Quantum channel | EPFL Graph Search

In quantum information theory, a quantum channel is a communication channel which can transmit quantum information, as well as classical information. An example of quantum information is the state of a qubit. An example of classical information is a text document transmitted over the Internet. More formally, quantum channels are completely positive (CP) trace-preserving maps between spaces of operators. In other words, a quantum channel is just a quantum operation viewed not merely as the reduced dynamics of a system but as a pipeline intended to carry quantum information. (Some authors use the term "quantum operation" to also include trace-decreasing maps while reserving "quantum channel" for strictly trace-preserving maps.) Memoryless quantum channel We will assume for the moment that all state spaces of the systems considered, classical or quantum, are finite-dimensional. The memoryless in the section title carries the same meaning as in classical information the

Engineering and characterizing nonclassical states of light in quantum optical networks

Kilian Robert Seibold

The exploration of open quantum many-body systems -systems of microscopic size exhibiting quantum coherence and interacting with their surrounding- has emerged as a key research area over the last years. The recent advances in controlling and preserving quantum coherence at the level of a single particle, developed in a wide variety of physical platforms, have been a major driving force in this field. The driven dissipative nature is a common characteristic of a wide class of modern experimental platforms in quantum science and technology, such as photonic systems, ultracold atoms, optomechanical systems, or superconducting circuits. The interplay between the coherent quantum dynamics and dissipation in open quantum systems leads to a wide range of novel out-of-equilibrium behaviours. Among them, the emergence in these systems of dynamical phases with novel broken symmetries, topological phases and the occurrence of dissipative phase transitions are of particular interest. This thesis aims at establishing a theoretical framework to engineer, characterize and control nonclassical states of light in photonic quantum optical networks in different regimes. The emphasis is put on its implementation, in particular with respect to integration and scalability in photonic platforms. In this thesis, we tackle some interesting aspects arising in the study of the dynamics of driven dissipative coupled nonlinear optical resonators. In that context, we consider the dynamics of two coupled nonlinear photonic cavities in the presence of inhomogeneous coherent driving and local dissipations using the Lindblad master equation formalism.We show that this simple open quantum many-body system can be subject to dynamical instabilities. In particular, our analysis shows that this system presents highly nonclassical properties and its dynamics exhibits dissipative Kerr solitons (DKSs), characterized by the robustness of its specific temporal or spatial waveform during propagation.In a second step, our intuition gained from this system composed of only few degrees of freedom is expanded to the study of systems of bigger size. In particular, we study DKSs originating from the parametric gain in Kerr microresonators. While DKSs are usually described using a classical mean-field approach, our work proposes a quantum-mechanical model formulated in terms of the truncated Wigner formalism. This analysis is motivated by the fact that technological implementations push towards the realization of DKSs in miniaturized integrated systems. These are operating at low power, a regime where quantum effects are expected to be relevant. Using the tools provided by the theory of open quantum systems, we propose a detailed investigation of the impact of quantum fluctuations on the spectral and dynamical properties of DKSs. We show that the quantum fluctuations arising from losses engender a finite lifetime to the soliton, and demonstrate that DKSs correspond to a specific class of dissipative time crystals.

EPFL2022

Near-field spectroscopy and theoretical modeling of disordered semiconductor quantum wires

