News & Events

Speaker: Amir O. Caldeira
Abstract Details: In this talk it is our intention to review the basic ideas of how entanglement relates to the so-called Schrödinger cat state and present a paradigmatic situation where states very similar to that one can be created. The example we have chosen is the SQUID ring which depending on the external bias allows us to implement a wealth of interesting physical situations to be treated. We shall argue that in these situations the question of dissipation is really relevant and the concept of decoherence naturally arises. Once we have accomplished that we discuss some possible implications of decoherence to the quantum theory of measurement . As a matter of fact, we shall employ an alternative measure of quantum correlation which goes beyond entanglement – the quantum discord – with the same purpose. We finally present recent experimental results performed with twin photons which corroborate our predictions. This Colloquium is jointly organised with the Center For Quantum Technologies, NUS.
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Speaker: Matthias Droth
Affiliation: University of Konstanz, Germany
Abstract Details: Armchair graphene nanoribbons (aGNR) are promising as a host material for electron spin qubits because of their potential for scalability and long coherence times [1]. The spin lifetime T1 is limited by spin relaxation, where the Zeeman energy is absorbed by lattice vibrations [2], mediated by spin-orbit and electron-phonon coupling. We have calculated T1 by treating all couplings analytically and find that T1 can be in the range of seconds for several reasons: (i) Van Vleck cancellation; (ii) weak spin-orbit coupling; (iii) low phonon density; (iv) vanishing coupling to out-of-plane modes due to the electronic structure of the aGNR. Owing to the vanishing nuclear spin of 12C, T1 is a good measure for overall coherence. These results and recent advances in the controlled production of graphene nanoribbons [3] make this system interesting for classical and quantum spin- tronics applications. [1] B. Trauzettel, D. V. Bulaev, D. Loss, and G. Burkard, Nature Phys. 3, 192-196 (2007). [2] M. Droth and G. Burkard, Phys. Rev. B 84, 155404 (2011). [3] X. Zhang et al., ACS Nano 7,198 (2013).
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Speaker: Paolo E. Trevisanutto
Affiliation: Graphene Centre & SSLS
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Speaker: Ma Ping Nang
Affiliation: Centre for Quantum Technologies, NUS
Abstract Details: Quantum Monte Carlo (QMC) algorithms based on a world-line representation are among the most powerful numerical-exact techniques for the simulation of non-frustrated spin models and of bosonic models at finite temperature. The directed worm algorithm (DWA) is one of these continuous-time numerical-exact QMC methods that proves to be very efficient. In this talk, I shall focus on the DWA simulation of boson Hubbard model, directly applicable to realistic bosonic optical lattices of actual sizes (~200^3). Its capability to directly compare with the experiments proves DWA to be in-challengable by any other numerical method. Last but not least, I shall discuss about the ALPS C++/Python implementation of DWA.
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Speaker: Tanmoy Das
Affiliation: LANL, USA & Graphene Centre
Abstract Details: The realization of most of the material properties such as the newly discovered ‘topological insulator’ and spin-orbit locked electronic states is limited to single compound synthesis with appropriate symmetries. Here we propose ways of artificially engineering such three dimensional (3D) bulk properties in layer by layer approaches. In the first example, we show that 3D `topological insulators’ (which act as insulator in the bulk while metallic on the surface) can be designed by growing bilayer of Rashba-type spin-orbit coupled 2D electronic gas on adjacent planes of bilayers.[1] Secondly, we propose two complementary design principles for engineering 3D Weyl semimetals and superconductors (which host relativistic electronic states dispersing in all three spatial directions with very high mobility). We show that such states can be engineered artificially in a layer-by-layer setup which includes even and odd parity orbitals in alternating layers.[2] Finally, we show how electronic interaction can be introduced and tuned in these highly functional 2D layered superlattices which renders a new form of phase, dubbed ‘spin-orbit density wave’.[3] Possible realizations and/ or experimental evidences of these proposals, and their fundamental implications will also be discussed. [1] Tanmoy Das, A. V. Balatsky, “Engineering three-dimensional topological insulators in Rashba-type spin-orbit coupled heterostructures”, Nat. Commun. 4, 1972 (2013). [2] Tanmoy Das, “Weyl semimetals and superconductors designed in an orbital selective superlattice”, Phys. Rev. B 88, 035444 (2013). [3] Tanmoy Das, “Interaction induced staggered spin-orbit order in two-dimensional electron gas”, Phys. Rev. Lett. 109, 246406 (2012).
