Talk @UFF, A. Bramati (Lab. Kastler Brossel, Paris), Friday May 24th, 11.00

This week’s talk at the Infoptics group @UFF will be given by our visitor from Paris, A. Bramati (Laboratoire Kastler Brossel, UPMC-Sorbonne 
Universités). His talk’s title is “Quantum fluids of light and nano-photonics at LKB”, and it’s scheduled for next Friday, May 24th, at 11.00 in room A5-01. All are welcome! For an abstract, please see below.

Data: Sexta-Feira 24/05/2019, Sala A5-01, 11 am.

Speaker: A. Bramati (Laboratoire Kastler Brossel, UPMC-Sorbonne 

Title: Quantum fluids of light and nano-photonics at LKB

Abstract: In this talk I will present an overview of the recent experiments performed in our group. The first part of the talk will be devoted to the so called quantum fluids of light [1], a wide family of systems where an effective photon-photon interaction can be engineered, resulting in a  hydrodynamical-like behavior. In particular I will focus on microcavity polaritons and atomic vapors. In polaritons, I will show how the development of flexible all-optical methods to inject a controlled orbital angular momentum in such systems results in the observation of
patterns of quantized vortices [2, 3]. These results open the way to the study of Abrikosov-like physics and new vortex collective phenomena. In Rubidium atomic vapors, I will present very recent experiments demonstrating the superfluid behavior in these media.

The second part of the talk will be devoted to our recent results on 
efficient room temperature single photon emitters based on core/shell colloidal semiconductor nanocrystals. By using asymmetric core/shell nanoparticles (dots-in-rods), blinking effects, multi-excitonic emission and 
polarization of the emitted photons can be simultaneously controlled by tuning the shell dimensions [4] allowing to achieve “state of the art” blinking-free sources of polarized single photons on-demand. Finally, I will discuss the different strategies we are pursuing to
develop hybrid photonic devices by coupling single nanocrystals with  various photonic structures like optical nanofibers, deep parabolic mirrors [5], liquid crystals [6] and semiconductor nanowires [7].


Talk @UFF, Ernesto Galvão, Friday May 17th, 11.00

I will be giving this week’s talk at our group at UFF. Please come to room A5-01, next Friday May 17th at 11.00 to hear me talk about “Dimension and coherence witnesses from SWAP tests”. For an abstract, please see below.

Room A5-01. Friday, May 17th, 2019, 11.00.

Speaker: Ernesto F. Galvão (UFF)

Title: Dimension and coherence witnesses from SWAP tests

Abstract: The SWAP test is a simple quantum circuit that projects onto the symmetric subspace of two quantum systems. When applied to two separable states, it results in a direct estimation of the two-state overlap Tr(rho sigma). Using many such tests, we can gather information about many pairwise overlaps of N states. I’ll show how this relational information can be used to witness quantum coherence, i.e. guarantee that the states are not diagonal in any single basis. I’ll also show how these tests can be used as a Hilbert space dimension witness. Finally, I’ll briefly describe how to implement these tests using linear optics.

Talk @UFF, Braian da Silva, May 10th, 11:00

This week’s talk at the Infoptics group (UFF) will be given by Braian Pinheiro da Silva (UFF), next Friday, May 10th, in room A5-01 at 11:00. His talk is titled “Self imaging from fractional Gouy phases in paraxial optics”, all are welcome! Please see below for an abstract.

Data: Sexta-Feira 10/05/2019, Sala A5-01, 11 am.

Palestrante: Braian Pinheiro Da Silva (UFF)

Title: Self imaging from fractional Gouy phases in paraxial optics.

Abstract: We investigate self-imaging structures composed by paraxial 
mode superpositions in optics. When different mode orders are combined, the resulting  pattern is not preserved under free propagation and gets transformed as a result of non  synchronized Gouy phases. However, it is possible to build self imaging structures in which the Gouy phases synchronize at specific longitudinal positions, recovering the initial pattern.

PNPD-CAPES postdoc scholarships @UFF

The Physics Institute at UFF has announced the opening of 3 PNPD-CAPES postdoc scholarships. The Infoptics group @UFF invites applications from candidates interested in working with us – click here to find out more about our research activities, or contact one of us directly. The deadline for applications is April 19th, and the monthly grant value is R$4100.

