Person: Hafezi, Mohammad
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Hafezi
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Mohammad
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Hafezi, Mohammad
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Publication Efficient All-Optical Switching Using Slow Light within a Hollow Fiber(American Physical Society, 2009) Bajcsy, M; Hofferberth, Sebastian; Balic, V; Peyronel, T; Hafezi, Mohammad; Zibrov, Alexander; Vuletic, V; Lukin, MikhailWe demonstrate a fiber-optical switch that is activated at tiny energies corresponding to a few hundred optical photons per pulse. This is achieved by simultaneously confining both photons and a small lasercooled ensemble of atoms inside the microscopic hollow core of a single-mode photonic-crystal fiber and using quantum optical techniques for generating slow light propagation and large nonlinear interaction between light beams.Publication Photonic Quantum Transport in a Nonlinear Optical Fiber(Institute of Physics, 2011) Hafezi, Mohammad; Chang, Darrick E.; Gritsev, Vladimir; Demler, Eugene; Lukin, MikhailWe theoretically study the transmission of few-photon quantum fields through a strongly nonlinear optical medium. We develop a general approach to investigate nonequilibrium quantum transport of bosonic fields through a finite-size nonlinear medium and apply it to a recently demonstrated experimental system where cold atoms are loaded in a hollow-core optical fiber. We show that when the interaction between photons is effectively repulsive, the system acts as a single-photon switch. In the case of attractive interaction, the system can exhibit either antibunching or bunching, associated with the resonant excitation of bound states of photons by the input field. These effects can be observed by probing statistics of photons transmitted through the nonlinear fiber.Publication Quantum Transport of Strongly Interacting Photons in a One-Dimensional Nonlinear Waveguide(American Physical Review, 2012) Hafezi, Mohammad; Chang, Darrick E.; Gritsev, Vladimir; Demler, Eugene; Lukin, MikhailWe present a theoretical technique for solving the quantum transport problem of a few photons through a one-dimensional, strongly nonlinear waveguide. We specifically consider the situation where the evolution of the optical field is governed by the quantum nonlinear Schrödinger equation. Although this kind of nonlinearity is quite general, we focus on a realistic implementation involving cold atoms loaded in a hollow-core optical fiber, where the atomic system provides a tunable nonlinearity that can be large even at a single-photon level. In particular, we show that when the interaction between photons is effectively repulsive, the transmission of multiphoton components of the field is suppressed. This leads to antibunching of the transmitted light and indicates that the system acts as a single-photon switch. On the other hand, in the case of attractive interaction, the system can exhibit either antibunching or bunching, which is in stark contrast to semiclassical calculations. We show that the bunching behavior is related to the resonant excitation of bound states of photons inside the system.Publication Robust Optical Delay Lines with Topological Protection(Nature Publishing Group, 2011) Hafezi, Mohammad; Demler, Eugene; Lukin, Mikhail; Taylor, JacobPhenomena associated with the topological properties of physical systems can be naturally robust against perturbations. This robustness is exemplified by quantized conductance and edge state transport in the quantum Hall and quantum spin Hall effects. Here we show how exploiting topological properties of optical systems can be used to improve photonic devices. We demonstrate how quantum spin Hall Hamiltonians can be created with linear optical elements using a network of coupled resonator optical waveguides (CROW) in two dimensions. We find that key features of quantum Hall systems, including the characteristic Hofstadter butterfly and robust edge state transport, can be obtained in such systems. As a specific application, we show that topological protection can be used to improve the performance of optical delay lines and to overcome some limitations related to disorder in photonic technologies.Publication Characterization of Topological States on a Lattice with Chern Number(Institute of Physics, 2008) Hafezi, Mohammad; Sørensen, Anders; Lukin, Mikhail; Demler, EugeneWe study Chern numbers to characterize the ground state of strongly interacting systems on a lattice. This method allows us to perform a numerical characterization of bosonic fractional quantum Hall (FQH) states on a lattice where the conventional overlap calculation with the known continuum case such as the Laughlin state, breaks down due to the lattice structure or dipole-dipole interaction. The non-vanishing Chern number indicates the existence of a topological order in the degenerate ground-state manifold.Publication Anyonic Interferometry and Protected Memories in Atomic Spin Lattices(Nature Publishing Group, 2008) Jiang, Liang; Brennen, Gavin; Gorshkov, Alexey; Hammerer, Klemens; Hafezi, Mohammad; Demler, Eugene; Luki, Mikhail; Zoller, PeterStrongly correlated quantum systems can exhibit exotic behavior called topological order which is characterized by non-local correlations that depend on the system topology. Such systems can exhibit remarkable phenomena such as quasi-particles with anyonic statistics and have been proposed as candidates for naturally fault-tolerant quantum computation. Despite these remarkable properties, anyons have never been observed in nature directly. Here we describe how to unambiguously detect and characterize such states in recently proposed spin lattice realizations using ultra-cold atoms or molecules trapped in an optical lattice. We propose an experimentally feasible technique to access non-local degrees of freedom by performing global operations on trapped spins mediated by an optical cavity mode. We show how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit. Furthermore, our technique can be used to probe statistics and dynamics of anyonic excitations.