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Narrow linewidth is a long-pursuing goal in precision measurement and sensing. We propose a parity-time (PT )-symmetric feedback method to narrow the linewidths of resonance systems. By using a quadrature measurement-feedback loop, we transform a dissipative resonance system into a PT-symmetric system. Unlike the conventional PT-symmetric systems which typically require two or more modes, here the PT-symmetric feedback system contains only a single resonance mode, which greatly extends the scope of applications. The method enables remarkable linewidth narrowing and enhancement of measurement sensitivity. We illustrate the concept in a thermal ensemble of atoms, achieving a 48-fold narrowing of the magnetic resonance linewidth. By applying the method in magnetometry, we realize a 22-times improvement of the measurement sensitivity. This work opens the avenue for studying non-Hermitian physics and high-precision measurements in resonance systems with feedback., 5 pages, 5 figures

Generation of mechanical squeezing has attracted a lot of interest for its nonclassical properties, applications in quantum information, and high-sensitivity measurement. Here we propose a detuning-switched method that can rapidly generate strong and stationary mechanical squeezing. The pulsed driving can dynamically transpose the optomechanical coupling into a linear optical force and maintain an effective mechanical frequency, which can introduce strong mechanical squeezing in a short time. Moreover, we show the obtained strong mechanical squeezing can be frozen by increasing the pulse intervals, leading to stationary mechanical squeezing with a fixed squeezing angle. Thus, our proposal provides fascinating insights and applications of modulated optomechanical systems.

Spin squeezing plays a crucial role in quantum metrology and quantum information science. Its generation is the prerequisite for further applications but still faces an enormous challenge since the existing physical systems rarely contain the required squeezing interactions. Here we propose a universal scheme to generate spin squeezing in coupled spin models with collective spin-spin interactions, which commonly exist in various systems. Our scheme can transform the coupled spin interactions into squeezing interactions, and reach the extreme squeezing with Heisenberg-limited measurement precision scaling as $1/N$ for $N$ particles. Only constant and continuous driving fields are required, which is accessible to a series of current realistic experiments. This work greatly enriches the variety of systems that can generate the Heisenberg-limited spin squeezing, with broad applications in quantum precision measurement.

Cavity optomechanical systems enable interactions between light and mechanical resonators, providing a platform both for fundamental physics of macroscopic quantum systems and for practical applications of precision sensing. The resonant enhancement of both mechanical and optical response in the cavity optomechanical systems has enabled precision sensing of multiple physical quantities, including displacements, masses, forces, accelerations, magnetic fields, and ultrasounds. In this article, we review the progress of precision sensing applications using cavity optomechanical systems. The review is organized in the following way: first we will introduce the physical principles of optomechanical sensing, including a discussion of the noises and sensitivity of the systems, and then review the progress in displacement sensing, mass sensing, force sensing, atomic force microscope (AFM) and magnetic resonance force microscope (MRFM), accelerometry, magnetometry, and ultrasound sensing, and introduce the progress of using quantum techniques especially squeezed light to enhance the performance of the optomechanical sensors. Finally, we give a summary and outlook.

Carbon dioxide is a common gas in the air which has been widely used in medical treatment. A carbon dioxide molecule consists of two oxygen atoms and one carbon atom through a covalent bond. In the body, carbon dioxide reacts with water to produce carbonic acid. In healthy people, carbon dioxide is maintained within a narrow range (35–45 mmHg) by physiological mechanisms. The role of hypocapnia (partial pressure of carbon dioxide < 35 mmHg) and hypercapnia (partial pressure of carbon dioxide > 45 mmHg) in the nervous system is intricate. Past researches mainly focus on the effect of hypocapnia to nerve protection. Nevertheless, Hypercapnia seems to play an important role in neuroprotection. The mechanisms of hypocapnia and hypercapnia in the nervous system deserve our attention. The purpose of this review is to summarize the effect of hypocapnia and hypercapnia in stroke and traumatic brain injury.

