Dicke quantum spin glass of atoms and photons

Recent studies of strongly interacting atoms and photons in optical cavities have rekindled interest in the Dicke model of atomic qubits coupled to discrete photon cavity modes. We study the multimode Dicke model with variable atom-photon couplings. We argue that a quantum spin-glass phase can appear, with a random linear combination of the cavity modes superradiant. We compute atomic and photon spectral response functions across this quantum phase transition, both of which should be accessible in experiments.     

光腔中两组分玻色-爱因斯坦凝聚体的受激辐射特性和量子相变

研究了单模光腔中两组分玻色-爱因斯坦凝聚的基态性质和相关的量子相变.通过利用自旋相干态变换将等效赝自旋哈密顿算符对角化并求得基态能量泛函.基态能量泛函对其经典场变量进行变分并取极小值,得到光子数解和相边界曲线.通过稳定性讨论发现系统除了出现正常相和超辐射相之外,还得到了多稳的宏观量子态;受激辐射来自于原子数反转的集体态,单组分的Dicke系统中并没有此现象;受激辐射只能从一组分的原子中产生,而另外的仍保持在普通超辐射状态.通过调整相关的原子-场耦合强度和频率失谐,超辐射和受激辐射态的顺序可以在原子的两个组分之间互换.     

光泵NH3分子远红外激光研究

运用量子系统的密度矩阵理论，采用迭代法计算了光泵ＮＨ３分子远红外超辐射和腔式激光的频谱特性和相应的最佳工作气压。理论计算表明相对于超辐射方式，光泵腔式ＮＨ３分子远红外激光具有宽带辐射以及纵向调谐特性，且总的输出光强在一定条件下得到提高，最佳工作气压也较低。在实验上对ＴＥＡＣＯ２－１０Ｒ（８）和９Ｒ（１６）泵浦的Ｎ 红外超辐射和腔式激光器产生的远红外辐射进行了测量。结果表明理论计算与实验结果相符。     

Spectra of optically pumped superradiant and cavity NH3 far-infrared laser

Based on the density matrix theory of quantum system, the spectra of optically pumped superradiant and cavity NH₃ far-infrared (FIR) lasers were calculated by means of iteration method. The calculation showed that, compared with the optically pumped superradiant FIR laser, the cavity laser had a wider band spectrum of FIR emission and a higher output power under certain condition, moreover, the spectra of cavity laser had multi longitudinal-mode structure. Supported by the Research Foundation of Hubei Province Education Committee, PRC     

Exploring symmetry breaking at the Dicke quantum phase transition

We study symmetry breaking at the Dicke quantum phase transition by coupling a motional degree of freedom of a Bose-Einstein condensate to the field of an optical cavity. Using an optical heterodyne detection scheme, we observe symmetry breaking in real time and distinguish the two superradiant phases. We explore the process of symmetry breaking in the presence of a small symmetry-breaking field and study its dependence on the rate at which the critical point is crossed. Coherent switching between the two ordered phases is demonstrated.     

Non-equilibrium dynamical phases of the two-atom Dicke model

In this paper, we investigate the non-equilibrium dynamical phases of the two-atom Dicke model, which can be realized in a two species Bose–Einstein condensate interacting with a single light mode in an optical cavity. Apart from the usual non-equilibrium normal and inverted phases, a non-equilibrium mixed phase is possible which is a combination of normal and inverted phase. A new kind of dynamical phase transition is predicted from non-superradiant mixed phase to the superradiant phase which can be achieved by tuning the two different atom–photon couplings. We also show that a dynamical phase transition from the non-superradiant mixed phase to the superradiant phase is forbidden for certain values of the two atom–photon coupling strengths.     

Plasmonic Dicke effect

We study cooperative emission by an ensemble of emitters, such as fluorescing molecules or semiconductor quantum dots, near a metal nanoparticle. The primary mechanism of cooperative emission is resonant energy transfer between emitters and plasmons rather than Dicke radiative coupling between emitters. The emission is dominated by three superradiant states with the same quantum yield as a single emitter, leading to a drastic reduction of ensemble radiated energy down to just thrice of that by a single emitter, the remaining energy being dissipated in the metal through subradiant states. We perform numerical calculations of system eigenstates and find that the plasmonic Dicke effect interactions affect is not impacted by the interactions between emitters or non-radiative losses in the metal.     

