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Cavity-mediated magnon-magnon coupling can lead to a transfer of spin-wave excitations between two spatially separated magnetic samples. We enunciate how the application of a two-photon parametric drive to the cavity can lead to stark amplification in this transfer efficiency. The recurrent multiphoton absorption by the cavity opens up an infinite ladder of accessible energy levels, which can induce higher-order transitions within the magnon Fock space. This is reflected in a heightened spin-current response from one of the magnetic samples when the neighboring sample is coherently pumped. The enhancement induced by the parametric drive can be considerably high within the stable dynamical region. Specifically, near the periphery of the stability boundary, the spin current is amplified by several orders of magnitude. Such striking enhancement factors are attributed to the emergence of parametrically induced strong coherences precipitated by a long-lived mode. While contextualized in magnonics, the generality of the principle would allow applications to energy transfer between systems contained in parametric cavities.

We find that stimulated Raman excitation of an atom by a two-photon pulse can be enhanced by orders of magnitude if the photons are simultaneously frequency correlated and spatially anticorrelated. That is, a correlated photon pair must have an inherent time delay between its constituent photons. This counterintuitive feature is a manifestation of the uncertainty principle, which yields that frequency-correlated photons cannot be time (spatially) correlated. This is opposite to two-photon absorption by a three-level atom, for which the enhancement occurs if photons in the pulse are frequency anticorrelated and spatially correlated, that is, photons in the pair simultaneously interact with the atom. Our findings can be useful for imaging and spectroscopy of biological samples which demand low illumination intensity.

Squeezing of quantum states is of great interest due to its application in high-precision measurement that can exceed standard quantum noise limit described by Heisenberg uncertainty relations. Here, we study the squeezing of quantum systems with $\text{SU}(3)$ symmetry via a one-axis twisting Hamiltonian and examine the intriguing connections between the squeezing of spin-1/2 and spin-1 systems. There are seven subalgebras of $\text{su}(3)$ Lie algebra. All these subalgebras are identical with $\text{su}(2)$ Lie algebra but not all of them have the same anticommutation relations as $\text{su}(2)$. Interestingly, squeezing parameters corresponding to spin-1 subalgebras depend not only on structure constants but also on anticommutation relations of the subalgebras. Our results are reported for the subalgebras with vanishing anticommutators and nematic squeezing, while in other cases our first-principle calculation recovers known results.

Decoherence remains one of the most serious challenges to the implementation of quantum technology. It appears as a result of the transformation over time of a quantum superposition state into a classical mixture due to the quantum system interacting with the environment. Since quantum systems are never completely isolated from their environment, decoherence therefore cannot be avoided in realistic situations. Decoherence has been extensively studied, mostly theoretically, because it has many important implications in quantum technology, such as in the fields of quantum information processing, quantum communication and quantum computation. Here we report a novel experimental scheme on the study of decoherence of a two-mode squeezed vacuum state via its second harmonic generation signal. Our scheme can directly extract the decoherence of the phase-sensitive quantum correlation $\langle \hat{a}\hat{b}\rangle$ between two entangled modes $a$ and $b$. Such a correlation is the most important characteristic of a two-mode squeezed state. More importantly, this is an experimental study on the decoherence effect of a squeezed vacuum state, which has been rarely investigated., Comment: 9 pages, 6 figures

We show how the Jaynes--Cummings--Rabi model of cavity quantum electrodynamics can be realized via an isomorphism to the Hamiltonian of a qubit inside a parametric amplifier cavity. This realization clears the way to observe the full spectrum of the Rabi model via a probe applied to a parametric amplifier cavity containing a qubit and a parametric oscillator operating below threshold. An important outcome of the isomorphism is that the actual frequencies are replaced by detunings which make it feasible to reach the ultra-strong coupling regime. We find that inside this regime the probed spectrum displays a narrow resonance peak that is traced back to the transition between ground and first excited states. The exact form of these states is given at an energy crossing and then extended numerically. At the crossing, the eigenstates are entangled states of field and atom where the field is found inside squeezed cat states.

