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/10.1007/9783030348823
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 highquality 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 JaynesCummingslike 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 degreesoffreedom 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 nonzero 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 manybody 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 timeevolution, and work with mixed states. We try to build a deeper understaing for the novel physics emerging from such nonunitary evolution, and what new phases of matter that may exist.
Forskningsprojekt
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)
ArtikelIn the Hamburg cold atom experiment with orbital states in an optical lattice, s and porbital atomic states hybridize between neighboring sites. In this work we show how this alternation of sites hosting s and porbital 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 spin1/2 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 sorbital states can be adiabatically eliminated to give an effective model for the porbital atoms. At the meanfield 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.

Monitoring the resonantly driven JaynesCummings oscillator by an external twolevel emitter
2020. Themistoklis K. Mavrogordatos, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 101 (5)
ArtikelWe 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 twolevel atom, while the first transforms from a driven empty cavity to a perturbative QED configuration and ultimately to a driven JaynesCummings (JC) oscillator through a varying lightmatter coupling strength. For our purpose, we appeal at first to the properties of resonance fluorescence in the statistical description of radiation emitted along two channelsthose of forward and sideways scatteringcomprising 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 badcavity 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 twolevel target, showing that coherent atomic oscillations of the target probe the onset of a secondorder dissipative quantum phase transition in the source.

Superradiance, bosonic Peierls distortion, and lattice gauge theory in a generalized RabiHubbard chain
2020. Axel Gagge, Jonas Larson. Physical Review A: covering atomic, molecular, and optical physics and quantum information 102 (6)
ArtikelWe investigate a onedimensional RabiHubbard 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 threelevel 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 secondorder 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 manybody superradiant phase. In the superradiant phase, a Z(2) symmetry is broken and the phase transition falls within the universality class of the transversefield 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.

Blochlike energy oscillations
2018. Axel Gagge, Jonas Larson. Physical Review A: covering atomic, molecular, and optical physics and quantum information 98 (5)
ArtikelWe 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 (timedependent) tilted singleband 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 sitelocalized 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 LandauZener 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.

Disordered cold atoms in different symmetry classes
Fernanda Pinheiro, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics
ArtikelWe consider an experimentally realizable model of noninteracting but randomly coupled atoms in a twodimensional optical lattice. By choosing appropriate real or complexvalued random fields and speciesdependent 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 nonzero 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 timereversal symmetry, or as vortices in the cases where timereversal 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 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 nonchiral cases we show how their eigenstates remain extended for larger values of the coupling with the disorder, if compared to the nonchiral ones.

Dissipationdriven 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)
ArtikelBy considering a solvable drivendissipative 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 firstorder phase transition into a continuous one due to large environmentinduced 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.

Liouvillian of the Open STIRAP Problem
2018. Thomas Mathisen, Jonas Larson. Entropy 20 (1)
ArtikelWith 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 timeevolution; 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 nonzero Liouvillian gap protects the system's (adiabatic) state from nonadiabatic excitations. In contrast to full Zeno freezing of the evolution, which is often found in many problems without explicit timedependence, here, the freezing takes place in the adiabatic basis such that the system still evolves but adiabatically.

Some remarks on 'superradiant' phase transitions in lightmatter systems
2017. Jonas Larson, Elinor K. Irish. Journal of Physics A 50 (17)
ArtikelIn this paper we analyze properties of the phase transition that appears in a set of quantum optical models; Dicke, TavisCummings, quantum Rabi, and finally the JaynesCummings model. As the lightmatter 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 meanfield 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 TavisCummings and JaynesCummings models, the continuous symmetry of these models implies that quantum fluctuations are not only negligible, but strictly zero. However, all models possess a nonanalyticity 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 nonequilibrium steady states remain critical, while the criticality for the open TavisCummings and JaynesCummings models is completely lost, i. e. in realistic settings one cannot expect a true critical behaviour for the two last models.

Quantum state engineering in hybrid open quantum systems
2016. Chaitanya Joshi, Jonas Larson, Timothy P. Spiller. Physical Review A 93 (4)
ArtikelWe investigate a possibility to generate nonclassical states in lightmatter coupled noisy quantum systems, namely, the anisotropic Rabi and Dicke models. In these hybrid quantum systems, a competing influence of coherent internal dynamics and environmentinduced 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 lightmatter 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 alloptical 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.

