Normal-mode splitting and output-field squeezing in a Kerr-down conversion optomechanical system

An investigation is reported of the effects of a Kerr-down conversion nonlinear crystal inside an intrinsically nonlinear optomechanical cavity on the dynamics of the oscillating mirror, the intensity and the squeezing spectra of the transmitted field. We show that in comparison with a bare optomechanical cavity, the combination of the cavity energy shift due to the weak Kerr nonlinearity and increase in the intracavity photon number due to the nonlinear gain medium can increase the normal mode splitting in the displacement spectrum of the oscillating mirror. Our study demonstrates that at high temperatures, when the thermal fluctuations in the system are important, the optomechanical and nonlinearity-induced resonances are distinguishable in the output field spectrum. However, at low temperatures, the presence of both nonlinearities enhances the amplitude of the mechanical-mode contribution to the spectrum and leads to the occurrence of normal-mode splitting in the transmitted field spectrum even for low values of the input power. Also, at low temperatures, the Kerr-down conversion nonlinearity increases the radiation pressure contribution to the degree of squeezing of the transmitted field more than that of a bare optomechanical cavity or a nonlinear cavity (in the absence of optomechanical coupling). Furthermore, we find that for the blue-detuned laser the Kerr nonlinearity extends the domain of the stability of the system and leads to the normal-mode splitting of the movable mirror and noise reduction in the range of frequencies in which a bare cavity is not stable.     

Controllable optical bistability and Fano line shape in a hybrid optomechanical system assisted by kerr medium: possibility of all optical switching

We theoretically analyse the optical and optomechanical nonlinearity present in a hybrid system consisting of a quantum dot(QD) coupled to an optomechanical cavity in the presence of a nonlinear Kerr medium, and show that this hybrid system can be used as an all optical switch. A high degree of control and tunability via the QD-cavity coupling strength, the Kerr and the optomechanical nonlinearity over the bistable behaviour shown by the mean intracavity optical field and the power transmission of the weak probe field can be achieved.The results obtained in this investigation has the potential to be used for designing efficient all-optical switch and high sensitive sensors for use in Telecom systems.     

Squeezing of the mechanical motion and beating 3 dB limit using dispersive optomechanical interactions

We study an optomechanical system consisting of an optical cavity and movable mirror coupled through dispersive linear optomechanical coupling (LOC) and quadratic optomechanical coupling (QOC). We work in the resolved side band limit with a high quality factor mechanical oscillator in a strong coupling regime. We show that the presence of QOC in the conventional optomechanical system (with LOC alone) modifies the mechanical oscillator’s frequency and reduces the back-action effects on mechanical oscillator. As a result of this the fluctuations in mechanical oscillator can be suppressed below standard quantum limit thereby squeeze the mechanical motion of resonator. We also show that either of the quadratures can be squeezed depending on the sign of the QOC. With detailed numerical calculations and analytical approximation we show that in such systems, the 3 dB limit can be beaten.     

Tunable double optomechanical induced transparency in an optomechanical system with Bose–Einstein condensate

We investigate the double optomechanically induced transparency (OMIT) of a weak problem field in a hybrid optomechanical system, composed of a Bose–Einstein condensate (BEC), a movable mirror and an optical cavity. Contrast to the single OMIT window in a traditional optomechanical system, the frequency difference between the BEC and the moving mirror in our system can lead to the splitting of the single OMIT window into two transparency windows. Interestingly, the splitting of the two windows varies near linearly with the frequency difference and is robust against the cavity decay. This property can be applied to detect the frequency of the movable mirror. Besides, the driving power and the BEC-cavity coupling strength play a key role in controlling the width of the two transparency windows.     

Negative effective mass,optical multistability and Fano line-shape control via mode tunnelling in double cavity optomechanical system

ABSTRACT

We theoretically investigate the optical and mechanical properties of a double cavity optomechanical system with one stationary and two harmonically bound mirrors. We show that it is possible for the mechanical mirrors in this system to possess negative effective mass. Working within the strong coupling and the resolved sideband regime, we show that the displacement of the middle resonator is multistable under certain constrains. We also point to the existence of optomechanically induced absorption (OMIA) and Fano resonance. Owing to the negative effective mass, our scheme can be exploited in the study of quantum optomechanical metamaterials.     

Mimicking a hybrid-optomechanical system using an intrinsic quadratic coupling in conventional optomechanical system

We consider an optical and mechanical mode interacting through both linear and quadratic dispersive couplings in a general cavity-optomechanical set-up. The parity and strength of an intrinsic quadratic optomechanical coupling (QOC) provides an opportunity to control the optomechanical (OM) interaction. We quantify this interaction by studying normal-mode splitting (NMS) as a function of the QOC's strength. The proposed scheme exhibits NMS features equivalent to a hybrid-OM system containing either an optical parametric amplifier or a Kerr medium. Such a system in reality could offer an alternative platform for devising state-of-art quantum devices with requiring no extra degrees-of-freedom as in hybrid-OM systems.     