$\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ is emerging as an excellent potential semiconductor for high power and optoelectronic devices. However, the successful development of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ in a wide range of applications requires a full understanding of the role and nature of its point and extended defects. In this work, high quality epitaxial $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ films were grown on sapphire substrates by metal-organic chemical vapor deposition and fully characterized in terms of structural, optical, and electrical properties. Then defects in the films were investigated by a combination of depth-resolved Doppler broadening and lifetime of positron annihilation spectroscopies and thermally stimulated emission (TSE). Positron annihilation techniques can provide information about the nature and concentration of defects in the films, while TSE reveals the energy level of defects in the bandgap. Despite very good structural properties, the films exhibit short positron diffusion length, which is an indication of high defect density and long positron lifetime, a sign for the formation of Ga vacancy related defects and large vacancy clusters. These defects act as deep and shallow traps for charge carriers as revealed from TSE, which explains the reason behind the difficulty of developing conductive $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ films on non-native substrates. Positron lifetime measurements also show nonuniform distribution of vacancy clusters throughout the film depth. Further, the work investigates the modification of defect nature and properties through thermal treatment in various environments. It demonstrates the sensitivity of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ microstructures to the growth and thermal treatment environments and the significant effect of modifying defect structure on the bandgap and optical and electrical properties of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$.

We demonstrate the emergence of transparent behavior in a chain of periodically spaced nonidentical quantum emitters coupled to a waveguide, in the special case when the nearest neighbor separation is a half-integral multiple of the resonant wavelength, i.e., $kL$ is an integral multiple of $\ensuremath{\pi}$, with $k$ being the wave number and $L$ the spatial periodicity. When equal but opposite frequency detunings are assigned in pairs to a system of even number of atoms, perfect transmission ensues. When the chain size is odd, a similar assignment leads to the disappearance of collective effects as the odd atom determines the spectral behavior. We also manifest the robustness of these features against dissipative effects and show how the spectral behavior hinges significantly on the relative detunings between the atoms as compared to the decay rate. A key distinction from the phenomenon of electromagnetically induced transparency is that in the waveguide case, the presence of an intrinsic waveguide-mediated phase coupling between the atoms strongly affects the transport properties. Furthermore, while reciprocity in single-photon transport does not generally hold due to the phase coupling, we observe an interesting exception for $kL=n\ensuremath{\pi}$ at which the waveguide demonstrates reciprocal behavior with regard to both the transmission and reflection coefficients.

When a laser beam passes through a rotating ground glass (RGG), the scattered light exhibits thermal statistics. This is extensively used in speckle imaging. This scattering process has not been addressed in photon picture and is especially relevant if non-classical light is scattered by the RGG. We develop the photon picture for the scattering process using the Bose statistics for distributing $N$ photons in $M$ pixels. We obtain analytical form for the P-distribution of the output field in terms of the P-distribution of the input field. In particular we obtain a general relation for the $n$-th order correlation function of the scattered light, i.e., $g_{\text{out}}^{(n)}\simeq n!\,g_{\text{in}}^{(n)}$, which holds for any order-$n$ and for arbitrary input states. This result immediately recovers the classical transformation of coherent light to pseudo-thermal light by RGG., Comment: 6.1 pages, 4 figures

We examine the directional characteristics of emission from a dipole on hyperbolic metamaterial (HMM) and we show the possibility of chiral emission which is strongly dependent on the polarization of the dipole. Using rigorous solutions of the Maxwell equations, we present analytical results on chirality which depends on the excitation of different evanescent modes in the HMM, some of which are easily accessible in current experiments. The origin of chirality lies in the near field interference of emission arising from parallel and perpendicular components of the dipole polarization especially if the two polarizations are out of phase by $\ensuremath{\pi}/2$. The dependence of chirality on the distance of the dipole from the HMM is obtained. The truly unidirectional emission is possible for some specific complex elliptic polarization of the dipole. Our analysis is based on the Green's function for the multilayered HMM. We also present results on chiral emission from emitters on metal surfaces. These results provide opportunities for controlling the excitation of the surface waves and can have potential applications in sensing molecules on HMM surfaces.

This paper shows a mechanism to produce entanglement between magnons via Kerr nonlinearity. The authors sudty this in a system of two YIG spheres and propose that the scheme can be extended to other systems.

Vector beams in optics play a vital role in a wide variety of fundamental studies and applications, including high-capacity communication, super-resolution imaging through biological tissue, quantum information encryption, surveillance, and sensing. Here the densely coded information and inseparability of spatial and polarization degrees of freedom complicate full characterization, and the lack of a technique for complete beam metrology hinders applications. The authors demonstrate a direct tomographic protocol that can reveal unambiguously the complete transverse field structure of a fully polarized vector beam, all with a single measurement.

