Open Systems & Quantum Information Seminars

If you want to join our seminar-list and receive the seminars announcements, please send an e-mail to stefano.olivares[at]fisica.unimi.it

It is possible to download the poster and the slides (as pdf files) of the seminars.

 

– Tue, March 4th 2014, 15:00 (Auletta Ottica Quantistica)

Roberta ZAMBRINI  (IFISC-UIB, Palma de Mallorca)

Synchronization, quantum correlations and entanglement in oscillator network

Abstract: Synchronization is one of the paradigmatic phenomena in the study of complex systems. It has been explored theoretically and experimentally mostly to understand natural phenomena, but also in view of technological applications. Although several mechanisms and conditions for synchronous behavior in spatially extended systems and networks have been identified, the emergence of this phenomenon has been largely unexplored in quantum systems until very recently. Here we discuss synchronization in quantum networks of different harmonic oscillators relaxing towards a stationary state, being essential the form of dissipation. By local tuning of one of the oscillators, we establish the conditions for synchronous dynamics, in the whole network or in a motif. Beyond the classical regime we show that synchronization between (even unlinked) nodes witnesses the presence of quantum correlations and entanglement. Furthermore, synchronization and entanglement can be induced between two different oscillators if properly linked to a random network.

 

– Thu, Feb 20th 2014, 15:00 (Auletta Ottica Quantistica)

  Abdelhakim GHARBI (University of Bejaia)

 Energy spectra of Hartmann and ring-shaped oscillator potentials using the quantum Hamilton Jacobi formalism

Abstract: We apply the exact quantization condition, introduced within the framework of Padgett and Leacock’s quantum Hamilton–Jacobi formalism, to angular and radial quantum action variables in the context of the Hartmann and the ring-shaped oscillator potentials which are separable and non-central. The energy spectra of the two systems are exactly obtained.

 

– Wednesday, May 22nd 2013, 14:30 (Aula Polvani)

  F. LEVI

  (Freiburg Insitute for Advanced Studies University of Freiburg)

  A hierarchy of Entanglement

Abstract: Entanglement is one of the most peculiar features of quantum mechanics. Not only of interest for fundamental reasons, a better understanding of these non-classical correlations is desirable in the framework of quantum information theory, where entanglement is considered a precious resource to harness for tasks such as teleportation, secure communications and quantum computing. The topic of this talk is the characterization of multipartite entanglement, i.e. entanglement between more than two subsystems. We present theoretical tools that allow us to estimate how many subsystems are participating to the entanglement present in a given state. More specifically, we construct a hierarchy of criteria that identify the different degrees of entanglement within mixed, n-partite systems. The lowest member in this hierarchy permits to assess whether a given n−partite state is at least bipartite entangled, and and the highest member detects exclusively genuine n-partite entanglement. We then conclude considering exemplary applications of our formal tools to systems of interest. In particular, the detection multipartite entanglement is used to explore the connection between quantum coherence and energy transfer efficiency in disordered networks; the onset of different degrees of multipartite entanglement is shown to allow a distinction between two-body and three-body  interactions. – POSTER  SLIDES

 

– Thursday, May 9th 2013, 14:00 (Aula Polvani)

  D. CHRUSCINSKI

  (Institute of Physics Nicolaus Copernicus University, Torun)

  Non-Markovian random unitary qubit dynamics

Abstract: I compare two approaches to non-Markovian quantum evolution: one based on the concept of divisible maps and the other one based on distinguishability of quantum states. The former concept is fully characterized in terms of local generator whereas it is in general not true for the latter one. A simple example of random unitary dynamics of a qubit shows the intricate difference between those approaches. Moreover, in this case both approaches are fully characterized in terms of local decoherence rates. – POSTER  SLIDES

 

– Wednesday, April 17th 2013, 11:00 (Sala riunione Ottica Quantistica)

  K. BANASZEK

  (University of Warsaw)

  Which way experiment with an internal degree of freedom

Abstract: We present an inequality relating visibility and which-way information for a particle equipped with an internal degree of freedom travelling through a Mach-Zehnder interferometer. The inequality paints an unexpectedly intricate picture of wave-particle duality in the general case. Strikingly, in some instances which-way information becomes erased by introducing classical uncertainty in the internal degree of freedom. Furthermore, even imperfect interference visibility measured for a suitable set of inputs can be sufficient to infer absence of which-way information. – POSTER  SLIDES

 

– Wednesday, February 20th 2013, 14.00 (Aula Caldirola)

  A. PORZIO

  (CNR–SPIN, Napoli)

