arXiv subjects

Helmut Ritsch

Publications and source records attributed to Helmut Ritsch.

At least 109 records · Page 6Linked to original sources

arXiv2014

Self-ordering and collective dynamics of transversely illuminated point-scatterers in a 1D trap

We study point-like polarizable particles confined in a 1D very elongated trap within the evanescent field of an optical nano-fiber or nano-structure. When illuminated transversely by coherent light, collective light scattering into propagating fiber modes induces long range interactions and eventually crystallisation of the particles into regular order. We develop a simple and intuitive scattering-matrix based approach to study these long-range interactions by collective scattering and the resulting light-induced self-ordering. For few particles we derive explicit conditions for self-consistent stable ordering. In the purely dispersive limit with negligible back-scattering, we recover the prediction of an equidistant lattice as previously found for effective dipole-dipole interaction models. We generalize our model to experimentally more realistic configurations including backscattering, absorption and a directional scattering asymmetry. For larger particle ensembles the resulting self-consistent particle-field equations can be numerically solved to study the formation of long-range order and stability limits.

physics.optics↗

arXiv2014

Self-ordering dynamics of ultracold atoms in multicolored cavity fields

We study light induced spatial crystallization of ultracold quantum particles confined along the axis of a high-$Q$ linear cavity via a transverse multicolor pump using numerical simulations. Whenever a pump frequency is tuned close to resonance with a longitudinal cavity mode, the dynamics favors bistable spatial particle ordering into a Bragg grating at a wavelength distance. Simultaneous pumping at several resonant frequencies fosters competition between the different spatial lattice orders, exhibiting complex nonlinear field dynamics involving several metastable atom-field states. For few particles even superpositions of different spatial orders entangled with different light mode amplitudes appear. By a proper choice of trap geometry and pump frequencies a broad variety of many particle Hamiltonians with a nontrivial long range coupling can be emulated in such a setup. When applying quantum Monte Carlo wave function simulations to study time evolution we find simultaneous super radiant scattering into several light modes and the buildup of strong non-classical atom field correlations.

quant-ph↗

arXiv2014

Optimized multifrequency light collection by adaptive self-ordering of scatterers in optical resonators

Mobile light scatterers in a high-Q optical cavity transversely illuminated by laser light close to a cavity resonance form ordered patterns, which maximize light scattering into the cavity and induce optical self-trapping. We show that a generalized form of such crystallization dynamics appears in multicolored pump fields with several cavity modes. Here the particles arrange in spatial patterns maximizing total light collection into the resonator. For changing input frequencies and strengths the particles dynamically adapt to the current illumination. Interestingly the system keeps some memory on past configurations, so that a later renewed application of the same pattern exhibits faster adaptation towards optimal collective scattering. In a noisy environment particles explore larger regions of configuration space spending most of the time close to optimum scattering configurations. This adaptive self-ordering dynamics should be implementable in a wide range of systems ranging from cold atoms in multimode cavities or nano-fiber traps to molecules or mobile nano-particles within an optical resonator.

quant-ph↗

arXiv2014

A superradiant clock laser on a magicwavelength optical lattice

An ideal superradiant laser on an optical clock transition of noninteracting cold atoms is predicted to exhibit an extreme frequency stability and accuracy far below mHz-linewidth. In any concrete setup sufficiently many atoms have to be confined and pumped within a finite cavity mode volume. Using a magic wavelength lattice minimizes light shifts and allows for almost uniform coupling to the cavity mode. Never- theless, the atoms are subject to dipole-dipole interaction and collective spontaneous decay which compromises the ultimate frequency stability. In the high density limit the Dicke superradiant linewidth enhancement will broaden the laser line and nearest neighbor couplings will induce shifts and fluctuations of the laser frequency. We estimate the magnitude and scaling of these effects by direct numerical simulations of few atom systems for different geometries and densities. For Strontium in a regularly filled magic wavelength configuration atomic interactions induce small laser frequency shifts only and collective spontaneous emission weakly broadens the laser. These interactions generally enhance the laser sensitivity to cavity length fluctuations but for optimally chosen operating conditions can lead to an improved synchronization of the atomic dipoles.

quant-ph↗

arXiv2014

Scattering approach to two-colour light forces and self-ordering of polarizable particles

