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Jonas Ola Oscar LarsonUniversitetslektor

Om mig

I huvudsak kretsar min forskning runt teoretisk kvantmekanik, från enskilda partiklar till mångpartikelsystem där kollektiva egenskaper blir viktiga. Jag anvnder mig ofta av verktyg utvecklade inom kvantinforamtion. Under senare år har jag fokuserat rätt mycket på s.k. öppna kvantsystem och kritiska egenskaper i dessa.

Undervisning

I nuläget undervisar jag kursen Analytisk mekanik (FK7049) som ges varje vår på fysikum, främst för masterstudenter. Kursen täcker standardmatierialet för motsvarande kurs, se https://link.springer.com/book/10.1007/978-3-030-34882-3

Jag är dessutom lärare på Kollokviumkursen for doktorander samt sitter med i kommitén som utvärderar alla bacheloruppsatser.

Forskning

  • Quantum optics - light matter interaction Quantum optics is a field studying how light and matter interact when both subsystems are treated quantum mechanically. One strategy to experimentally reach the regimes where a full quantum description is essential is to use atoms confined within high-quality optical resonators. It is here possible to single out single atomic electronic transitions as well as single photon cavity modes. These systems are well described by various types of Jaynes-Cummings-like models. In modern times the atoms can be replaced by superconducting devices and the resonators by "transmission line resonators", such that everything can be contained on electronic chips. These are typically what is employed by Google and others in order to build early versions of quantum comupters. I have approached these systems in somewhat unconeventional views, like thinking of them in terms simple "molecules" where the photon degrees-of-freedom are serving as vibrational phonons of a molecule. Recently I am also describing these models as exotic lattice models with interesting topological properties.
  • Quantum phase transitions and quantum simulators Traditionally, a continous phase transition is accompamied by a spontaneous symmetry breaking; in the normal phase the state is symmetric with a vanishing order parameter, while in the symmetry broken phase it has a non-zero order parameter that specifies the broken symmetry. The presence of such phase transitions in the clssical world are due to thermal fluctuations that cause the state to spontaneously pick a "direction". Quantum systems possess inherent fluctuations thanks to the Heisenberg uncertainty principle, and it turns out that these can also cause spontaneous symmetry breaking - quantum phase transitions. A relatively new field is to study well known quantum many-body models in the realm of highly controllable experimentally relevant systems. These are tailored quantum systems that serve the purpose of simulating another system that is difficult to access experientally - a quantum simulator. In a way it is type of a quantum comuter, however not a universal one. Quantum simulators are often considered when studying quantum critical models (phase transitions), for example to map out the phase diagram of some interesting Hamiltonian. We consider different realizations of quantum simulators, but one particularly important one is formed from ultracold atoms held in optical lattices. These are very robust, clean and versatile systems, ideal for realizing quantum simulators.
  • Open quantum systems Quantum physics provides many advantages over classical physics, i.e. entanglement and superpositions. However, it comes with a high prize; these properties are extremely fragile for and imperfections. The greatest difficulty being the coupling of the system to its surrounding environment. This invitebly leads to decoherence and the loss of ''quantumness''. To describe the effects of an environment one must give up unitary time-evolution, and work with mixed states. We try to build a deeper understaing for the novel physics emerging from such non-unitary evolution, and what new phases of matter that may exist.

Publikationer

I urval från Stockholms universitets publikationsdatabas

  • Magnetic phases of orbital bipartite optical lattices

    2020. Pil Saugmann, Jonas Larson. New Journal of Physics 22 (2)

    Artikel

    In the Hamburg cold atom experiment with orbital states in an optical lattice, s- and p-orbital atomic states hybridize between neighboring sites. In this work we show how this alternation of sites hosting s- and p-orbital states gives rise to a plethora of different magnetic phases, quantum and classical. We focus on phases whose properties derive from frustration originating from a competition between nearest and next nearest neighboring exchange interactions. The physics of the Mott insulating phase with unit filling is described by an effective spin-1/2 Hamiltonian showing great similarities with the J(1)-J(2) model. Based on the knowledge of the J(1)-J(2) model, supported by numerical simulations, we discuss the possibility of a quantum spin liquid phase in the present optical lattice system. In the superfluid regime we consider the parameter regime where the s-orbital states can be adiabatically eliminated to give an effective model for the p-orbital atoms. At the mean-field level we derive a generalized classical XY model, and show that it may support maximum frustration. When quantum fluctuations can be disregarded, the ground state should be a spin glass.

    Läs mer om Magnetic phases of orbital bipartite optical lattices
  • Monitoring the resonantly driven Jaynes-Cummings oscillator by an external two-level emitter

    2020. Themistoklis K. Mavrogordatos, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 101 (5)

    Artikel

    We address the consequences of back action in the unidirectional coupling of two cascaded open quantum subsystems connected to the same reservoir at different spatial locations. In the spirit of H. J. Carmichael [Phys. Rev. Lett. 70, 2273 (1993)], the second subsystem is a two-level atom, while the first transforms from a driven empty cavity to a perturbative QED configuration and ultimately to a driven Jaynes-Cummings (JC) oscillator through a varying light-matter coupling strength. For our purpose, we appeal at first to the properties of resonance fluorescence in the statistical description of radiation emitted along two channels-those of forward and sideways scattering-comprising the monitored output. In the simplest case of an empty cavity coupled to an external atom, we derive analytical results for the nonclassical fluctuations in the fields occupying the two channels, pursuing a mapping to the bad-cavity limit of the JC model to serve as a guide for the description of the more involved dynamics. Finally, we exemplify a conditional evolution for the composite system of a critical JC oscillator on resonance coupled to an external monitored two-level target, showing that coherent atomic oscillations of the target probe the onset of a second-order dissipative quantum phase transition in the source.

