λJ J J λJ J J λJ J J

It is well known that light quanta (photons) can interact with each other in nonlinear media, much like massive particles do, but in practice these interactions are usually very weak. Here we describe a novel approach to realize strong nonlinear interactions at the single-photon level. Our method makes use of recently demonstrated efficient coupling between individual optical emitters and tightly confined, propagating surface plasmon excitations on conducting nanowires. We show that this system can act as a nonlinear two-photon switch for incident photons propagating along the nanowire, which can be coherently controlled using quantum optical techniques. As a novel application, we discuss how the interaction can be tailored to create a single-photon transistor,where the presence or absence of a single incident photon in a "gate" field is sufficient to completely control the propagation of subsequent "signal" photons.

We describe a general technique that allows one to induce and control strong interaction between spin states of neighboring atoms in an optical lattice. We show that the properties of spin exchange interactions, such as magnitude, sign, and anisotropy, can be designed by adjusting the optical potentials. We illustrate how this technique can be used to efficiently ''engineer'' quantum spin systems with desired properties, for specific examples ranging from scalable quantum computation to probing a model with complex topological order that supports exotic anyonic excitations.

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Understanding relaxation processes is an important unsolved problem in many areas of physics. A key challenge is the scarcity of experimental tools for the characterization of complex transient states. We used measurements of full quantum mechanical probability distributions of matter-wave interference to study the relaxation dynamics of a coherently split one-dimensional Bose gas and obtained comprehensive information about the dynamical states of the system. After an initial rapid evolution, the full distributions reveal the approach toward a thermal-like steady state characterized by an effective temperature that is independent from the initial equilibrium temperature of the system before the splitting process. We conjecture that this state can be described through a generalized Gibbs ensemble and associate it with prethermalization.

In this paper, we study transport properties of non-equilibrium systems under the application of light in many-terminal measurements, using the Floquet picture. We propose and demonstrate that the quantum transport properties can be controlled in materials such as graphene and topological insulators, via the application of light. Remarkably, under the application of off-resonant light, topological transport properties can be induced; these systems exhibits quantum Hall effects in the absence of a magnetic field with a near quantization of the Hall conductance, realizing so-called quantum Hall systems without Landau levels first proposed by Haldane.

† These two authors contributed equally to this work.Geometric phases that characterize the topological properties of Bloch bands play a fundamental role in the modern band theory of solids. Here we report on the direct measurement of the geometric phase acquired by cold atoms moving in one-dimensional optical lattices. Using a combination of Bloch oscillations and Ramsey interferometry, we extract the Zak phase -the Berry phase acquired during an adiabatic motion of a particle across the Brillouin zone -which can be viewed as an invariant characterizing the topological properties of the band. For a dimerized optical lattice, which models polyacetylene, we measure a difference of the Zak phase equal to δϕ Zak = 0.97(2)π for the two possible polyacetylene phases with different dimerization. This indicates that the two dimerized phases belong to different topological classes, such that for a filled band, domain walls have fractional quantum numbers. Our work establishes a new general approach for probing the topological structure of Bloch bands in optical lattices. u-,k Clockwise WindingAnti-Clockwise Winding

Fermionic alkaline-earth atoms have unique properties that make them attractive candidates for the realization of novel atomic clocks and degenerate quantum gases. At the same time, they are attracting considerable theoretical attention in the context of quantum information processing. Here we demonstrate that when such atoms are loaded in optical lattices, they can be used as quantum simulators of unique many-body phenomena. In particular, we show that the decoupling of the nuclear spin from the electronic angular momentum can be used to implement many-body systems with an unprecedented degree of symmetry, characterized by the SU(N) group with N as large as 10. Moreover, the interplay of the nuclear spin with the electronic degree of freedom provided by a stable optically excited state allows for the study of spin-orbital physics. Such systems may provide valuable insights into strongly correlated physics of transition metal oxides, heavy fermion materials, and spin liquid phases.The interest in fermionic alkaline-earth atoms [1, 2, 3, 4, 5, 6, 7, 8] stems from their two key features: (1) the presence of a metastable excited state 3 P 0 coupled to the ground 1 S 0 state via an ultranarrow doubly-forbidden transition [1] and (2) the almost perfect decoupling [1] of the nuclear spin I from the electronic angular momentum J in these two states, since they both have J = 0. This decoupling implies that s-wave scattering lengths involving states 1 S 0 and 3 P 0 are independent of the nuclear spin, aside from the restrictions imposed by fermionic antisymmetry. We show that the resulting SU(N) spin symmetry (where N = 2I + 1 can be as large as 10) together with the possibility of combining (nuclear) spin physics with (electronic) orbital physics open up a wide field of extremely rich many-body systems with alkaline-earth atoms.In what follows, we derive the two-orbital SU(N)-symmetric Hubbard model describing alkaline-earth atoms in 1 S 0 and 3 P 0 states trapped in an optical lattice. We focus on specific parameter regimes characterized by full or partial atom localization due to strong atomic interactions, where simpler effective spin Hamiltonians can be derived. The interplay between orbital and spin degrees of freedom in such effective models is a central topic in quantum magnetism and has attracted tremendous interest in the condensed matter community. Alkaline earth atoms thus provide, on the one hand, a unique opportunity for the implementation of some of these models for the first time in a defect-free and fully controllable environment. On the other hand, they open a new arena to study a wide range of models, many of which have not been discussed previously, even theoretically. We demonstrate, in particular, how to implement the KugelKhomskii model studied in the context of transition metal oxides [9, 10,11,12,13], the Kondo lattice model [14,15,16,17,18,19,20,21,22,23,24,25,26] [27,28,29,30,31,32,33,34]. For example, we discuss how, by appropriately choosing the initial state, a single alkaline-earth atom s...