Collective coherent scattering of laser light by an ensemble of polarizable point particles creates long range interactions, whose properties can be tailored by choice of injected laser powers, frequencies and polarizations. We use a transfer matrix approach to study the forces induced by non-interfering fields of orthogonal polarization or different frequencies in a 1D geometry and find long range self-ordering of particles without a prescribed order. Adjusting laser frequencies and powers allows to tune inter-particle distances and provides a wide range of possible dynamical couplings not accessible in usual standing light wave geometries with prescribed order. In this work we restrict the examples to two frequencies and polarisations but the framework also allows to treat multicolour light beams with random phases. These dynamical effects should be observable in existing experimental setups with effective 1D geometries such as atoms or nanoparticles coupled to the field of an optical nanofibre or transversely trapped in counterpropagating Gaussian beams.

quant-ph↗

arXiv2013

Subrecoil cavity cooling towards degeneracy: A numerical study

We present a detailed numerical analysis of the temperature limit and timescale of cavity cooling of a dilute gas in the quantum regime for particles and light. For a cavity with a linewidth smaller than the recoil frequency efficient cooling towards quantum degeneracy is facilitated by applying a tailored sequence of laser pulses transferring the particles towards lower momenta. Two-particle Monte Carlo wave function simulations reveal strongly improved cooling properties for a ring versus a standing-wave geometry. Distinct quantum correlations and cooling limits for bosons and fermions demonstrate quantum statistical effects. In particular, in ring cavities the photon-mediated long-range interaction favours momentum-space pairing of bosons, while fermion pairs exhibit anti-correlated or uncorrelated momenta. The results are consistent with recent experiments and give encouraging prospects to achieve sufficient conditions for the condensation of a wide class of polarisable particles via cavity cooling.

quant-ph↗

arXiv2013

Seeding patterns for self-organization of photons and atoms

When atoms scatter photons from a transverse laser into a high-finesse optical cavity, they form crystalline structures which maximize the intracavity light field and trap the atoms in the ordered array. Stable organization occurs when the laser field amplitude exceeds a certain threshold. For planar single-mode cavities there exist two equivalent possible atomic patterns, which determine the phase of the intracavity light field. Under these premises, we show that the effect of an additional laser pumping the cavity makes one pattern more favorable than the other and that it can dynamically force the system into a predetermined configuration. This is an instance of pattern formation and seeding in a nonlinear quantum-optical system.

quant-ph↗

arXiv2013

Attractive Optical Forces from Blackbody Radiation

Blackbody radiation around hot objects induces ac Stark shifts of the energy levels of nearby atoms and molecules. These shifts are roughly proportional to the fourth power of the temperature and induce a force decaying with the third power of the distance from the object. We explicitly calculate the resulting attractive blackbody optical dipole force for ground state hydrogen atoms. Surprisingly, this force can surpass the repulsive radiation pressure and actually pull the atoms against the radiation energy flow towards the surface with a force stronger than gravity. We exemplify the dominance of the "blackbody force" over gravity for hydrogen in a cloud of hot dust particles. This overlooked force appears relevant in various astrophysical scenarios, in particular, since analogous results hold for a wide class of other broadband radiation sources.

quant-ph↗

arXiv2013

Protected state enhanced quantum metrology with interacting two-level ensembles

Ramsey interferometry is routinely used in quantum metrology for the most sensitive measurements of optical clock frequencies. Spontaneous decay to the electromagnetic vacuum ultimately limits the interrogation time and thus sets a lower bound to the optimal frequency sensitivity. In dense ensembles of two-level systems the presence of collective effects such as superradiance and dipole-dipole interaction tends to decrease the sensitivity even further. We show that by a redesign of the Ramsey-pulse sequence to include different rotations of individual spins that effectively fold the collective state onto a state close to the center of the Bloch sphere, partial protection from collective decoherence and dephasing is possible. This allows a significant improvement in the sensitivity limit of a clock transition detection scheme over the conventional Ramsey method for interacting systems and even for non-interacting decaying atoms.

quant-ph↗

arXiv2013

Light induced crystallization of cold atoms in a thin 1D optical tube

Collective off resonant scattering of coherent light by a cold gas induces long-range interactions via interference of light scattered by different particles. In a 1D configuration these interactions grow particularly strong for particles trapped along an optical nanofiber. We show that there exists a threshold pump laser intensity, above which the gas can be found in a crystalline, selfsustained order. In the nonabsorbing regime we determine the critical condition for the onset of order as well as the forms of particle density and scattered field patterns along the fiber above threshold. Surprisingly, there can coexist multiple stationary solutions with distinct density and field profiles.