    Läs mer om Monitoring the resonantly driven Jaynes-Cummings oscillator by an external two-level emitter
  • Superradiance, bosonic Peierls distortion, and lattice gauge theory in a generalized Rabi-Hubbard chain

    2020. Axel Gagge, Jonas Larson. Physical Review A: covering atomic, molecular, and optical physics and quantum information 102 (6)

    Artikel

    We investigate a one-dimensional Rabi-Hubbard type of model, arranged such that a quantum dot is sandwiched between every cavity. The role of the quantum dot is twofold, to transmit photons between neighboring cavities and simultaneously act as an effective photon nonlinearity. We consider three-level quantum dots in the A configuration, where the left and right leg couple exclusively to the left or right cavity. This noncommuting interaction leads to two highly entangled incompressible phases, separated by a second-order quantum phase transition; the degrees of freedom of the quantum dots can be viewed as a dynamical lattice for the photons which spontaneously breaks Z(2) symmetry due to a bosonic Peierls instability, leading to a phase with dimerized order. Additionally, we find a normal insulating phase and a superfluid phase that acts as a quantum many-body superradiant phase. In the superradiant phase, a Z(2) symmetry is broken and the phase transition falls within the universality class of the transverse-field Ising model. Finally, we show that the model can be interpreted as a Z(2) lattice gauge theory in the absence of a dipolar field on the lower qutrit levels.

    Läs mer om Superradiance, bosonic Peierls distortion, and lattice gauge theory in a generalized Rabi-Hubbard chain
  • Bloch-like energy oscillations

    2018. Axel Gagge, Jonas Larson. Physical Review A: covering atomic, molecular, and optical physics and quantum information 98 (5)

    Artikel

    We identify a type of periodic evolution that appears in driven quantum systems. Provided that the instantaneous (adiabatic) energies are equidistant we show how such systems can be mapped to (time-dependent) tilted single-band lattice models. Having established this mapping, the dynamics can be understood in terms of Bloch oscillations in the instantaneous energy basis. In our lattice model the site-localized states are the adiabatic ones, and the Bloch oscillations manifest as a periodic repopulation among these states, or equivalently a periodic change in the system's instantaneous energy. Our predictions are confirmed by considering two different models: a driven harmonic oscillator and a Landau-Zener grid model. To strengthen the link between our energy Bloch oscillations and the original spatial Bloch oscillations we add a random disorder that breaks the translational invariance of the spectrum. This verifies that the oscillating evolution breaks down and instead turns into a diffusive spreading. Finally, we consider a trapped ion setup and demonstrate how the mechanism can be utilized to prepare motional cat state of the ion.

    Läs mer om Bloch-like energy oscillations
  • Disordered cold atoms in different symmetry classes

    Fernanda Pinheiro, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics

    Artikel

    We consider an experimentally realizable model of non-interacting but randomly coupled atoms in a two-dimensional optical lattice. By choosing appropriate real or complex-valued random fields and species-dependent energy offsets, this system can be used to analyze effects of disorder in four different classes: The chiral BDI and AIII, and the A and AI symmetry classes. These chiral classes are known to support a metallic phase at zero energy, which here, due to the inevitable finite size of the system, should also persist in a neighborhood of non-zero energies. As we discuss, this is of particular interest for experiments involving quenches. Away from the centre of the spectrum, we find that excitations appear as domain walls in the cases with time-reversal symmetry, or as vortices in the cases where time-reversal symmetry is absent. Therefore, a quench in a system with uniform density would lead to the formation of either vortices or domain walls depending on the symmetry class. For the non-chiral models in the A and AI classes, a population imbalance between the two atomic species naturally occurs. In these cases, one of the two species is seen to favour a more uniform density. We also study the onset of localization as the disorder strength is increased for the different classes, and by deriving an effective model for the non-chiral cases we show how their eigenstates remain extended for larger values of the coupling with the disorder, if compared to the non-chiral ones.

    Läs mer om Disordered cold atoms in different symmetry classes
  • Dissipation-driven quantum phase transitions and symmetry breaking

    2018. Julia Hannukainen, Jonas Larson. Physical Review A: covering atomic, molecular, and optical physics and quantum information 98 (4)

    Artikel

    By considering a solvable driven-dissipative quantum model, we demonstrate that continuous phase transitions in dissipative systems may occur without an accompanying symmetry breaking. As such, the underlying mechanism for this type of transition is qualitatively different from that of continuous equilibrium phase transitions. In our model, the transition is solely a result of the interplay between Hamiltonian and dissipative dynamics and manifests as a nonanalyticity in the steady state. (rho) over cap (ss) in the thermodynamic limit. Based on knowledge from critical classical models we suggest that this behavior derives from a rounding of a first-order phase transition into a continuous one due to large environment-induced fluctuations. Despite being conceptually different from the traditional continuous transitions, we show that expectations of local observables can still be characterized by a set of critical exponents.