We examine quantum interference effects due to absorption and emission from multiple atoms coupled to a waveguide and highlight the modifications they entail in regards to single-photon transport properties. A prominent upshot of these interference phenomena is the resonant suppression of the reflection amplitude, which leads to the observation of multiple Fano minima in the reflection spectrum. Such minima determine the points at which transparency is induced in the system. By taking recourse to the real-space Hamiltonian framework, we calculate analytically the reflectivity and transmissivity for a one-dimensional waveguide that evanescently couples to a chain of equally spaced quantum emitters. The inter-emitter spacing relative to the wavelength of the propagating photon, leading to a waveguide-mediated "phase-coupling" between the atoms, is found to fundamentally affect the existence of Fano minima. For a chain of $N$ atoms, the number of minima can be at most $N-1$. However, suitable choices of the phase can suppress the discernibility of the full range of roots in the reflection spectrum. A principal observation for the case of multiple emitters is the emergence of super-Gaussian characteristics close to zero-detuning and consequently, a plateau-shaped broadband spectrum in the region of high reflectivity. For a large chain size, the plateau gets transformed into a flat-topped quasi-rectangular profile in the frequency domain., Comment: 10 pages, 4 figures

Fr\"ohlich discovered the remarkable condensation of polar vibrations into the lowest frequency mode when the system is pumped externally. For a full understanding of the Fr\"ohlich condensate one needs to go beyond the mean field level to describe critical behavior as well as quantum fluctuations. The energy redistribution among vibrational modes with nonlinearity included is shown to be essential for realizing the condensate and the phonon-number distribution, revealing the transition from quasi-thermal to super-Poissonian statistics with the pump. We further study the spectroscopic properties of the Fr\"ohlich condensate, which are especially revealed by the narrow linewidth. This gives the long-lived coherence and the collective motion of the condensate. Finally we show that the proteins such as Bovine Serum Albumin (BSA) and lysozyme are most likely the candidates for observing such collective modes in THz regime by means of Raman or infrared (IR) spectroscopy., Comment: 5 pages, 4 figures + 8 pages Supplementary Material

Radiative dipolar resonance with Lorentzian line-shape induces the otherwise dark quadrupolar resonances resulting in electromagnetically induced transparency (EIT). The two interfering excitation pathways of the dipole are earlier shown to result in a Fano line shape with a high figure of merit suitable for sensing. In metamaterials made of metal nanorods or antennas, the plasmonic EIT (PIT) efficiency depends on the overlap of the dark and bright mode spectra as well as the asymmetry resulting from the separation between the monomer (dipole) and dimer (quadrupole) that governs the coupling strength. Increasing asymmetry in these structures leads to the reduction of the figure of merit due to a broadening of the Fano resonance. We demonstrate a PIT system in which the simultaneous excitation of two dipoles result in double PIT. The corresponding two quadrupoles interact and control the quality factor (Q) of the PIT resonance. We show an antiresonancelike symmetric line shape with nonzero asymmetry factors. The PIT resonance vanishes due to quadrupole-quadrupole coupling. A Q factor of more than 100 at 0.977 THz is observed, which is limited by the experimental resolution of 6 GHz. From polarization-dependent studies we show that the broadening of the Lorentzian resonance is due to scattering-induced excitation of orthogonally oriented dipoles in the monomer and dimer bars in the terahertz regime. The high Q factors in the terahertz frequency region demonstrated here are interesting for sensing application.

The $\mathcal{PT}$ symmetry breaking threshold in discrete realizations of systems with balanced gain and loss is determined by the effective coupling between the gain and loss sites. In one dimensional chains, this threshold is maximum when the two sites are closest to each other or the farthest. We investigate the fate of this threshold in the presence of parallel, strongly coupled, Hermitian (neutral) chains, and find that it is increased by a factor proportional to the number of neutral chains. We present numerical results and analytical arguments for this enhancement. We then consider the effects of adding neutral sites to $\mathcal{PT}$ symmetric dimer and trimer configurations and show that the threshold is more than doubled, or tripled by their presence. Our results provide a surprising way to engineer the $\mathcal{PT}$ threshold in experimentally accessible samples., 7 pages, 6 figures

Calculation of the entropy of an ideal Bose-Einstein condensate (BEC) in a three-dimensional trap reveals unusual, previously unrecognized, features of the canonical ensemble. It is found that, for any temperature, the entropy of the Bose gas is equal to the entropy of the excited particles although the entropy of the particles in the ground state is nonzero. We explain this by considering the correlations between the ground-state particles and particles in the excited states. These correlations lead to a correlation entropy which is exactly equal to the contribution from the ground state. The correlations themselves arise from the fact that we have a fixed number of particles obeying quantum statistics. We present results for correlation functions between the ground and excited states in a Bose gas, so as to clarify the role of fluctuations in the system. We also report the sub-Poissonian nature of the ground-state fluctuations.

