基于金刚石氮-空位色心自旋系综与超导量子电路混合系统的量子节点纠缠

量子纠缠是实现量子计算和量子通信的核心基础,本文提出了在金刚石氮-空位色心(NV centers)自旋系综与超导量子电路耦合的混合系统中实现两个分离量子节点之间纠缠的理论方案.在该混合系统中,把金刚石NV centers自旋系综和与之耦合的超导共面谐振器视为一个量子节点,两个量子节点之间通过一个空的超导共面谐振器连接.具有较长相干时间的NV centers自旋系综作为一个量子存储器,用于制备、存储和发送量子信息;易于外部操控的超导量子电路可执行量子逻辑门操作,快速调控量子信息.为了实现两个分离量子节点之间的纠缠,首先对系统的哈密顿量进行正则变换,将其等价为两个NV centers自旋系综与同一个超导共面谐振器之间的JC耦合;然后采用NV centers自旋-光子混合比特编码的方式,通过调节超导共面谐振器的谐振频率,精确控制体系演化时间,高保真度地实现了两个分离量子节点之间的量子纠缠.本方案还可以进一步扩展和集成,用于构建多节点纠缠的分布式量子网络.     

双光腔耦合下机械振子的基态冷却

机械振子的基态冷却是腔量子光力学中的基本问题之一.所谓的基态冷却就是让机械振子的稳态声子数小于1.本文通过光压涨落谱和稳态声子数研究双光腔光力系统(标准单光腔光力系统中引入第二个光腔,并与第一个光腔直接耦合)的基态冷却.首先得到系统的有效哈密顿量,然后给出朗之万方程和速率方程,最后分别给出空腔和原子腔的光压涨落谱、冷却率和稳态声子数.通过光压涨落谱、冷却率和稳态声子数表达式,重点讨论空腔时机械振子的基态冷却,发现当满足最佳参数条件(机械振子的冷却跃迁速率对应光压涨落谱的最大值,而加热跃迁速率对应光压涨落谱的最小值)时,机械振子可以被冷却到稳态声子数足够少.此外分析:当辅助腔内注入原子系综时,若参数选择恰当可能更利于基态冷却.     

Finite-amplitude standing acoustic waves in a cubically nonlinear medium

The acoustic field in a resonator filled with a cubically nonlinear medium is investigated. The field is represented as a linear superposition of two strongly distorted counterpropagating waves. Unlike the case of a quadratically nonlinear medium, the counterpropagating waves in a cubically nonlinear medium are coupled through their mean (over a period) intensities. Free and forced standing waves are considered. Profiles of discontinuous oscillations containing compression and expansion shock fronts are constructed. Resonance curves, which represent the dependences of the mean field intensity on the difference between the boundary oscillation frequency and the frequency of one of the resonator modes, are calculated. The structure of the profiles of strongly distorted “forced” waves is analyzed. It is shown that discontinuities are formed only when the difference between the mean intensity and the detuning takes certain negative values. The discontinuities correspond to the jumps between different solutions to a nonlinear integro-differential equation, which, in the case of small dissipation, degenerates into a third-degree algebraic equation with an undetermined coefficient. The dependence of the intensity of discontinuous standing waves on the frequency of oscillations of the resonator boundary is determined. A nonlinear saturation is revealed: at a very large amplitude of the resonator wall oscillations, the field intensity in the resonator ceases depending on the amplitude and cannot exceed a certain limiting value, which is determined by the nonlinear attenuation at the shock fronts. This intensity maximum is reached when the frequency smoothly increases above the linear resonance. A hysteresis arises, and a bistability takes place, as in the case of a concentrated system at a nonlinear resonance.     

Natural frequency for exponential shaped thermoacoustic resonator

The two-dimensional nonlinear acoustic field of eleven exponential shaped resonators was simulated with a computational fluid dynamics software Fluent.The influence of driving frequency and driving intensity on pressure in resonator as well as its natural frequency was investigated.The relationship between natural frequency and theoretical calculation resonance frequency was also explored.It is found that beating phenomena can be observed in the resonator when the driving frequency deviates from the natural frequency.Moreover,the natural frequency of resonator increases with the increasing of driving intensity,which shows a hard spring effect.However,the driving intensity plays little effect on natural frequency and the natural frequencies are smaller than the theoretical calculation values in any driving intensity.Meanwhile,a formula between the natural frequency and its first-order resonance frequency from theoretical calculation was obtained by linear fitting for all these exponential shaped resonators under consideration.It is also found that the highest pressure amplitude and highest pressure ratio can be obtained from the exponential shaped resonator of m=2.8 under the same driving intensity.Moreover,the relation between natural frequency and the theoretical resonance frequency for m=2.8-tube is slightly different from other tubes in consideration.     