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  • Liouvillian of the Open STIRAP Problem

    2018. Thomas Mathisen, Jonas Larson. Entropy 20 (1)

    Artikel

    With the corresponding Liouvillian as a starting point, we demonstrate two seemingly new phenomena of the STIRAP problem when subjected to irreversible losses. It is argued that both of these can be understood from an underlying Zeno effect, and in particular both can be viewed as if the environment assists the STIRAP population transfer. The first of these is found for relative strong dephasing, and, in the language of the Liouvillian, it is explained from the explicit form of the matrix generating the time-evolution; the coherence terms of the state decay off, which prohibits further population transfer. For pure dissipation, another Zeno effect is found, where the presence of a non-zero Liouvillian gap protects the system's (adiabatic) state from non-adiabatic excitations. In contrast to full Zeno freezing of the evolution, which is often found in many problems without explicit time-dependence, here, the freezing takes place in the adiabatic basis such that the system still evolves but adiabatically.

    Läs mer om Liouvillian of the Open STIRAP Problem
  • Some remarks on 'superradiant' phase transitions in light-matter systems

    2017. Jonas Larson, Elinor K. Irish. Journal of Physics A 50 (17)

    Artikel

    In this paper we analyze properties of the phase transition that appears in a set of quantum optical models; Dicke, Tavis-Cummings, quantum Rabi, and finally the Jaynes-Cummings model. As the light-matter coupling is increased into the deep strong coupling regime, the ground state turns from vacuum to become a superradiant state characterized by both atomic and photonic excitations. It is pointed out that all four transitions are of the mean-field type, that quantum fluctuations are negligible, and hence these fluctuations cannot be responsible for the corresponding vacuum instability. In this respect, these are not quantum phase transitions. In the case of the Tavis-Cummings and Jaynes-Cummings models, the continuous symmetry of these models implies that quantum fluctuations are not only negligible, but strictly zero. However, all models possess a non-analyticity in the ground state in agreement with a continuous quantum phase transition. As such, it is a matter of taste whether the transitions should be termed quantum or not. In addition, we also consider the modifications of the transitions when photon losses are present. For the Dicke and Rabi models these non-equilibrium steady states remain critical, while the criticality for the open Tavis-Cummings and Jaynes-Cummings models is completely lost, i. e. in realistic settings one cannot expect a true critical behaviour for the two last models.

    Läs mer om Some remarks on 'superradiant' phase transitions in light-matter systems
  • Quantum state engineering in hybrid open quantum systems

    2016. Chaitanya Joshi, Jonas Larson, Timothy P. Spiller. Physical Review A 93 (4)

    Artikel

    We investigate a possibility to generate nonclassical states in light-matter coupled noisy quantum systems, namely, the anisotropic Rabi and Dicke models. In these hybrid quantum systems, a competing influence of coherent internal dynamics and environment-induced dissipation drives the system into nonequilibrium steady states (NESSs). Explicitly, for the anisotropic Rabi model, the steady state is given by an incoherent mixture of two states of opposite parities, but as each parity state displays light-matter entanglement, we also find that the full state is entangled. Furthermore, as a natural extension of the anisotropic Rabi model to an infinite spin subsystem, we next explored the NESS of the anisotropic Dicke model. The NESS of this linearized Dicke model is also an inseparable state of light and matter. With an aim to enrich the dynamics beyond the sustainable entanglement found for the NESS of these hybrid quantum systems, we also propose to combine an all-optical feedback strategy for quantum state protection and for establishing quantum control in these systems. Our present work further elucidates the relevance of such hybrid open quantum systems for potential applications in quantum architectures.

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  • Cavity-Assisted Generation of Sustainable Macroscopic Entanglement of Ultracold Gases

    2015. Chaitanya Joshi, Jonas Larson. Atoms 3 (3), 348-366

    Artikel

    Prospects for reaching persistent entanglement between two spatially-separated atomic Bose-Einstein condensates are outlined. The system setup comprises two condensates loaded in an optical lattice, which, in return, is confined within a high-Q optical resonator. The system is driven by an external laser that illuminates the atoms, such that photons can scatter into the cavity. In the superradiant phase, a cavity field is established, and we show that the emerging cavity-mediated interactions between the two condensates is capable of entangling them despite photon losses. This macroscopic atomic entanglement is sustained throughout the time-evolution apart from occasions of sudden deaths/births. Using an auxiliary photon mode and coupling it to a collective quadrature of the two condensates, we demonstrate that the auxiliary mode's squeezing is proportional to the atomic entanglement, and as such, it can serve as a probe field of the macroscopic entanglement.

    Läs mer om Cavity-Assisted Generation of Sustainable Macroscopic Entanglement of Ultracold Gases
  • Dicke-type phase transition in a multimode optomechanical system

    2015. Jesse Mumford, D. H. J. O'Dell, Jonas Larson. Annalen der Physik 527 (1-2), 115-130

    Artikel

    We consider the membrane in the middle optomechanical model consisting of a laser pumped cavity which is divided in two by a flexible membrane that is partially transmissive to light and subject to radiation pressure. Steady state solutions at the mean-field level reveal that there is a critical strength of the light-membrane coupling above which there is a symmetry breaking bifurcation where the membrane spontaneously acquires a displacement either to the left or the right. This bifurcation bears many of the signatures of a second order phase transition and we compare and contrast it with that found in the Dicke model. In particular, by studying limiting cases and deriving dynamical critical exponents using the fidelity susceptibility method, we argue that the two models share very similar critical behaviour. For example, the obtained critical exponents indicate that they fall within the same universality class. Away from the critical regime we identify, however, some discrepancies between the two models. Our results are discussed in terms of experimentally relevant parameters and we evaluate the prospects for realizing Dicke-type physics in these systems.

