for generating a fully atom-referenced stabilized soliton microcomb.The pump light around 1560.48 nm is locked to an ultra-low-expansion(ULE)cavity.This pump light is then frequency-doubled and referenced to the atomic transition of87Rb.The repetition rate of the soliton microcomb is injection-locked to an atomic-clockstabilized radio frequency(RF)source,leading to mHz stabilization at 1 s.As a result,all comb lines have been frequencystabilized based on the atomic reference and the ULE cavity,achieving a very high precision of approximately 18 Hz at 1 s,corresponding to the frequency stability of 9.5×10^(-14).Our approach provides a fully stabilized microcomb experiment scheme with no requirement of f-2f technique,which could be easily implemented and generalized to various photonic platforms,thus paving the way towards the ultraprecise optical sources for high precision spectroscopy.

Stimulated Brillouin interaction between guided phonons and photons in a lithium niobate waveguide

The Brillouin scattering is one of the strongest nonlinear optical effects in dielectrics[1].In particular,the backward Brillouin scattering(BBS)provides an efficient method to achieve low-noise optical gain for active photonics[2]and the coherent interconnects between 10 GHz phonons and photons[3].However,previous studies on BBS are primarily limited to the unconfined phonons[1]or phonon confined in suspended structures[4-6].

Frequency stabilization and tuning of breathing solitons in Si3N4 microresonators

Dissipative Kerr solitons offer broadband coherent and low-noise frequency combs and stable temporal pulse trains,having shown great potential applications in spectroscopy,communications,and metrology.Breathing solitons are a particular kind of dissipative Kerr soliton in which the pulse duration and peak intensity show periodic oscillation.Here we have investigated the breathing dissipative Kerr solitons in silicon nitride(Si3N4)microrings,while the breathing period shows uncertainties of around megahertz(MHz)order in both simulation and experiments.This instability is the main obstacle for future applications.By applying a modulated signal to the pump laser,the breathing frequency can be injection locked to the modulation frequency and tuned over tens of MHz with frequency noise significantly suppressed.Our demonstration offers an alternative knob for the control of soliton dynamics in microresonators and paves a new avenue towards practical applications of breathing solitons.

Emerging material platforms for integrated microcavity photonics

Many breakthroughs in technologies are closely associated with the deep understanding and development of new material platforms.As the main material used in microelectronics,Si also plays a leading role in the development of integrated photonics.The indirect bandgap,absence ofχ(2)nonlinearity and the parasitic nonlinear absorptions at the telecom band of Si imposed technological bottlenecks for further improving the performances and expanding the functionalities of Si microcavities in which the circulating light intensity is dramatically amplified.The past two decades have witnessed the burgeoning of the novel material platforms that are compatible with the complementary metal-oxide-semiconductor(COMS)process.In particular,the unprecedented optical properties of the emerging materials in the thin film form have resulted in revolutionary progress in microcavity photonics.In this review article,we summarize the recently developed material platforms for integrated photonics with the focus on chip-scale microcavity devices.The material characteristics,fabrication processes and device applications have been thoroughly discussed for the most widely used new material platforms.We also discuss open challenges and opportunities in microcavity photonics,such as heterogeneous integrated devices,and provide an outlook for the future development of integrated microcavities.

Fabrication of the high-Q Si_(3)N_(4) microresonators for soliton microcombs

The microresonator-based soliton microcomb has shown a promising future in many applications.In this work,we report the fabrication of high quality[Q]Si_(3)N_(4)microring resonators for soliton microcomb generation.By developing the fabri-cation process with crack isolation trenches and annealing,we can deposit thick stoichiometric Si3N4 film of 800 nm without cracks in the central area.The highest intrinsic Q of the Si_(3)N_(4)microring obtained in our experiments is about 6×10^(6),corresponding to a propagation loss as low as 0.058 dBm/cm.With such a high Q film,we fabricate microrings with the anomalous dispersion and demonstrate the generation of soliton microcombs with 100 mW on-chip pump power,with an optical parametric oscillation threshold of only 13.4 mW.Our Si_(3)N_(4)integrated chip provides an ideal platform for researches and applications of nonlinear photonics and integrated photonics.