Non-Hermitian topological effects are of crucial importance both in fundamental physics and applications. Here we discover the gain-loss-induced hybrid second-order skin-topological effect and the PT phase transition in skin-topological modes. By studying a non-Hermitian Haldane model, we find that the topological edge modes are localized on a special type of corner, while the bulk modes remain extended. Such an effect originates from the interplay between gain, loss, and the chiral edge currents induced by the nonlocal flux, which can be characterized by considering the properties of the edge sites as a one-dimensional chain. We establish a relation between the skin-topological effect and the PT symmetries belonging to different edges. Moreover, we discover the PT phase transition with the emergence of exceptional points between pairs of skin-topological modes. Our results pave the way for the investigation of non-Hermitian topological physics and PT phase transition in higher-dimensional systems.

The existence of exotic spin-dependent forces may shine light on new physics beyond the Standard Model. We utilize two iron shielded SmCo$_5$ electron-spin sources and two optically pumped magnetometers to search for exotic long-range spin-spin-velocity-dependent force. The orientations of spin sources and magnetometers are optimized such that the exotic force is enhanced and common-mode noise is effectively subtracted. We set direct limit on proton-electron interaction in the force range from 1\,cm to 1\,km. Our experiment represents more than ten orders of magnitude improvement than previous works.

Ground-state cooling of multiple mechanical resonators is an important goal in the study of quantum optomechanics. Here, we propose a dynamic dissipative synchronous cooling method for the simultaneous cooling of the two mechanical resonators in the strong optomechanical coupling regime. The synchronized modulation of the cavity dissipation can significantly accelerate the cooling process, with both of the resonators reaching the ground state. We derive the analytical cooling limits, which agree with the numerical simulations. The present scheme opens a new prospect for the research of multiple-mode ground-state cooling of mechanical resonators.

The synchronization of the motion of microresonators has attracted considerable attention. In previous studies, the microresonators for synchronization were studied mostly in the linear regime. While the important problem of synchronizing nonlinear microresonators was rarely explored. Here we present theoretical methods to synchronize the motions of chaotic optical cavity modes in an optomechanical system, where one of the optical modes is strongly driven into chaotic motion and transfers chaos to other weakly driven optical modes via a common mechanical resonator. This mechanical mode works as a common force acting on each optical mode, which, thus, enables the synchronization of states. We find that complete synchronization can be achieved in two identical chaotic cavity modes. For two arbitrary nonidentical chaotic cavity modes, phase synchronization can also be achieved in the strong-coupling small-detuning regime.

Nonreciprocity is important in both optical information processing and topological photonics studies. Conventional principles for realizing nonreciprocity rely on magnetic fields, spatiotemporal modulation, or nonlinearity. Here we propose a generic principle for generating nonreciprocity by taking advantage of energy loss, which is usually regarded as harmful. The loss in a resonance mode induces a phase lag, which is independent of the energy transmission direction. When multichannel lossy resonance modes are combined, the resulting interference gives rise to nonreciprocity, with different coupling strengths for the forward and backward directions, and unidirectional energy transmission. This study opens a new avenue for the design of nonreciprocal devices without stringent requirements.

We show that photons propagating through a Rydberg-dressed atomic ensemble can exchange its spin state with a single atom. Such a spin-exchange collision exhibits both dissipative and coherent features, depending on the interaction strength. For strong interaction, the collision dissipatively drives the system into an entangled dark state of the photon with an atom. In the weak interaction regime, the scattering coherently flips the spin of a single photon in the multi-photon input pulse, demonstrating a generic single-photon subtracting process. An analytic analysis of this process reveals a universal trade-off between efficiency and purity of the extracted photon, which applies to a wide class of single-photon subtractors. We show that such a trade-off can be optimized by adjusting the scattering rate under a novel phase-matching condition., 6+6 pages, 4+3 figures