    Läs mer om Dicke-type phase transition in a multimode optomechanical system
  • Disordered cold atoms in different symmetry classes

    2015. Fernanda Pinheiro, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 92 (2)

    Artikel

    We consider an experimentally realizable model of noninteracting but randomly coupled atoms in a two-dimensional optical lattice. By choosing appropriate real or complex-valued random fields and species-dependent energy offsets, this system can be used to analyze effects of disorder in four different symmetry classes: the chiral BDI and AIII and the nonchiral A and AI. These chiral classes are known to support a metallic phase at zero energy, which here, due to the inevitable finite size of the system, should also persist in a neighborhood of nonzero energies. As we discuss, this is of particular interest for experiments involving quenches. Away from the center of the spectrum, we find that excitations appear as domain walls in the cases with time-reversal symmetry or as vortices in the cases where time-reversal symmetry is absent. Therefore, a quench in a system with uniform density would lead to the formation of either vortices or domain walls depending on the symmetry class. For the nonchiral models in classes A and AI, a population imbalance between the two atomic species naturally occurs. In these cases, one of the two species is seen to favor a more uniform density. We also study the onset of localization as the disorder strength is increased for the different classes, and by deriving an effective model for the nonchiral cases we show how their eigenstates remain extended for larger values of the coupling with the disorder when compared to the nonchiral ones.

    Läs mer om Disordered cold atoms in different symmetry classes
  • Multiple-time-scale Landau-Zener transitions in many-body systems

    2015. Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 91 (1)

    Artikel

    Motivated by recent cold-atom experiments in optical lattices, we consider a lattice version of the Landau-Zener problem. Every single site is described by a Landau-Zener problem, but due to particle tunneling between neighboring lattice sites this on-site single-particle Landau-Zener dynamics couples to the particle motion within the lattice. The lattice, apart from having a dephasing effect on single-site Landau-Zener transitions, also implies, in the presence of a confining trap, an intersite particle flow induced by the Landau-Zener sweeping. This gives rise to an interplay between intra-and intersite dynamics. The adiabaticity constraint is therefore not simply given by the standard one, the Hamiltonian rate of change relative to the gap of the on-site problem. In experimentally realistic situations, the full system evolution is well described by Franck-Condon physics; e.g., nonadiabatic excitations are predominantly external ones characterized by large phononic vibrations in the atomic cloud, while internal excitations are very weak as close-to-perfect on-site transitions take place.

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  • Phases of d-orbital bosons in optical lattices

    2015. Fernanda Pinheiro, Jani-Petri Martikainen, Jonas Larson. New Journal of Physics 17

    Artikel

    We explore the properties of bosonic atoms loaded into the d bands of an isotropic square optical lattice. Following the recent experimental success reported in [Y. Zhai et al., Phys. Rev. A 87, 063638 (2013)], in which populating d bands with a 99% fidelity was demonstrated, we present a theoretical study of the possible phases that can appear in this system. Using the Gutzwiller ansatz for the three d band orbitals we map the boundaries of the Mott insulating phases. For not too large occupation, two of the orbitals are predominantly occupied, while the third, of a slightly higher energy, remains almost unpopulated. In this regime, in the superfluid phase we find the formation of a vortex lattice, where the vortices come in vortex/anti-vortex pairs with two pairs locked to every site. Due to the orientation of the vortices time-reversal symmetry is spontaneously broken. This state also breaks a discrete Z2-symmetry. We further derive an effective spin-1/2 model that describe the relevant physics of the lowest Mott-phase with unit filling. We argue that the corresponding two dimensional phase diagram should be rich with several different phases. We also explain how to generate antisymmetric spin interactions that can give rise to novel effects like spin canting.

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  • A diabatic representation of the two lowest electronic states of Li-3

    2014. Elham Nour Ghassemi, Jonas Larson, Åsa Larson. Journal of Chemical Physics 140 (15), 154304

    Artikel

    Using the Multi-Reference Configuration Interaction method, the adiabatic potential energy surfaces of Li-3 are computed. The two lowest electronic states are bound and exhibit a conical intersection. By fitting the calculated potential energy surfaces to the cubic E circle times epsilon Jahn-Teller model we extract the effective Jahn-Teller parameters corresponding to Li-3. These are used to set up the transformationmatrix which transforms from the adiabatic to a diabatic representation. This diabatization method gives a Hamiltonian for Li-3 which is free from singular non-adiabatic couplings and should be accurate for large internuclear distances, and it thereby allows for bound dynamics in the vicinity of the conical intersection to be explored.

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  • Impurity in a bosonic Josephson junction

    2014. Jesse Mumford, Jonas Larson, D. H. J. O'Dell. Physical Review A. Atomic, Molecular, and Optical Physics 89 (2), 023620

    Artikel

    We study a model describing N identical bosonic atoms trapped in a double-well potential together with a single-impurity atom, comparing and contrasting it throughout with the Dicke model. As the boson-impurity coupling strength is varied, there is a symmetry-breaking pitchfork bifurcation which is analogous to the quantum phase transition occurring in the Dicke model. Through stability analysis around the bifurcation point, we show that the critical value of the coupling strength has the same dependence on the parameters as the critical coupling value in the Dicke model. We also show that, like the Dicke model, the mean-field dynamics goes from being regular to chaotic above the bifurcation and macroscopic excitations of the bosons are observed. Although the boson-impurity system behaves like a poor man's version of the Dicke model, we demonstrate a self-trapping phenomenon which thus far has not been discussed in the realm of the Dicke model.

