Effect of quantum interference on a three-level atom driven by two laser fields

Abstract

We study the effect of quantum interference on the population distribution and absorptive properties of a V-type three-level atom driven by two lasers of unequal intensities and different angular frequencies. Three coupling configurations of the lasers to the atom are analysed: (a) both lasers coupled to the same atomic transition, (b) each laser coupled to different atomic transition and (c) each laser coupled to both atomic transitions. Dressed states for the three coupling configurations are identified, and the population distribution and absorptive properties of the weaker field are interpreted in terms of transition dipole moments and transition frequencies among these dressed states. In particular, we find that in the first two cases there is no population inversion between the bare atomic states, but the population can be trapped in a superposition of the dressed states induced by quantum interference and the stronger field. We show, that the trapping of the population, which results from the cancellation of transition dipole moments, does not prevent the weaker field to be coupled to the cancelled (dark) transitions. As a result, the weaker field can be strongly amplified on transparent transitions. In the case of each laser coupled to both atomic transitions the population can be trapped in a linear superposition of the excited bare atomic states leaving the ground state unpopulated in the steady state. Moreover, we find that the absorption rate of the weaker field depends on the detuning of the strong field from the atomic resonances and the splitting between the atomic excited states. When the strong field is resonant to one of the atomic transitions a quasi-trapping effect appears in one of the dressed states. In the quasi-trapping situation all the transition dipole moments are different from zero, which allows the weaker field to be amplified on the inverted transitions. When the strong field is tuned halfway between the atomic excited states, the population is completely trapped in one of the dressed states and no amplification is found for the weaker field.     

Shape of the particulate beam attenuation spectrum and its inversion to obtain the shape of the particulate size distribution

The link between the spectral shape of the beam attenuation spectrum and the shape of the particle size distribution (PSD) of oceanic particles is revisited to evaluate the extent to which one can be predicted from the other. Assuming a hyperbolic (power-law) PSD, N(D) ? D(-xi), past studies have found for an infinite distribution of nonabsorbing spheres with a constant index of refraction that the attenuation spectrum is hyperbolic and that the attenuation spectral slope gamma is related to the PSD slope xi by xi = gamma + 3. Here we add a correction to this model because of the finite size of the biggest particle in the population. This inversion model is given by xi = gamma + 3 - 0.5 exp(-6gamma). In most oceanic observations xi > 3, and the deviation between these two models is negligible. To test the robustness of this inversion, we perturbed its assumptions by allowing for populations of particles that are nonspherical, or absorbing, or with an index of refraction that changes with wavelength. We found the model to provide a good fit for the range of parameters most often encountered in the ocean. In addition, we found that the particulate attenuation spectrum, c(p)(lambda), is well described by a hyperbolic relation to the wavelength c(p) ? lambda(-gamma) throughout the range of the investigated parameters, even when the inversion model does not apply. This implies that knowledge of the particulate attenuation at two visible wavelengths could provide, to a high degree of accuracy, the particulate attenuation at other wavelengths in the visible spectrum.     

Multiple target detection using split spectrum processing and groupdelay moving entropy

The split spectrum processing technique obtains a frequency-diverse ensemble of narrow-band signals through a filterbank then recombines them nonlinearly to improve target visibility. Although split spectrum processing is an effective method for suppressing grain noise in ultrasonic nondestructive testing, its application was mainly limited to the detection of single targets or multiple targets having similar spectral characteristics. In this paper, the group delay moving entropy technique is introduced primarily to enhance the performance of split spectrum processing in detecting multiple targets which exhibit different spectral characteristics (i.e., variations in target signal center frequency and bandwidth). This is likely to occur in complex, dispersive, and nonhomogeneous media such as composites, layered, and clad materials, etc. The analysis shows that the group delay moving entropy method can be used effectively to select the optimal frequency region for split spectrum processing when detecting such targets. Based on an iterative procedure that combines group delay moving entropy and split spectrum processing, multiple targets can be identified one at a time, and subsequently eliminated by using time domain windows. The removal of the dominant target improves the detection of the remaining weaker targets. Simulation results are presented which demonstrate the feasibility of the multistep split spectrum processing technique for detecting multiple targets in such materials     

Spectral light extinction to characterize fast fog formation

Two supplementary methods for time-dependent droplet sizing, both based on the spectral dependence of light extinction, are applied to an adiabatically expanding vapor in which droplets are formed as a result of heterogeneous condensation. First, by measuring the extinction coefficients at three different wavelengths, we obtain time-dependent values of the modal radius, the size variance, and the droplet number density, with a typical time resolution of 1 μs. The shape of the size-distribution function is investigated by a second method. Using a white-light source in combination with a spectrometer and a CCD array, we obtain the full visible light attenuation spectrum with a time resolution of 1.5 ms. By applying an inversion technique based on trial size distributions, we find that the zeroth-order log-normal distribution describes the fog adequately. Both methods yield the same droplet growth curves and droplet number densities.     