quant-ph↗

arXiv2013

Excitons and Cavity Polaritons for Optical Lattice Ultracold Atoms

Ultracold atoms uniformly filling an optical lattice can be treated like an artificial crystal. An implementation including the atomic occupation of a single excited atomic state can be represented by a two-component Bose-Hubbard model. Its phase diagram exhibits a quantum phase transition from a superfluid to a Mott insulator phase. The dynamics of electronic excitations governed by electrostatic dipole-dipole interactions in the ordered regime can be well described by wave-like collective excitations called excitons. Here we present an extensive study of such excitons for a wide range of geometries and dimensionality. Their lifetimes can vary over many orders of magnitude from metastable propagation to superradiant decay. Particularly strong effects occur in one dimensional atomic chains coupled to tapered optical fibers. For an optical lattice within a cavity the excitons are coupled to cavity photons and the resulting collective cavity QED model can be efficiently formulated in terms of polaritons. Their properties are explicitly calculated for different lattices and they constitute a non-destructive monitoring tool for important system properties. Even the formation of molecules in optical lattices manifests itself in modified polariton properties as e.g. an anisotropic optical spectrum. Partial dissipation of the exciton energy in the lattice leads to heating, which can be microscopically understood through a mechanism transferring atoms into higher Bloch bands via a resonant excitation transfer among neighboring lattice sites. The presence of lattice defects like vacancies in the Mott insulator induces a characteristic scattering of polaritons, which can be optically observed to monitor the lattice integrity. Our models can be applied to simulate and understand corresponding collective phenomena in solid crystals, where many effects are often masked by noise and disorder.

cond-mat.quant-gas↗

arXiv2012

Quantum-correlated motion and heralded entanglement of distant optomechanically coupled objects

The motion of two distant trapped particles or mechanical oscillators can be strongly coupled by light modes in a high finesse optical resonator. In a two mode ring cavity geometry, trapping, cooling and coupling is implemented by the same modes. While the cosine mode provides for trapping, the sine mode facilitates ground state cooling and mediates non-local interactions. For classical point particles the centre-of-mass mode is strongly damped and the individual momenta get anti-correlated. Surprisingly, quantum fluctuations induce the opposite effect of positively-correlated particle motion, which close to zero temperature generates entanglement. The non-classical correlations and entanglement are dissipation-induced and particularly strong after detection of a scattered photon in the sine mode. This allows for heralded entanglement by post-selection. Entanglement is concurrent with squeezing of the particle distance and relative momenta, while the centre-of-mass observables acquire larger uncertainties.

quant-ph↗

arXiv2012

Optomechanical deformation and strain in elastic dielectrics

Light forces induced by scattering and absorption in elastic dielectrics lead to local density modulations and deformations. These perturbations in turn modify light propagation in the medium and generate an intricate nonlinear response. We generalise an analytic approach where light propagation in one-dimensional media of inhomogeneous density is modelled as a result of multiple scattering between polarizable slices. Using the Maxwell stress tensor formalism we compute the local optical forces and iteratively approach self-consistent density distributions where the elastic back-action balances gradient- and scattering forces. For an optically trapped dielectric we derive the nonlinear dependence of trap position, stiffness and total deformation on the object's size and field configuration. Generally trapping is enhanced by deformation, which exhibits a periodic change between stretching and compression. This strongly deviates from qualitative expectations based on the change of photon momentum of light crossing the surface of a dielectric. We conclude that optical forces have to be treated as volumetric forces and that a description using the change of photon momentum at the surface of a medium is inappropriate.