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Speaker: Joel K.W. Yang
Affiliation: IMRE and SUTD
Abstract Details: The field of Nanoplasmonics focuses on the phenomenon of light interaction with metal structures that support resonances at optical frequencies. The collective oscillations of charges in these structures give rise to resonances and enhanced fields that depend strongly on the precise geometry, metal gap sizes, and surrounding dielectric media. Due to the significant dimensional dependence of nanoplasmonic structures down to the few-nanometer scale, highly precise nanofabrication capabilities are imperative. In this talk we will discuss some advances in nanofabrication processes involving electron-beam lithography [1], self-assembly [2], and chemical synthesis [3] to control features in these nanostructures that affect their interaction with light. Examples include the control of nanostructure sizes to produce color prints [4], gap sizes for strong localization of light [5], atomic-scale grain boundaries, and nanoscale contacts between metal nanostructures and a dielectric substrates as sources of damping. References [1] H. Duan, H. Hu, H.K. Hui, Z. Shen, J.K.W. Yang, “Free-standing sub-10 nm nanostencils for the definition of gaps in plasmonic antennas”, Nanotechnology 2013, 24, 185301 [2] M. Asbahi, K.T.P. Lim, F. Wang, H. Duan, N. Thiyagarajah, V. Ng, and J.K.W. Yang , “Directed Self-Assembly of Densely Packed Gold Nanoparticles”, Langmuir 2012, 28, 6725-17216 [3] H. Hu, Y.A. Akimov, H. Duan, X. Li, M. Liao, R. Lee, L. Wu, H. Chen, H. Fan, P. Bai, P.S. Lee, J.K.W. Yang and Z. Shen*, “Photoluminescence via gap plasmons between single silver nanowires and a thin gold film”, Nanoscale 2013, 5, 12086-12091 [4] K. Kumar, H. Duan, R. Hegde, S. Koh, J. Wei, J.K.W. Yang, “Printing Color at the Optical Diffraction Limit”, Nature Nanotechnology 2012 7, 557–561 [5] H. Duan, A.I. Fernandez-Dominguez, M. Bosman, S.A. Maier, J.K.W. Yang, “Nanoplasmonics: Classical Down To The Nanometer Scale”, Nano Letters 2013, 12 (3), 1683-1689
About the Speaker: Joel K.W. Yang is an Assistant Professor at the Singapore University of Technology and Design. He received his SM (2005) and PhD (2009) degrees from the Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science. During his PhD, he worked on superconducting nanowire single-photon detectors, and developed capability for sub-10-nm patterning using electron-beam lithography and block-copolymer self assembly. He holds a joint appointment at the Institute of Materials Research and Engineering (IMRE) of A*STAR where he continues to lead the efforts on Nanoplasmonics, and Nanopatterning. He is recognized for pioneering color printing at the highest-possible resolutions (100,000 dpi) using scattering off metal nanostructures. His research interests include sub-10-nm resolution lithography, directed self assembly, and in interfacing plasmonic with electronics. He was the recipient of the MIT Technology Review TR35@Singapore award, and the Singapore Young Scientist Award.
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Speaker: Gareth W. Jones
Affiliation: NUS
Abstract Details: Elastic plate theory can be used to model many important and interesting systems, from atomically-thin graphene, to soft gel sheets and two-dimensional biological membranes such as epithelia. The ability of these materials to bend out of plane in response to internal and external forces has led them to be used in a wide variety of applications, including MEMS devices such as piezoelectric actuators. In particular, the ability of hydrogels to change thermal and chemical energy into motion has led to their use in biocompatible devices like valves and programmable actuators. However, the nonlinear nature of the governing equations hinders the shape optimization of these structures for their desired purpose. In this talk I present a computational method that can be used to find the deformation or mechanical properties of an elastic plate that optimizes their efficacy in applications. Two examples are provided, namely the optimization of the shape of a microscopic graphene pressure switch, and the swelling of a thin gel sheet causing it to adopt a desired shape with useful mechanical properties.
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Speaker: Anthony Leggett
Affiliation: UIUC and NUS
Abstract Details: I give an introduction to the general idea of topologically protected quantum computing. I then review some of the systems, both naturally occurring and purpose-engineered, which have been proposed for its implementation, and try to assess the likelihood of success in each case.
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Abstract Details:

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Speaker: Ashley DaSilva
Affiliation: U. Texas at Austin, USA
Abstract Details: Graphene's response to electromagnetic excitation shows promise for a variety of applications including near perfect light absorption and radiative energy transfer control. At the heart of this response are collective oscillations of the carriers of graphene, which are known as plasmons. I will discuss the plasmon modes of weakly coupled graphene multilayers. These plasmon modes are associated with peaks in the transmission coefficient of electromagnetic waves. A radiative molecule placed near to the surface of such a graphene multilayer will radiate into these modes, thus decaying more rapidly, provided the energy of the radiated modes match the energy range of the plasmon modes. This ability to control radiative energy transfer is quantified by the Purcell factor, which is greatly enhanced in graphene multilayer systems in the THz to IR regimes of the electromagnetic spectrum. I will compare this behavior to that of metallic superlattices, which show enhanced Purcell factor in the optical part of the electromagnetic spectrum. Tuning the graphene Fermi level provides a knob with which to control the plasmon energies. This tunabilitiy as well as the novel energy regime (THz to IR) makes graphene multilayers an exciting system in which to study optical properties.
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