Stochastic Formation of Polariton Condensates in Two Degenerate Orbital States

We explore the exciton-polariton condensation in the two degenerate orbital states. In the honeycomb lattice potential, at the third band we have two degenerate vortex-antivortex lattice states at the inequivalent K and K'-points. We have observed energetically degenerate condensates within the linewidth ~ 0.3 meV, and directly measured the vortex-antivortex lattice phase order of the order parameter. We have also observed the intensity anticorrelation between polariton condensates at the K- and K'-points. We relate this intensity anticorrelation to the dynamical feature of polariton condensates induced by the stochastic relaxation from the common particle reservoir.Comment: 5 pages, 4 figure

Quantum simulation of Fermi-Hubbard models in semiconductor quantum dot arrays

We propose a device for studying the Fermi-Hubbard model with long-range Coulomb interactions using an array of quantum dots defined in a semiconductor two-dimensional electron gas system. Bands with energies above the lowest energy band are used to form the Hubbard model, which allows for an experimentally simpler realization of the device. We find that depending on average electron density, the system is well described by a one- or two-band Hubbard model. Our device design enables the control of the ratio of the Coulomb interaction to the kinetic energy of the electrons independently to the filling of the quantum dots, such that a large portion of the Hubbard phase diagram may be probed. Estimates of the Hubbard parameters suggest that a metal-Mott insulator quantum phase transition and a d-wave superconducting phase should be observable using current fabrication technologies.Comment: 12 pages, 3 figures, 1 table

Highly excited exciton-polariton condensates

This research was supported by the Japan Society for the Promotion of Science (JSPS) through its FIRST Program and KAKENHI Grant Numbers 24740277 and 25800181, a Space and Naval Warfare Systems (SPAWAR) Grant Number N66001-09-1-2024, the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), the National Institution of Information and Communication Technology (NICT), the joint study program at Institute for Molecular Science. T. H. acknowledges the support of Toray Science Foundation, KDDI Foundation, the Asahi Glass Foundation, ther Murata Science Foundation, and REFEC. T.B. acknowledges the support of the Shanghai Research Challenge Fund, New York University Global Seed Grants for Collaborative Research, National Natural Science Foundation of China (Grant No. 61571301), the Thousand Talents Program for Distinguished Young Scholars (Grant No. D1210036A), and the NSFC Research Fund for International Young Scientists (Grant No. 11650110425).Exciton polaritons are a coherent electron-hole-photon (e-h-p) system where condensation has been observed in semiconductor microcavities. In contrast to equilibrium Bose-Einstein condensation (BECs) for long lifetime systems, polariton condensates have a dynamical nonequilibrium feature owing to the similar physical structure that they have to semiconductor lasers. One of the distinguishing features of a condensate to a laser is the presence of strong coupling between the matter and photon fields. Irrespective of its equilibrium or nonequilibrium nature, exciton polaritons have been observed to maintain strong coupling. We show that by investigating the high-density regime of exciton-polariton condensates, the negative branch is directly observed in photoluminescence. This is evidence that the present e-h-p system is still in the strong-coupling regime, contrary to past results where the system reduced to standard lasing at high density.PostprintPostprintPeer reviewe

