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Viewing upcoming talks containing the keyword: 19
Physics and Astronomy Colloquia
Speaker: Pratap Raychaudhuri (Tata Institute of Fundamental Research)
In 1969, working on a theoretical problem out of pure mathematical curiosity, David Thouless stumbled upon a new kind of phase transition, across which physical properties show abrupt change but the free energy varies smoothly. Very soon, Michael Kosterlitz and David Thouless realised that this kind of phase transition could be ubiquitous across 2-dimensional (2D) systems. For the particular case of a 2-dimensional crystalline solid, the (Berezinski)-Kosterlitz-Thouless (BKT) theory predicts that the solid melts via a novel intermediate state, called the hexatic fluid, which possesses the orientational order of a solid but the flow properties of a fluid. Recently, using a combination of real space imaging and transport measurements we unraveled the hexatic vortex fluid state in a thin film of the amorphous superconductor, MoGe . In this talk I will discuss the properties of this hexatic vortex fluid and also discuss its extreme sensitivity to external electromagnetic perturbations [2,3].
1. Melting of the Vortex Lattice through Intermediate Hexatic Fluid in an a-MoGe Thin Film, Indranil Roy, Surajit Dutta, Aditya N. Roy Choudhury, Somak Basistha, Ilaria Maccari, Soumyajit Mandal, John Jesudasan, Vivas Bagwe, Claudio Castellani, Lara Benfatto, and Pratap Raychaudhuri, Phys. Rev. Lett. 122, 047001 (2019).
2. Extreme sensitivity of the vortex state in a-MoGe films to radio-frequency electromagnetic perturbation, Surajit Dutta, Indranil Roy, Soumyajit Mandal, John Jesudasan, Vivas Bagwe, and Pratap Raychaudhuri, Phys. Rev. B 100, 214518 (2019).
3. An inertial model of vortices to explain the extreme sensitivity of hexatic vortex fluid to low frequency ac excitation, Surajit Dutta and Pratap Raychaudhuri, Physica C 578, 1353740 (2020).
On: September 18, 2020 From: 10h00 To: 11h00View talk
Physics and Astronomy Colloquia
Speaker: Jonathan Pritchard (University of Strathclyde)
Quantum mechanics offers a revolutionary approach to how information is processed, with unprecedented levels of security through quantum networking and exponential speed up with quantum computing. Neutral atoms provide an excellent platform for quantum computing, enabling large numbers of identical qubits to be cooled and trapped overcoming major barriers to scaling experienced by competing architectures . A crucial ingredient for quantum computing is the ability to perform two-qubit gate operations, for which the strong, long-range dipole-dipole interaction between Rydberg atoms can be exploited to create a ‘dipole blockade’ which prevents the excitation of more than one Rydberg atom within a radius R < 10 μm . Using this effect we have previously demonstrated ground-state entanglement between a pair of atoms with a fidelity of 81% , however when scaling to larger qubit numbers the collective enhancement of the many-body state leads to number dependent pulse areas that reduce gate fidelity.We show recent results demonstrating an alternative mesoscopic gate scheme based on electromagnetically induced transparency (EIT), originally proposed by Müller et al. . This protocol provides a scalable approach to performing entanglement of large number of target qubits using a single control atom whilst circumventing challenges of the collective Rabi frequency. The resulting CNOT^N gate protocol is therefore robust against number fluctuations and provides a route to creating useful entangled states for high-precision measurements beyond the standard quantum limit as well as providing a key gate for implementing error correction in neutral atom arrays.
An alternative route to scalable quantum computing involves hybrid quantum systems based on coupling atomic qubits to superconducting microwave cavities via the strong Rydberg atom electric dipole moment. As well as extending the interaction range from microns to centimeters, this provides a versatile quantum interface enabling generation, storage and entanglement of photons in both microwave an optical domains using a scalable on-chip design. We present progress towards a hybrid quantum interface based on superconducting niobium nitride coplanar waveguide resonators, optimized for operation at 4 K to allow strong coupling to Rydberg atom qubits.
 C.S. Adams, J.D. Pritchard and J. Shaffer, “Rydberg Atom Quantum Technologies,” J. Phys. B (in press) (2019).
 M. Saffman, T.G. Walker and K. Mølmer, “Quantum information with Rydberg atoms,” Rev. Mod. Phys. 82, 2313 (2010).
 M. Müller et al., “Mesoscopic Rydberg Gate Based on Electromagnetically Induced Transparency,” Phys. Rev. Lett. 102, 170502 (2009).
 C.J. Picken, R. Legaie, K. McDonnell and J.D. Pritchard, “Entanglement of neutral-atom qubits with long ground-Rydberg coherence times,” Quantum Sci. Technol. 4, 015011 (2018).