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  • Interaction-induced Landau-Zener transitions

    2014. Jonas Larson. Europhysics letters 107 (3), 30007

    Artikel

    By considering a quantum-critical Lipkin-Meshkov-Glick model we analyze a new type of Landau-Zener transitions where the population transfer is mediated by interaction rather than from a direct diabatic coupling. For this scenario, at a mean-field level the dynamics is greatly influenced by quantum interferences. In particular, regardless of how slow the Landau-Zener sweep is, for certain parameters almost no population transfer occurs, which is in stark contrast to the regular Landau-Zener model. For moderate system sizes, this counterintuitive mean-field behaviour is not duplicated in the quantum case. This can be attributed to quantum fluctuations and to the fact that multi-level Landau-Zener-Stuckelberg interferences have a dephasing effect on the above-mentioned phenomenon. We also find a discrepancy between the quantum and mean-field models in terms of how the transfer probabilities scale with the sweep velocity.

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  • Anomalous molecular dynamics in the vicinity of a conical intersection

    2013. Jonas Larson, Elham Nour Ghassemi, Åsa Larson. Europhysics letters 101 (4)

    Artikel

    Conical intersections between molecular electronic potential energy surfaces can greatly affect molecular dynamics and chemical properties. Molecular gauge theory is capable of explaining many of these often unexpected phenomena deriving from the physics of the conical intersection. Here we will give an example of anomalous dynamics in the paradigm E x epsilon Jahn-Teller model, which does not allow for a simple explanation in terms of standard molecular gauge theory. By introducing a dual gauge theory, we unwind this surprising behavior by identifying it with an intrinsic spin Hall effect. Thus, this work link knowledge of condensed-matter theories with non-adiabatic molecular dynamics. Furthermore, via ab initio calculations of potential energy surfaces, the findings are as well demonstrated to appear in a realistic system such as the Li-3 molecule.

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  • Chaos in circuit QED

    2013. Jonas Larson, Duncan H. J. O'Dell. Journal of Physics B 46 (22), 224015

    Artikel

    We study the open system dynamics of a circuit quantum electrodynamics (QED) model operating in the ultrastrong coupling regime. If the resonator is pumped periodically in time the underlying classical system is chaotic. Indeed, the periodically driven Jaynes-Cummings model in the Born-Oppenheimer approximation resembles a Duffing oscillator which in the classical limit is a well-known example of a chaotic system. Detection of the field quadrature of the output field acts as an effective position measurement of the oscillator. We address how such detection affects the quantum chaotic evolution in this bipartite system. We differentiate between single measurement realizations and ensembles of repeated measurements. In the former case a measurement/decoherence induced localization effect is encountered, while in the latter this localization is almost completely absent. This is in marked contrast to numerous earlier works discussing the quantum-classical correspondence in measured chaotic systems. This lack of a classical correspondence under relatively strong measurement induced decoherence is attributed to the inherent quantum nature of the qubit subsystem and in particular to the quantum correlations between the qubit and the field which persist despite the decoherence.

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  • Chaos-driven dynamics in spin-orbit-coupled atomic gases

    2013. Jonas Larson, Brandon M. Anderson, Alexander Altland. Physical Review A. Atomic, Molecular, and Optical Physics 87 (1), 013624

    Artikel

    The dynamics, appearing after a quantum quench, of a trapped, spin-orbit coupled, dilute atomic gas is studied. The characteristics of the evolution is greatly influenced by the symmetries of the system, and we especially compare evolution for an isotropic Rashba coupling and for an anisotropic spin-orbit coupling. As we make the spin-orbit coupling anisotropic, we break the rotational symmetry and the underlying classical model becomes chaotic; the quantum dynamics is affected accordingly. Within experimentally relevant time scales and parameters, the system thermalizes in a quantum sense. The corresponding equilibration time is found to agree with the Ehrenfest time, i.e., we numerically verify a similar to ln ( (h) over bar (-1)) scaling. Upon thermalization, we find that the equilibrated distributions show examples of quantum scars distinguished by accumulation of atomic density for certain energies. At shorter time scales, we discuss nonadiabatic effects deriving from the spin-orbit-coupled induced Dirac point. In the vicinity of the Dirac point, spin fluctuations are large and, even at short times, a semiclassical analysis fails.

    Läs mer om Chaos-driven dynamics in spin-orbit-coupled atomic gases
  • Integrability versus quantum thermalization

    2013. Jonas Larson. Journal of Physics B 46 (22), 224016

    Artikel

    Non-integrability is often taken as a prerequisite for quantum thermalization. Still, a generally accepted definition of quantum integrability is lacking. With the basis in the driven Rabi model we discuss this careless usage of the term 'integrability' in connection to quantum thermalization. The model would be classified as non-integrable according to the most commonly used definitions, for example, the only preserved quantity is the total energy. Despite this fact, a thorough analysis conjectures that the system will not thermalize. Thus, our findings suggest first of all (i) that care should be paid when linking non-integrability with thermalization, and secondly (ii) that the standardly used definitions for quantum integrability are unsatisfactory.