Currently epsilon near zero materials (ENZ) have become important for controlling the propagation of light and enhancing by several orders of magnitude the Kerr and other nonlinearities. Given this advance it is important to examine the quantum electrodynamic processes and information tasks near ENZ materials. We study the entanglement between two two-level systems near ENZ materials and compare our results with the case where the ENZ material is replaced by a metal. It is shown that with ENZ materials substantial entanglement can be achieved over larger distances than for metal films. We show that this entanglement over large distances is due to the fact that one can not only have large emission rates but also large energy transmission rates at the epsilon-nearzero wavelength. This establishes superiority of ENZ materials for studying processes specifically important for quantum information tasks.

Several recent experiments have reported a variety of new collective behaviors of two atoms in a cavity. An important experimental result was that the back-reaction in a high-quality cavity leads to subradiance rather than superradiance. Further investigation by changing the parameter domain has shown the possibility of hyperradiance. In this paper we present a model of hyperradiance which is based on the idea of vacuum Rabi splittings when the driving field is detuned from the atom or cavity resonance so that it hits one of the dressed states. We show that this hyperradiance is accompanied by significant nonclassicality of the output. Some aspects, but not nonclassicality, of the hyperradiance when the driving field is very weak can be understood semiclassically. Our mechanism of hyperradiance applies to atoms coupled symmetrically to the cavity mode, whereas in the work of M. Pleinert et al. [Optica 4, 779 (2017)] atoms have to be coupled almost asymmetrically to the cavity.

With Maxwell-Bloch equations how the superradiance can lead to amplification and gain at a frequency much larger than the pumping frequency has been shown. This remarkable effect has been examined in terms of a simpler model involving two coupled oscillators, with one of them parametrically driven. We show that this coupled-oscillator model has a hidden parity-time ($\mathcal{PT}$) symmetry for quantum amplification by superradiant emission of radiation (QASER); we thus bring $\mathcal{PT}$ symmetry to the realm of parametrically coupled resonators. Moreover, we find that the QASER gain arises from the broken-$\mathcal{PT}$-symmetry phase. We then quantize the simplified version of QASER using quantum Langevin equations. The quantum description enables us to understand how the system starts from quantum fluctuations.

We present a study of collective multi-photon blockade in coherently driven atoms in a single mode cavity. Considering two atoms strongly coupled to an optical cavity, we show that the two-photon blockade with two-photon anti-bunching, and the three-photon blockade with three-photon anti-bunching can be observed simultaneously. The three-photon blockade probes both dressed states in the two photon and three photon spaces. The two photon and three photon blockades strongly depend on the location of two atoms in the strong coupling regime. The asymmetry in the atom-cavity coupling constants opens new pathways for multiphoton blockade which is also shown to be sensitive to the atomic decay and pumping strengths. The work presented here predicts many new quantum statistical features due to the collective behavior of atoms and can be useful to generate non-classical photon pairs.

We introduce a method for the verification of nonclassical light which is independent of the complex interaction between the generated light and the material of the detectors. This is accomplished by means of a multiplexing arrangement. Its theoretical description yields that the coincidence statistics of this measurement layout is a mixture of multinomial distributions for any classical light field and any type of detector. This allows us to formulate bounds on the statistical properties of classical states. We apply our directly accessible method to heralded multiphoton states which are detected with a single multiplexing step only and two detectors, which are in our work superconducting transition-edge sensors. The nonclassicality of the generated light is verified and characterized through the violation of the classical bounds without the need for characterizing the used detectors., Comment: close to published version