On: September 25, 2020 From: 10h00 To: 11h00View talk
Cond Mat Seminars
Speaker: Eric Heller (Harvard)
The missing theory of resistivity is exposed, found hiding on the other side of the wave-particle duality for phonons. Using a coherent state representation of phonons sets the lattice free to vibrate as semiclassical electrons quasi-elastically deflect off lattice density fluctuations. Metallic conduction electrons are found to be traversing a confused sea of nearly classical forces. Superlattices cause channeling and flat band propagation providing a seeming escape from electron-phonon interactions. Bloch waves take on only a supporting role.
On: September 30, 2020 From: 14h00 To: 15h00View talk
Theoretical treatment of spin-forbidden processes and molecular magnetism: Quantitative tools and qualitative analysis
Speaker: Anna Krylov (University of Southern California)
This lecture will describe theoretical aspects of spin-related phenomena, such as spin-forbidden processes, which play an important role in photovoltaics, and molecular magnetism, which is of interest for quantum information science. Recent methodological developments within equation-of-motion coupled cluster theory will be discussed and illustrated by examples.1. D. Casanova and A. I. Krylov, Spin-flip methods in quantum chemistryPhys. Chem. Chem. Phys. 22, 4326 (2020).2. P. Pokhilko and A. I. Krylov, Quantitative El-Sayed rules for many-body wavefunctions from spinless transition density matrices, J. Phys. Chem. Lett. 10, 4857 (2019).3. P. Pokhilko and A. I. Krylov, Effective Hamiltonians derived from equation-of-motion coupled-cluster wave-functions: Theory and application to the Hubbard and Heisenberg Hamiltonians, J. Chem. Phys. 152, 094108 (2020).
On: September 30, 2020 From: 16h00 To: 17h00View talk
Physics and Astronomy Colloquia
Speaker: Nicholas Wright (University of Keele)
The formation and evolution of young star clusters and OB associations is fundamental to our understanding of the star formation process, the conditions faced by young binary and planetary systems, and the formation of long-lived open and globular clusters. I will present 3D kinematic studies of multiple young star clusters and OB associations that constrain their current dynamical state, probe how they formed and reveal how they are currently dispersing. Our results challenge the classical picture of star cluster and OB association formation and suggest a new picture whereby star clusters and OB associations originate as extended systems.
On: October 2, 2020 From: 10h00 To: 11h00View talk
Cond Mat Seminars
Speaker: Claire Donnelly (University of Cambridge)
Three dimensional magnetic systems promise significant opportunities for applications, for example providing higher density devices and new functionality associated with complex topology and greater degrees of freedom [1,2]. For the experimental realisation of these new properties, appropriate characterisation techniques are required to determine both the three-dimensional magnetic structure, and its response to external excitations. For three-dimensional magnetic imaging, we have developed X-ray magnetic nanotomography , combining a new iterative reconstruction algorithm  with a dual rotation axis experimental setup, therefore providing access to the three-dimensional magnetic configuration at the nanoscale. In a first demonstration, we have determined the complex three-dimensional magnetic structure within the bulk of a micrometre-sized soft magnetic pillar and observed a magnetic configuration that consists of vortices and antivortices, as well as Bloch point singularities . In addition to the magnetic structure, the dynamic response of the 3D magnetic configuration to excitations is key to our understanding of both fundamental physics, and applications. With our recent development of X-ray magnetic laminography [5,6], it is now possible to determine the magnetisation dynamics of a three-dimensional magnetic system  with spatial and temporal resolutions of 50 nm and 70 ps, respectively. A final challenge concerns the identification of nanoscale topological objects within the complex reconstructed magnetic configurations. To address this, we have recently implemented calculations of the magnetic vorticity [7,8], that make possible the location and identification of 3D magnetic solitons, leading to the first observation of magnetic vortex rings . These new experimental capabilities of X-ray magnetic imaging open the door to the elucidation of complex three-dimensional magnetic structures, and their dynamic behaviour. Fernández-Pacheco et al., “Three-dimensional nanomagnetism” Nature Communications 8, 15756 (2017) Donnelly and V. Scagnoli, “Imaging three-dimensional magnetic systems with X-rays” J. Phys. D: Condensed Matter (2019). Donnelly et al., “Three-dimensional magnetization structures revealed with X-ray vector nanotomography” Nature 547, 328 (2017).  Donnelly et al., “Tomographic reconstruction of a three-dimensional magnetization vector field” New Journal of Physics 20, 083009 (2018).  Donnelly et al., “Time-resolved imaging of three-dimensional nanoscale magnetization dynamics”, Nature Nanotechnology 15, 356 (2020).  Witte, et al., “From 2D STXM to 3D Imaging: Soft X-ray Laminography of Thin Specimens”, Nano Letters 20, 1305 (2020).  Cooper, “Propagating magnetic vortex rings in ferromagnets.” PRL. 82, 1554 (1999). Donnelly et al., “Experimental observation of vortex rings in a bulk magnet” Nature Physics Accepted (2020)
On: November 11, 2020 From: 13h00 To: 14h00View talk