There are fundamental rules and principles setting the limits of physical systems. It triggers an interesting thought—can we break the limits under specific circumstances? A realistic system can only provide limited functionalities because its performance is physically constrained by some fundamental principles. ‘Breaking the limit’, which usually implies that the capability of a system could be enhanced significantly, is intriguing in many research areas such as high-precision metrology, imaging and nanophotonics. The claim of ‘breaking the limit’ can be generally divided into three categories. Firstly, the broken limit (such as standard quantum limit) is just a technical limit, but not a fundamental limit governed by basic physical principles (Fig. 1a). Secondly, some limits (e.g. diffraction limits in optical focusing and imaging) are broken at the cost of sacrificing other performances of a system. Thirdly, most limits are generally derived with some prerequisites or assumptions. Once the working scenarios are changed or go beyond the prerequisite condition, the previous limit becomes invalid so that it can be broken. It would lead to some misunderstandings if the limits, such as the Heisenberg limit, time-bandwidth limit and efficiency limit, of metasurfaces were not elaborated precisely.

We theoretically investigate the optical second-order sideband generation (OSSG) in an optical parity-time (PT) symmetric system, which consists of a passive cavity trapping the atomic ensemble and an active cavity. Compared with the double-passive system, it is found that near the exceptional point (EP), the efficiency of the OSSG increases sharply not only for the blue probepump detuning resonant case but also for the red one. Using experimentally achievable parameters, we study the effect of the atomic ensemble on the efficiency of the OSSG in the PT-symmetric system. The numerical results show that the efficiency of the OSSG is 30% higher than that of the first-order sideband, which is realized easily by simultaneously modulating the atom-cavity coupling strength and detuning. Moreover, the efficiency of the OSSG can also be tuned effectively by the pump power, and the efficiency is robust when the pump power is strong enough. This study may have some guidance for modulating the nonlinear optical properties and controlling light propagation, which may stimulate further applications in optical communications.

Optical nonreciprocity is an essential property for a wide range of applications, such as building nonreciprocal optical devices that include isolators and circulators. The realization of optical nonreciprocity relies on breaking the symmetry associated with Lorentz reciprocity, which typically requires stringent conditions such as introducing a strong magnetic field or a high-finesse cavity with nonreciprocal coupling geometry. Here we discover that the collision effect of thermal atoms, which is undesirable for most studies, can induce broadband optical nonreciprocity. By exploiting the thermal atomic collision, we experimentally observe magnet-free and cavity-free optical nonreciprocity, which possesses a high isolation ratio, ultrabroad bandwidth, and low insertion loss simultaneously. The maximum isolation ratio is close to 40 dB, while the insertion loss is less than 1 dB. The bandwidth for an isolation ratio exceeding 20 dB is over 1.2 GHz, which is 2 orders of magnitude broader than typical resonance-enhanced optical isolators. Our work paves the way for the realization of high-performance optical nonreciprocal devices and provides opportunities for applications in integrated optics and quantum networks.

Optical forces, generally arising from changes of field gradients or linear momentum carried by photons, form the basis for optical trapping and manipulation. Advances in optical forces help to reveal the nature of light-matter interactions, giving answers to a wide range of questions and solving problems across various disciplines, and are still yielding new insights in many exciting sciences, particularly in the fields of biological technology, material applications, and quantum sciences. This review focuses on recent advances in optical forces, ranging from fundamentals to applications for biological exploration. First, the basics of different types of optical forces with new light-matter interaction mechanisms and near-field techniques for optical force generation beyond the diffraction limit with nanometer accuracy are described. Optical forces for biological applications from in vitro to in vivo are then reviewed. Applications from individual manipulation to multiple assembly into functional biophotonic probes and soft-matter superstructures are discussed. At the end future directions for application of optical forces for biological exploration are provided.