Andrea Feltrin

In this thesis work, we report scanning near-field optical photoluminescence spectroscopy measurements with at best 200 nm spatial resolution performed on disordered semiconductor V-groove AlGaAs/GaAs quantum wires. In order to interpret the results of these measurements, we develop two different theoretical models. In the first part we study the relationship between disorder and exciton thermalization processes. To do this, we measure the temperature evolution of the excitonic photoluminescence emission lines in near-field spectra at low excitation intensity. The emission peaks in these spectra arise from the radiative recombination of quantum wire excitons localized in potential dips of the confinement potential, due to monolayer width fluctuations along the quantum wire direction. We observe in a non negligible fraction of the measured spectra an increase of the emission line intensities on the low energy side of the spectrum for increasing temperature, which is in contrast with the picture of a thermalized exciton population. In addition to this, the statistical analysis of hundreds of such near-field spectra in terms of autocorrelation functions reveals a shoulder at about 2 meV, which we interpret as a level repulsion feature, related to the energetic and spatial correlation of excitonic states in a disordered system. We develop a theoretical model for the simulation of near-field photoluminescence spectra. In order to do this we model the disordered potential starting from the real interface structure of V-groove quantum wires. The Schr®odinger equation for the exciton is then solved in the center of mass approximation. We calculate the intrinsic radiative recombination rates and the phonon scattering rates, necessary to solve the Boltzmann equation for the exciton population. The statistical analysis of the simulated spectra shows a similar feature in the autocorrelation function. However a good agreement can be obtained only if the diagonal exciton phonon interaction is taken into account, which leads to a modification of the excitonic spectral lineshape and to the appearance of a broad photoluminescence emission background. The temperature dependence of the simulated spectra shows very similar features to the ones observed experimentally. The model for the simulation of steady state near-field photoluminescence spectra is improved in order to simulate time resolved far-field photoluminescence. We study the temperature dependence of the photoluminescence decay time for different disorder configurations and relate it to the exciton population dynamics. We show that at the lowest temperatures the pinning of the decay time at values of several hundreds of picoseconds corresponds to a non thermalized exciton population. On the other hand the square root like temperature dependence, expected in an ideal quantum wire, corresponds to a thermalized exciton population. We also discuss the limits of extracting the intrinsic exciton radiative lifetime from this square like dependence. Taking into account dissociated electron hole states in our model modifies the temperature behaviour, leading to a quasi exponential temperature dependence of the photoluminescence decay time. In the second part, we study the relationship between disorder and biexciton binding energies in quantum wires. We succeed in isolating several extended quantum wire domains, where one single exciton peak dominates the photoluminescence spectrum at low excitation intensity. By increasing the excitation we systematically observe in these large domains the appearance of an additional line in the spectrum, which we attribute to the radiative recombination of the biexciton. The biexcitonic binding energy slightly increases from 2.1 meV for the spatially most extended peaks to 2.3 meV for biexcitons associated to excitons whose localization length is smaller than the experimental resolution. We also observe quantum wire domains with more than one excitonic peak, indicating shorter exciton localization lengths in the order of tens of nanometers. In these domains the appearance of the biexcitonic emission line depends on the number of excitonic peaks observed at low excitation intensity in a near-field photoluminescence spectrum. In spectra containing a few exciton emission lines, biexciton peaks appear associated to each exciton peak. The binding energies show larger variations and range between 2 meV and 2.6 meV. We observe also one biexciton peak appearing on the high energy side of an exciton peak with a negative binding energy of -0.9 meV. On the contrary in spectra containing a larger number of exciton peaks, either only a few additional biexciton peaks appear or no biexcitonic emission lines at all are observed upon increasing the excitation intensity. In order to explain these results we solve the Schr®odinger equation for four one dimensional particles interacting through the Coulomb potential and confined in single particle potential boxes of variable size and barrier height. The results show that the biexciton binding energy first increases as the dip size decreases, reaches a maximum and finally decreases for the smallest sizes as a result of the Coulomb repulsion between the holes in the biexciton. However our model does not account for the disapperance of the biexciton lines, nor for the experimentally measured negative biexciton binding energy.

EPFL2004

Observation of the Decay B-c(+) -> B-s(0)pi(+)

Vladislav Balagura, Aurelio Bay, Marc-Olivier Bettler, Frédéric Blanc, Joël Bressieux, Daniel Hugo Cámpora Pérez, Lucía Castillo García, Peter Clarke, Victor Coco, Greig Alan Cowan, Adam Davis, Michel De Cian, Hans Dijkstra, Frédéric Guillaume Dupertuis, Paolo Durante, Christoph Frei, Luis Alberto Granado Cardoso, Guido Haefeli, Pierre Jaton, Ilya Komarov, Yiming Li, Johan Luisier, Pietro Marino, Maurizio Martinelli, Andrei Martynov, Raluca Anca Muresan, Bastien Luca Muster, Tatsuya Nakada, Matthew Needham, Niko Neufeld, Luca Pescatore, Cédric Potterat, Jessica Prisciandaro, Albert Puig Navarro, Barinjaka Rakotomiaramanana, Gerhard Raven, Julien Rouvinet, Olivier Schneider, Liang Sun, Frédéric Teubert, Mark Tobin, Minh Tâm Tran, Jian Wang, Jean Wicht, Songmei Wu, Lei Zhang, Yi Zhang

The result of a search for the decay B-c(+) -> B-s(0)pi(+) is presented, using the B-s(0) -> D-s(-)pi(+) and B-s(0) -> J/psi phi channels. The analysis is based on a data sample of pp collisions collected with the LHCb detector, corresponding to an integrated luminosity of 1 fb(-1) taken at a center-of-mass energy of 7 TeV, and 2 fb(-1) taken at 8 TeV. The decay B-c(+) -> B-s(0)pi(+) is observed with significance in excess of 5 standard deviations independently in both decay channels. The measured product of the ratio of cross sections and branching fraction is inverted right perpendicular sigma(B-c(+))/sigma(B-s(0))inverted left perpendicular x B(B-c(+) -> B-s(0)pi(+)) = inverted right perpendicular2.37 +/- 0.31 (stat) +/- 0.11 (syst)(-0.13)(+0.17) (tau(Bc+))inverted left perpendicular x 10(-3) the pseudorapidity range 2 < eta(B) < 5, where the first uncertainty is statistical, the second is systematic, and the third is due to the uncertainty on the B-c(+) lifetime. This is the first observation of a B meson decaying to another B meson via the weak interaction.