指数型热声谐振管的固有频率

采用计算流体力学软件Fluent模拟研究了11种不同形状参数的指数型热声谐振管内二维非线性声场特性,分析了驱动频率和驱动强度对管内声压演化过程及固有频率的影响,并探索了指数管的固有频率与理论计算谐频之间的关系.研究发现:当驱动频率偏离谐振管固有频率时,管内将出现明显的"拍"现象;指数管的固有频率随驱动强度的增加而增加,呈现硬弹簧效应,但驱动强度对固有频率的影响较小并且在任何驱动下指数管的固有频率均小于理论计算谐频.针对所研究的指数型管,获得了其固有频率与理论计算谐频之间的关系式.结果表明,相同驱动下,形状参数m值约等于2.8的指数管所能获得的压力幅值及压比最大,且m=2.8指数管的固有频率与理论计算谐频之间的关系式与其他管型略有不同.      

Uniform radio frequency fields in loop-gap resonators for EPR spectroscopy

At high frequencies, e.g., Q- and W-bands, it is advantageous to make the axial length of loop-gap resonators (LGRs) at least as long as a free-space wavelength. The opposite scaling of capacitance and inductance with LGR length suggests that the length of an LGR can be increased without limit, with the axial radio frequency (rf) field profiles and resonance frequency independent of length. This scaling is accurate for resonator dimensions much less than one free-space wavelength. When the resonator length approaches one-tenth of a free-space wavelength, the rf field uniformity degrades. From one-tenth to one free-space wavelength, computer simulations and experimental measurements show that the axial magnetic field energy density profile is peaked in the center of the LGR, gradually decreases from 25 to 50% at a distance one radius from the end, and rapidly there-after. The nonuniformity is of two types. One type, in the vicinity of one radius of the end, is caused by the flaring of the field as it curves from the central loop to the end region, into the larger return loop(s). The other type, in the central part of the resonator, is caused by impedance mismatch at the ends of the LGR. The LGR may be viewed as a strongly reentrant (ridge) waveguide nearly open at both ends and supporting a standing wave. A transmission line model relates the central nonuniformity to the fringing capacitance and inductance at the ends of the resonator. This nonuniformity can be eliminated in several ways including modifying the ends of the LGR by adding a small metal bridge or a dielectric ring. These uniformity trimming elements increase the fringing capacitance and/or decrease the fringing inductance. With trimmed ends, LGRs can be made many free-space wavelengths long. The maximum resonator length is determined by the proximity in frequency of the fundamental LGR mode to the next highest frequency mode as well as the quality factor. Results of this theory are compared and conformed with finite-element simulations. This theory connects the uniform LGR with the uniform field cavity resonators previously introduced by this laboratory.     

Manipulation of soliton ensembles by spectral filters

It was shown recently that an ensemble of solitons can be created in a driven optical fiber resonator. The ensemble can exist in either an ordered or a disordered state; these have been referred to as soliton solid or gas (fluid), respectively. We demonstrate through numerical simulation that the transition from one state to the other or vice versa, and further manipulation of the ensemble, can be effected through the insertion of spectral filters. Received: 7 July 2000 / Revised version: 1 September 2000 / Published online: 30 November 2000     

The ion cyclotron resonator in the magnetosphere

Our concern here is to present the idea of the ion cyclotron resonator in the planetary magnetosphere and to discuss briefly the experimental status of the corresponding theory. The resonator confines the ion cyclotron waves to a thin equatorial zone, so that it keeps the wave field from coming into contact with the ionosphere, resulting in a decrease in energy losses. The properties of the resonator are illustrated by adopting a plausible distribution of the magnetic field in the equatorial zone, which yields an expression for the discrete spectrum of the waves just above the gyrofrequency of heavy ions. We show that the resonator is remarkable for many reasons, including the frequency dependence of its size and specific structure of the spectrum.     

Strong coupling of a spin ensemble to a superconducting resonator

We report the realization of a quantum circuit in which an ensemble of electronic spins is coupled to a frequency tunable superconducting resonator. The spins are nitrogen-vacancy centers in a diamond crystal. The achievement of strong coupling is manifested by the appearance of a vacuum Rabi splitting in the transmission spectrum of the resonator when its frequency is tuned through the nitrogen-vacancy center electron spin resonance.     

