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 Physics , 2001, DOI: 10.1088/0305-4470/34/47/324 Abstract: We discuss a generalization to 2 qubits of the standard Bloch sphere representation for a single qubit, in the framework of Hopf fibrations of high dimensional spheres by lower dimensional spheres. The single qubit Hilbert space is the 3-dimensional sphere S3. The S2 base space of a suitably oriented S3 Hopf fibration is nothing but the Bloch sphere, while the circular fibres represent the qubit overall phase degree of freedom. For the two qubits case, the Hilbert space is a 7-dimensional sphere S7, which also allows for a Hopf fibration, with S3 fibres and a S4 base. A main striking result is that suitably oriented S7 Hopf fibrations are entanglement sensitive. The relation with the standard Schmidt decomposition is also discussed
 Physics , 2003, DOI: 10.1088/0305-4470/37/4/022 Abstract: Physical constraints such as positivity endow the set of quantum states with a rich geometry if the system dimension is greater than two. To shed some light on the complicated structure of the set of quantum states, we consider a stratification with strata given by unitary orbit manifolds, which can be identified with flag manifolds. The results are applied to study the geometry of the coherence vector for n-level quantum systems. It is shown that the unitary orbits can be naturally identified with spheres in R^{n^2-1} only for n=2. In higher dimensions the coherence vector only defines a non-surjective embedding into a closed ball. A detailed analysis of the three-level case is presented. Finally, a refined stratification in terms of symplectic orbits is considered.
 Pierre Cladé Physics , 2014, Abstract: In Paris, we are using an atom interferometer to precisely measure the recoil velocity of an atom that absorbs a photon. In order to reach a high sensitivity, many recoils are transferred to atoms using the Bloch oscillations technique. In this lecture, I will present in details this technique and its application to high precision measurement. I will especially describe in details how this method allows us to perform an atom recoil measurement at the level of $1.3 \times 10^{-9}$. This measurement is used in the most precise determination of the fine structure constant that is independent of quantum electrodynamics.
 Physics , 2007, Abstract: We present three different matrix bases that can be used to decompose density matrices of d--dimensional quantum systems, so-called qudits: the generalized Gell-Mann matrix basis, the polarization operator basis, and the Weyl operator basis. Such a decomposition can be identified with a vector --the Bloch vector, i.e. a generalization of the well known qubit case-- and is a convenient expression for comparison with measurable quantities and for explicit calculations avoiding the handling of large matrices. We consider the important case of an isotropic two--qudit state and decompose it according to each basis. Investigating the geometry of entanglement of special parameterized two--qubit and two--qutrit states, in particular we calculate the Hilbert--Schmidt measure of entanglement, we find that the Weyl operator basis is the optimal choice since it is closely connected to the entanglement of the considered states.
 Mihai Ivan Mathematics , 2014, Abstract: Dynamics of Maxwell-Bloch top system, that includes Maxwell-Bloch and Lorenz-Hamilton equations as particular cases, is studied in the framework Poisson geometry. Constants of motion as well as the relation of solution to that of pendulum are presented. Equilibrium states are determined and their complete stability analysis are performed. Results are applied to an optimal control problem on the Lie group G4.
 Physics , 2002, DOI: 10.1016/S1386-9477(02)01076-7 Abstract: We comment on the relation between Berry phase and quantized Hall conductivities for charge and spin currents in some Bloch states, such as Bloch electrons in the presence of electromagnetic fields and quasiparticles in the vortex states of superfluid $^3$He. One can find out that the arguments presented here are closely related to the spontaneous polarization in crystalline dielectrics and the adiabatic pumping.
 Physics , 2011, DOI: 10.1103/PhysRevA.85.013639 Abstract: We present a local measurement of gravity combining Bloch oscillations and atom interferometry. With a falling distance of 0.8 mm, we achieve a sensitivity of 2x10-7 g with an integration time of 300 s. No bias associated with the Bloch oscillations has been measured. A contrast decay with Bloch oscillations has been observed and attributed to the spatial quality of the laser beams. A simple experimental configuration has been adopted where a single retro-reflected laser beam is performing atoms launch, stimulated Raman transitions and Bloch oscillations. The combination of Bloch oscillations and atom interferometry can thus be realized with an apparatus no more complex than a standard atomic gravimeter.
 Physics , 2008, Abstract: We use Bloch oscillations to transfer coherently many photon momenta to atoms. Then we can measure accurately the recoil velocity $\hbar k/m$ and deduce the fine structure constant $\alpha$. The velocity variation due to Bloch oscillations is measured using atom interferometry. This method yields to a value of the fine structure constant $\alpha^{-1}= 137.035 999 45 (62)$ with a relative uncertainty of about $4.5 \times 10^{-9}$.
 Physics , 2010, Abstract: We report on experiments investigating quantum transport and band interferometry of an atomic Bose-Einstein condensate in an optical lattice with a two-band miniband structure, realized with a Fourier-synthesized optical lattice potential. Bloch-Zener oscillations, the coherent superposition of Bloch oscillations and Landau-Zener tunneling between the two bands are observed. When the relative phase between paths in different bands is varied, an interference signal is observed, demonstrating the coherence of the dynamics in the miniband system. Measured fringe patterns of this St\"uckelberg interferometer allow to interferometrically map out the band structure of the optical lattice over the full Brillouin zone.
 Craig Hogan Physics , 2012, Abstract: All existing experimental results are currently interpreted using classical geometry. However, there are theoretical reasons to suspect that at a deeper level, geometry emerges as an approximate macroscopic behavior of a quantum system at the Planck scale. If directions in emergent quantum geometry do not commute, new quantum-geometrical degrees of freedom can produce detectable macroscopic deviations from classicality: spatially coherent, transverse position indeterminacy between any pair of world lines, with a displacement amplitude much larger than the Planck length. Positions of separate bodies are entangled with each other, and undergo quantum-geometrical fluctuations that are not describable as metric fluctuations or gravitational waves. These fluctuations can either be cleanly identified or ruled out using interferometers. A Planck-precision test of the classical coherence of space-time on a laboratory scale is now underway at Fermilab.
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