A coherently driven two-level atom inside a parametric oscillator

We consider a coherently driven two-level atom in an optical parametric oscillator operating below threshold. Applying the Hiesenberg and quantum Langevin equations, we study the power spectrum and the normalised second-order correlation function of the fluorescent light in the weak and strong driving light limits. We also calculate the atomic inversion in the two limiting cases. We find that the widths of the Mollow triplets increases as the degree of squeezing increases and even the side peaks disappear as the amount of squeezing approaches its maximum value. Moreover, the oscillatory nature of the second-order correlation function and the atomic inversion in the strong driving light limit damps out as the degree of squeezing increases.     

Aerosol properties from spectral extinction and backscatter estimated by an inverse Monte Carlo method

The feasibility of using a generalized stochastic inversion methodology to estimate aerosol size distributions accurately by use of spectral extinction, backscatter data, or both is examined. The stochastic method used, inverse Monte Carlo (IMC), is verified with both simulated and experimental data from aerosols composed of spherical dielectrics with a known refractive index. Various levels of noise are superimposed on the data such that the effect of noise on the stability and results of inversion can be determined. Computational results show that the application of the IMC technique to inversion of spectral extinction or backscatter data or both can produce good estimates of aerosol size distributions. Specifically, for inversions for which both spectral extinction and backscatter data are used, the IMC technique was extremely accurate in determining particle size distributions well outside the wavelength range. Also, the IMC inversion results proved to be stable and accurate even when the data had significant noise, with a signal-to-noise ratio of 3.     

Two-level atom in a squeezed vacuum with finite bandwidth

Abstract

The resonance fluorescence of a two-level atom driven by a coherent laser field and damped by a finite bandwidth squeezed vacuum is analysed. We extend the Yeoman and Barnett technique to a non-zero detuning of the driving field from the atomic resonance and discuss the role of squeezing bandwidth and the detuning in the level shifts, widths and intensities of the spectral lines. The approach is valid for arbitrary values of the Rabi frequency and detuning but for the squeezing bandwidths larger than the natural line-width in order to satisfy the Markoff approximation. The narrowing of the spectral lines is interpreted in terms of the quadrature-noise spectrum. We find that, depending on the Rabi frequency, detuning and the squeezing phase, different factors contribute to the line narrowing. For a strong resonant driving field there is no squeezing in the emitted field and the fluorescence spectrum exactly reveals the noise spectrum. In this case the narrowing of the spectral lines arises from the noise reduction in the input squeezed vacuum. For a weak or detuned driving field the fluorescence exhibits a large squeezing and, as a consequence, the spectral lines have narrowed linewidths. Moreover, the fluorescence spectrum can be asymmetric about the central frequency despite the symmetrical distribution of the noise. The asymmetry arises from the absorption of photons by the squeezed vacuum which reduces the spontaneous emission. For an appropriate choice of the detuning some of the spectral lines can vanish despite that there is no population trapping. Again this process can be interpreted as arising from the absorption of photons by the squeezed vacuum. When the absorption is large it may compensate the spontaneous emission resulting in the vanishing of the fluorescence lines.     