EXCITON POLARITON CONDENSATION IN TWO DIMENSIONAL PERIODIC LATTICE POTENTIALS

Bose-Einstein condensation (BEC) was first predicted by Bose and Einstein, and originally defined as a macroscopic occupation of bosonic particles in the ground state. After first demonstration of a nearly ideal Bose-Einstein condensate in 1995 with ultra cold atoms, a great amount of research has been done, such as Bose-Einstein condensates in two-dimensional system, and the observation of a quantised vortex. Exciton polaritons (EPs) are quasi-particles resulting from the strong coupling between (cavity) photons and (quantum well) excitons, and have been considered as a candidate for BEC in solids. Because of their photonic component, their mass is much lighter than that of atoms or excitons, which leads to a much higher critical temperature for BEC in EP systems. The observation of BEC in EP systems has been reported by several groups. Because EPs have a short lifetime of the order of ～ps they may decay from the system before reaching the thermal equilibrium. Continuous pumping replenishes EPs in the system, and as aresult phase coherence for a time longer than their lifetime. Therefore the system has been considered to undergo dynamic condensation of EPs. In atomic systems, another research direction that has attracted a large amount of attention in recent years is quantum simulation. The aim of quantum simulation is to give another way to investigating difficult quantum many-body problems by simulating quantum models experimentally on other quantum devices. This is advantageous because it is often easier to control the model parameters in the artificially fabricated device, rather than the original system being examined. Currently, most researchers working on quantum simulation have been working with cold atom systems and ion trap systems. To perform the quantum simulation, how to implement the desired model is an important question. In cold atom systems, optical lattices provide a suitable way to create periodic lattice potentials. On the other hand, in EP systems, several ways to create potentials have been suggested and demonstrated. Currently weak one dimensional lattice potentials have been demonstrated experimentally. In this thesis, we implemented several two dimensional lattice potentials in EP systems. The dynamic nature of EPs gives us rich physics, especially due to metastable condensation. The EP systems have the advantage over atomic system in that it is easier for excited state condensations to be formed. The periodic lattice potentials are implemented by depositing patterned thin metals. The metal changes the boundary condition of photons and make the cavity resonance higher in energy, which leads to the potential barriers for EPs of the order of ～200μeV. In two dimensional square lattice potentials, we have observed d-orbital wave condensates at M-points. The order parameter of this meta-stable condensation has 2D atomic d-orbital symmetry, and two-fold rotational symmetry against the trap center. We detect this d-orbital wave through the momentum space distribution, and the real field distribution. In two dimensional triangular lattice potentials, we observed several results indicating the formation of vortex-antivortex lattices, which originates from the single particle wave function of the meta-stable state at M-points.報告番号: 甲27285 ; 学位授与年月日: 2011-03-24 ; 学位の種別: 課程博士 ; 学位の種類: 博士(情報理工学) ; 学位記番号: 博情第323号 ; 研究科・専攻: 情報理工学系研究科電子情報学専

2次元周期格子ポテンシャル中のエキシトンポラリトン凝縮

Bose-Einstein condensation (BEC) was first predicted by Bose and Einstein, and originally defined as a macroscopic occupation of bosonic particles in the ground state. After first demonstration of a nearly ideal Bose-Einstein condensate in 1995 with ultra cold atoms, a great amount of research has been done, such as Bose-Einstein condensates in two-dimensional system, and the observation of a quantised vortex. Exciton polaritons (EPs) are quasi-particles resulting from the strong coupling between (cavity) photons and (quantum well) excitons, and have been considered as a candidate for BEC in solids. Because of their photonic component, their mass is much lighter than that of atoms or excitons, which leads to a much higher critical temperature for BEC in EP systems. The observation of BEC in EP systems has been reported by several groups. Because EPs have a short lifetime of the order of ～ps they may decay from the system before reaching the thermal equilibrium. Continuous pumping replenishes EPs in the system, and as aresult phase coherence for a time longer than their lifetime. Therefore the system has been considered to undergo dynamic condensation of EPs. In atomic systems, another research direction that has attracted a large amount of attention in recent years is quantum simulation. The aim of quantum simulation is to give another way to investigating difficult quantum many-body problems by simulating quantum models experimentally on other quantum devices. This is advantageous because it is often easier to control the model parameters in the artificially fabricated device, rather than the original system being examined. Currently, most researchers working on quantum simulation have been working with cold atom systems and ion trap systems. To perform the quantum simulation, how to implement the desired model is an important question. In cold atom systems, optical lattices provide a suitable way to create periodic lattice potentials. On the other hand, in EP systems, several ways to create potentials have been suggested and demonstrated. Currently weak one dimensional lattice potentials have been demonstrated experimentally. In this thesis, we implemented several two dimensional lattice potentials in EP systems. The dynamic nature of EPs gives us rich physics, especially due to metastable condensation. The EP systems have the advantage over atomic system in that it is easier for excited state condensations to be formed. The periodic lattice potentials are implemented by depositing patterned thin metals. The metal changes the boundary condition of photons and make the cavity resonance higher in energy, which leads to the potential barriers for EPs of the order of ～200μeV. In two dimensional square lattice potentials, we have observed d-orbital wave condensates at M-points. The order parameter of this meta-stable condensation has 2D atomic d-orbital symmetry, and two-fold rotational symmetry against the trap center. We detect this d-orbital wave through the momentum space distribution, and the real field distribution. In two dimensional triangular lattice potentials, we observed several results indicating the formation of vortex-antivortex lattices, which originates from the single particle wave function of the meta-stable state at M-points.University of Tokyo (東京大学