CavityAssisted Generation of Sustainable Macroscopic Entanglement of Ultracold Gases
2015. Chaitanya Joshi, Jonas Larson. Atoms 3 (3), 348366
ArtikelProspects for reaching persistent entanglement between two spatiallyseparated atomic BoseEinstein condensates are outlined. The system setup comprises two condensates loaded in an optical lattice, which, in return, is confined within a highQ 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 cavitymediated interactions between the two condensates is capable of entangling them despite photon losses. This macroscopic atomic entanglement is sustained throughout the timeevolution 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.

Dicketype phase transition in a multimode optomechanical system
2015. Jesse Mumford, D. H. J. O'Dell, Jonas Larson. Annalen der Physik 527 (12), 115130
ArtikelWe 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 meanfield level reveal that there is a critical strength of the lightmembrane 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 Dicketype physics in these systems.

Disordered cold atoms in different symmetry classes
2015. Fernanda Pinheiro, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 92 (2)
ArtikelWe consider an experimentally realizable model of noninteracting but randomly coupled atoms in a twodimensional optical lattice. By choosing appropriate real or complexvalued random fields and speciesdependent 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 timereversal symmetry or as vortices in the cases where timereversal 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.

Multipletimescale LandauZener transitions in manybody systems
2015. Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 91 (1)
ArtikelMotivated by recent coldatom experiments in optical lattices, we consider a lattice version of the LandauZener problem. Every single site is described by a LandauZener problem, but due to particle tunneling between neighboring lattice sites this onsite singleparticle LandauZener dynamics couples to the particle motion within the lattice. The lattice, apart from having a dephasing effect on singlesite LandauZener transitions, also implies, in the presence of a confining trap, an intersite particle flow induced by the LandauZener sweeping. This gives rise to an interplay between intraand 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 onsite problem. In experimentally realistic situations, the full system evolution is well described by FranckCondon 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 closetoperfect onsite transitions take place.

Phases of dorbital bosons in optical lattices
2015. Fernanda Pinheiro, JaniPetri Martikainen, Jonas Larson. New Journal of Physics 17
ArtikelWe 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/antivortex pairs with two pairs locked to every site. Due to the orientation of the vortices timereversal symmetry is spontaneously broken. This state also breaks a discrete Z2symmetry. We further derive an effective spin1/2 model that describe the relevant physics of the lowest Mottphase 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.

A diabatic representation of the two lowest electronic states of Li3
2014. Elham Nour Ghassemi, Jonas Larson, Åsa Larson. Journal of Chemical Physics 140 (15), 154304
ArtikelUsing the MultiReference Configuration Interaction method, the adiabatic potential energy surfaces of Li3 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 JahnTeller model we extract the effective JahnTeller parameters corresponding to Li3. These are used to set up the transformationmatrix which transforms from the adiabatic to a diabatic representation. This diabatization method gives a Hamiltonian for Li3 which is free from singular nonadiabatic 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.

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
ArtikelWe study a model describing N identical bosonic atoms trapped in a doublewell potential together with a singleimpurity atom, comparing and contrasting it throughout with the Dicke model. As the bosonimpurity coupling strength is varied, there is a symmetrybreaking 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 meanfield dynamics goes from being regular to chaotic above the bifurcation and macroscopic excitations of the bosons are observed. Although the bosonimpurity system behaves like a poor man's version of the Dicke model, we demonstrate a selftrapping phenomenon which thus far has not been discussed in the realm of the Dicke model.

Interactioninduced LandauZener transitions
2014. Jonas Larson. Europhysics letters 107 (3), 30007
ArtikelBy considering a quantumcritical LipkinMeshkovGlick model we analyze a new type of LandauZener transitions where the population transfer is mediated by interaction rather than from a direct diabatic coupling. For this scenario, at a meanfield level the dynamics is greatly influenced by quantum interferences. In particular, regardless of how slow the LandauZener sweep is, for certain parameters almost no population transfer occurs, which is in stark contrast to the regular LandauZener model. For moderate system sizes, this counterintuitive meanfield behaviour is not duplicated in the quantum case. This can be attributed to quantum fluctuations and to the fact that multilevel LandauZenerStuckelberg interferences have a dephasing effect on the abovementioned phenomenon. We also find a discrepancy between the quantum and meanfield models in terms of how the transfer probabilities scale with the sweep velocity.