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  • On the rotating wave approximation in the adiabatic limit

    2013. Jonas Larson. Physica Scripta T153, 014040

    Artikel

    I revisit a longstanding question in quantum optics; when is the rotating wave approximation justified? In terms of the Jaynes-Cummings and Rabi models I demonstrate that the approximation in general breaks down in the adiabatic limit regardless of system parameters. This is explicitly shown by comparing Berry phases of the two models, where it is found that this geometrical phase is strictly zero in the Rabi model contrary to the non-trivial Berry phase of the Jaynes-Cummings model. The source of this surprising result is traced back to different topologies in the two models.

    Läs mer om On the rotating wave approximation in the adiabatic limit
  • XYZ Quantum Heisenberg Models with p-Orbital Bosons

    2013. Fernanda Pinheiro (et al.). Physical Review Letters 111 (20)

    Artikel

    We demonstrate how the spin-1/2 XYZ quantum Heisenberg model can be realized with bosonic atoms loaded in the p band of an optical lattice in the Mott regime. The combination of Bose statistics and the symmetry of the p-orbital wave functions leads to a nonintegrable Heisenberg model with antiferro-magnetic couplings. Moreover, the sign and relative strength of the couplings characterizing the model are shown to be experimentally tunable. We display the rich phase diagram in the one-dimensional case and discuss finite size effects relevant for trapped systems. Finally, experimental issues related to preparation, manipulation, detection, and imperfections are considered.

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  • Absence of Vacuum Induced Berry Phases without the Rotating Wave Approximation in Cavity QED

    2012. Jonas Larson. Physical Review Letters 108 (3)

    Artikel

    We revisit earlier studies on Berry phases suggested to appear in certain cavity QED settings. It has been especially argued that a nontrivial geometric phase is achievable even in the situation of no cavity photons. We, however, show that such results hinge on imposing the rotating wave approximation (RWA), while without the RWA no Berry phases occur in these schemes. A geometrical interpretation of our results is obtained by introducing semiclassical energy surfaces which in a simple way brings out the phase-space dynamics. With the RWA, a conical intersection between the surfaces emerges and encircling it gives rise to the Berry phase. Without the RWA, the conical intersection is absent and therefore the Berry phase vanishes. It is believed that this is a first example showing how the application of the RWA in the Jaynes-Cummings model may lead to false conclusions, regardless of the mutual strengths between the system parameters.

    Läs mer om Absence of Vacuum Induced Berry Phases without the Rotating Wave Approximation in Cavity QED
  • Confined p-band Bose-Einstein condensates

    2012. Fernanda Pinheiro, Jani-Petri Martikainen, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 85 (3)

    Artikel

    We study bosonic atoms on the p band of a two-dimensional optical square lattice in the presence of a confining trapping potential. Using a mean-field approach, we show how the anisotropic tunneling for p-band particles affects the cloud of condensed atoms by characterizing the ground-state density and the coherence properties of the atomic states both between sites and atomic flavors. In contrast to the usual results based on the local-density approximation, the atomic density can become anisotropic. This anisotropic effect is especially pronounced in the limit of weak atom-atom interactions and of weak lattice amplitudes, i.e., when the properties of the ground state are mainly driven by the kinetic energies. We also investigate how the trap influences known properties of the nontrapped case. In particular, we focus on the behavior of the antiferromagnetic vortex-antivortex order, which for the confined system is shown to disappear at the edges of the condensed cloud.

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  • Entanglement of distant optomechanical systems

    2012. C. Joshi (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 85 (3), 033805

    Artikel

    We theoretically investigate the possibility to generate nonclassical states of optical and mechanical modes of optical cavities, distant from each other. A setup comprised of two identical cavities, each with one fixed and one movable mirror and coupled by an optical fiber, is studied in detail. We show that with such a setup there is potential to generate entanglement between the distant cavities, involving both optical and mechanical modes. The scheme is robust with respect to dissipation, and nonlocal correlations are found to exist in the steady state at finite temperatures.

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  • Fractional domain walls from on-site softening in dipolar bosons

    2012. Emma Wikberg (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 85 (3), 033607

    Artikel

    We study dipolar bosons in a 1D optical lattice and identify a region in parameter space-strong coupling but relatively weak on-site repulsion-hosting a series of stable charge-density-wave (CDW) states whose low-energy excitations, built from fractional domain walls, have remarkable similarities to those of non-Abelian fractional quantum Hall states. Here, a conventional domain wall between translated CDW's may be split by inserting strings of degenerate, but inequivalent, CDW states. Outside these insulating regions, we find numerous supersolids as well as a superfluid regime. The mentioned phases should be accessible experimentally and, in particular, the fractional domain walls can be created in the ground state using single-site addressing, i.e., by locally changing the chemical potential.

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  • Multiorbital bosons in bipartite optical lattices

    2012. Jani-Petri Martikainen, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 86 (2), 023611

    Artikel

    We study interacting bosons in a two-dimensional square bipartite optical lattice. By focusing on the regime where the first three excited bands are nearly degenerate (i.e., the first-excited p bands in one sublattice are nearly degenerate with the s band of the other sublattice), we derive a multi-orbital tight-binding model which captures the most relevant features of the band structure. In addition, we also derive a corresponding generalized Bose-Hubbard model and solve it numerically under different situations, both with and without a confining trap. It is especially found that the hybridization between sublattices can strongly influence the phase diagrams and, in a trap, enable even appearances of condensed phases intersecting the same Mott insulating plateaus.