  Evoluzione sperimentale di stati entangled bipartiti in canali

  Gaussiani: i fenomeni di decoerenza in canali di trasmissione reali

Abstract: La decoerenza e’ dovuta alla inevitabile interazione dei sistemi “coerenti” con il mondo esterno. Questo fenomeno, noto in ottica ben prima dell’insorgere della meccanica quantistica, è uno dei maggiori ostacoli alla realizzazione di protocolli di comunicazione quantistica a grande distanza e con elevata fidelity. A questo inevitabile consumarsi delle proprieta’ quantistiche dei sistemi entangled si aggiungono i problemi di rivelazione delle proprietà quantistiche di tali sistemi in regime di pochi fotoni. In questo seminario si discuterà il comportamento dei sistemi Gaussiani bipartiti trasmessi a grande distanza e si valuteranno i limiti ai quali sia  ancora possibile rivelare le proprietà di entanglement. La parte più tecnica del seminario verra’ preceduta da una parte introduttiva a carattere tutoriale per quanto riguarda le procedure di generazione e caratterizzazione dell’entanglement a variabili continue. – POSTER  SLIDES

 

– Wednesday, February 20th 2013, 14.30 (Aula Caldirola)

  D. BUONO

  (Università di Salerno e CNR–SPIN, Napoli)

  Criteri di Entanglement e non località di Bell

  per stati bipartiti Gaussiani

Abstract: La probabilità di successo dei protocolli di Informazione e  Comunicazione Quantistica realizzati con risorse gaussiane dipende fortemente dalla qualità dell’entanglement delle risorse. Risulta  dunque interessante stabilire quali criteri di entanglement, tra i  numerosi presenti in letteratura, siano i più adatti a valuare  l’effettivo grado di entanglement delle risorse. A tale scopo,  discuteremo alcuni dei principali criteri generalmente adottati per la  valutazione dell’entanglement di stati bipartiti Gaussiani ed il legame  tra entanglement e purezza degli stati. Discuteremo inoltre le  differenti implicazioni di entanglement e non-località alla Bell. – POSTER  SLIDES

 

– Thursday, January 17th 2013, 11.30 (Aula Polvani)

  M. BONDANI

  (Istituto di Fotonica e Nanotecnologie, CNR, and CNISM UdR Como)

  High-order photon-number correlations: a resource for the

  characterization and the application of quantum states

Abstract: Correlations play a fundamental role in the investigation of optical coherence, in connection to quantum state characterization, to define nonclassicality and for the enhancement of ghost-imaging protocols. Usually, correlation functions are defined in terms of normal ordered operators and result in expressions that are not accessible by realistic direct detection schemes. We thus define and derive the analytical expression of correlation functions at any order by only using quantities that can be experimentally accessed by direct detection, taking into account the non-unit quantum efficiency of the detection scheme. We show their usefulness in fully characterizing a multimode twin-beam state in comparison with classical states and, in particular, we introduce a nonclassicality criterion based on a simple linear combination of high-order correlation functions. From the experimental point of view, we implement a direct detection scheme based on hybrid photodetectors to experimentally investigate high-order correlations for detected photons. Our scheme is self-consistent, allowing the estimation of all the involved parameters (quantum efficiency, number of modes and average energy) directly from the same experimental data. Results are in very good agreement with theory, thus suggesting the exploitation of our scheme for reliable state characterization in quantum technology. In particular, we show that high-order correlations actually represent a useful discriminating tool of the nature of the state and demonstrate that, at increasing correlation order, the differences between classical and quantum states become more and more evident. – POSTER  SLIDES

 

– Wednesday, November 14th 2012, 15.30 (Aula Polvani)

  A. S. HOLEVO

  (Steklov Mathematical Institute, Moskow)

 Extreme bosonic channels

Abstract: The set of all channels with fixed input and output is convex. We first givea convenient formulation of necessary and sufficient condition for a channel to be extreme point of this set in terms of complementary channel, a notion of big importance in quantum information theory. This formulation is based on the general approach to extremality of completely positive maps in an operator algebra due to Arveson. We then apply this formulation to prove the main result: under certain nondegeneracy conditions, purity of the environment is necessary and sufficient for extremality of Bosonic quasi-free channel. It follows that Gaussian channel between finite-mode Bosonic systems is extreme if and only if it has minimal noise (quantum-limited). – POSTER  SLIDES

 

 – Tuesday, November 6th 2012, 14.30 (Aula Polvani)

  H.-P. BREUER

  (University of Freiburg)

 Quantification and control of non-Markovian dynamics

 in open quantum systems

Abstract: Realistic quantum mechanical systems are always exposed to an external environment. The presence of the environment often gives rise to a Markovian process in which the system irretrievably loses information to its surroundings. However, many quantum systems exhibit a pronounced non-Markovian behavior which is characterized by a flow of information from the environment back to the open system. This backflow of information implies the presence of memory effects and represents the key feature of non-Markovian quantum dynamics. We introduce a general measure for the degree of non-Markovian behavior in open quantum systems and discuss its physical and mathematical properties. Moreover, we will discuss recent experiments which allow to control the information flow between system and environment, to monitor the transition from the Markovian to the non-Markovian regime, and to directly determine the measure for non-Markovianity by measurements on the open system. – POSTER  SLIDES