Anomalous molecular dynamics in the vicinity of a conical intersection
2013. Jonas Larson, Elham Nour Ghassemi, Åsa Larson. Europhysics letters 101 (4)
ArtikelConical 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 JahnTeller 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 condensedmatter theories with nonadiabatic 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 Li3 molecule.

Chaos in circuit QED
2013. Jonas Larson, Duncan H. J. O'Dell. Journal of Physics B 46 (22), 224015
ArtikelWe 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 JaynesCummings model in the BornOppenheimer approximation resembles a Duffing oscillator which in the classical limit is a wellknown 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 quantumclassical 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.

Chaosdriven dynamics in spinorbitcoupled atomic gases
2013. Jonas Larson, Brandon M. Anderson, Alexander Altland. Physical Review A. Atomic, Molecular, and Optical Physics 87 (1), 013624
ArtikelThe dynamics, appearing after a quantum quench, of a trapped, spinorbit 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 spinorbit coupling. As we make the spinorbit 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 spinorbitcoupled induced Dirac point. In the vicinity of the Dirac point, spin fluctuations are large and, even at short times, a semiclassical analysis fails.

Integrability versus quantum thermalization
2013. Jonas Larson. Journal of Physics B 46 (22), 224016
ArtikelNonintegrability 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 nonintegrable 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 nonintegrability with thermalization, and secondly (ii) that the standardly used definitions for quantum integrability are unsatisfactory.

On the rotating wave approximation in the adiabatic limit
2013. Jonas Larson. Physica Scripta T153, 014040
ArtikelI revisit a longstanding question in quantum optics; when is the rotating wave approximation justified? In terms of the JaynesCummings 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 nontrivial Berry phase of the JaynesCummings model. The source of this surprising result is traced back to different topologies in the two models.

XYZ Quantum Heisenberg Models with pOrbital Bosons
2013. Fernanda Pinheiro (et al.). Physical Review Letters 111 (20)
ArtikelWe demonstrate how the spin1/2 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 porbital wave functions leads to a nonintegrable Heisenberg model with antiferromagnetic 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 onedimensional case and discuss finite size effects relevant for trapped systems. Finally, experimental issues related to preparation, manipulation, detection, and imperfections are considered.

Absence of Vacuum Induced Berry Phases without the Rotating Wave Approximation in Cavity QED
2012. Jonas Larson. Physical Review Letters 108 (3)
ArtikelWe 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 phasespace 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 JaynesCummings model may lead to false conclusions, regardless of the mutual strengths between the system parameters.

Confined pband BoseEinstein condensates
2012. Fernanda Pinheiro, JaniPetri Martikainen, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 85 (3)
ArtikelWe study bosonic atoms on the p band of a twodimensional optical square lattice in the presence of a confining trapping potential. Using a meanfield approach, we show how the anisotropic tunneling for pband particles affects the cloud of condensed atoms by characterizing the groundstate density and the coherence properties of the atomic states both between sites and atomic flavors. In contrast to the usual results based on the localdensity approximation, the atomic density can become anisotropic. This anisotropic effect is especially pronounced in the limit of weak atomatom 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 vortexantivortex order, which for the confined system is shown to disappear at the edges of the condensed cloud.

Entanglement of distant optomechanical systems
2012. C. Joshi (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 85 (3), 033805
ArtikelWe 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.

Fractional domain walls from onsite softening in dipolar bosons
2012. Emma Wikberg (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 85 (3), 033607
ArtikelWe study dipolar bosons in a 1D optical lattice and identify a region in parameter spacestrong coupling but relatively weak onsite repulsionhosting a series of stable chargedensitywave (CDW) states whose lowenergy excitations, built from fractional domain walls, have remarkable similarities to those of nonAbelian 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 singlesite addressing, i.e., by locally changing the chemical potential.

Multiorbital bosons in bipartite optical lattices
2012. JaniPetri Martikainen, Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 86 (2), 023611
ArtikelWe study interacting bosons in a twodimensional square bipartite optical lattice. By focusing on the regime where the first three excited bands are nearly degenerate (i.e., the firstexcited p bands in one sublattice are nearly degenerate with the s band of the other sublattice), we derive a multiorbital tightbinding model which captures the most relevant features of the band structure. In addition, we also derive a corresponding generalized BoseHubbard 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.

