Topological insulators are a striking example of materials in which topological invariants are manifested in robustness against perturbations [1,2]. Their most prominent feature is the emergence of topological edge states with reduced dimension at the boundary between areas with distinct topological invariants. The observable physical effect is unidirectional robust transport, unaffected by defects or disorder. Topological insulators were originally observed in the integer quantum Hall effect [3], and subsequently suggested [4-6] and observed [7] even in the absence of magnetic field. These were fermionic systems of correlated electrons. However, during the past decade the concepts of topological physics have been introduced into numerous fields beyond condensed matter, ranging from microwaves [8,9] and photonic systems [10-12] to cold atoms [13,14], acoustics [15,16] and even mechanics [17,18]. Recently, topological insulators were proposed [19-21] in exciton-polariton systems organized as honeycomb (graphene-like) lattices, under the influence of a magnetic field. Topological phenomena in polaritons are fundamentally different from all topological effects demonstrated experimentally thus far: exciton-polaritons are part-light part-matter quasiparticles emerging from the strong coupling of quantum well excitons and cavity photons [22]. Here, we demonstrate experimentally the first exciton-polariton topological insulator. This constitutes the first symbiotic light-matter topological insulators. Our polariton lattice is excited non-resonantly, and the chiral topological polariton edge mode is populated by a polariton condensation mechanism. We use scanning imaging techniques in real-space and in Fourier-space to measure photoluminescence, and demonstrate that the topological edge mode avoids defects, and that the propagation direction of the mode can be reversed by inverting the applied magnetic field. Our exciton-polariton topological insulator paves the way for a variety of new topological phenomena, as they involve light-matter interaction, gain, and perhaps most importantly -exciton-polaritons interact with one another as a nonlinear many-body system.Microcavity exciton-polaritons (polaritons) are composite bosons originating from the strong coupling of quantum well excitons to microcavity photons. While the excitonic fraction provides a strong non-linearity, the photonic part results in a low effective mass, allowing the formation of a driven-dissipative Bose-Einstein condensate [23,24] and a superfluid phase [25], making polaritons being referred to as "quantum fluids of light" [26]. For the epitaxially well-controlled III-V semiconductor material system, a variety of techniques are available to micropattern such cavities in order to precisely engineer the potential landscapes of polaritons [27]. With the recent advances of bringing topological effects to the realms of photonics [8][9][10][11][12]28], several avenues to realize topological edge propagation with polaritons have been suggested [19][20][21], wi...

No abstract

Future communication and computation technologies that exploit quantum information require robust and well-isolated qubits. Electron spins in III-V semiconductor quantum dots, while promising candidates, see their dynamics limited by undesirable hysteresis and decohering effects of the nuclear spin bath. Replacing electrons with holes should suppress the hyperfine interaction and consequently eliminate strong nuclear effects. Using picosecond optical pulses, we demonstrate coherent control of a single hole qubit and examine both free-induction and spin-echo decay. In moving from electrons to holes, we observe significantly reduced hyperfine interactions, evidenced by the reemergence of hysteresis-free dynamics, while obtaining similar coherence times, limited by non-nuclear mechanisms. These results demonstrate the potential of optically controlled, quantum dot hole qubits. arXiv:1106.5676v1 [quant-ph]

We study the rectification of voltage fluctuations in a system consisting of two Coulomb-coupled quantum dots. The first quantum dot is connected to a reservoir where voltage fluctuations are supplied and the second one is attached to two separate leads via asymmetric and energy-dependent transport barriers. We observe a rectified output current through the second quantum dot depending quadratically on the noise amplitude supplied to the other Coulomb-coupled quantum dot. The current magnitude and direction can be switched by external gates, and maximum output currents are found in the nA region. The rectification delivers output powers in the pW region. Future devices derived from our sample may be applied for energy harvesting on the nanoscale beneficial for autonomous and energy-efficient electronic applications. DOI: 10.1103/PhysRevLett.114.146805 PACS numbers: 73.23.-b, 73.50.Td, 73.61.Ey, 85.30.-z Extracting work from random fluctuations by energy conversion to a unidirectional particle flow is a key enabling technology and has consequently triggered substantial experimental and theoretical work [1][2][3][4]. The exploitation of temperature and fluctuation gradients for energy harvesting has led to new concepts such as Brownian and Büttiker-Landauer motors [5][6][7][8], phonon rectifiers [9,10], and piezoelectric nanogenerators [11][12][13]. Challenging factors in miniaturizing heat engines are an efficient energy conversion and the maintenance of welldefined hot and cold spots [14]. Quantum dot structures are among the smallest possible heat engines conceived thus far. Pioneering work in this field was conducted by, among others, Molenkamp et al., who measured the Seebeck voltage of single quantum dots (QDs) and quantum point contacts (QPCs) [15][16][17]. In recent years, research concerning heat engines based on QDs and QPCs followed [18][19][20][21][22]. Furthermore, Coulomb-coupled systems attracted attention due to their ability to generate currents in unbiased wires via the Coulomb drag [23][24][25]. A striking proposal combining rectifying effects with QDs was recently made by Sánchez et al., who showed that two capacitively coupled QDs connected to electron reservoirs operated in the Coulomb-blockade regime can act as a rectifier that transfers each energy quantum that passes from one to the other QD to the motion of single electrons (i.e., to charge quanta) [26]. Notably, the heat and charge current directions are decoupled in the proposed system. Later, Sothmann et al. investigated a similar design based on open QD systems (with conductances higher than the conductance quantum) exhibiting higher output currents, which makes this proposal more accessible to experimental realization [27,28]. Furthermore, it combines maximum output power as well as maximum efficiency at the same electrostatic configuration, which is in contrast to Coulomb-blockade systems, where maximum efficiency theoretically occurs at zero output power [26].In this Letter, we present a system that converts voltage fluctuation...