Stability of symmetry breaking states in finite-size Dicke model with photon leakage

We investigate the finite-size Dicke model with photon leakage. It is shown that the symmetry breaking states, which are characterized by non-vanishing $〈\stackrel{?}{a}〉\ne 0$ and correspond to the ground states in the superradiant phase in the thermodynamic limit, are stable, while the eigenstates of the isolated finite-size Dicke Hamiltonian conserve parity symmetry. We introduce and analyze an effective master equation that describes the dynamics of a pair of the symmetry breaking states that are the degenerate lowest energy eigenstates in the superradiant region with photon leakage. It becomes clear that photon leakage is essential to stabilize the symmetry breaking states and to realize the superradiant phase without the thermodynamic limit. Our theoretical analysis provides an alternative interpretation using the finite-size model to explain results from cold atomic experiments showing superradiance with the symmetry breaking in an optical cavity.     

Experimental Study of Pulsed Optically Pumped Superradiant and Cavity NH3 Far-Infrared Laser

Using a grating tuned TEA-CO2 laser with 10R(8) and 9R(16) line to pumped superradiant NH3 far-infrared (FIR) laser which operated in the mode of amplified stimulated emission and Fabry-Perot cavity NH3 FIR laser with a sample tube of 100cm and 20cm in length respectively, the corresponding pulsed FIR lasers were obtained successfully. Meantime, the experimental measure method was improved. Besides, the operating performance and the spectral characteristics of superradiant and cavity NH3-OPFIRL were studied experimentally.     

Superradiance and subradiance in an inhomogeneously broadened ensemble of two-level systems coupled to a low-Q cavity

The collective spontaneous emission of a fully inverted inhomogeneously broadened ensemble of N two-level systems coupled to a single-mode low-Q cavity is investigated numerically using Monte Carlo wave function technique. An intrinsically bi-exponential emission dynamics is found when the time scales of superradiance tau(sr) and inhomogeneous dephasing T2* approximately 1/Deltaomega(inh) become comparable: a fast superradiant is followed by a slow subradiant decay. Experimental configurations using ensembles of quantum dots coupled to optical microcavities are proposed as possible candidates to observe the combined superradiant and subradiant energy relaxation.     

Superradiant emission of LaF<Subscript>3</Subscript>:Pr<Superscript>3+</Superscript> in resonator

Experimental data concerning superradiant emission from the LaF₃ medium doped with impurity praseodymium ions are presented in the cases of a free medium and when the medium is placed into an optical resonator. The spike structure of superradiance is registered and studied. When the medium is placed inside a cavity, a new channel of energy removal by superradiance related to the cavity mode appears; the old noncavity channels are preserved. The duration of superradiance in the cavity channel is decreased, and regular modulation arises. These peculiarities of superradiance induced by the presence of an optical resonator are explained.     

Experimental observation of photon echoes and power-efficiency analysis in a cavity environment

We present what is, to our best knowledge, the first experimental demonstration of echo signals generated from a sample in an optical cavity. The configuration allows us to use an optically thin sample and still have high effective absorption as well as relatively uniform excitation in the longitudinal direction within the sample. We observed echo signal power of up to twice that of certain input fields. The observed efficiency is much greater than the normally expected few percent. By using an input field encoded with a bit sequence, we also show that the cavity does not introduce significant distortion into the reproduced signal.     

Superradiance-Driven Phonon Laser

We propose to enhance the generation of a phonon laser by exploiting optical superradiance. In our scheme, the optomechanical cavity contains a movable membrane, which supports a mechanical mode, and the superradiance cavity can generate the coherent collective light emissions by applying a transverse pump to an ultracold intracavity atomic gas. The superradiant emission turns out to be capable of enhancing the phonon laser performance.This indicates a new way to operate a phonon laser with the assistance of coherent atomic gases trapped in a cavity or lattice potentials.     