physics.optics↗

arXiv2012

Cold atoms in cavity-generated dynamical optical potentials

We review state-of-the-art theory and experiment of the motion of cold and ultracold atoms coupled to the radiation field within a high-finesse optical resonator in the dispersive regime of the atom-field interaction with small internal excitation. The optical dipole force on the atoms together with the back-action of atomic motion onto the light field gives rise to a complex nonlinear coupled dynamics. As the resonator constitutes an open driven and damped system, the dynamics is non-conservative and in general enables cooling and confining the motion of polarizable particles. In addition, the emitted cavity field allows for real-time monitoring of the particle's position with minimal perturbation up to sub-wavelength accuracy. For many-body systems, the resonator field mediates controllable long-range atom-atom interactions, which set the stage for collective phenomena. Besides correlated motion of distant particles, one finds critical behavior and non-equilibrium phase transitions between states of different atomic order in conjunction with superradiant light scattering. Quantum degenerate gases inside optical resonators can be used to emulate opto-mechanics as well as novel quantum phases like supersolids and spin glasses. Non-equilibrium quantum phase transitions, as predicted by e.g. the Dicke Hamiltonian, can be controlled and explored in real-time via monitoring the cavity field. In combination with optical lattices, the cavity field can be utilized for non-destructive probing Hubbard physics and tailoring long-range interactions for ultracold quantum systems.

cond-mat.quant-gas↗

arXiv2012

REVIEW. Quantum optics with ultracold quantum gases: towards the full quantum regime of the light-matter interaction

Although the study of ultracold quantum gases trapped by light is a prominent direction of modern research, the quantum properties of light were widely neglected in this field. Quantum optics with quantum gases closes this gap and addresses phenomena, where the quantum statistical nature of both light and ultracold matter play equally important roles. First, light can serve as a quantum nondemolition (QND) probe of the quantum dynamics of various ultracold particles from ultracold atomic and molecular gases to nanoparticles and nanomechanical systems. Second, due to dynamic light-matter entanglement, projective measurement-based preparation of the many-body states is possible, where the class of emerging atomic states can be designed via optical geometry. Light scattering constitutes such a quantum measurement with controllable measurement back-action. As in cavity-based spin squeezing, atom number squeezed and Schroedinger cat states can be prepared. Third, trapping atoms inside an optical cavity one creates optical potentials and forces, which are not prescribed but quantized and dynamical variables themselves. Ultimately, cavity QED with quantum gases requires a self-consistent solution for light and particles, which enriches the picture of quantum many-body states of atoms trapped in quantum potentials. This will allow quantum simulations of phenomena related to the physics of phonons, polarons, polaritons and other quantum quasiparticles.

cond-mat.quant-gas↗

arXiv2011

Kinetic theory of cavity cooling and self-organisation of a cold gas

We study spatial self-organisation and dynamical phase-space compression of a dilute cold gas of laser-illuminated polarisable particles in an optical resonator. Deriving a non-linear Fokker--Planck equation for the particles' phase-space density allows us to treat arbitrarily large ensembles in the far-detuning limit and explicitly calculate friction forces, momentum diffusion and steady-state temperatures. In addition, we calculate the self-organisation threshold in a self-consistent analytic form. For a homogeneous ensemble below threshold the cooling rate for fixed laser power is largely independent of the particle number. Cooling leads to a $q$-Gaussian velocity distribution with a steady-state temperature determined by the cavity linewidth. Numerical simulations using large ensembles of particles confirm the analytical threshold condition for the appearance of an ordered state, where the particles are trapped in a periodic pattern and can be cooled to temperatures close to a single vibrational excitation.

quant-ph↗

arXiv2011

Cavity QED with magnetically coupled collective spin states

We report strong coupling between an ensemble of nitrogen-vacancy center electron spins in diamond and a superconducting microwave coplanar waveguide resonator. The characteristic scaling of the collective coupling strength with the square root of the number of emitters is observed directly. Additionally, we measure hyperfine coupling to 13C nuclear spins, which is a first step towards a nuclear ensemble quantum memory. Using the dispersive shift of the cavity resonance frequency, we measure the relaxation time T1 of the NV center at millikelvin temperatures in a non-destructive way.

cond-mat.mes-hall↗

arXiv2011

Optical Properties of Collective Excitations for Finite Chains of Trapped Atoms

Resonant dipole-dipole interaction modifies the energy and decay rate of electronic excitations for finite one dimensional chains of ultracold atoms in an optical lattice. We show that collective excited states of the atomic chain can be divided into dark and bright modes, where a superradiant mode with an enhanced collective effective dipole dominates the optical scattering. Studying the generic case of two chain segments of different length and position exhibits an interaction blockade and spatially structured light emission. Ultimately, an extended system of several interfering segments models a long chain with randomly distributed defects of vacant sites. The corresponding emission pattern provides a sensitive tool to study structural and dynamical properties of the system.

quant-ph↗