Electromagnetically induced transparency (EIT) is a quantum interference effect arising from different transition pathways of optical fields. Within the transparency window, both absorption and dispersion properties strongly change, which results in extensive applications such as slow light and optical storage. Due to the ultrahigh quality factors, massive production on a chip and convenient all-optical control, optical microcavities provide an ideal platform for realizing EIT. Here we review the principle and recent development of EIT in optical microcavities. We focus on the following three situations. First, for a coupled-cavity system, all-optical EIT appears when the optical modes in different cavities couple to each other. Second, in a single microcavity, all-optical EIT is created when interference happens between two optical modes. Moreover, the mechanical oscillation of the microcavity leads to optomechanically induced transparency. Then the applications of EIT effect in microcavity systems are discussed, including light delay and storage, sensing, and field enhancement. A summary is then given in the final part of the paper.

Quantum information processing based on Rydberg atoms emerged as a promising direction two decades ago. Recent experimental and theoretical progresses have shined exciting light on this avenue. In this concise review, we will briefly introduce the basics of Rydberg atoms and their recent applications in associated areas of neutral atom quantum computation and simulation. We shall also include related discussions on quantum optics with Rydberg atomic ensembles, which are increasingly used to explore quantum computation and quantum simulation with photons., Comment: This review briefly summarizes recent progresses on Rydberg atom-based quantum computation and quantum simulation

The nanoscale wavelength and polarization router, which can simultaneously separate wavelength and polarization modes, is an essential component of on-chip nanophotonic devices. Here, an on-chip wavelength and polarization router is realized experimentally based on a three-layer hybrid waveguide of Au-SiO

On-chip light sources are essential components for integrated photonic circuits and quantum information processing chips. High directionality, high collection efficiency, and ultrasmall feature size are the most significant features for a light source in the wavelength range for optical communication, around 1550 nm. The authors use nanomanipulation to create an ultrasmall, unidirectional on-chip light source based on PbS quantum dots and an epsilon-near-zero (ENZ) material. This work not only shows the way to integrated photonic devices based on ENZ materials, but also provides an advanced method for the precise assembly of composite functional nanostructures.

A gravitational-wave detector in space can register signals in the low-frequency band below 1 Hz. The signal laser from a remote satellite has a typical power of only several picowatts, which hinders precise detection of the laser's phase. With this in mind, the authors present an ultranarrow-band amplifier with high gain and low noise, which exploits four-wave mixing induced by radiation pressure. This mechanically mediated amplifier can work at the quantum noise limit, in principle. Moreover, this approach may allow one to generate quantum entanglement between the amplified and conjugate fields.

An on-chip integrated wavelength filter and router device is realized using two-dimensional metal/dielectric nanostructures. The device can filter wavelengths of light from an incident broadband beam, and further route the filtered signals to different ports on the same chip. The footprint of the entire device is only 3.4 μm × 7.3 μm. Both the number of wavelength channels and the central wavelength of each channel can be tuned by adjusting the structure parameters, or by using a pumped laser. This work demonstrates an ultracompact and robust integrated multifunctional device, and provides a novel and flexible method for the integration of nanophotonic devices.

Optical cavities with high figure of merit Q / V is essential to enhance the interaction of light and matter. Here, a hybrid photonic-plasmonic nano-cavity, consisting of an L3 photonic crystal nano-cavity and plasmonic bowtie nano-antennas, is proposed to have an ultrahigh figure of merit Q / V of 8.4 × 10 6 ( λ / n ) − 3 , which is the highest value ever demonstrated for all previous works about L3-type photonic crystal nano-cavities. The value of Q / V is enhanced by more than 25 times compared to that in a bare L3 photonic crystal nano-cavity and is 60 times greater than plasmonic bowtie nano-antennas. As a result, the single-atom cooperativity parameter is improved by 26 times with respect to a bare L3 photonic crystal nano-cavity, and strong coupling between light and a single emitter is achieved. The proposed structure provides a new platform to achieve strong coupling between light and a single emitter, which holds great potential for applications in quantum optics, quantum information, and nonlinear optics.