New paper: Quantum and classical bounds for unknown two-state overlaps (Galvão, Brod)

I and Daniel Brod have just posted a new preprint to the arXiv: “Quantum and classical bounds for unknown two-state overlaps“. In it, we address a very general problem about state distinguishability. The seed for these ideas were experiments to characterize multiphoton sources, done in collaboration with Fabio Sciarrino’s group in Rome. In this post I’ll try to summarize the main ideas of the paper.

Suppose we have 3 objects A, B, and C, but can only do pairwise comparisons. We find out that B=A and C=A. Logic alone then dictates that B=C (after all, equality is transitive). Note that we deduced that B=C without ever directly comparing them. Now, when can we infer equality in the situation where these objects are drawn from probabilistic ensembles? What about the situation in which A, B, C are described by quantum states?

Classical case. We consider 3 independent processes that output objects A, B, and C according to three arbitrary probability distributions. We show how logical coherence give us bounds on the probabilities that the objects drawn are the same. One example:

p(B=C) \ge p(A=B) + p(A=C) -1.

Quantum case. Let’s use the two-state overlap r_{AB}= \left|<A|B> \right|^2 to quantify the similarity of states |A> and |B>. So the question becomes: if we have 3 unknown states, but know the two overlaps r_{AB} and r_{AC}, what can we say about the unknown r_{BC}? In the paper we prove attainable bounds for r_{AB}. One example: if r_{AB}+r_{AC}>1, then

r_{BC} \ge \left( \sqrt{r_{AB}r_{AC}} - \sqrt{(1-r_{AB})(1-r_{AC})}\right)^2.

Check an illustration of these bounds in the Bloch sphere below. The bounds we found are a bit more involved than this, as the 3 states in general span a 3-dimensional Hilbert space.

With |A> in the north pole, fixing r_{AB} puts |B> at a certain latitude; fixing r_{AC} puts |C> at another latitude. Then |B> and |C> can be as close as shown in the figure, or as far as |B> and |C'>.

Comparing quantum with classical. At first sight, it may seem that the two results above are incomparable. The key to making the comparison is “quantizing” the classical probabilistic processes. This can be done by fixing one Hilbert-space basis, and finding diagonal states and observables that give us the classical probabilistic processes we want. These classical models feature no superpositions, only classical, incoherent probabilistic mixtures. For classical models, it is easy to show that the overlap reduces to the probability that the drawn objects are equal. This enables us to directly compare the quantum and classical inequalities we have derived, describe the quantum violations of the classical inequalities, and so on.

Interestingly, the overlap can be measured directly, without ever getting any information on each individual state, using a SWAP test. In photonic experiments, this corresponds to a Hong-Ou-Mandel interferometric test.

More than 3 states, applications. We also show how to use our 3-state results to prove quantum bounds on overlaps involving any number of pure states. We describe applications in characterizing multiphoton indistinguishability (our original inspiration for this work), and Hilbert-space dimensionality witnesses. Besides these applications, I hope to unearth more connections between our results and contextuality/non-locality/coherence theory. I also guess our results may help put limits on epistemic models for quantum theory.

If you’ve read this far, I hope you’ll be curious enough to read our preprint!

II Workshop on Quantum Information and Thermodynamics – IIP Natal – 11 to 22/03 (via SPIN off QuBIT)

Official website: OVERVIEW During the last decades, the field known as Quantum Thermodynamics has attracted high interest from the scientific community, not only from physicists, but also from computer science and engineering. The recent litereature witnessed a significant increase in the volume of articles on the thermodynamics of small systems, where quantum mechanics become […]

via II Workshop on Quantum Information and Thermodynamics – IIP Natal – 11 to 22/03 — SPIN off QuBIT

Talk @UFF, Tiago Mendes (ICTP), Friday Dec. 14th, 11:00

This week’s talk at UFF will be given by Tiago Mendes (ICTP), and its title is “Detecting universal properties of lattice models with entanglement Hamiltonians”. It’ll be next Friday, Dec. 14th, at 11:00 in room A5-01, all are welcome to attend. For an abstract, please see below. Read the rest of this entry »