The spectrum of atomic excess free volume in grain boundaries

The grain boundary (GB) excess volume is an important structural factor that is strongly correlated with various thermodynamic and kinetic properties of GBs such as GB energy, GB mobility, GB diffusivity, and GB segregation energy, etc. However, the excess volume is usually reported as an average value of the entire GB. Such simplification does not consider the spectral nature of the excess volume in a GB, which cannot be used to describe the atomic mechanisms of some kinetic process, such as GB migration, that involves only a few atoms at a time. Here, we explore the spectrum of atomic excess volume in representative nanocrystalline Ni and Al samples as well as 388 Ni bicrystals based on the Olmsted dataset by using atomistic simulations. It is found that the nanocrystalline Ni and Al models show a skew-normal distribution in the spectrum of both the atomic excess volume and the atomic excess energy in the GBs, which show a weak inverse correlation between them. This is in stark contrast to the widely reported positive correlation between GB energy and excess volume based on the average value. We further show based on the statistical analysis that the correlation between the atomic excess volume and excess energy strongly depends on the GB type and a universal trend between them does not exist. While low ∑ Ni GBs generally shows a strong inverse linear correlation between these two properties, such correlation is weak for high ∑ Ni GBs. Moreover, we find that the spectrum of the excess volume shows characteristics distribution in some special Ni GBs. For example, twist GBs generally show a symmetrical unimodal distribution while most surveyed ∑3 Ni GBs with anti-thermal behavior show an apparent bimodal distribution. Nevertheless, a strong correlation is found between the atomic excess volume and the segregation energy based on the nanocrystalline Al model with Mg impurity, which implies a possible universal trend between the two properties. The current study thus shows that the excess volume provides useful insights in revealing the elemental structure–property correlations in GBs, which may be used as a structural variable in future thermodynamic modeling of GBs.

     

Quantum Amplifier Performance of Strongly Driven Atomic Systems

Abstract

In this paper we consider the gain obtained in strongly driven and damped atomic systems within the framework of quantum amplifier theory. The atomic system suffices to break the time-reversal symmetry and no rigged reservoirs or continuous spectra are needed. We consider both the driven two-level system and Λ-type systems. We derive master equations for the boson mode using adiabatic elimination techniques and use these equations to determine the amplifier performance of the systems. We find that in both cases the quantum limit on the noise can be approached without introducing any quantum features of the strong driving field. The basic interaction event between the amplified bosons and the atomic system suffices to saturate the noise limit. Ideal amplifier performance is, however, found only in the limit when the driving field is far off resonance, and then the ensuing gain becomes very small. The result is understandable because in this limit little population is accumulated on the upper level which drives the noise through spontaneous emission. We believe our method and main conclusions to be quite universal, and they provide insight into the operation of quantum amplifiers.     

Optical Amplification and Spontaneous Emission in an Ar + Discharge

The relationship between optical amplification and spontaneous emission noise has been studied in an argon ion travelling wave amplifier by beat spectrum analysis. Different optical arrangements have been examined with both single and double pass amplification. The results establish that when coherent light travels through a gain tube, the amplified beam acquires intensity fluctuation noise which is greater than that of a Poissonian distribution (shot noise). The noise power is shown to be the same as that of an equivalent unamplified beam mixed with the amplified spontaneous emission from the gain tube. It is shown that such measurements provide a practical technique for the otherwise difficult determination of the degree of population inversion in the gain medium. They also give insight into the potential of optical amplifiers for enhancement of lidar and system performance.     

Cooperative Atomic Behaviour and Oscillator Formation in a Squeezed Vacuum

Abstract

Time-dependent (numerical) results are presented for super-radiant behaviour in the Dicke model of N _a = 2, 3 atoms in a broad band squeezed vacuum. This concerns the fluctuations and the intensity of the fluorescent radiation as well as the atomic population inversion of the system with atoms initially in an atomic coherent state. In the steady state, and in the N _a → ∞, we show that the ‘atomic’ Dicke model behaves like a ‘giant quantum oscillator’, in which the number of excited atoms asymptotically approaches the average number of photons in the resonant mode of the squeezed vacuum, just as in the thermally driven case.     

Unconventional geometric phase gates using two four-level rf-SQUIDs with cyclic population transfer in a cavity

We propose a scheme for realizing two-qubit quantum phase gates via an unconventional geometric phase shift using two four-level superconducting artificial atoms with cyclic population transfer (broken symmetry) in a cavity. In this scheme, the logical gates' operation only involves the metastable states of each artificial atom. Moreover, under certain conditions, the atoms are disentangled with the cavity mode. Thus, the gate is insensitive to both the atomic spontaneous emission and the cavity decay.     

Quantum coherence effects in a Raman amplifier

We have studied optical pulse propagation in a Raman fiber amplifier doped with a three-level medium and driven by a control laser pulse. We analyze the spatial-temporal dynamics of pulse propagation for different atomic initial conditions. The propagation of an optical pulse through the amplifier can be sustained by a control laser that induces transparency via quantum coherence, which is useful for extending the distance between optical repeaters. Under certain conditions, amplification is achieved without population inversion. The results could be useful for laser control of optical pulses in amplifiers and waveguides.     