Anomalous decoherence and absence of thermalization in a photonic manybody system
2011. Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 83 (5), 052103
ArtikelThe 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 collapserevival 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.

Bandstructure 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
ArtikelWe calculate the band structure of ultracold atoms located inside a laserdriven optical cavity. For parameters where the atomcavity system exhibits bistability, the atomic band structure develops loop structures akin to the ones predicted for BoseEinstein condensates in ordinary (noncavity) optical lattices. However, in our case the nonlinearity derives from the cavity backaction 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 meanfield 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)].

Diode for BoseEinstein condensates
2011. Jonas Larson. Europhysics letters 96 (5), 50004
ArtikelGiven 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.

Loading of bosons in optical lattices into the p band
2011. Jonas Larson, JaniPetri Martikainen. Physical Review A. Atomic, Molecular, and Optical Physics 84 (2), 023621
ArtikelWe 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 WannierStark ladder problem. By adjusting the acceleration constant, a situation of resonant tunneling between the s and p bands is achievable. Within a meanfield model, considering (87)Rb atoms, we demonstrate population transfer from the s to the p bands with around 95% efficiency. Nonlinear effects deriving from atomatom interactions, as well as coupling of the quasibound WannierStark states to the energy continuum, are considered.

Multiparticle entanglement of charge qubits coupled to a nanoresonator
2011. M. AbdelAty (et al.). Physica. E, LowDimensional systems and nanostructures 43 (9), 16251630
ArtikelThe 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 multiparticle 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 manybody systems. For instance, due to intrinsic qubitqubit couplings our model is related to a driven XY spin model.

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
ArtikelA 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 meanfield approach, we identify the various regimes and corresponding phase diagram. At the short time scale, chaos is demonstrated to prevent regular selftrapping, while for longer times a dissipationinduced selftrapping effect is possible.

Analog of the spinorbitinduced anomalous Hall effect with quantized radiation
2010. Jonas Larson. Physical Review A. Atomic, Molecular, and Optical Physics 81 (5), 51803
ArtikelWe demonstrate how the term describing the interaction between a single twolevel 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 spinorbit induced anomalous Hall effect. The effective magnetic field, deriving from the nonAbelian 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.

Circuit QED scheme for the realization of the LipkinMeshkovGlick model
2010. Jonas Larson. Europhysics letters 90 (5), 54001
ArtikelWe propose a scheme in which the LipkinMeshkovGlick 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 longrange order of the LipkinMeshkovGlick model is here mediated by the cavity field. For a closed qubit system, the inherent secondorder phase transition of the qubits is reflected in the intensity of the output cavity field. In the broken symmetry phase, the manybody ground state is highly entangled. Relaxation of the qubits is analyzed within a meanfield treatment. The secondorder phase transition is lost, while new bistable regimes occur.

Dark solitons near the Mottinsulatorsuperfluid phase transition
2010. Konstantin V. Krutitsky, Jonas Larson, Maciej Lewenstein. Physical Review A. Atomic, Molecular, and Optical Physics 82 (3), 33618
ArtikelDark solitons of ultracold bosons in the vicinity of the Mottinsulatorsuperfluid 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 onsite and offsite modes. For the onsite modes, there are small areas between the Mottinsulator regions where the lifetime is very large, and in particular much larger than that for the offsite modes.

Quantum ground state of selforganized atomic crystals in optical resonators
2010. Sonia FernandezVidal (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 81 (4), 43407
ArtikelCold atoms, driven by a laser and simultaneously coupled to the quantum field of an optical resonator, may selforganize 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 selforganization by mapping the atomic dynamics of the selforganized crystal to a BoseHubbard model. We find that the quantum ground state of the selforganized 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 intracavityfield intensity is maximum and one would expect a Mottinsulator state, we find intervals of parameters where the phase is compressible. These states could be realized in existing experimental setups.

