Yves CAUDANO, Université de Namur

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On Friday October 06, Yves CAUDANO, from Université de Namur, Belgium, will give a seminar about Quantum weak measurements in optics : from geometric phases to amplification and sensing at 09:30 a.m. in Pierre Cotton’s room.

Abstract : Quantum pre- and post-selected weak measurements have garnered significant attention for practical and fundamental reasons. On one hand, they can amplify minute phenomena that would otherwise remain undetectable. For instance, they enable investigating subtle deviations from the laws of geometric optics upon reflection and transmission of optical beams, such as the spatial and angular Goos-Hänchen shifts. On the other hand, they induce a negligible state perturbation, making them ideal to study foundational issues of quantum mechanics : for example, probing quantum superpositions, performing consecutive quantum measurements, or sensing particle trajectories in interference patterns, as in the double-slit experiment. Observations in weak measurements depend on a complex number, called a weak value, that is not bounded to the eigenvalue range. Relatedly, weak values enable direct reconstruction of the complex quantum wavefunction.
Most often, weak values are interpreted in terms of their real and imaginary parts, as they provide the observed shift in the position and momentum representations of the measuring device wavefunction. However, we show that the polar decomposition of the weak values offers another fruitful point of view, due to the appearance of geometric phases. In the case of qubit systems, there is an immediate connection to solid angles on the Bloch sphere. However, for qudits, the geometry of the state manifold is more complicated. In particular, we illustrate how geodesic triangles defining the geometric phases of weak values can be described in multiple ways : using a generalized Bloch (hyper)sphere, Majorana’s stellar representation, or a projection on a spherical octant that was developed previously to represent qutrit states.
Additionally, when the modulus of weak value becomes large, an appropriate post-selection can effectively amplify the experimental sensitivity in the presence of technical limitations. However, dissipation can compromise the effectiveness of amplification. Thus, we examine how the dissipation incurred by an atom or molecule after its interaction with a cavity can be probed by measuring the cavity field in a post-selected weak measurement. Our theoretical model thoroughly explains the connection between weak measurement observations and the particle properties. We also demonstrate generally how dissipation affects the weak value, and we propose a method for extracting information about a system’s dissipative dynamics using weak values at short dissipation times.

Biography :

Contact : Yves CAUDANO, Research Unit Lasers and Spectroscopies, Department of Physics, Namur Institute for Complex Systems (naXys) and Namur Institute for Structured Matter (NISM)

Invitation : NCO seminar