Self-Oscillating Mode of a Nanoresonator

The paper proposes a new graphene resonator circuit which operates on the principle of a self-oscillator and has no drawbacks typical of nanoresonators as mass detectors and associated with their law quality factor, eigenfrequency errors (measurements from resonance curves), and dependence of quench frequency on oscillation frequency (curves with quenching for nonlinear systems). The proposed circuit represents a self-oscillator comprising an amplifier, a graphene resonator, and a positive feedback loop with a graphene oscillation transducer, and its major advantage is in self-tuning to resonance frequency at slowly varying resonator parameters, compared to oscillation periods. The graphene layer with a conducting substrate beneath it forms a capacitor which is recharged by a dc voltage source as its capacitance varies due to graphene deformation, and the recharge current is an oscillation- dependent signal transmitted from the transducer to the amplifier input. The graphene layer is placed in a magnetic field and is deformed when a current from the amplifier output is passed through. By properly choosing the magnetic field direction and the amplifier gain, it is possible to provide swinging oscillation whose amplitude is limited by the amplifier nonlinearity. For the proposed system we present an electromechanical model, dimensionless equations of motion, and numerical data demonstrating the generation of steady-state oscillations with eigenfrequency. Also presented is an analysis showing that the system can have only one limit cycle and that this cycle is always stable. The proposed resonator circuit can be used as a mass detector which determines the added mass from a change in self-oscillation frequency.     

金刚石氮空位色心耦合机械振子和腔场系统中方差压缩研究

研究了金刚石氮空位中心(NV色心)同时耦合腔场和机械振子系统中声子场的方差压缩动力学特性,分析了金刚石NV色心初态和NV色心与机械振子耦合强度对声子场方差压缩影响.结果发现:可以制备压缩时间长、压缩幅度大的声子场压缩态,其物理原因是机械振子具有最大相干性,并且通过调控NV色心初态以及磁场梯度可以实现对机械振子方差压缩非经典特性的操控,从而在理论上提供了一种调控声子场方差压缩的方式.     

Nonreciprocal ground-state cooling of mechanical resonator in a spinning optomechanical system

We theoretically present a scheme for nonreciprocal ground-state cooling in a double-cavity spinning optomechanical system which is consisted of an optomechanical resonator and a spinning optical harmonic resonator with directional driving. The optical Sagnac effect generated by the whispering-gallery cavity (WGC) rotation creates frequency difference between the WGC mode, we found that the mechanical resonator (MR) can be cooled to the ground state when the propagation direction of driving light is opposite to the spin direction of the WGC, but not from the other side, vice versa, so that the nonreciprocal cooling is achieved. By appropriately selecting the system parameters, the heating process can be completely suppressed due to the quantum interference effect. The proposed approach provides a platform for quantum manipulation of macroscopic mechanical devices beyond the resolved sideband limit.     

Photocount statistics of chaotic lasers

We derive the photocount statistics of the radiation emitted from a chaotic laser resonator in the regime of single-mode lasing. Random spatial variations of the resonator eigenfunctions lead to strong mode-to-mode fluctuations of the laser emission. The distribution of the mean photocount over an ensemble of modes changes qualitatively at the lasing transition, and displays up to three peaks above the lasing threshold.     

Acoustic impedance at the interface between a plain and a perforated pipe

This paper considers the effective impedance that pertains as low frequency sound in a plain pipe radiates into a general perforated pipe of equal diameter. A previous theory that considered only the reactance is extended to also include resistance. Experimental measurements are made of the response of a Helmholtz resonator to an external sound field, where the neck of the Helmholtz resonator has both plain and perforated pipe sections. A complete theoretical model of this resonator allows for comparison between measured and predicted results of transfer functions from the external to internal sound fields of the resonator. The Nyquist plot of the admittance transfer function is extremely sensitive to the small resistance values, whereas the pressure transfer function gives more accurate results for resonant frequency and hence reactance than the usual method. In particular the results for resistance are so sensitive that it becomes possible to infer which of the current models for aperture resistance within the perforate is the most appropriate.     

A Field-Probing Approach and Its Application to a Power Distribution Network

This study presents a means to probe the electric-field distribution in a microstrip resonator using the existing power/ground pins. The strength of the electric-field could be correlated with the reflection coefficient. Physical mechanism for the position-depended reflection effect is interpreted from the perspective of probe coupling. An application of this method is demonstrated through three phases: determining the maximum field of a microstrip resonator, shaping the geometry at the field bulk, and detuning the resonant frequency of a power distribution network. The results reveal that this approach can be employed to achieve a resonance-free operating environment.     

Electromechanical models of nanoresonators

The goal of this study is to construct simple electromechanical models of nanoresonators as mass detectors. A major obstacle in the achievement of sufficient measurement accuracy for the resonant frequency associated with the adsorption of additional mass onto the graphene layer is a low quality factor of the oscillatory system containing the graphene layer. A graphene resonator can be considered as an elastic system with distributed parameters. The application of the Galerkin method to study nearly resonant vibrational modes reduces the problem to considering an oscillatory system with a few degrees of freedom with pronounced nonlinear properties. These properties are, first of all, due to the nonlinear dependence of the forces produced by the electric field on the graphene deflection and, second, due to the nonlinear dependence of the graphene layer tension on its deflection. Taking into account the nonlinear properties leads to the appearance of characteristic drops in the resonance curve which allow for a more accurate resonant frequency measurement. Resonance curves with such characteristic drops can be obtained using a demonstration experimental macromodel of the resonator. Two absolutely new layouts are proposed, such as a differential resonator and resonator with parametric excitation. The oscillations excited in the differential resonator that contains two graphene layers resemble beats. In this case, small changes in the mass of the main layer correspond to significant changes in the frequency of the envelope. This effect is illustrated by oscillograms obtained for an experimental macromodel of the differential resonator. The parametric resonator has one graphene layer between two conducting surfaces. Parametric excitation of steady-state high amplitude oscillations is possible in this resonator only in a narrow frequency band close to the eigenfrequency. The band width reduces with a decrease in the quality factor of the oscillatory system. The latter fact can be useful for the improvement of eigenfrequency measurement accuracy at a low quality factor of the oscillatory system.     