We theoretically investigate optical detection of a weak classical force acting on a free particle in a dissipative coupling optomechanical system with a degenerate parametric amplifier (PA). We show that the PA allows one to achieve the force sensitivity far better than the standard quantum limit (SQL) over a broad range of the detection frequencies. The improvement depends on the parametric gain and the driving power. Moreover, we discuss the effects of the mechanical damping and the thermal noise on the force sensitivity. We find that the robustness of the force sensitivity much better than the SQL against the mechanical damping and the thermal noise is achievable in the presence of the PA with a high parametric gain. For the temperature $T = 1$ K, the improvement in sensitivity is better by a factor of about 7 when the driving power is set at a value corresponding to the SQL with no PA., Comment: 10 pages, 6 figures, 2 tables

It has been shown that perfect photon absorption can occur in the linear excitation regime of cavity quantum electrodynamics (CQED), in which photons from two identical light fields coupled into two ends of the cavity are completely absorbed and result in excitation of the polariton state of the CQED system. The output light from the cavity is totally suppressed by the destructive interference and the polariton state can only decay incoherently back to the ground state. Here we analyze the perfect photon absorption and onset of optical bistability in the nonlinear regime of the CQED and show that the perfect photon absorption persists in the nonlinear regime of the CQED below the threshold of the optical bistability. Therefore the perfect photon absorption is a phenomenon that can be observed in both linear and nonlinear regimes of CQED. Furthermore, our study reveals for the first time that the optical bistability is influenced by the input-light interference and can be manipulated by varying the relative phase of the two input fields., Comment: 10 pages, 4 figures

The fluctuation theorem has a very special place in the study of nonequilibrium dynamics of physical systems. The form in which it is used most extensively is the Gallavoti-Cohen fluctuation theorem which is in terms of the distribution of the work p(W)/p(-W)=exp(αW). We derive the general form of the fluctuation theorems for an arbitrary multidimensional Gaussian Markov process. Interestingly, the parameter α is by no means universal, hitherto taken for granted in the case of linear Gaussian processes. As a matter of fact, conditions under which α does become a universal parameter 1/KT are found to be rather restrictive. As an application we consider fluctuation theorems for classical cyclotron motion of an electron in a parabolic potential. The motion of the electron is described by four coupled Langevin equations and thus is nontrivial. The generalized theorems are equally valid for nonequilibrium steady states and could be especially important in the presence of anisotropic diffusion.

Superradiance typically requires preparation of atoms in highly entangled multi-particle states, the so-called Dicke states. In this paper we discuss an alternative route where we prepare such states from initially uncorrelated atoms by a measurement process. By measuring higher order intensity intensity correlations we demonstrate that we can simulate the emission characteristics of Dicke superradiance by starting with atoms in the fully excited state. We describe the essence of the scheme by first investigating two excited atoms. Here we demonstrate how via Hanbury Brown and Twiss type of measurements we can produce Dicke superradiance and subradiance displayed commonly with two atoms in the single excited symmetric and antisymmetric Dicke states, respectively. We thereafter generalize the scheme to arbitrary numbers of atoms and detectors, and explain in detail the mechanism which leads to this result. The approach shows that Hanbury Brown and Twiss type intensity interference and the phenomenon of Dicke superradiance can be regarded as two sides of the same coin. We also present a compact result for the characteristic functional which generates all order intensity intensity correlations., 8 pages, 3 figures

Readout and retrieval processes are proposed for efficient, high-fidelity quantum state transfer between a matter qubit, encoded in the level structure of a single atom or ion, and a photonic qubit, encoded in a time-reversal-symmetric single-photon wave packet. They are based on controlling spontaneous photon emission and absorption of a matter qubit on demand in free space by stimulated Raman adiabatic passage. As these processes do not involve mode selection by high-finesse cavities or photon transport through optical fibers, they offer interesting perspectives as basic building blocks for free-space quantum-communication protocols., Comment: 5 pages, 5 figures

Superradiance is one of the outstanding problems in quantum optics since Dicke introduced the concept of enhanced directional spontaneous emission by an ensemble of identical two-level atoms. The effect is based on correlated collective Dicke states which turn out to be highly entangled. Here we show that enhanced directional emission of spontaneous radiation can be produced also with statistically independent incoherent sources via the measurement of higher order correlation functions of the emitted radiation. Our analysis is applicable to a wide variety of quantum systems like trapped atoms, ions, quantum dots or NV-centers, and is also valid for statistically independent incoherent classical emitters. This is experimentally confirmed with up to eight independent thermal light sources., 5 pages, 4 figures

We adopt the Ramsey's method of separated oscillatory fields to study coherences of the mechanical system in an optomechanical resonator. The high resolution Ramsey fringes are observed in the emission optical field, when two pulses separated in time are applied. We develop a theory to describe the transient optomechanical behavior underlying the Ramsey fringes. We also perform the experimental demonstration using a silica microresonator. The method is versatile and can be adopted to different types of mechanical resonators, electromechanical resonators.