Ultracold atoms in a cavitymediated doublewell system
2010. Jonas Larson, JaniPetri Martikainen. Physical Review A. Atomic, Molecular, and Optical Physics 82 (3), 33606
ArtikelWe study groundstate properties and dynamics of a dilute ultracold atomic gas in a doublewell potential. The Gaussian barrier separating the two wells derives from the interaction between the atoms and a quantized field of a driven FabryPerot cavity. Due to intrinsic atomfield 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 selforganize and the intracavityfield 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 selftrapping of BoseEinstein condensates in doublewell setups. This pseudoselftrapping effect is studied within a meanfield treatment valid for large atom numbers. For small numbers of atoms, we consider the analogous manybody problem and demonstrate a collapserevival structure in the Josephson oscillations.

A Fock state lattice approach to quantum optics
Pil Saugmann, Jonas Larson.
We analyze a set of models frequently appearing in quantum optical settings by expressing their Hamiltonians in terms of Fockstate lattices. The few degreesoffreedom of such models, together with the system symmetries, make the emerging Fockstate lattices rather simple such that they can be linked to known lattice models from the condensed matter community. This sheds new light on known quantum optical systems. While we provide a rather long list of models and their corresponding Fockstate lattices, we pick a few ones in order to demonstrate the strength of the method. The threemode boson model, for example, is shown to display a fractal spectrum, and chiral evolution in the Fockstate lattice characterized by localized distributions traversing along symmetric trajectories. In a second example we consider the central spin model which generates a Fockstate lattice reminiscent of the SSHmodel hosting topological edge states. We further demonstrate how the phenomena of flat bands in lattice models can manifest in related Fockstate lattices, which can be linked to so called dark states.

Exploring phononlike interactions in onedimensional BoseFermi mixtures
Axel Gagge, Jonas Larson, Themis Mavrogordatos.
With the objective of simulating the physical behaviour of electrons moving in a dynamical background, we study a cold atomic BoseFermi mixture in an optical lattice potential felt only by the bosons. The bosons, assumed to be in the deep superfluid regime, inherit the periodicity of the optical lattice and subsequently act as a dynamical potential for the polarized fermions. Due to the atomphonon interaction between the fermions and the condensate, the coupled system displays a BerezinskiiKosterlitzThouless transition from a Luttinger liquid to a Peierls phase. For sufficiently strong BoseFermi interaction, however, the Peierls phase becomes unstable and is succeeded by either a collapsed or a separated phase. We find that the main role of the optical lattice amounts to stabilizing the Peierls phase. Furthermore, the presence of a confining harmonic trap leads to a rich physical behaviour beyond what is observed for either bosons or fermions separately trapped. In particular, for an attractive BoseFermi interaction, the insulating phase may develop a fermionic weddingcake like configuration reflecting the dynamical nature of the underlying lattice potential. For repulsive interaction, on the other hand, we conclude that the trap destabilizes the Peierls phase and the two species separate.

Route towards classical frustration and band flattening via optical lattice distortion
Pil Saugmann (et al.).
We propose and experimentally explore a method for realizing frustrated lattice models usinga BoseEinstein condensate held in an optical square lattice. A small lattice distortion opens upan energy gap such the lowest band splits into two. Along the edge of the first Brillouin zone forboth bands a nearly flat energymomentum dispersion is realized. For the excited band a highlydegenerate energy minimum arises. By loading ultracold atoms into the excited band, a classicallyfrustrated XY model is formed, describing rotors on a square lattice with competing nearest andnext nearest tunnelling couplings. Our experimental optical lattice provides a regime, where a fullycoherent BoseEinstein condensate is observed, and a regime where frustration is expected. If weadiabatically tune from the condensate regime to the regime of frustration, the momentum spectrashows a complete loss of coherence. Upon slowly tuning back to the condensate regime, coherenceis largely restored. Good agreement with model calculations is obtained.

Fockstatelattice approach to quantum optics
2023. Pil Saugmann, Jonas Larson. Physical Review A: covering atomic, molecular, and optical physics and quantum information 108 (3)
ArtikelWe analyze a set of models frequently appearing in quantum optical settings by expressing their Hamiltonians in terms of Fockstate lattices (FSLs). The few degreesoffreedom of such models, together with the system symmetries, make the emerging FSLs relatively simple such that they can be linked to known lattice models from the condensedmatter community. Thus, the FSLs may shed new light on known quantum optical systems. While we provide a rather long list of models and their corresponding FSLs, we pick a few to demonstrate the method's strength. The threemode boson model, for example, is shown to display a fractal spectrum and chiral evolution in the FSL characterized by localized distributions traversing along symmetric trajectories. In a second example, we consider the central spin model, which generates an FSL reminiscent of the SuSchriefferHeeger model hosting topological edge states. We further demonstrate how the phenomenon of flat bands in lattice models can manifest in related FSLs, which can be linked to socalled dark states.