A differential graphene-based resonator

We describe a new, in principle, layout of a graphene resonator—a differential resonator, which makes it possible to increase substantially its sensitivity to the mass deposited on it. The differential resonator consists of two parallel graphene films, which are fastened in insulating supports; the lower film is arranged over the conducting surface. The force coupling between the films is performed by the electrostatic field in the space between them. Several equilibrium positions are possible in such a mechanical system. Small free oscillations near the stable equilibrium position are considered. The field strength is selected so that the mechanical system of two graphene films would have two close eigenfrequencies. The free oscillations of such a system have the form of intrinsic frequencies of the system much lower that the partial frequency of each film. When depositing the particle on the upper film, the partial eigenfrequency of this film decreases. In this case, the characteristic envelope frequency also decreases, and a small variation in the partial eigenfrequency leads to considerable variation in the characteristic envelope frequency. This provides higher sensitivity to the mass of the revealed particle for the differential resonator compared with the resonator based on one film.     

Design of a pulsed spatial neutron magnetic spin resonator

It is well known that upon passage through a spatially alternating transverse magnetic field, produced by a meander-shaped conducting foil, in its rest frame each neutron creates its individual frequency which depends on its velocity and the period of the meander. A resonant spin flip process takes place, if this frequency equals the Larmor frequency determined by a homogeneous vertical field. Clearly, this effect can be used to monochromatise a polarised neutron beam. Here we propose a novel design of such a magnetic resonator consisting of a sequence of separate compact modules, which provide high homogeneity of the transversal field oscillations and allow rapid beam chopping since they meet the specifications of fast electronic switching. The wavelength resolution of this resonator device can be changed in an instant and likewise an arbitrary amplitude modulation of the transversal field can be established, which is required for an efficient suppression of subsidiary maxima of the wavelength-dependent spin flip probability.     

1.1-GHz continuous-wave EPR spectroscopy with a frequency modulation method

Continuous-wave EPR spectroscopy using a frequency modulation (FM) scheme was developed. An electronically tunable resonator and an automatic tuning control (ATC) system were used. Using the FM scheme instead of magnetic field modulation, we detected EPR absorption at the first derivative mode. We used a microwave frequency of 1.1 GHz in the present experiment. Similar signal-to-noise ratios were obtained with conventional field modulation and the FM method, and a low-quality factor EPR resonator was not necessary to suppress the significant microwave reflection from the resonator. The FM method with a tunable resonator may be an alternative solution to achieving phase-sensitive detection, when the side-effects of magnetic field modulation, such as microphonic noise and mechanical vibration, are detrimental for EPR detection.     

激光调制参数对光纤环型谐振腔光谱特性影响

谐振光学环型腔作为光学陀螺的核心敏感单元,其光学调制谱和与之对应的鉴频曲线的特性成为提高光学陀螺系统检测灵敏度的关键。为了研究光学陀螺的调制和鉴频谱线特性,优化陀螺性能,设计并搭建了实验测试系统,光纤环形谐振腔采用分光比为50∶50、直径17 cm的保偏光纤,总长2.2 m。使用直流高压放大器扫描窄线宽激光器（线宽小于1 kHz）的压电转化模块,扫描频率和电压分别选取20 Hz和1 V,使用模拟比例积分电路进行锁频并反馈给激光器的压电转化模块,使激光器的输出频率跟踪谐振腔实时变化。研究分析了光纤环型谐振腔在两种情况下所对应的透射谱和鉴频曲线:第一种情况为调制电压分别为2 V和4 V,对应调制频率从100 kHz到4 MHz变化;第二种情况为当调制频率为900 kHz,调制电压从2 V到10 V变化。通过实验,得到了不同调制参数下光学陀螺谱线的谐振深度、半高全宽、线性带宽、动态范围、品质因数、标度因数以及对应的锁频精度七种物理量的详细变化情况,并进一步得到了静态测试条件下三种陀螺的最佳调制频率及与之所匹配的调制电压。为进一步研究激光调制对光纤环型谐振腔光谱的影响提供指导。