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  • Anomalous decoherence and absence of thermalization in a photonic many-body system

    2011. Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 83 (5), 052103

    Artikel

    The intention of this work is twofold: first, to present a most simple system capable of simulating the intrinsic bosonic Josephson effect with photons and, second, to study various outcomes deriving from inherent or external decoherence. A qubit induces an effective coupling between two externally pumped cavity modes. Without cavity losses and in the dispersive regime, intrinsic Josephson oscillations of photons between the two modes occurs. In this case, contrary to regular Markovian decoherence, the qubit purity shows a Gaussian decay and recurrence of its coherence. Due to intrinsic nonlinearities, both the Josephson oscillations as well as the qubit properties display a rich collapse-revival structure, where, however, the complexity of the qubit evolution is in some sense stronger. The qubit as a meter of the photon dynamics is considered, and it is shown that qubit dephasing, originating, for example, from nondemolition measurements, results in an exponential destruction of the oscillations which manifests the collectiveness of the Josephson effect. Nonselective qubit measurements, on the other hand, render a Zeno effect seen in a slowing down of the Josephson oscillations. Contrary to dephasing, cavity dissipation results in a Gaussian decay of the scaled Josephson oscillations. Finally, following Ponomarev et al. [Phys. Rev. Lett. 106, 010405 (2011)], we analyze aspects of thermalization. In particular, despite similarities with the generic model studied by Ponomarev et al., our system does not seem to thermalize.

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  • Band-structure loops and multistability in cavity QED

    2011. B. Prasanna Venkatesh, Jonas Larson, D. H. J. O'Dell. Physical Review A. Atomic, Molecular, and Optical Physics 83 (6), 063606

    Artikel

    We calculate the band structure of ultracold atoms located inside a laser-driven optical cavity. For parameters where the atom-cavity system exhibits bistability, the atomic band structure develops loop structures akin to the ones predicted for Bose-Einstein condensates in ordinary (noncavity) optical lattices. However, in our case the nonlinearity derives from the cavity back-action rather than from direct interatomic interactions. We find both bi- and tristable regimes associated with the lowest band, and show that the multistability we observe can be analyzed in terms of swallowtail catastrophes. Dynamic and energetic stability of the mean-field solutions is also discussed, and we show that the bistable solutions have, as expected, one unstable and two stable branches. The presence of loops in the atomic band structure has important implications for proposals concerning Bloch oscillations of atoms inside optical cavities [Peden et al., Phys. Rev. A 80, 043803 (2009); Prasanna Venkatesh et al., Phys. Rev. A 80, 063834 (2009)].

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  • Diode for Bose-Einstein condensates

    2011. Jonas Larson. Europhysics letters 96 (5), 50004

    Artikel

    Given a quantum state at some instant of time t, the underlying system Hamiltonian can not only predict how the state will evolve, but also the history of the state prior to t. Thereby, in order to have a directed motion, like in a diode, some sort of irreversibility must be considered. For the atom diode, this has been achieved by spontaneous decay of excited atomic levels. For an atomic condensate, it is clear, however, that such decay will cause both heating and decoherence of the condensate. To overcome this complication we introduce a different setup where the dissipation does not act directly on the atoms. The excited atoms are stimulatedly driven back to the ground state by exciting a cavity mode, which in return decays to the vacuum via photon losses. The efficiency of the method utilizing experimental parameters is shown to be almost perfect within large parameter regimes.

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  • Loading of bosons in optical lattices into the p band

    2011. Jonas Larson, Jani-Petri Martikainen. Physical Review A. Atomic, Molecular, and Optical Physics 84 (2), 023621

    Artikel

    We present a method for transferring bosonic atoms residing on the lowest s band of an optical lattice to the first excited p bands. Our idea hinges on resonant tunneling between adjacent sites of accelerated lattices. The acceleration effectively shifts the quasibound energies on each site such that the system can be cast into a Wannier-Stark ladder problem. By adjusting the acceleration constant, a situation of resonant tunneling between the s and p bands is achievable. Within a mean-field model, considering (87)Rb atoms, we demonstrate population transfer from the s to the p bands with around 95% efficiency. Nonlinear effects deriving from atom-atom interactions, as well as coupling of the quasibound Wannier-Stark states to the energy continuum, are considered.

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  • Multi-particle entanglement of charge qubits coupled to a nanoresonator

    2011. M. Abdel-Aty (et al.). Physica. E, Low-Dimensional systems and nanostructures 43 (9), 1625-1630

    Artikel

    The dynamics of charge qubits coupled to a nanomechanical resonator under the influence of both a phonon bath in contact with the resonator and irreversible decay of the qubits is considered. The focus of our analysis is devoted to multi-particle entanglement and the effects arising from the coupling to the reservoir. Even in the presence of the reservoirs, the inherent entanglement is found to be rather robust. Due to this fact, together with control of system parameters, the system may, therefore, be especially suited for quantum information processing. Our findings also shed light on the evolution of open quantum many-body systems. For instance, due to intrinsic qubit-qubit couplings our model is related to a driven XY spin model.