Experimental repetitive quantum channel simulation

Universal control of quantum systems is a major goal to be achieved for quantum information processing,which demands thorough understanding of fundamental quantum mechanics and promises applications of quantum technologies. So far, most studies concentrate on ideally isolated quantum systems governed by unitary evolutions, while practical quantum systems are open and described by quantum channels due to their inevitable coupling to environment. Here, we experimentally simulate arbitrary quantum channels for an open quantum system, i.e. a single photonic qubit in a superconducting quantum circuit.The arbitrary channel simulation is achieved with minimum resource of only one ancilla qubit and measurement-based adaptive control. By repetitively implementing the quantum channel simulation,we realize an arbitrary Liouvillian for a continuous evolution of an open quantum system for the first time. Our experiment provides not only a testbed for understanding quantum noise and decoherence,but also a powerful tool for full control of practical open quantum systems.

Bosonic quantum error correction codes in superconducting quantum circuits

Quantum information is vulnerable to environmental noise and experimental imperfections,hindering the reli-ability of practical quantum information processors.Therefore,quantum error correction(QEC)that can pro-tect quantum information against noise is vital for universal and scalable quantum computation.Among many different experimental platforms,superconducting quantum circuits and bosonic encodings in superconducting microwave modes are appealing for their unprecedented potential in QEC.During the last few years,bosonic QEC is demonstrated to reach the break-even point,i.e.the lifetime of a logical qubit is enhanced to exceed that of any individual components composing the experimental system.Beyond that,universal gate sets and fault-tolerant operations on the bosonic codes are also realized,pushing quantum information processing towards the QEC era.In this article,we review the recent progress of the bosonic codes,including the Gottesman-Kitaev-Preskill codes,cat codes,and binomial codes,and discuss the opportunities of bosonic codes in various quantum applications,ranging from fault-tolerant quantum computation to quantum metrology.We also summarize the challenges associated with the bosonic codes and provide an outlook for the potential research directions in the long terms.

Tuneable red,green,and blue single-mode lasing in heterogeneously coupled organic spherical microcavities

Tuneable microlasers that span the full visible spectrum,particularly red,green,and blue(RGB)colors,are of crucial importance for various optical devices.However,RGB microlasers usually operate in multimode because the mode selection strategy cannot be applied to the entire visible spectrum simultaneously,which has severely restricted their applications in on-chip optical processing and communication.Here,an approach for the generation of tuneable multicolor single-mode lasers in heterogeneously coupled microresonators composed of distinct spherical microcavities is proposed.With each microcavity serving as both a whispering-gallery-mode(WGM)resonator and a modulator for the other microcavities,a single-mode laser has been achieved.The colors of the single-mode lasers can be freely designed by changing the optical gain in coupled cavities owing to the flexibility of the organic materials.Benefiting from the excellent compatibility,distinct color-emissive microspheres can be integrated to form a heterogeneously coupled system,where tuneable RGB single-mode lasing is realized owing to the capability for optical coupling between multiple resonators.Our findings provide a comprehensive understanding of the lasing modulation that might lead to innovation in structure designs for photonic integration.

Experimental demonstration of dissipative sensing in a self-interference microring resonator

The dissipative sensing based on a self-interference microring resonator composed of a microring resonator and a U-shaped feedback waveguide is demonstrated experimentally. Instead of a frequency shift induced by the phase shift of the waveguide or the microcavity, the dissipative sensing converts the phase shift to the effective external coupling rate, which leads to the change of linewidth of the optical resonance and the extinction ratio in the transmission spectrum. In our experiment, the power dissipated from a microheater on the feedback waveguide is detected by the dissipative sensing mechanism, and the sensitivity of our device can achieve0.22 d B/m W. This dissipative sensing mechanism provides another promising candidate for microcavity sensing applications.

Observation of high-Q optomechanical modes in the mounted silica microspheres

An efficient method to mount a coupled silica microsphere and tapered fiber system is proposed and demonstrated experimentally. For the purpose of optomechanical studies, high-quality-factor optical(Q_o～ 10~8) and mechanical modes(Q_m～ 0.87 × 10~4) are maintained after the mounting process. For the mounted microsphere, the coupling system is more stable and compact and, thus, is beneficial for future studies and applications based on optomechanical interactions. Especially, the packaged optomechanical system, which is tested in a vacuum chamber,paves the way toward quantum optomechanics research in cryostat.

Single-sideband microwave-to-optical conversion in high-Q ferrimagnetic microspheres

Coherent conversion of microwave and optical photons can significantly expand the capabilities of information processing and communications systems.Here,we experimentally demonstrate the microwave-to-optical frequency conversion in a magneto-optical whispering gallery mode microcavity.By applying a magnetic field parallel to the microsphere equator,the intracavity optical field will be modulated when the magnon is excited by the microwave drive,leading to a microwave-to-optical conversion via the magnetic Stokes and anti-Stokes scattering processes.The observed single-sideband conversion phenomenon indicates a nontrivial optical photon–magnon interaction mechanism derived from the magnon that induced both the frequency shift and modulated coupling rate of optical modes.In addition,we demonstrate the single-sideband frequency conversion with an ultrawide tuning range up to 2.5 GHz,showing its great potential in microwave-to-optical conversion.