Macroscopic order appears as the collective behaviour of many interacting particles. Prime examples are superfluidity in helium 1 , atomic Bose-Einstein condensation 2 , s-wave 3 and d-wave superconductivity 4 and metal-insulator transitions 5. Such physical properties are tightly linked to spin and charge degrees of freedom and are greatly enriched by orbital structures 6. Moreover, high-orbital states of bosons exhibit exotic orders distinct from the orders with real-valued bosonic ground states 7. Recently, a wide range of related phenomena have been studied using atom condensates in optical lattices 8-10 , but the experimental observation of highorbital orders has been limited to momentum space 11,12. Here we establish microcavity exciton-polariton condensates as a promising alternative for exploring high-orbital orders. We observe the formation of d-orbital condensates on a square lattice and characterize their coherence properties in terms of population distributions both in real and momentum space. Exciton-polaritons emerge from the strong light-matter coupling in semiconductor quantum wells embedded in a planar microcavity structure. They behave as degenerate Bose gases in the low-density and low-temperature limit 13. Exciton-polaritons have undergone a dynamic phase transition, in which a macroscopic number of particles are accumulated in the lowest-energy single-particle state with a long-range order 14-17. Owing to their very light effective mass, the phase transition temperatures of exciton-polaritons are eight to nine orders of magnitude higher than those of atomic Bose-Einstein condensates. Coherence properties of exciton-polariton condensates have been characterized by the direct optical access in spatial and momentum spaces 14-16,18. Modern solid-state physics has studied quantum many-body phenomena whose properties reflect exotic orbital nature, another intrinsic degree of freedom, which interplays with charge and spin degrees of freedom. Its energy degeneracy and spatial anisotropy generate rich dynamics in weakly interacting many-body systems. For example, a key role of d-orbital nature has been actively investigated in salient phenomena including metal-insulator transitions 5 , colossal magnetoresistance 6,19 , and recently discovered iron-pnictide superconductors 20,21. These orbital ordering phenomena originate from the strong correlation effects of electrons in the anisotropic degenerate d-orbitals. Theoretical modelling of such

We create a large exciton-polariton condensate and employ a Michelson interferometer setup to characterize the short-and longdistance behavior of the first order spatial correlation function. Our experimental results show distinct features of both the twodimensional and nonequilibrium characters of the condensate. We find that the gaussian short-distance decay is followed by a powerlaw decay at longer distances, as expected for a two-dimensional condensate. The exponent of the power law is measured in the range 0.9-1.2, larger than is possible in equilibrium. We compare the experimental results to a theoretical model to understand the features required to observe a power law and to clarify the influence of external noise on spatial coherence in nonequilibrium phase transitions. Our results indicate that Berezinskii-KosterlitzThouless-like phase order survives in open-dissipative systems.quantum well excitons | semiconductor microcavities T he spatial correlation function quantifies the coherence properties of a system (1). In a 3D Bose-condensed gas, long range order is observed, and the correlation function decays toward a plateau at large distances (2, 3). In the homogeneous 2D Bose gas (4), however, no long range order can be established (5). Instead, Berezinskii-Kosterlitz-Thouless (BKT) theory of the equilibrium interacting gas predicts a transition to a low-temperature superfluid phase, which shows a power-law decay of the correlation function (6, 7). Unfortunately, it is frequently hard to directly measure this, and only very recently (8) was indication of the power-law decay of coherence seen in a 2D atomic gas. It has been theoretically predicted (9, 10) that power-law decay of coherence survives in the nonequilibrium problem, and it is this prediction that the current experiment sets out to test.Exciton polaritons are short-lived quasiparticles formed in a semiconductor quantum well strongly coupled to a planar microcavity (11). Each one is a superposition of a quantum well exciton and a microcavity photon, and they behave as 2D bosons below the Mott density. Above a threshold particle density, condensation is observed (12). Due to the nonequilibrium nature of polariton condensation, understanding its coherence properties is quite revealing regarding the different roles of fluctuations in the equilibrium and nonequilibrium problems.Previous measurements on polariton condensates have demonstrated coherence at large distances but were limited by large experimental uncertainties (13) or highly disordered samples (14,15), and the long-distance behavior could not be fully extracted. Recently, the correlation function at large distances was studied in 1D condensates confined in a quantum wire (16) and in a valley of the disorder potential (17). In ref. 16, the data was energy-resolved so that excited states were filtered out, while in ref. 17 a rare area on the sample was chosen in which a single mode condensate is seen. The purpose of both those experiments was to investigate how long the coherence length o...

No abstract

We report the efficient coherent photon scattering from a semiconductor quantum dot embedded in a pillar microcavity. We show that a surface acoustic wave can periodically modulate the energy levels of the quantum dot, but has a negligible effect on the cavity mode. The scattered narrow-band laser is converted to a pulsed single-photon stream, displaying an anti-bunching dip characteristic of single-photon emission. Multiple phonon sidebands are resolved in the emission spectrum, due to the absorption and emission of vibrational quanta in each scattering event.