Opto-Mechanical Effects in Superradiant Light Scattering by Bose-Einstein Condensate in an Optical Cavity

We investigate the effects of a movable mirror (cantilever) of an optical cavity on the superradiant light scattering from a Bose-Einstein condensate (BEC) in an optical lattice. We show that the mirror motion has a dynamic dispersive effect on the cavity-pump detuning. Varying the intensity of the pump beam, one can switch between the pure superradiant regime and the Bragg scattering regime. The mechanical frequency of the mirror strongly influences the time interval between two Bragg peaks. We find that when the system is in the resolved side band regime for mirror cooling, the superradiant scattering is enhanced due to coherent energy transfer from the mechanical mirror mode to the cavity field mode.     

Opto-Mechanical Effects in Superradiant Light Scattering by Bose-Einstein Condensate in an Optical Cavity

We investigate the effects of a movable mirror (cantilever) of an optical cavity on the superradiant light scattering from a Bose-Einstein condensate (BEC) in an optical lattice. We show that the mirror motion has a dynamic dispersive effect on the cavity-pump detuning. Varying the intensity of the pump beam, one can switch between the pure superradiant regime and the Bragg scattering regime. The mechanical frequency of the mirror strongly influences the time interval between two Bragg peaks. We find that when the system is in the resolved side band regime for mirror cooling, the superradiant scattering is enhanced due to coherent energy transfer from the mechanical mirror mode to the cavity field mode.     

Dicke Quantum Phase Transition for a Bose-Einstein Condensate in a Two-Mode Optical Cavity

In this paper we mainly discuss the ground state properties of the two-mode Dicke model, which is realized in an ensemble of two-level atoms interacting simultaneously with two quantized cavity fields. We reveal rich phase diagrams and discover the second-order quantum phase transition from the normal phase to the superradiant phase by means of the spin-coherent-state variational method. While the critical phase transition point can be shifted by the detuning of the cavity mode or the atom-field coupling imbalance parameter. The collective atom-field coupling imbalance parameter can make the phase transition point symmetrically shift left or right in the resonance or non-resonance. If the two collective atom-photon coupling strengths are not equal in the resonance, the system may be in different phases, while the phases occupied are completely symmetrical. When one of the coupling constants vanishes or two couplings are equal, the ground-states’s properties and related QPT reduce to that of a standard or an ordinary Dicke model.

     

Exchange dipole interaction in a multilevel cooperative system of atoms

Atomic systems with three or more equidistant energy levels, in which a cascade process is possible, are considered. Hamiltonians obtained for such systems are analogous to Heisenberg Hamiltonians, but for systems with integral spins. For Dicke states in multilevel systems, quantum-mechanical mean values of the energy of a cooperative system are calculated taking into account weak interactions between atoms. The type of emission preceding superradiant avalanche emission of the system is analyzed. It can be expected that a coherent state may be formed, in which collective processes affect one another not only via population of the intermediate common layer, but also via phasing of pure quantum states. The single superfluorescence pulse that can be formed in this case is not a simple superposition of two or more pulses of sequential superradiant transitions in two-level systems.     

Analysis of the effect of small-scale inhomogeneities on the electrophysical characteristics of a thin layer by intracavity laser spectroscopy

A mathematical model is constructed, which makes it possible to vary the characteristic sizes of roughness, the electrophysical parameters of the biological sample under investigation, and its geometrical characteristics and to establish the relations between these parameters and biological properties of the biological tissue being modeled, as well as to calculate theoretically the absorption spectra of optically thin biological samples placed into the cavity of an optical resonator.     

Holographic storage of multiple coherence gratings in a Bose-Einstein condensate

We demonstrate superradiant conversion between a two-mode collective atomic state and a single-mode light field in an elongated cloud of Bose-condensed atoms. Two off-resonant write beams induce superradiant Raman scattering, producing two independent coherence gratings with different wave vectors in the cloud. By applying phase-matched read beams after a controllable delay, the gratings can be selectively converted into the light field also in a superradiant way. Because of the large optical density and the small velocity width of the condensate, a high conversion efficiency of >70% and a long storage time of >120 micros were achieved.     

Quantum correlations among superradiant Bose-Einstein condensate atoms

Quantum correlations among atoms in superradiant Bose-Einstein condensates are discussed. It is shown that atoms in the superradiant atomic condensate can exhibit continuous variable quantum entanglement analogous to Einstein-Podolsky-Rosen (EPR)-type quantum correlations. Comparison to quantum entanglement in the Dicke model in thermal equilibrium is provided.