Realization of quantum ground state in an optomechanical crystal cavity

Ground-state cooling of the mechanical degree of freedom is a fundamental requirement for quantum state transfer between relevant optical and mechanical systems.Here,we fabricate optomechanical crystal cavities with co-localized mechanical and optical modes on a monolithic chip.The typical linewidth of the optical modes at telecom wavelength is as low as 0.95 GHz,and the mechanical resonant frequency is around 5 GHz,which means the optomechanical system can be operated in the resolved sideband regime.With the statistics of the asymmetry in the scattering rates of red and blue detuning driving processes,we initialize and characterize the mechanical system in its quantum ground-state of motion,with a mean thermal phonon occupancy nth=0.018±0.0034,corresponding to a mode temperature of 57.3 m K.It is a general platform for quantum state transfer between relevant optical and mechanical systems,as well as the quantum entanglement between these systems.

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.

Vernier effect facilitates integrated lithium niobate single-mode lasers

A research group from Nankai University in China has demonstrated an integrated whispering gallery mode(WGM)microcavity laser on erbium-doped thin-film lithium niobate with a pump threshold of about 200 μW and a side-mode suppression ratio exceeding 26 d B [1]. The Vernier effect is applied to realize the single-mode lasing in a photonic molecule. The work provides a feasible scheme for on-chip single-mode laser for integrated lithium niobate on insulator(LNOI) photonic circuits, and more active devices in LNOI will be developed inspired by this work.

Design of a micrometer-long superconducting nanowire perfect absorber for efficient high-speed single-photon detection

Despite very efficient superconducting nanowire single-photon detectors(SNSPDs)reported recently,combining their other performance advantages such as high speed and ultralow timing jitter in a single device still remains challenging.In this work,we present a perfect absorber model and the corresponding detector design based on a micrometer-long NbN nanowire integrated with a 2D photonic crystal cavity of ultrasmall mode volume,which promises simultaneous achievement of near-unity absorption,gigahertz counting rates,and broadband optical response with a 3 dB bandwidth of 71 nm.Compared to previous stand-alone meandered and waveguideintegrated SNSPDs,this perfect absorber design addresses the trade space in size,efficiency,speed,and bandwidth for realizing large on-chip single-photon detector arrays.

Highly tunable broadband coherent wavelength conversion with a fiber-based optomechanical system

Modern information networks are built on hybrid systems working at disparate optical wavelengths.Coherent interconnects for converting photons between different wavelengths are highly desired.Although coherent interconnects have conventionally been realized with nonlinear optical effects,those systems require demanding experimental conditions,such as phase matching and/or cavity enhancement,which not only bring difficulties in experimental implementation but also set a narrow tuning bandwidth(typically in the MHz to GHz range as determined by the cavity linewidth).Here,we propose and experimentally demonstrate coherent information transfer between two orthogonally propagating light beams of disparate wavelengths in a fiber-based optomechanical system,which does not require phase matching or cavity enhancement of the pump beam.The coherent process is demonstrated by interference phenomena similar to optomechanically induced transparency and absorption.Our scheme not only significantly simplifies the experimental implementation of coherent wavelength conversion but also extends the tuning bandwidth to that of an optical fiber(tens of THz),which will enable a broad range of coherent-optics-based applications,such as optical sensing,spectroscopy,and communication.

Hybrid superconducting photonic-phononic chip for quantum information processing

The integration of qubits with long coherence times and functional quantum devices on a single chip,and thus the realization of an allsolidstate quantum computing chip,is an important goal in current experimental research on quantum information processing.Among various quantum platforms,a series of significant progresses have been made in photonic quantum chips and superconducting quantum chips,while both the number of qubits and the complexity of quantum circuits have been increasing.Although these two chip platforms have respective unique advantages and potentials,their shortcomings have been gradually revealed and need to be solved.By introducing phonon-integrated devices,it is possible to combine all unsuspended phononic,photonic,and superconducting quantum devices organi-cally on the same chip to achieve coherent coupling among them.Here,we provide a prospect and a short review on the integrated photonic,superconducting,and hybrid quantum chips for quantum information processing.