The classic lattice XY model is one of the universal models of statistical mechanics appearing in a broad variety of optical and condensed matter systems. One of its possible realizations is a system of tunnel-coupled spinor polariton condensates, where phases of individual condensates play a role of the two- dimensional spins. We show that the account of the polarization degree of freedom of cavity polaritons adds a new twist to the problem, modifying in particular the structure of the ground state. We formulate the corresponding classical spin Hamiltonian, which couples phase and polarization dynamics, and consider several particular geometries, demonstrating the principal differences between the scalar and spinor cases. Possible analog of spin Meissner effect for coupled condensates is discussed.
The classic lattice XY model is one of the universal models of statistical mechanics appearing in a broad variety of optical and condensed matter systems. One of its possible realizations is a system of tunnel-coupled spinor polariton condensates, where phases of individual condensates play a role of the two- dimensional spins. We show that the account of the polarization degree of freedom of cavity polaritons adds a new twist to the problem, modifying in particular the structure of the ground state. We formulate the corresponding classical spin Hamiltonian, which couples phase and polarization dynamics, and consider several particular geometries, demonstrating the principal differences between the scalar and spinor cases. Possible analog of spin Meissner effect for coupled condensates is discussed.
We theoretically investigate the interplay between Zeeman splitting and TE-TM-induced spin-flip tunneling in coupled exciton-polariton condensates systems and its impact on the spin-Meissner effect. We demonstrate that although a single condensate exhibits the effect of full paramagnetic screening via spin-anisotropic interactions, the inter-site spin-flip tunneling can dramatically alter this behavior. The geometry of the system is shown to play a crucial role. In particular, in a dyad, the chemical potential reveals quadratic scaling with the magnetic field. In a triangle, the competition between Zeeman and TE-TM splittings produces a rich phase diagram that features asymmetric polarization states corresponding to both positive and negative magnetic susceptibility. In a square configuration, the symmetry of the network can restore the spin-Meissner effect, so that the condensate emission frequency becomes magnetic field independent in an extended parameter range. These findings not only shed light on the fundamental physics of polariton lattices but also suggest promising avenues for engineering robust spin-controlled photonic devices and polaritonic simulators.
We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the ``tunneling'' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.
In this thesis we study various aspects of spinor Bose-Einstein condensates in optical superlattices using a Bose-Hubbard Hamiltonian that takes spin effects into account. We decouple the unit cells of the superlattice via a mean-field approach and take into account the dynamics within the unit cell exactly. In this way we derive the ground-state phase diagram of spinor bosons in superlattices. The system supports Mott-insulating as well as superfluid phases. The transitions between these phases are second-order for spinless bosons and second- or first-order for spin-1 bosons. Antiferromagnetic interactions energetically penalize high-spin configurations and elongate all Mott lobes, especially the ones corresponding to an even atom number on each lattice site. We find that the quadratic Zeeman effect lifts the degeneracy between different polar superfluid phases leading to additional metastable phases and first-order phase transitions. A change of magnetic fields can drive quantum phase transitions in the same way as a change in the tunneling amplitude does. Furthermore we study the physics of spin-1 atoms in superlattices deep in the Mott insulating phase when the superlattice decomposes into isolated double-well potentials. Assuming that a small number of spin-1 bosons is loaded in an optical double-well potential, we study single-particle tunneling that occurs when one lattice site is ramped up relative to a neighboring site. Spin-dependent effects modify the tunneling events in a qualitative and quantitative way. Depending on the asymmetry of the double well different types of magnetic order occur, making the system of spin-1 bosons in an optical superlattice a model for mesoscopic magnetism with an unprecedented control of the parameters. Homogeneous and inhomogeneous magnetic fields are applied and the effects of the linear and the quadratic Zeeman shifts are examined. We generalize the concept of bosonic staircases to connected double-well potentials. We show that an energy offset between the two sites of the unit cell in an extended superlattice induces a staircase of single-atom resonances in \nthe same way as in isolated double well. We also examine single-atom resonances in the superfluid regime and find clear fingerprints of them in the superfluid density. We also investigate the bipartite entanglement between the sites and construct states of maximal entanglement. The entanglement in our system is due to both orbital and spin degrees of freedom. We calculate the contribution of orbital and \nspin entanglement and show that the sum of these two terms gives a lower bound for the total entanglement.