We present a scheme for coherently manipulating quantum states of photons by incorporating multiple photonic modes in a system with long-range interactions. The presence of nonlocal photon-photon interactions destroys the energy or momentum matching conditions between distinct propagating polaritons, and consequently gives rise to blockaded effective coupling between the corresponding polaritons. Such a blockade mechanism protects the system from interaction-induced dissipations and enables highly tunable few-photon nonlinearities. Taking Rydberg atomic ensemble as an example, we illustrate several intriguing phenomena based on the proposed scheme, e.g., the deterministic generation of entangled photon pairs, the nonlinear beam splitting, as well as the establishment of a tunable dressed interaction between individual photons., Comment: 12 pages, 7 figures; accepted by Phys. Rev. A (https://journals.aps.org/pra/accepted/3207dYabHbd15562788742495523c7bccf59ce158)

Quantum ground-state cooling of macroscopic mechanical resonators is of essential importance to both fundamental physics and applied science. Conventional method of laser cooling is limited by the quantum backaction, which requires mechanical sideband resolved in order to cool to ground state. This work presents an idea to break the quantum backaction limit by engineering intracavity optical squeezing. It gives rise to quantum interference for all the dissipation channels, and under certain circumstances can totally remove the influence of the cavity dissipation and the resultant quantum backaction, with much lower cooling limit irrespective of the sideband resolution. We show that our scheme enables ground-state cooling in the highly unresolved sideband limit and it also works beyond the weak coupling regime, which provides the opportunity for quantum manipulation of macroscopic mechanical systems., Comment: 14pages, 4 figures

Spin squeezing (SS) is a recognized resource for realizing measurement precision beyond the standard quantum limit $\propto 1/\sqrt{N}$. The rudimentary one-axis twisting (OAT) interaction can facilitate SS and has been realized in diverse experiments, but it cannot achieve extreme SS for precision at Heisenberg limit $\propto 1/{N}$. Aided by deep reinforcement learning (DRL), we discover size-independent universal rules for realizing nearly extreme SS with OAT interaction using merely a handful of rotation pulses. More specifically, only 6 pairs of pulses are required for up to $10^4$ particles, while the time taken to reach extreme SS remains on the same order of the optimal OAT squeezing time, which makes our scheme viable for experiments that reported OAT squeezing. This study highlights the potential of DRL for controlled quantum dynamics., Comment: 5 pages, 4 figures

Volatile organic compounds (VOCs) are important precursors of O

The adsorption efficiencies of equimolar amount of beta–cyclodextrin (-CD) on aqueous solution of 14 commonly volatile organic compounds (VOCs), including benzene, toluene, p-xylene, m-xylene, o-xylene, chlorobenzene, bromobenzene, ethyl benzene, styrene, 1,1-dichloroethene, trans-1,2-dichloroethene, cis-1,2-dichloroethene, trichloroethene and tetrachloroethene, were studied by UV-vis spectrophotometry, respectively. The results showed that the adsorption coefficient (1) of -CD on benzene series had a good exponential relationship with its solubility in water (co), 1 = 1.104co –0.44, R2 = 0.868. The adsorption efficiency of -CD on chloroalkenes was better than that of benzene series. Moreover, the higher the terminal distribution of charge of VOC molecule was, the higher the adsorption efficiency of -CD on them. However, adsorption efficiency of bamboo charcoal on these VOCs was much lower than that of -CD. Depending on the biotrickling filteration equipment containing bamboo charcoal as the growth medium for microorganism, the treatment efficiencies of styrene and odor by adding -CD were increased to 95.47% and 71.77%, and increasing rates by 4.50% and 3.76%, respectively. This study provides a new synergistic idea and method for the treatment of VOCs by biological trickling filter combined with -CD.

We have studied multimode four-wave mixing (FWM) in an unresolved sideband cavity optomechanical system. The radiation pressure coupling between the cavity fields and multiple mechanical modes results in the formation of a series of tripod-type energy-level systems, which induce the multimode FWM phenomenon. The FWM mechanism enables remarkable amplification of a weak signal field accompanied by the generation of an FWM field when only a microwatt-level pump field is applied. For proper system parameters, the amplified signal and FWM fields have equal intensity with opposite phases. The gain and frequency response bandwidth of the signal field can be dynamically tuned by varying the pump intensity, optomechanical coupling strength, and additional feedback control. Under certain conditions, the frequency response bandwidth can be very narrow and reaches the level of hertz.

