Entangling the motion of two optically trapped objects via time-modulated driving fields

We study entanglement of the motional degrees of freedom of two tethered and optically trapped microdisks inside a single cavity. By properly choosing the position of the trapped objects in the optical cavity and driving proper modes of the cavity it is possible to equip the system with linear and quadratic optomechanical couplings. We show that a parametric coupling between the fundamental vibrational modes of two tethered mircodiscs can be generated via a time modulated input laser. For a proper choice of the modulation frequency, this mechanism can drive the motion of the microdisks into an inseparable state in the long time limit via a two-mode squeezing process. We numerically confirm the performance of our scheme for current technology and briefly discuss an experimental setup which can be employed for detecting this entanglement by employing the quadratic coupling. We also comment on the perspectives for generating such entanglement between the oscillations of optically levitated nanospheres.Comment: 9 pages, 3 figure

The Bose-Hubbard model with localized particle losses

We consider the Bose-Hubbard model with particle losses at one lattice site. For the non-interacting case, we find that half of the bosons of an initially homogeneous particle distribution, are not affected by dissipation that only acts on one lattice site in the center of the lattice. A physical interpretation of this result is that the surviving particles interfere destructively when they tunnel to the location of the dissipative defect and therefore never reach it. Furthermore we find for a one-dimensional model that a fraction of the particles can propagate across the dissipative defect even if the rate of tunneling between adjacent lattice sites is much slower than the loss rate at the defect. In the interacting case, the phase coherence is destroyed and all particles eventually decay. We thus analyze the effect of small interactions and small deviations from the perfectly symmetric setting on the protection of the particles against the localized losses. A possible experimental realization of our setup is provided by ultracold bosonic atoms in an optical lattice, where an electron beam on a single lattice site ionizes atoms that are then extracted by an electrostatic field.Comment: 10 pages, 5 figures, minor revisions to previous versio

Floquet engineering in superconducting circuits: from arbitrary spin-spin interactions to the Kitaev honeycomb model

We derive a theory for the generation of arbitrary spin-spin interactions in superconducting circuits via periodic time modulation of the individual qubits or the qubit-qubit interactions. The modulation frequencies in our approach are in the microwave or radio frequency regime so that the required fields can be generated with standard generators. Among others, our approach is suitable for generating spin lattices that exhibit quantum spin liquid behavior such as Kitaev's honeycomb model.Comment: 21 pages, 9 figure

Interfirm co-operation and learning within SME networks - two cases from the Styrian Automotive cluster

Recent publications in the cluster-related literature have shown that interfirm links imply the potential to foster higher forms of learning within clusters. Especially networks deserve in this context a particular focus of attention. The purpose of this paper is to show essential conditions that should be present at cluster level in order to enable such forms of learning between the firms. This will be done in order to give a first advice for public and semi-public cluster institutions to facilitate interfirm collaborations and cluster related activities. Two case-studies of SME-networks selected from Styrian clusters will give the opportunity to get deeper insights into the conditions that enable clusters to bring forth double loop learning activities. In a first step particular criteria for the presence of double-loop learning will be established. In a second step the conditions for this specific type of learning will be dealt with. Among the categories of conditions that will be examined in detail are the relations and interactions in the network, the types of joint projects carried out between the firms and the specific organizational culture that prevails at the individual firm level. Key words: SME-networks, learning, organizational culture.

Migration of bosonic particles across a Mott insulator to superfluid phase interface

We consider a boundary between a Mott insulator and a superfluid region of a Bose-Hubbard model at unit filling. Initially both regions are decoupled and cooled to their respective ground states. We show that, after switching on a small tunneling rate between both regions, all particles of the Mott region migrate to the superfluid area. This migration takes place whenever the difference between the chemical potentials of both regions is less than the maximal energy of any eigenmode of the superfluid. We verify our results numerically with DMRG simulations and explain them analytically with a master equation approximation, finding good agreement between both approaches. Finally we carry out a feasibility study for the observation of the effect in coupled arrays of micro-cavities and optical lattices.Comment: 5 pages, 6 figures, to appear in Phys. Rev. Let

Material and immaterial dimensions of clusters. Cooperation and learning as infrastructure for innovation

The paper concentrates on forms of cooperation in a learning context and presents theory-based empirical results of interactive learning processes in different clusters. A general outline of institutional aspects of clusters and networks is given and more specific theories of interactive learning are focussed. An extensive comparison of forms of such learning processes is undertaken, and finally policy conclusions are be drawn.

Correlator expansion approach to stationary states of weakly coupled cavity arrays

We introduce a method for calculating the stationary state of a translation invariant array of weakly coupled cavities in the presence of dissipation and coherent as well as incoherent drives. Instead of computing the full density matrix our method directly calculates the correlation functions which are relevant for obtaining all local quantities of interest. It considers an expansion of the correlation functions and their equations of motion in powers of the photon tunneling rate between adjacent cavities, leading to an exact second order solution for any number of cavities. Our method provides a controllable approximation for weak tunneling rates applicable to the strongly correlated regime that is dominated by nonlinearities in the cavities and thus of high interest.Comment: contribution to J. Phys. B special issue celebrating Jaynes-Cummings physic

Nonclassical Radiation from Thermal Cavities in the Ultrastrong Coupling Regime

Thermal or chaotic light sources emit radiation characterized by a slightly enhanced probability of emitting photons in bunches, described by a zero-delay second-order correlation function $g^{(2)}(0) = 2$. Here we explore photon-coincidence counting statistics of thermal cavities in the ultrastrong coupling regime, where the atom-cavity coupling rate becomes comparable to the cavity resonance frequency. We find that, depending on the system temperature and coupling rate, thermal photons escaping the cavity can display very different statistical behaviors, characterised by second-order correlation functions approaching zero or greatly exceeding two.Comment: results on frequency resolved photon correlations added, to appear in Phys. Rev. Let