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  • Photonic Josephson effect, phase transitions, and chaos in optomechanical systems

    2011. Jonas Larson, Mats Horsdal. Physical Review A. Atomic, Molecular, and Optical Physics 84 (2), 021804

    Artikel

    A photonic analog of the Josephson effect is analyzed for a system formed by a partly transparent mechanical membrane dividing an optical cavity into two halves. Photons tunneling between the two subcavities constitute the coherent Jospehson current. The force acting upon the membrane due to the light pressure induces a nonlinearity, which results in a rich dynamical structure. For example, contrary to standard bosonic Josephson systems, we encounter chaos. By means of a mean-field approach, we identify the various regimes and corresponding phase diagram. At the short time scale, chaos is demonstrated to prevent regular self-trapping, while for longer times a dissipation-induced self-trapping effect is possible.

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  • Analog of the spin-orbit-induced anomalous Hall effect with quantized radiation

    2010. Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 81 (5), 51803

    Artikel

    We demonstrate how the term describing the interaction between a single two-level atom and two cavity field modes may attain a Rashba form. As an outcome, cavity QED provides a testbed for studies of phenomena reminiscent of the spin-orbit induced anomalous Hall effect. The effective magnetic field, deriving from the non-Abelian gauge potentials rendered by the Rashba coupling, induces a transverse force acting on the phase space distributions. Thereby, the phase space distributions build up a transverse motion manifesting itself in spiral trajectories, rather than circular ones obtained for a zero magnetic field as one would acquire for the corresponding Abelian gauge potentials. Utilizing realistic experimental parameters, the phenomenon is numerically verified, ascertain that it should be realizable with current techniques.

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  • Circuit QED scheme for the realization of the Lipkin-Meshkov-Glick model

    2010. Jonas Larson. Europhysics letters 90 (5), 54001

    Artikel

    We propose a scheme in which the Lipkin-Meshkov-Glick model is realized within a circuit QED system. An array of N superconducting qubits interacts with a driven cavity mode. In the dispersive regime, the cavity mode is adiabatically eliminated generating an effective model for the qubits alone. The characteristic long-range order of the Lipkin-Meshkov-Glick model is here mediated by the cavity field. For a closed qubit system, the inherent second-order phase transition of the qubits is reflected in the intensity of the output cavity field. In the broken symmetry phase, the many-body ground state is highly entangled. Relaxation of the qubits is analyzed within a mean-field treatment. The second-order phase transition is lost, while new bistable regimes occur.

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  • Dark solitons near the Mott-insulator-superfluid phase transition

    2010. Konstantin V. Krutitsky, Jonas Larson, Maciej Lewenstein. Physical Review A. Atomic, Molecular, and Optical Physics 82 (3), 33618

    Artikel

    Dark solitons of ultracold bosons in the vicinity of the Mott-insulator-superfluid phase transition are studied. Making use of the Gutzwiller ansatz we have found antisymmetric eigenstates corresponding to standing solitons, as well as propagating solitons created by phase imprinting. Near the phase boundary, superfluidity has either a particle or a hole character depending on the system parameters, which greatly affects the characteristics of both types of solitons. Within the insulating Mott regions, soliton solutions are prohibited by lack of phase coherence between the lattice sites. Linear and modulational stability show that the soliton solutions are sensitive to small perturbations and, therefore, unstable. In general, their lifetimes differ for on-site and off-site modes. For the on-site modes, there are small areas between the Mott-insulator regions where the lifetime is very large, and in particular much larger than that for the off-site modes.

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  • Quantum ground state of self-organized atomic crystals in optical resonators

    2010. Sonia Fernandez-Vidal (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 81 (4), 43407

    Artikel

    Cold atoms, driven by a laser and simultaneously coupled to the quantum field of an optical resonator, may self-organize in periodic structures. These structures are supported by the optical lattice, which emerges from the laser light they scatter into the cavity mode and form when the laser intensity exceeds a threshold value. We study theoretically the quantum ground state of these structures above the pump threshold of self-organization by mapping the atomic dynamics of the self-organized crystal to a Bose-Hubbard model. We find that the quantum ground state of the self-organized structure can be the one of a Mott insulator, depending on the pump strength of the driving laser. For very large pump strengths, where the intracavity-field intensity is maximum and one would expect a Mott-insulator state, we find intervals of parameters where the phase is compressible. These states could be realized in existing experimental setups.

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  • Ultracold atoms in a cavity-mediated double-well system

    2010. Jonas Larson, Jani-Petri Martikainen. Physical Review A. Atomic, Molecular, and Optical Physics 82 (3), 33606

    Artikel

    We study ground-state properties and dynamics of a dilute ultracold atomic gas in a double-well potential. The Gaussian barrier separating the two wells derives from the interaction between the atoms and a quantized field of a driven Fabry-Perot cavity. Due to intrinsic atom-field nonlinearity, several interesting phenomena arise which are the focus of this work. For the ground state, there is a critical pumping amplitude in which the atoms self-organize and the intra-cavity-field amplitude drastically increases. In the dynamical analysis, we show that the Josephson oscillations depend strongly on the atomic density and may be greatly suppressed within certain regimes, reminiscent of self-trapping of Bose-Einstein condensates in double-well setups. This pseudo-self-trapping effect is studied within a mean-field treatment valid for large atom numbers. For small numbers of atoms, we consider the analogous many-body problem and demonstrate a collapse-revival structure in the Josephson oscillations.

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