A general theory is presented to describe optomechanical interactions of acoustic phonons, having extremely long lifetimes in superfluid $^{4}\mathrm{He}$, with optical photons in the medium placed in a suitable electromagnetic cavity. The acoustic nonlinearity in the fluid motion is included to consider processes beyond the usual linear process involving the absorption or emission of one phonon at a time. We first apply our formulation to the simplest one-phonon process involving the usual resonant anti-Stokes upconversion of an incident optical mode. However, when the allowed optical cavity modes are such that there is no single-phonon mode in the superfluid, which can give rise to a resonant allowed anti-Stokes mode, we must consider the possibility of two-phonon upconversion. For such a case, we show that the two-step two-phonon process could be dominant. We present arguments for a large two-step process and negligible single-step two-phonon contribution. The two-step process also shows interesting quantum interference among different transition pathways.

We propose a scheme for the generation of a robust stationary squeezed state of a mechanical resonator in a quadratically coupled optomechanical system, driven by a pulsed laser. The intracavity photon number presents periodic intense peaks suddenly stiffening the effective harmonic potential felt by the mechanical resonator. These "optical spring kicks" tend to squeeze the resonator position, and due to the interplay with fluctuation-dissipation processes one can generate a stationary state with more than 13 dB of squeezing even starting from moderately "pre-cooled" initial thermal states., Comment: 5 pages, 3 figures. This paper supersedes an earlier version arXiv:1109.3905

We present the results of a comprehensive investigation of the quantum-noise properties of a continuous-wave phase-conjugate mirror (PCM) formed using backward-four-wave mixing in potassium vapor. We characterize the quantum-noise properties of the PCM as functions of the vapor density, pump detuning from resonance, and relative frequency detuning of the signal beam from the probe beam. We compare the noise measurements of the PCM with the predicted noise of an ideal quantum-noise-limited PCM and determine the value of the minimum signal that can be used to perform phase conjugation with unity signal-to-noise ratio. For the range of vapor densities studied, we find that the PCM operates nearest the quantum-noise limit and that the value of the minimum signal is lowest under conditions in which the reflectivity is maximized. These results demonstrate that it is possible to perform phase conjugation with signals as weak as 14 fW with near-unity reflectivity. Our measurements are in qualitative agreement with the predictions of a quantum theory of phase conjugation via nearly degenerate four-wave mixing in a two-level system.

We consider the possibility of wave propagation in the normally disallowed (band-gap) region of a resonant optical medium, that is, in the band of frequencies near resonance where the real part of the frequency-dependent dielectric function is negative. We demonstrate that wave propagation can become allowed by the application of a strong electromagnetic field resonant with some additional transition of the material system. The frequencies at which these effects can occur are strongly influenced by local-field effects.

We have used a generic two-mode Hamiltonian with two associated time scales to study the evolution of generalized SU(1,1) coherent states, in particular the pair and Perelomov coherent states, which have been realized in many systems such as radiation fields, trapped ions, and phonons. We have found that their dynamics does not depend on the ratio of these time scales but is instead determined crucially by the difference in photon numbers of the two modes. This is in stark contrast to the previously studied harmonic-oscillator coherent states and can be attributed to the different nature of the underlying algebra. We provide analytical results for their revival and fractional revival along with numerical plots of the autocorrelation function and the quadrature distribution and demonstrate the formation of Schrodinger cats. The results are extremely sensitive to the Casimir invariant and the complex parameters characterizing SU(1,1) coherent states.

I consider the interaction of a superposition of mesoscopic coherent states and its approach to a mixed state as a result of a suitably controlled environment. I show how the presence of a gain medium in a cavity can lead to diagonalization in coherent state basis in contrast to the standard model of decoherence. I further show how the new model of decoherence can lead to the generation of $s$ ordered quasi distributions., 10 pages, two figure pages, RevTeX

We suggest a scheme to reconstruct a two-mode entangled state in cavity QED by using the interaction of a V-configuration three-level atom and by driving the cavity field. After the atomic interaction with the cavity fields, the probability of the atom being in its initial ground state is found to be directly related to the two-mode Wigner characteristic function. The Wigner function and the four-dimensional density-matrix elements can be obtained for the two-mode entangled field by simple transformations. We consider both the cases where two entangled modes are prepared in one cavity or in two spatially separated cavities.

