The macroscopic shape of liquid-crystalline elastomers strongly depends on the order parameter of the mesogenic groups. This order can be manipulated if photo-isomerisable groups, e.g. containing N=N bonds, are introduced into the material. We have explored the large photo-mechanical response of such an azobenzene-containing nematic elastomer at different temperatures, using force and optical birefringence measurements, and focusing on fundamental aspects of population dynamics and the related speed and repeatability of the response. The characteristic time of "on" and "off" regimes strongly depends on temperature, but is generally found to be very long. We were able to verify that the macroscopic relaxation of the elastomer is determined by the nematic order dynamics and not, for instance, by the polymer network relaxation.

We show that high squeezing and entanglement can be generated at the output of a cavity containing atoms interacting with two fields in a Coherent Population Trapping situation, on account of a non-linear Faraday effect experienced by the fields close to a dark-state resonance in a cavity. Moreover, the cavity provides a feedback mechanism allowing to reduce the quantum fluctuations of the ground state spin, resulting in strong steady state spin-squeezing. Close to the CPT (or dark-) resonance the fields experience a strong dispersive effect, but little absorption [3]. Consequently, several schemes have been studied which take advantage of this strong non-linearity to generate quantum correlations and squeezing in the fields under EIT or CPT conditions [7,8,9].Following the recent squeezed and entangled light states generation with cold atoms [10, 11], we study in this Letter the interaction of atoms inside an optical cavity with two field modes close to a dark-resonance. We first show that a multistable behavior for the intracavity light may occur close to the CPT resonance on account of a non-linear polarization self-rotation effect experienced by the light [12,13]. Such polarization instabilities have been observed with cold atoms [10] and thermal vapor cells [13]. We then calculate the field noise spectra and predict that strong correlations exist between the fields exiting the cavity and that squeezing and entanglement can be generated close to the switching threshold and for a wide range of parameters. In contrast with the experiments of Refs. [10,11,14] the squeezing and the entanglement are not deteriorated by excess atomic noise due to optical pumping processes and could be efficiently generated either with cold atoms or thermal vapor cells. Last, we show that under appropriate conditions on the cavity induced feedback the ground state atomic spin fluctuations may also be strongly squeezed, and subsequently read out using techniques developed within the context of quantum memory [15].We consider N Λ-like atoms with ground-states 1 and 2, interacting with two modes of the field, A 1 and A 2 . To simplify the discussion we turn to the symmetrical case of incident fields with equal power and will choose the parameters so that the Rabi frequencies of both transitions are equal: Ω i = Ω (i = 1, 2) and close to one-photon resonance. This situation corresponds to Coherent Population Trapping, since the atoms are pumped into a superposition of levels 1 and 2 -the so-called dark-statewhich is decoupled from the fields [2]. For Ω 1 = Ω 2 , this dark-state corresponds to a state with maximum ground state coherence: |D = (|1 − |2 )/ √ 2. Although this is not essential, let us note that, if levels 1 and 2 are Zeeman sublevels, the incident field can then be considered as being linearly polarized. As we will see later this facilitates the physical discussion and provides a simple picture in terms of the Stokes polarization vector and the collective spin formalism.If the fields are symmetrically detuned with respect t...

We investigate the mechanical properties of a static dense granular assembly in response to a local forcing. To this end, a small cyclic displacement is applied on a grain in the bulk of a 2D disordered packing under gravity and the displacement fields are monitored. We evidence a dominant long range radial response in the upper half part above the sollicitation and after a large number of cycles the response is 'quasi-reversible' with a remanent dissipation field exhibiting long range streams and vortex-like symmetry.

We propose a dynamical approach to quantum memories using an oscillator-cavity model. This overcomes the known difficulties of achieving high quantum input-output fidelity with long storage times compared to the input signal duration. We use a generic model of the memory response, which is applicable to any linear storage medium ranging from a superconducting device to an atomic medium. The temporal switching or gating of the device may either be through a control field changing the coupling, or through a variable detuning approach, as in more recent quantum memory experiments. An exact calculation of the temporal memory response to an external input is carried out. This shows that there is a mode-matching criterion which determines the optimum input and output pulse time evolution. This optimum pulse shape can be modified by changing the gate characteristics. In addition, there is a critical coupling between the atoms and the cavity that allows high fidelity in the presence of long storage times. The quantum fidelity is calculated both for the coherent state protocol, and for a completely arbitrary input state with a bounded total photon number. We show how a dynamical quantum memory can surpass the relevant classical memory bound, while retaining long storage times.