Weak measurements with orbital angular momentum pointer states

Weak measurements are a unique tool for accessing information about weakly interacting quantum systems with minimal back action. Joint weak measurements of single-particle operators with pointer states characterized by a two-dimensional Gaussian distribution can provide, in turn, key information about quantum correlations which can be of relevance for quantum information applications. Here we demonstrate that by employing two-dimensional pointer states endowed with orbital angular momentum (OAM), it is possible to extract weak values of the higher order moments of single-particle operators, an inaccessible quantity with Gaussian pointer states only. We provide a specific example that illustrates the advantages of our method both, in terms of signal enhancement, and information retrieval.Comment: 5 pages, accepted for publication in Phys. Rev. Let

Maximally Entangled Mixed-State Generation via Local Operations

We present a general theoretical method to generate maximally entangled mixed states of a pair of photons initially prepared in the singlet polarization state. This method requires only local operations upon a single photon of the pair and exploits spatial degrees of freedom to induce decoherence. We report also experimental confirmation of these theoretical results.Comment: 5 pages, 2 figures, to be published in Physical Review

Optimal experiment design revisited: fair, precise and minimal tomography

Given an experimental set-up and a fixed number of measurements, how should one take data in order to optimally reconstruct the state of a quantum system? The problem of optimal experiment design (OED) for quantum state tomography was first broached by Kosut et al. [arXiv:quant-ph/0411093v1]. Here we provide efficient numerical algorithms for finding the optimal design, and analytic results for the case of 'minimal tomography'. We also introduce the average OED, which is independent of the state to be reconstructed, and the optimal design for tomography (ODT), which minimizes tomographic bias. We find that these two designs are generally similar. Monte-Carlo simulations confirm the utility of our results for qubits. Finally, we adapt our approach to deal with constrained techniques such as maximum likelihood estimation. We find that these are less amenable to optimization than cruder reconstruction methods, such as linear inversion.Comment: 16 pages, 7 figure

Spin-Hall effect and circular birefringence of a uniaxial crystal plate

The linear birefringence of uniaxial crystal plates is known since the 17th century, and it is widely used in numerous optical setups and devices. Here we demonstrate, both theoretically and experimentally, a fine lateral circular birefringence of such crystal plates. This effect is a novel example of the spin-Hall effect of light, i.e., a transverse spin-dependent shift of the paraxial light beam transmitted through the plate. The well-known linear birefringence and the new circular birefringence form an interesting analogy with the Goos-H\"anchen and Imbert-Fedorov beam shifts that appear in the light reflection at a dielectric interface. We report the experimental observation of the effect in a remarkably simple system of a tilted half-wave plate and polarizers using polarimetric and quantum-weak-measurement techniques for the beam-shift measurements. In view of great recent interest in spin-orbit interaction phenomena, our results could find applications in modern polarization optics and nano-photonics.Comment: 16 pages, 8 figures, to appear in Optic

Classical and quantum scattering in optical systems

The central topic of the Thesis concerns light scattering experiments with entangled photons. Specifically, we study the effect of scattering processes on polarization-entanglement of twin-photons. The main idea is that scattering generally couples polarization and spatial degrees of freedom of photons. The details of this coupling depend on the characteristics of the scattering medium. Such coupling, in turn, can reduce the entanglement of twin-photons, if the photon pairs are detected in a momentum-insensitive way. We have investigated a broad range of optical scattering media ranging from milk to polymer fibers. By manipulating the parameters of these samples we were able to generate a broad range of quantum states, proving for the first time that scattering processes are a substantial tool for mixed-state engineering. This is of great importance for quantum information since any real-world application thereof has to deal with mixed (as opposed to pure) states.UBL - phd migration 201

Entangled mixed-state generation by twin-photon scattering

We report novel experimental results on mixed-state generation by multi-mode scattering of polarization-entangled photons. By using a large variety of scattering media we obtain two markedly different classes of scattered states; namely Werner-like and sub-Werner-like states. Our experimental findings are in excellent agreement with a phenomenological model based upon the description of a scattering process as a quantum map

Ray splitting in paraxial optical cavities

We present a numerical investigation of the ray dynamics in a paraxial optical cavity when a ray splitting mechanism is present. The cavity is a conventional two-mirror stable resonator and the ray splitting is achieved by inserting an optical beam splitter perpendicular to the cavity axis. We show that depending on the position of the beam splitter the optical resonator can become unstable and the ray dynamics displays a positive Lyapunov exponent.Comment: 13 pages, 7 figures, 1 tabl