Route toward classical frustration and band flattening via optical lattice distortion
2022. Pil Saugmann (et al.). Physical Review A: covering atomic, molecular, and optical physics and quantum information 106 (4)
ArtikelWe propose and experimentally explore a method for realizing frustrated lattice models using a BoseEinstein condensate held in an optical square lattice. A small lattice distortion opens up an energy gap such that the lowest band splits into two. Along the edge of the first Brillouin zone for both bands, a nearly flat energymomentum dispersion is realized. For the excited band, a highly degenerate energy minimum arises. By loading ultracold atoms into the excited band, a classically frustrated XY model is formed, describing rotors on a square lattice with competing nearest and nextnearest tunneling couplings. Our experimental optical lattice provides a regime where a fully coherent BoseEinstein condensate is observed and a regime where frustration is expected. If we adiabatically tune from the condensate regime to the regime of frustration, the momentum spectra show a complete loss of coherence. Upon slowly tuning back to the condensate regime, coherence is largely restored. Good agreement with model calculations is obtained.

The Jaynes–Cummings Model and Its Descendants: Modern research directions
2021. Jonas Larson, Themistoklis Mavrogordatos.
BokThe Jaynes–Cummings (JC) model has been at the forefront of quantum optics for almost six decades to date, providing one of the simplest yet intricately nonlinear formulations of lightmatter interaction in modern physics. Laying most of the emphasis to the omnipresence of the model a crossa range of disciplines, this monograph brings up the fundamental generality of its formalism, looking at a wide gamut of applications in specific physical systems among several realms, including atomic physics, quantum optics, solidstate physics and quantum information science. When bringing the various pieces together to assemble our narrative, we have primarily targeted researchers in quantum physics and quantum optics. The monograph also comprises an accessible introduction for graduate students engaged with nonequilibrium quantum phase transitions, quantum computing and simulation, and quantum manybody physics. In that framework, we aim to reveal the common ground between physics and applications scattered across literature and different technological advancements. The exposition guides the reader through a vibrant field interlacing quantum optics and condensedmatter physics. All sections are devoted to the strong interconnection between theory and experiment, historically linked to the development of the various modern research directions stemming from JC physics. This is accompanied by a comprehensive list of references to the key publications that have shaped its evolution since the early 1960s. Finally, we have endeavoured to keep the presentation of such a multisided material as concise as possible, interspersing continuous text with various illustrations alongside an economical use of mathematical expressions.

Conical Intersections in Physics: An Introduction to Synthetic Gauge Theories
2020. Jonas Larson, Sjöqvist Erik, Öhberg Patrik.
BokThis concise book introduces and discusses the basic theory of conical intersections with applications in atomic, molecular and condensed matter physics.
Conical intersections are linked to the energy of quantum systems. They can occur in any physical system characterized by both slow and fast degrees of freedom  such as e.g. the fast electrons and slow nuclei of a vibrating and rotating molecule  and are important when studying the evolution of quantum systems controlled by classical parameters. Furthermore, they play a relevant role for understanding the topological properties of condensed matter systems.
Conical intersections are associated with many interesting features, such as a breakdown of the BornOppenheimer approximation and the appearance of nontrivial artificial gauge structures, similar to the AharonovBohm effect.
Some applications presented in this book include
 Molecular Systems: some molecules in nonlinear nuclear configurations undergo JahnTeller distortions under which the molecule lower their symmetry if the electronic states belong to a degenerate irreducible representation of the molecular point group.
 Solid State Physics: different types of Berry phases associated with conical intersections can be used to detect topologically nontrivial states of matter, such as topological insulators, Weyl semimetals, as well as Majorana fermions in superconductors.
 Cold Atoms: the motion of cold atoms in slowly varying inhomogeneous laser fields is governed by artificial gauge fields that arise when averaging over the fast internal degrees of freedom of the atoms. These gauge fields can be Abelian or nonAbelian, which opens up the possibility to create analogs to various relativistic effects at low speed.
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