Arrays of bosonic condensates of exciton-polaritons have emerged as a promising platform for simulating classical XY models, capable of rapidly reaching phase-locked states that may be mapped to arrays of two-dimensional classical spins. However, it remains unclear whether these states genuinely minimize the corresponding XY Hamiltonian and how the convergence time scales with the system size. Here, we develop an analytical model revealing that an array of N condensates possesses N stable phase configurations. The system selectively amplifies a specific configuration dependent on the pump power: at low power, the state with the smallest eigenvalue of an effective XY Hamiltonian is favored, while at high power, the state with the largest eigenvalue prevails. At intermediate pump powers, the system visits all eigenstates of the Hamiltonian. Crucially, the formation rate for any of these phase-locked states remains on the order of 100 ps, independent of the size of the array, demonstrating the exceptional speed and scalability of polariton-based XY simulators.
Exciton-polariton Bose-Einstein condensates (BECs) are newly emerged quantum systems that are capable of showing macroscopic quantum phenomena with intrinsic open-dissipative nature. The spatial distribution of the polariton density, without any external potential, can be controlled by the geometric shape of the pumping laser, enabling the investigation of polariton dynamics with topologically non-trivial configurations. Meanwhile, exciton-polaritons have spin degrees of freedom inherited from excitons and photons, making it a candidate for the realization of quantum logic gates. In this thesis, we will investigate theoretically the nonlinear dynamics of exciton-polariton BECs involving both polaritons' spatial degrees of freedom and spin degrees of freedom, and interactions between them. This thesis is organised as follows: In Chapter 1, we will present an overall review of exiton-polariton systems and important properties of polariton BECs and then introduce the dynamical equations with various interactions that will serve as the main theoretical tool for subsequent chapters. Several polariton pumping and trapping techniques appearing in later chapters will also be introduced. In Chapter 2, we will investigate the superfluidity properties of a single-component polariton condensate under an incoherent annular pumping configuration. By studying the stability properties of polariton persistent currents, we find that the persistent currents can exhibit dynamical instability and energetic-like instability according to different parameter region. A stability phase diagram will be given and its relation with the Landau's criterion will be discussed. In Chapter 3, we will investigate the spin dynamics of a two-component polariton condensate under a homogeneous pumping configuration. Owing to the Josephson coupling, there exist multiple steady state solutions that allow of controlled spin state switching. A desynchronized region where there exists no stable steady solution is found. In the desynchronized region, a desynchronized state beating periodically over time can exist, which will serve as a building block of spin waves presented in the next chapter. In Chapter 4, by combining results from the previous two chapters we will investigate generally the nonlinear dynamics of polariton condensates under an annular pumping configuration. The spin-orbit interaction provided by the Josephson coupling supports azimuthon states that have simultaneous modulations in both amplitude and phase. The azimuthon states, when viewed in a different polarization basis, form rotating spin waves that can be referred to as the optical ferris wheel. In Chapter 5, results from previous chapters will be extended to micocavities that support the anisotropic TE-TM splitting interaction. Rotating singularities (small-scale vortices) are found as a result. Their properties and experimental observation techniques will be discussed. Chapter 2-5 provide a theoretical framework for the nonlinear dynamics of polariton condensates. They rely mostly on optical trapping techniques and are ready to be tested in experiments. In Chapter 6, polaritons trapped by an engineered periodic mesa potential will be discussed. We will investigate the band structure of polaritons under the influence of the periodic potential together with discussions on the phase-modulated interference pattern which corresponds to the polariton Talbot patterns observed in experiments.
Investigation of the spin-cavity polaritons (SCP) is important since they play a decisive role in creating long-storage quantum memories and optical interfaces, polaritonic chemistry, and masers, especially those which operate at room temperature, being of great significance for a lot of different applications. In this report, we suggest an almost completely classical model for the description of the polaritonic peaks conditioned both by free and forced Rabi oscillations of the SCP system. Our investigation is based on the linearized coupled differential equations we derived semi-classically for the magnetization component of the transition of the spin triplet states and the varying field in a cavity when the empty cavity is exactly tuned to the transition. The normal frequencies of this system of equations were found, and the corresponding graphs were constructed for the appropriate values of the parameters. It is shown that the increase in the spin-photon coupling can cause both the pushing apart of normal frequencies at a mutual decay rate (which is the known effect) and the merging together of normal decay rates at a mutual frequency (which is the predicted effect here). Polariton peaks, observable by the cavity transmission function and by EPR in the form of absorption and dispersion signals, were also investigated, and corresponding graphs were constructed. At that, instead of a transition of spin triplet states, a variety of other two-level systems (atoms, molecules, excitons, and so on) can act as emitters. Since the value of the spin-photon coupling is of great interest for applied scientists, obtaining this value by comparing our results with experimental data is of great importance.