Exponential localization of moving end mirror in optomechanics

We discuss the dynamics of moving end mirror of an optomechanical system that consists of a Fabry-Perot cavity loaded with dilute condensate and driven by a single-mode optical field. It is shown that quantum mechanical phenomenon of dynamical localization occurs both in position and momentum space for moving end mirror in the system. The parametric dependencies of dynamical localization are discussed. We also provide a set of parameters which makes this phenomenon experimentally feasible.     

Optomechanics with silicon nanowires by harnessing confined electromagnetic modes

The optomechanical coupling that emerges in an optical cavity in which one of the mirrors is a mechanical resonator has allowed sub-Kelvin cooling with the prospect of observing quantum phenomena and self-sustained oscillators with very high spectral purity. Both applications clearly benefit from the use of the smallest possible mechanical resonator. Unfortunately, the optomechanical coupling largely decays when the size of the mechanical system is below the light wavelength. Here, we propose to exploit the optical resonances associated to the light confinement in subwavelength structures to circumvent this limitation, efficiently extending optomechanics to nanoscale objects. We demonstrate this mechanism with suspended silicon nanowires. We are able to optically cool the mechanical vibration of the nanowires from room temperature to 30-40 K or to obtain regenerative mechanical oscillation with a frequency stability of about one part per million. The reported optomechanical phenomena can be exploited for developing cost-optimized mass sensors with sensitivities in the zeptogram range.     

Effects of a highly dispersive atomic medium inside an optical ring cavity

Atomic media inside an optical cavity can significantly alter the spectral response of the cavity. Both theoretical and experimental examinations are made of the cavity transmission with a highly dispersive intracavity multilevel atomic medium. It is found, owing to the reduced absorption and steep dispersion change accompanying electromagnetically induced transparency in such a multi-level atomic medium, that the cavity linewidth can be made much narrower than the empty cavity linewidth. Cavity linewidth narrowing is measured as a function of both the coupling beam power and the atomic density. These experimental results are in good agreement with the theoretical predictions.     

The synchronization and entanglement of optomechanical systems

We investigate synchronization and entanglement in two coupled cavity optomechanical systems. The classical synchronization, quantum synchronization and entanglement of the two cavity fields and the two mechanical oscillators are analysed, respectively. Our results show that the two cavity resonators are synchronization without entanglement, while the two mechanical oscillators are entangled with quantum-phase synchronization. We conclude that the quantum synchronization and entanglement have no affirmatory relationship although they are both signature of correlation.     

Optomechanical effect on the Dicke quantum phase transition and quasi-particle damping in a Bose–Einstein condensate: a new tool to measure weak force

Abstract

We make a semi-classical steady state analysis of the influence of mirror motion on the quantum phase transition for an optomechanical Dicke model in the thermodynamic limit. An additional external mechanical pump is shown to modify the critical value of atom–photon coupling needed to observe the quantum phase transition. We further show how to choose the mechanical pump frequency and cavity–laser detuning to produce extremely cold condensates. The present system can be used as a quantum device to measure weak forces.     

Coherent coupling between an optomechanical membrane and an interacting photon Bose–Einstein condensate

In this paper, we study theoretically the optomechanical interaction of an interacting condensate of photons with an oscillating mechanical membrane in a microcavity. We show that in the Bogoliubov approximation, due to the large number of photons in the condensate, there is a linear strong effective coupling between the Bogoliubov mode of the photonic Bose–Einstein condensate (BEC) and the mechanical motion of the membrane which depends on the photon–photon scattering potential. This coupling leads to the cooling of the mechanical motion, the normal mode splitting (NMS), the squeezing of the output field and the entanglement between the excited mode of the cavity and the mechanical mode. Since the photon condensation occurs at room temperature, this hybrid system can be potentially considered as a room temperature source of squeezed light as well as a suited candidate for exploring the quantum effects. We show that, on one hand, the non-linearity of the photon gas increases the degree of the squeezing of the output field of the microcavity and the efficiency of the cooling process at high temperatures. On the other hand, it reduces the NMS in the displacement spectrum of the oscillating membrane and the degree of the optomechanical entanglement. In addition, the temperature of the photonic BEC can be used to control the above-mentioned phenomena.     

Generation of motional entangled coherent state in an optomechanical system in the single photon strong coupling regime

The single-photon strong coupling in the deep-resolved sideband of the optomechanical system induces photon blockade (PB) effect. For the PB cavity, an initial mechanical coherent state evolves into superposition of phonon cat states entangled with the cavity Fock states. Measurement of the cavity photon number states produces phonon even and odd cat states. The information leakage effect of two photon states on the fidelity of cat states is calculated, it is shown that for low average phonon number this effect is negligible and decreases by increasing the two photon cavity state. The Lindblad equation is solved numerically to obtain the environmental effects on the fidelity of cat states.     

