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Heisenberg-limited sensitivity with decoherence-enhanced measurements ; Martin, John in Nature Communications (2011), 2(223), 1-9 Quantum-enhanced measurements use quantum mechanical effects to enhance the sensitivity of the measurement of classical quantities, such as the length of an optical cavity. The major goal is to beat the ... [more ▼] Quantum-enhanced measurements use quantum mechanical effects to enhance the sensitivity of the measurement of classical quantities, such as the length of an optical cavity. The major goal is to beat the standard quantum limit (SQL), that is, an uncertainty of order 1/ N, where N is the number of quantum resources (for example, the number of photons or atoms used), and to achieve a scaling 1/N, known as the Heisenberg limit. So far very few experiments have demonstrated an improvement over the SQL. The required quantum states are generally highly entangled, difficult to produce, and very prone to decoherence. Here, we show that Heisenberg- limited measurements can be achieved without the use of entangled states by coupling the quantum resources to a common environment that can be measured at least in part. The method is robust under decoherence, and in fact the parameter dependence of collective decoherence itself can be used to reach a 1/N scaling. [less ▲] Detailed reference viewed: 81 (25 ULg)Multiqubit symmetric states with a high geometric entanglement Martin, John ; ; et al Poster (2010, July) Detailed reference viewed: 24 (9 ULg)Multifractality in the kicked rotator Martin, John ; ; et al Poster (2010, July) Detailed reference viewed: 34 (9 ULg)Multifractality in quantum maps Martin, John ; ; et al Poster (2010, March) Detailed reference viewed: 11 (4 ULg)Multiqubit symmetric states with high geometric entanglement Martin, John ; ; et al in Physical Review. A (2010), 81(6), 0623471-6 We propose a detailed study of the geometric entanglement properties of pure symmetric N-qubit states, focusing more particularly on the identification of symmetric states with a high geometric ... [more ▼] We propose a detailed study of the geometric entanglement properties of pure symmetric N-qubit states, focusing more particularly on the identification of symmetric states with a high geometric entanglement and how their entanglement behaves asymptotically for large N. We show that much higher geometric entanglement with improved asymptotical behavior can be obtained in comparison with the highly entangled balanced Dicke states studied previously. We also derive an upper bound for the geometric measure of entanglement of symmetric states. The connection with the quantumness of a state is discussed. [less ▲] Detailed reference viewed: 38 (6 ULg)Multifractal wave functions of simple quantum maps Martin, John ; ; et al in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2010), 82 We study numerically multifractal properties of two models of one-dimensional quantum maps: a map with pseudointegrable dynamics and intermediate spectral statistics and a map with an Anderson-like ... [more ▼] We study numerically multifractal properties of two models of one-dimensional quantum maps: a map with pseudointegrable dynamics and intermediate spectral statistics and a map with an Anderson-like transition recently implemented with cold atoms. Using extensive numerical simulations, we compute the multifractal exponents of quantum wave functions and study their properties, with the help of two different numerical methods used for classical multifractal systems (box-counting and wavelet methods). We compare the results of the two methods over a wide range of values. We show that the wave functions of the Anderson map display a multifractal behavior similar to eigenfunctions of the three-dimensional Anderson transition but of a weaker type. Wave functions of the intermediate map share some common properties with eigenfunctions at the Anderson transition (two sets of multifractal exponents, with similar asymptotic behavior), but other properties are markedly different (large linear regime for multifractal exponents even for strong multifractality, different distributions of moments of wave functions, and absence of symmetry of the exponents). Our results thus indicate that the intermediate map presents original properties, different from certain characteristics of the Anderson transition derived from the nonlinear sigma model. We also discuss the importance of finite-size effects. [less ▲] Detailed reference viewed: 26 (8 ULg)Entanglement of random localized and multifractal states Martin, John ; ; Conference (2009, August) Detailed reference viewed: 6 (0 ULg)Time reversal of Bose-Einstein condensates Martin, John ; ; Poster (2009, July) Detailed reference viewed: 11 (1 ULg)Decoherence-enhanced measurements ; Martin, John Conference (2009, May) Detailed reference viewed: 13 (0 ULg)Entropy of entanglement and multifractal exponents for random states ; Martin, John ; in Physical Review. A (2009), 79 We relate the entropy of entanglement of ensembles of random vectors to their generalized fractal dimensions. Expanding the von Neumann entropy around its maximum we show that the first order only depends ... [more ▼] We relate the entropy of entanglement of ensembles of random vectors to their generalized fractal dimensions. Expanding the von Neumann entropy around its maximum we show that the first order only depends on the participation ratio, while higher orders involve other multifractal exponents. These results can be applied to entanglement behavior near the Anderson transition. [less ▲] Detailed reference viewed: 40 (13 ULg)Chaotic dynamics of a Bose-Einstein condensate coupled to a qubit Martin, John ; ; in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2009), 79 We study numerically the coupling between a qubit and a Bose-Einstein condensate (BEC) moving in a kicked optical lattice, using Gross-Pitaevskii equation. In the regime where the BEC size is smaller than ... [more ▼] We study numerically the coupling between a qubit and a Bose-Einstein condensate (BEC) moving in a kicked optical lattice, using Gross-Pitaevskii equation. In the regime where the BEC size is smaller than the lattice period, the chaotic dynamics of the BEC is effectively controlled by the qubit state. The feedback effects of the nonlinear chaotic BEC dynamics preserve the coherence and purity of the qubit in the regime of strong BEC nonlinearity. This gives an example of an exponentially sensitive control over a macroscopic state by internal qubit states. At weak nonlinearity quantum chaos leads to rapid dynamical decoherence of the qubit. The realization of such coupled systems is within reach of