Electromagnetically induced squeezing of atomic spin

We present a special quantum effect named electromagnetically induced squeezing of atomic spin in the conventional Λ-type atomic system driven by a strong pump field and a relatively weak probe field in an atomic ensemble. Nearly perfect atomic spin squeezing in the y component can be achieved with fully on resonance at high pump-field intensity, large optical depth of the atomic ensemble, and a low coherence decay rate between the two lower levels. This method would greatly facilitate the generation of atomic spin squeezing by using only coherent laser fields, and may find potential applications in realistic quantum metrology, quantum computation, and quantum information processing.     

太阳模拟器中光谱修正滤光片的研制

在太阳模拟器中,鉴于氙灯的优点及其光谱能量分布与太阳光谱的相近性,通常使用氙灯模拟太阳光谱,但由于氙灯光谱与太阳光谱分布存在差异,所以研制一种滤光片进行氙灯光谱分布的修正.考虑到滤光片工作在强光辐照务件下,所以选用的薄膜材料必须具有低的吸收和高的抗能量阈值.采用电子束和离子辅助沉积技术进行膜层的制备,选择合理的沉积方式并通过优化工艺参数使膜层表面光滑、牢固且通过抗能量测试,最终研制出的光谱修正滤光片满足使用要求.     

Simultaneous determination of refractive index, extinction coefficient, and void distribution of titanium dioxide thin film by optical methods

The refractive index n(λ) and the extinction coefficient k(λ) of a TiO(2) film prepared by electron-beam evaporation are determined in the spectral region 1.5-5.5 eV. The transmission spectrum of the TiO(2) film on a vitreous silica specimen is inverted to get the k(λ) of TiO(2) in its interband transition region. Above 3.5 eV, k(λ) is used to get the coefficients of the quantum mechanically derived dispersion relation of an amorphous TiO(2). These coefficients and n(∞) are used to determine n(λ). The modeling procedure is applied to spectroscopic ellipsometry data of a TiO(2) film on a c-Si specimen, and the void distribution of the film is revealed. With spectroscopic ellipsometry data above the fundamental band gap, valuable information about surface roughness is obtained. The effective thickness of this rough surface layer is confirmed by an atomic force microscopy measurement.     

New frequency-domain technique for joint measurement of nonlineartransition shift and partial erasure

Nonlinear transition shift (NLTS) and partial erasure have been measured independently by traditional time- or frequency-domain techniques without considering the interference between the two. In some cases, such interference can severely distort the measurement results. To solve this problem, a new frequency-domain technique is proposed that measures NLTS and partial erasure jointly. At each given density, three specially constructed periodic data patterns are written and certain harmonics of the readback signals are measured and used to calculate the amount of NLTS and partial erasure simultaneously. The method is simple and easy to implement with a spectrum analyzer     

Excited-state spectroscopy on an individual quantum dot using atomic force microscopy

We present a new charge sensing technique for the excited-state spectroscopy of individual quantum dots, which requires no patterned electrodes. An oscillating atomic force microscope cantilever is used as a movable charge sensor as well as gate to measure the single-electron tunneling between an individual self-assembled InAs quantum dot and back electrode. A set of cantilever dissipation versus bias voltage curves measured at different cantilever oscillation amplitudes forms a diagram analogous to the Coulomb diamond usually measured with transport measurements. The excited-state levels as well as the electron addition spectrum can be obtained from the diagram. In addition, a signature which can result from inelastic tunneling by phonon emission or a peak in the density of states of the electrode is also observed, which demonstrates the versatility of the technique.     

Tensilely Strained Germanium Nanomembranes as Infrared Optical Gain Media

The use of tensilely strained Ge nanomembranes as mid‐infrared optical gain media is investigated. Biaxial tensile strain in Ge has the effect of lowering the direct energy bandgap relative to the fundamental indirect one, thereby increasing the internal quantum efficiency for light emission and allowing for the formation of population inversion, until at a strain of about 1.9% Ge is even converted into a direct‐bandgap material. Gain calculations are presented showing that, already at strain levels of about 1.4% and above, Ge films can provide optical gain in the technologically important 2.1–2.5 μm spectral region, with transparency carrier densities that can be readily achieved under realistic pumping conditions. Mechanically stressed Ge nanomembranes capable of accommodating the required strain levels are developed and used to demonstrate strong strain‐enhanced photoluminescence. A detailed analysis of the high‐strain emission spectra also demonstrates that the nanomembranes can be pumped above transparency, and confirms the prediction that biaxial‐strain levels in excess of only 1.4% are required to obtain significant population inversion.