Academic year 2013 - 2014
9 July Alberto Amo, CNRS-Laboratoire de Photonique et Nanostructures, Marcoussis, Francia
"Condensados de polaritones en microstructuras: de agujeros negros acústicos al estudio de la física del grafeno"
Los polaritones son quasipartículas mitad luz, mitad materia que surgen del acoplamiento fuerte entre excitones y fotones confinados en una microcavidad semiconductora. Gracias a su componente fotónica, los polaritones pueden ser fácilmente creados, manipulados y detectados utilizando técnicas ópticas estándar. Por otro lado, su componente excitónica les confiere interacciones polaritón-polaritón extremadamente fuertes. Estas características junto a su carácter bosónico han permitido estudiar fascinantes efectos a muchos cuerpos como la condensación [1] o la superfluidez [2], y permiten la descripción de un gas de polaritones como un fluido cuántico hidrodinámico [3].
En el “Laboratoire de Photonique et Nanostructures” del CNRS hemos desarrollado una técnica de grabado de la estructura semiconductora que nos permite esculpir el potencial sentido por los polaritones de cavidad. De esta forma hemos construido canales unidimensionales en los que hemos realizado un agujero negro acústico para polaritones, un sistema con un fuerte potencial para estudiar la radiación de Hawking en el laboratorio. Asimismo hemos fabricado potenciales en forma de panal de abeja que confieren a los polaritones una dispersión análoga a la de los electrones en grafeno [4]. Estos sistemas análogos, fácilmente manipulables, abren interesantes perspectivas para estudiar el efecto de interacciones en redes tipo grafeno, o la implementación de campos de gauge y efectos spin-órbita en fotones.
[1] J. Kasprzak, M. Richard, S. Kundermann, et al., Nature 443, 409 (2006).
[2] A. Amo, J. Lefrère, S. Pigeon, et al., Nature Phys. 5, 805 (2009).
[3] A. Amo, S. Pigeon, D. Sanvitto, et al., Science 332, 1167 (2011).
[4] T. Jacqmin, I. Carusotto, I. Sagnes, et al., Phys. Rev. Lett. 112, 116402 (2014).
4 June Antonio Fernández-Domínguez, Dpto de Física Teórica de la Materia Condensada, UAM
"Sub-wavelength control of light with transformation optics"
Transformation optics (TO) is a recently developed theoretical framework that makes possible an unprecedented control over electromagnetic fields at a deep sub-wavelength scale [1]. In the context of metamaterial technology, this tool provides a direct link between a desired electromagnetic phenomenon, such as perfect lensing or invisibility cloaking, and the material response required for its occurrence. TO has also been revisited as a powerful method to describe surface-plasmon-assisted effects in complex nanostructures. In this talk, I will illustrate this approach through the treatment of paradigmatic plasmonic geometries such as spherical dimers [2] or crescent-shaped nanoparticles [3]. I will also show how TO can be pushed beyond macroscopic electromagnetics through the implementation of spatially dispersive metal permittivities [4]. Finally, I will discuss recent experimental reports [5] which demonstrate the validity of TO predictions even below the nanoscale.
1. J. B. Pendry et al., Science 3, 337 (2012).
2. J. B. Pendry et al., Nature Phys. 9, 518 (2013).
3. A. I. Fernández-Domínguez et al., Nano Lett. 12, 5946 (2012).
4. A. I. Fernández-Domínguez et al., Phys. Rev. Lett. 108, 106802 (2012).
5. C. Ciraci et al., Science 337, 1072 (2012).
28 April Anton Ramsak, Faculty of Mathematics and Physics, University of Ljubljana, and J. Stefan Institute, 1000 Ljubljana, Slovenia
"Exact nonadiabatic non-Abelian geometric phases"
First we will present an exact solution for the wavefunction of an electron in a semiconductor quantum wire with spin-orbit interaction and driven by external time-dependent harmonic confining potential [1]. The motivation is the manipulation of electron spin by locally applying an external electric field -- in the absence of magnetic fields which in practice can not selectively be applied in spatially small regions. Next, the solution will further be extended to a more general system, where also the spin-orbit interaction can be time dependent. This additional time dependent degree of freedom enables a holonomic non-Abelian qubit manipulation [2]. For a broad class of driving functions one can by the virtue of the exact solution also in the non-adiabatic regime construct analytically the corresponding dynamical and the geometric Anandan phase [3] or in the adiabatic limit the Wilczek-Zee phase [4]. By breaking the time reversal symmetry the results lead to the corresponding Aharonov-Anandan phase [5] which in the adiabatic limit reduces to the usual Berry phase [6]. A short introduction to the concept of geometric phases in quantum mechanics will be given.