current experimental techniques. [less ▲] Detailed reference viewed: 63 (17 ULg)Entanglement and Localization of Wavefunctions ; ; Martin, John in Complex Phenomena in Nanoscale Systems, NATO Science for Peace and Security Series B: Physics and Biophysics, Volume . ISBN 978-90-481-3118-1. Springer Netherlands, 2009, p. 51 (2009) We review recent works that relate entanglement of random vectors to their localization properties. In particular, the linear entropy is related by a simple expression to the inverse participation ratio ... [more ▼] We review recent works that relate entanglement of random vectors to their localization properties. In particular, the linear entropy is related by a simple expression to the inverse participation ratio, while next orders of the entropy of entanglement contain information about e.g. the multifractal exponents. Numerical simulations show that these results can account for the entanglement present in wavefunctions of physical systems. [less ▲] Detailed reference viewed: 22 (0 ULg)Interference in the light emitted by a single tunneling atom Martin, John ; Poster (2008, July) Detailed reference viewed: 6 (1 ULg)Time Reversal of Atomic Matter Waves Martin, John ; ; Poster (2008, May) Detailed reference viewed: 6 (0 ULg)Spontaneous emission from a tunneling atom ; Martin, John Conference (2008, March) Detailed reference viewed: 6 (0 ULg)Multifractality and intermediate statistics in quantum maps Martin, John ; ; in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2008), 77 We study multifractal properties of wave functions for a one-parameter family of quantum maps displaying the whole range of spectral statistics intermediate between integrable and chaotic statistics. We ... [more ▼] We study multifractal properties of wave functions for a one-parameter family of quantum maps displaying the whole range of spectral statistics intermediate between integrable and chaotic statistics. We perform extensive numerical computations and provide analytical arguments showing that the generalized fractal dimensions are directly related to the parameter of the underlying classical map, and thus to other properties such as spectral statistics. Our results could be relevant for Anderson and quantum Hall transitions, where wave functions also show multifractality. [less ▲] Detailed reference viewed: 20 (5 ULg)Spontaneous emission from a two-level atom tunneling in a double-well potential ; Martin, John in Physical Review. A (2008), 77 We study a two-level atom in a double-well potential coupled to a continuum of electromagnetic modes (black-body radiation in three dimensions at zero absolute temperature). Internal and external degrees ... [more ▼] We study a two-level atom in a double-well potential coupled to a continuum of electromagnetic modes (black-body radiation in three dimensions at zero absolute temperature). Internal and external degrees of freedom of the atom couple due to recoil during emission of a photon. We provide a full analysis of the problem in the long wavelengths limit up to the border of the Lamb-Dicke regime, including a study of the internal dynamics of the atom (spontaneous emission), the tunneling motion, and the electric field of the emitted photon. The tunneling process itself may or may not decohere depending on the wavelength corresponding to the internal transition compared to the distance between the two wells of the external potential, as well as on the spontaneous emission rate compared to the tunneling frequency. Interference fringes appear in the emitted light from a tunneling atom, or an atom in a stationary coherent superposition of its center-of-mass motion, if the wavelength is comparable to the well separation, but only if the external state of the atom is post selected. [less ▲] Detailed reference viewed: 32 (11 ULg)Cooling by Time Reversal of Atomic Matter Waves Martin, John ; ; in Physical Review Letters (2008), 100 We propose an experimental scheme which allows us to realized approximate time reversal of matter waves for ultracold atoms in the regime of quantum chaos. We show that a significant fraction of the atoms ... [more ▼] We propose an experimental scheme which allows us to realized approximate time reversal of matter waves for ultracold atoms in the regime of quantum chaos. We show that a significant fraction of the atoms return back to their original state, being at the same time cooled down by several orders of magnitude. We give a theoretical description of this effect supported by extensive numerical simulations. The proposed scheme can be implemented with existing experimental setups. [less ▲] Detailed reference viewed: 25 (8 ULg)Detuning effects in the vertical cold-atom micromaser Martin, John ; Bastin, Thierry in European Physical Journal D -- Atoms, Molecules, Clusters & Optical Physics (2008), 48 The quantum theory of the cold atom micromaser including the effects of gravity is established in the general case where the cavity mode and the atomic transition frequencies are detuned.We show that ... [more ▼] The quantum theory of the cold atom micromaser including the effects of gravity is established in the general case where the cavity mode and the atomic transition frequencies are detuned.We show that atoms which classically would not reach the interaction region are able to emit a photon inside the cavity. The system turns out to be extremely sensitive to the detuning and in particular to its sign. A method to solve the equations of motion for non resonant atom-field interaction and arbitrary cavity modes is presented. [less ▲] Detailed reference viewed: 30 (13 ULg)Coherent control of atomic tunnelling Martin, John ; in Journal of Physics B Atomic Molecular Physics (2008), 41 We study the tunnelling of a two-level atom in a double well potential while the atom is coupled to a single electromagnetic field mode of a cavity. The coupling between internal and external degrees of ... [more ▼] We study the tunnelling of a two-level atom in a double well potential while the atom is coupled to a single electromagnetic field mode of a cavity. The coupling between internal and external degrees of freedom, due to the mechanical effect on the atom from photon emission into the cavity mode, can dramatically change the tunnelling behaviour. We predict that in general the tunnelling process becomes quasiperiodic. In a certain regime of parameters a collapse and revival of the tunnelling occurs. Accessing the internal degrees of freedom of the atom with a laser allows us to coherently manipulate the atom position, and in particular to prepare the atom in one of the two wells. [less ▲] Detailed reference viewed: 24 (7 ULg) |
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