We find that dissipation can be utilized as a resource to enhance optomechanically induced transparency in the unresolved sideband regime. This positive effect of dissipation holds potential for applications including high-precision measurements and slow light.

Ground state cooling of massive mechanical objects remains a difficult task restricted by the unresolved mechanical sidebands. We propose an optomechanically-induced-transparency cooling scheme to achieve ground state cooling of mechanical motion without the resolved sideband condition in a pure optomechanical system with two mechanical modes coupled to the same optical cavity mode. We show that ground state cooling is achievable for sideband resolution $\omega_{m}/\kappa$ as low as 0.003. This provides a new route for quantum manipulation of massive macroscopic devices and high-precision measurements., Comment: 6 pages, 5 figures

Anti-Stokes one-photon luminescence from a single gold nanorod is experimentally investigated. The anti-Stokes emission of gold nanorods is enhanced and strongly modulated by localized surface plasmon resonance (LSPR). It is found that the polarization dependence of the anti-Stokes emission is in strong correlation with that of the Stokes emission. Further experiments provide evidence that LSPR significantly enhanced both excitation and emission processes. Moreover, the line shape of the anti-Stokes emission is dependent on the surface temperature, which is related to the distribution of free electrons near the Fermi level. This discovery provides an effective method in principle to probe localized temperature at nanoscale dimension. Here, the reported results about the anti-Stokes emission provide more understanding for the photoemission process from the plasmonic nanostructures.

This corrects the article DOI: 10.1103/PhysRevLett.119.233901.

We show that anti-$\mathcal{PT}$ symmetry can be realized in dissipatively coupled optical systems. Its emergence gives rise to spontaneous phase transitions for the guided and localized photonic eigenmodes in the waveguide and cavity systems studied, respectively. As a ubiquitous feature we demonstrate that constant refraction [analogous to unit refraction in [Nat. Phys. 12, 1139 (2016)]] occurs in the $\mathcal{PT}$-symmetric phase, which leads to several interesting properties for the photonic system, such as flat broadband light transport and dispersion-induced dissipation.

Quantum manipulation is challenging in localized-surface plasmon resonances (LSPRs) due to strong dissipations. To enhance quantum coherence, here we propose to engineer the electromagnetic environment of LSPRs by placing metallic nanoparticles (MNPs) in optical microcavities. An analytical quantum model is first built to describe the LSPR-microcavity interaction, revealing the significantly enhanced coherent radiation and the reduced incoherent dissipation. Furthermore, when a quantum emitter interacts with the LSPRs in the cavity-engineered environment, its quantum yield is enhanced over 40 times and the radiative power over one order of magnitude, compared to those in the vacuum environment. Importantly, the cavity-engineered MNP-emitter system can enter the strong coupling regime of cavity quantum electrodynamics, providing a promising platform for the study of quantum plasmonics, quantum information processing, precise sensing, and spectroscopy.

The interesting point is important features in images, which is frequently used for image registration, scene analysis, object tracking. Many algorithms for detecting the interesting points have been developed up to now. Among them, the Harris interesting point detector is proved most stable against rotation and noise, while it is also very sensitive to scale change. In order to solve this problem, a new multi-scale Harris interesting point detection algorithm is proposed in this paper. The algorithm consists of two main stages. In stage one, we build a multi-scale representation for Harris measure and find candidate interesting points at each scale level. In stage two, a novel measure is defined to measure the stability of each point. Based on the measure, the final interesting point is selected from candidates. The experiments on both optical and sar data sets have shown that, compared with stand Harris interesting point detector and some other multi-scale interesting point detectors, our algorithm is more robust.