1. T. Cadez, J.H. Jefferson, and A. Ramsak, New J. Phys. 15, 013029 (2013).
2. T. Cadez, J.H. Jefferson, and A. Ramsak, Phys. Rev. Lett. 112, 150402 (2014).
3. J. Anandan, Physics Letters A 133, 171 (1988).
4. F. Wilczek and A. Zee, Phys. Rev. Lett. 52, 2111 (1984).
5. Y. Aharonov and J. Anandan, Phys. Rev. Lett. 58, 1593 (1987).
6. M. V. Berry, Proc. Roy. Soc. Lon., A. Mathematical and Physical Sciences 392, 45 (1984).
7 February Abolfazl Bayat, University College London
"An order parameter for impurity systems at quantum criticality"
The issue of quantum criticality in quantum impurity systems is an interesting problem. In contrast to ordinary bulk quantum phase transitions, the notion of a conventional order parameter which exhibits scaling is notably missing at an impurity quantum critical point. We propose a spin chain emulation of two-impurity Kondo model which exhibits a quantum phase transition (beyond the Landau-Ginzburg paradigm). This quantum phase transition leaves its finger prints in entanglement, quantified by negativity, between different constituents of the system. We also explore the possibility to use the Schmidt gap, which is an observable obtained from the entanglement spectrum, as an order parameter. It faithfully captures the scaling behaviour by correctly predicting the critical exponent of the dynamically generated length scale at the quantum critical point.
6 February David Cubero, Universidad de Sevilla
"Vibrational mechanics in an optical lattice and quasiperiodicity in a nonlinear system"
First, we will show results that demonstrate theoretically and experimentally the phenomenon of vibrational resonance in a periodic potential, using cold atoms in an optical lattice as a model system. A high-frequency (HF) drive, with frequency much larger than any characteristic frequency of the system, is applied, leading to the renormalization of the potential. The experiments demonstrate that ratchet transport can be controlled by the HF drive via potential renormalization.
Secondly, we will analyze the relationship between irrationality and quasiperiodicity in a nonlinear driven system whose steady-state response is very sensitive to the periodic or quasiperiodic character of the input signal. More precisely, the system is spatially periodic and symmetric, showing finite ratchet currents due to symmetry-breaking driving forces. In the infinite time limit, an input signal consisting of two incommensurate frequencies will be recognized by the system as quasiperiodic. We will show that this is in general not true in the case of finite interaction times. An irrational ratio of the driving frequencies of the input signal is not sufficient for it to be recognized by the nonlinear system as quasiperiodic. Thus, the system response depends on the nature of the irrational ratio, as well as the observation time. We will show a condition for the input signal to be identified by the system as quasiperiodic. Such a condition also takes into account the sub-Fourier response of the nonlinear system.
14 January Maciej Lewenstein, ICFO
"Report from the frontiers of atomic, molecular and optical physics and quantum information"
I will talk about some hot topics of AMO and QI by presenting examples of several lines of research, concentrating on theoretical and experimental challenges. The particular subjects will include: a) quantum simulators, i.e. systems capable of simulation of non-trivial and hard to treat quantum many body problems; b) novel kind of numerical and theoretical approaches to many body systems (tensor network states), employing the role of entanglement in many body problems; c) hybrid systems, combining nano-physics with quantum optics.
(this seminar will take place at 12:30)
26 November Renaud Parentani, Paris-Sud Orsay
"Observability of quantum entanglement due to pair creation effects"
In cosmology, it is well known that the expansion of the scale factor induces spontaneous pair creation effects. In particular, in the inflationary scenario, this mechanism explains the observed statistical properties of the large scale structures. Similarly, in cond-mat systems, a sudden temporal change induces pair creation effects. We emphasize that the state so produced is quantum mechanically entangled, and that this property can be observationally verified by accurate measurements of the 2-point correlation functions. We shall also explain that the notion of non-separability (Werner, 1989) provides a simple operational definition of "quantum mechanical entanglement".
Seminar based on arXiv:1305.6841 and arXiv:1311.3507.