Deterministic generation of singlet state of N atoms in coupled cavities via adiabatic passage of a dark state

In this paper, we propose an efficient scheme to generate the N-atom singlet state via adiabatic passage of a dark state in cavity quantum electrodynamics (QED) system. Appropriate Rabi frequencies of the classical fields are selected to realize the present scheme. We discuss the influence of decoherence induced by cavity decay and atomic spontaneous emission by numerical calculation. The result shows that the scheme is insensitive to atomic spontaneous emission as the atomic excited state is hardly populated in adiabatic evolution.     

Investigation of Fano resonances in the optomechanical cavity via a magnetic field

We theoretically investigate Fano resonances in a single-cavity optomechanical system, which is driven by an external force arisen from a passed current through one end of a mirror in a magnetic field. It is revealed that the asymmetric Fano shape in the optomechanical system strongly depended on the strength of the magnetic field and current intensity. Further, we study the phase of the transmitted probe light and find a tunable switch from slow to fast light and vice versa via manipulating the magnetic field.     

Femtogram doubly clamped nanomechanical resonators embedded in a high-Q two-dimensional photonic crystal nanocavity

We demonstrate a new optomechanical device system which allows highly efficient transduction of femtogram nanobeam resonators. Doubly clamped nanomechanical resonators with mass as small as 25 fg are embedded in a high-finesse two-dimensional photonic crystal nanocavity. Optical transduction of the fundamental flexural mode around 1 GHz was performed at room temperature and ambient conditions, with an observed displacement sensitivity of 0.94 fm/Hz(1/2). Comparison of measurements from symmetric and asymmetric double-beam devices reveals hybridization of the mechanical modes where the structural symmetry is shown to be the key to obtain a high mechanical quality factor. Our novel configuration opens the way for a new category of "NEMS-in-cavity" devices based on optomechanical interaction at the nanoscale.     

Generating EPR-type entanglement of degenerate optomechanical parametric oscillators

Einstein–Podolski–Rosen (EPR) entanglement states are achievable by combining two single-mode position and momentum squeezed states at a 50:50 beam splitter (BS). To generate the EPR mechanical entanglement, we consider the system consisted of two parametric optomechanical resonators, where two mechanical oscillators are linearly coupled. The linear coupling forms the symmetric and antisymmetric combinations of two mechanical modes, parallel to a 50:50 BS mixing. In the weak optomechanical coupling regime and via applying the opposite phases of parametric interactions, the symmetric and antisymmetric mechanical modes can be position and momentum squeezed, respectively. Therefore, two original mechanical modes are EPR entangled. Moreover, the mechanical thermal noise can decrease the entanglement. But with the parametric interaction enhanced optomechanical cooling, the influence of thermal noise on entanglement can be significantly suppressed, and the mechanical entanglement can be generated under a relatively high temperature. We also discuss the critical thermal occupation where the entanglement disappears, which is proportional to the optomechanical cooperativity parameter.     

N-level Atom Interacting with Single-mode Radiation Field: An Exactly Solvable Model with Multiphoton Transitions and Intensity-dependent Coupling

Abstract

We study the dynamics of an N-level atom coupled in a lossless cavity to a single-mode near-resonant quantized field. The atomic levels are coupled by the multiphoton transitions and the coupling constants between the field and the atomic levels are supposed to be intensity dependent. We find the exact solution for the state vector describing the dynamics of the atom-plus-field system. As an illustration we use the model for studying (i) the time evolution of the atomic occupation probability with the initially coherent field and (ii) the light squeezing, when the cavity field is initially in the vacuum state and the atom is prepared in the atomic ‘coherent state’ (a superposition of atomic states).     

主动光钟研究进展

自2005年至今，主动光钟经过了近20年的发展。主动光钟利用原子系综作为增益介质，其受激辐射可直接作为钟激光信号。因为主动光钟工作在坏腔区域，因此具有腔牵引抑制和窄线宽两个显著的优点，可以有效克服被动光钟存在的腔长热噪声问题。由于其优越的性能，主动光钟受到了国内外同行的广泛关注。根据实现方式不同，本文将主动光钟划分为原子束型主动光钟、基于激光冷却和光晶格囚禁的主动光钟、原子束及光晶格“复合型”主动光钟、法拉第主动光钟、离子阱囚禁型主动光钟以及热原子气室型主动光钟。对于不同类型的主动光钟，本文详细介绍了其实验及理论研究进展，并分析其优劣。最后，分析了主动光钟在精密测量领域的应用并展望了主动光钟的发展方向，为推动主动光钟的广泛应用提供借鉴。