Stokes vector characterization by strongly measuring weak values.

We report the implementation of a non-standard procedure to perform Stokes polarimetry, which was recently proposed by considering weak value measurements. Our procedure is not restricted to weak measurements but applies for both weak and strong couplings between the observable being measured; the polarization (spin) vector; and the measuring device, the "pointer." In optics, the polarization-pointer coupling is usually implemented with a birefringent crystal. This applies in the weak coupling regime. We overcame this limitation by using an alternative setup in which one can go from weak to strong couplings by tuning a moveable mirror. We carried out our proof-of-concept experiments with a laser beam, the image of which was recorded and processed on a charge-coupled device. Our results illustrate that some concepts, originally introduced in a quantum context, in fact refer to properties that are common to both quantum and classical phenomena.

Wave-particle duality in tripartite systems.

Quantum objects, sometimes called quantons, often display a characteristic feature referred to as wave-particle duality (WPD). Lately, this and other quantum traits have been subjected to intensive research, mainly motivated by the development of quantum information science. As a consequence, the scopes of some concepts have been extended, and it has been realized that they are not in the exclusive domain of quantum physics. This is particularly clear in optics, where qubits may show up as Jones vectors and WPD has its counterpart as wave-ray duality. WPD was originally addressed by focusing on a single qubit, which was afterwards supplemented with a second one playing the role of a path-marker in an interferometer setup. Fringe contrast, a sign of wave-like behavior, was proved to be diminished in connection with the effectiveness of the marker, the inducer of particle-like behavior. Going from bipartite to tripartite states is a natural and necessary step towards better understanding of WPD. This step is what we have accomplished in this work. We report some constraints ruling WPD for tripartite systems, as well as their experimental display with single photons.

Experimental display of generalized wave-particle duality.

The quantification of wave-particle duality (WPD) by means of measurable features associated to it, such as fringe visibility ($\mathcal {V}$) and path distinguishability ($\mathcal {D}$), led to the establishment of the constraint $\mathcal {V}^{2}+\mathcal {D}^{2} \leq \,1$. The two involved quantities refer to so-called "quantons", physical objects that are capable of generating an interferometric pattern, while being at least partially localizable. Any quanton's internal degree of freedom (DOF) can in principle be used as a path-marker. When the quanton and its internal DOF are simultaneously engaged, new constraints can be derived and experimentally tested. Generalized constraints show how $\mathcal {V}$ and $\mathcal {D}$ relate to other quantifiers and bring to light coherences that might remain otherwise hidden in both quantum and classical light. We submitted two-qubit constraints to experimental tests, using optical light beams. This shows that, despite the rather contrived nature of the constraints, linear optics setups are appropriate to test them. Our experimental results are in very good agreement with theoretical predictions related to the tested constraints. Our results also show that quantifiers such as $\mathcal {V}$ and $\mathcal {D}$ help not only to quantify, but also to generalize the concept of WPD.