We examine the revival features in wave packet dynamics of a particle confined in a finite square well potential. The possibility of tunneling modifies the revival pattern as compared to an infinite square well potential. We study the dependence of the revival times on the depth of the square well and predict the existence of superrevivals. The nature of these superrevivals is compared with similar features seen in the dynamics of wavepackets in an anharmonic oscillator potential., 8 pages in Latex two-column format with 5 figures (eps). To appear in Physical Review A

We analyze the probe absorption spectra of a strongly driven $\ensuremath{\Lambda}$ system with arbitrary spacing between the two lower levels. Under the condition that the spacing between the two lower levels is of the order of a strong-pump-field Rabi frequency, important interference effects are observed due to same-field coupling with both the transitions. We report the possibility of a significant gain from the cross talk among optical transitions, and a strong dependence of gain and absorption on the relative polarizations of the pump and probe beams. We present an analysis based on dressed states to explain our numerical results.

We present physical reasoning for the missing interference [Zhu, Narducci, and Scully, Phys. Rev. A 52, 4791 (1995)] in emission in a laser-driven ∨ system. We demonstrate how the interference effects can be restored by considering an additional channel of spontaneous emission. Analytical results are given to identify the various pathways contributing to this interference. The interference terms show up in the form of dispersive contributions to the line shape.

We develop a nonperturbative approach to study the generation of nonlinear signals in a coherently prepared medium. Specifically, we study the process ${\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}+({\ensuremath{\omega}}_{2}+\ensuremath{\Omega})$, in a system prepared in a coherent-population-trapping state. We calculate enhancement factors of the order of $\ensuremath{\sim}{10}^{2}$ in the generated signal, under a variety of conditions thus supplementing the work of Jain et al. [Phys. Rev. Lett. 77, 4326 (1996)]. Our approach also enables us to study other related issues. We demonstrate the existence of pulse matching at higher probe powers.

We examine the dynamical evolution of wave packets in a large class of quantum systems where two quantum numbers determine the energy spectrum and consequently the dynamical behavior. Using a generic Hamiltonian, we study the formation of coherent structures. The ratio of two time scales as well as the symmetry of the initial wave packet decisively determines the revival and fractional revivals of the system for which we give analytical results. The theory is applicable to a wide class of systems from diatomic molecules to ions in two-dimensional traps and two interacting Bose condensates.

Near-degenerate lower levels in a $\ensuremath{\Lambda}$ system have an additional coherence term due to interaction with the vacuum of the radiation field. We report the effects of this spontaneously generated coherence on the formation of a trapped state in the presence of two coherent fields of arbitrary intensity. We show that such coherence preserves both electromagnetically induced transparency and coherent population trapping (CPT) phenomena. However, it changes the time scales associated with the formation of the CPT state, and brings about quantitative changes in the line profiles. We present a clear analytical explanation for our numerical results. We also report the dependence of line shapes on the relative phase between the two applied fields.

We predict the existence of cavity-induced two-photon absorption in unidentical atoms, and demonstrate the nonclassical character of this two-photon absorption. We show the effects of photon statistics on cavity induced two-photon absorption. We also note the possibility of trapping states in the present system.

We calculate Raman scattering of light from a Bose condensate of dilute gas confined in a harmonic trap. We examine the changes in the Raman line shape as a function of temperature. The linewidth exhibits a remarkable transition as the system is cooled down from the noncondensed to the condensed phase. We also present results for sidebands arising from deeper traps. We show that all these results on Raman line shapes are especially pronounced in the backward direction.

We consider the feasibility of controlling the energy transfer between an unexcited and an excited atom using microcavities. The excitation-transfer probability is related to the strength of the dipole-dipole interaction and thus the excitation-transfer rates could be increased by enhancing the dipole-dipole interaction. We demonstrate this enhancement with an explicit three-dimensional calculation for a metallic cavity, which we characterize with a realistic dielectric function. We also demonstrate the shifts in cavity resonances from their ideal positions $m\ensuremath{\lambda}/2$ ($m$ is an integer and \ensuremath{\lambda} is the wavelength).