We consider quantum metrology with arbitrary prior knowledge of the parameter. We demonstrate that a single sensing two-level system can act as a virtual multilevel system that offers increased sensitivity in a Bayesian single-shot metrology scenario, and that allows one to estimate (arbitrary) large parameter values by avoiding phase wraps. This is achieved by making use of additional degrees of freedom or auxiliary systems not participating in the sensing process. The joint system is manipulated by intermediate control operations in such a way that an effective Hamiltonian, with an arbitrary spectrum, is generated that mimics the spectrum of a multisystem interacting with the field. We show how to use additional internal degrees of freedom of a single trapped ion to achieve a high-sensitivity magnetic field sensor for fields with arbitrary prior knowledge.

1 aSekatski, P.1 aSkotiniotis, M.1 aDuer, W. uhttp://grupsderecerca.uab.cat/giq/node/85101747nas a2200133 4500008003900000020002200039245007000061210006900131300001400200520131300214100001901527700002001546856004701566 2017 d a978-1-5386-2913-000aInformation Theoretic Principles of Universal Discrete Denoising0 aInformation Theoretic Principles of Universal Discrete Denoising a205–2103 aSocial media platforms make tremendous amounts of data available. Often times, the same information is behind multiple different available data sets. This lends growing importance to latent variable models that try to learn the hidden information from the available imperfect versions. For example, social media platforms can contain an abundance of pictures of the same person, yet all of which are taken from different perspectives. In a simplified scenario, one may consider pictures taken from the same perspective, which are distorted by noise. This latter application allows for a rigorous mathematical treatment, which is the content of this contribution. We apply a recently developed method of dependent component analysis to image denoising when multiple distorted copies of one and the same image are available, each being corrupted by a different and unknown noise process. In a simplified scenario, one may assume such a distorted image to be corrupted by noise that acts independently on each pixel. We answer completely the question of how to perform optimal denoising, when at least three distorted copies are available: First we define optimality of an algorithm in the presented scenario, and then we describe an aymptotically optimal universal discrete denoising algorithm (UDDA).

1 aNoetzel, Janis1 aWinter, Andreas uhttp://grupsderecerca.uab.cat/giq/node/85601522nas a2200145 4500008003900000022001400039245004500053210004500098490000800143520111500151100001901266700002401285700002001309856004701329 2017 d a1364-502100aInterferometric visibility and coherence0 aInterferometric visibility and coherence0 v4733 aRecently, the basic concept of quantum coherence (or superposition) has gained a lot of renewed attention, after Baumgratz et al. (Phys. Rev. Lett. 113, 140401. (doi: 10.1103/PhysRevLett.113.140401)), following Aberg (http://arxiv.org/abs/quant-ph/0612146), have proposed a resource theoretic approach to quantify it. This has resulted in a large number of papers and preprints exploring various coherence monotones, and debating possible forms for the resource theory. Here, we take the view that the operational foundation of coherence in a state, be it quantum or otherwise wave mechanical, lies in the observation of interference effects. Our approach here is to consider an idealized multi-path interferometer, with a suitable detector, in such a way that the visibility of the interference pattern provides a quantitative expression of the amount of coherence in a given probe state. We present a general framework of deriving coherence measures from visibility, and demonstrate it by analysing several concrete visibility parameters, recovering some known coherence measures and obtaining some new ones.1 aBiswas, Tanmoy1 aDiaz, Maria, Garcia1 aWinter, Andreas uhttp://grupsderecerca.uab.cat/giq/node/84700422nas a2200121 4500008003900000245007400039210006900113100002200182700002200204700001800226700002000244856003600264 2015 d00aImplementing unitary 2-designs using random diagonal-unitary matrices0 aImplementing unitary 2designs using random diagonalunitary matri1 aNakata, Yoshifumi1 aHirche, Christoph1 aMorgan, Ciara1 aWinter, Andreas uhttp://arxiv.org/abs/1502.0751400516nas a2200157 4500008003900000022001400039245005400053210005400107260001100161490000800172100002000180700002300200700002000223700001800243856009700261 2015 d a1079-711400aIndividual Quantum Probes for Optimal Thermometry0 aIndividual Quantum Probes for Optimal Thermometry c6/20150 v1141 aCorrea, Luis, A1 aMehboudi, Mohammad1 aAdesso, Gerardo1 aSanpera, Anna uhttp://grupsderecerca.uab.cat/giq/publications/individual-quantum-probes-optimal-thermometry01679nas a2200181 4500008003900000022001300039245006200052210006200114260001200176300001400188490000800202520111700210100001701327700001801344700001701362700002001379856009801399 2014 d a0024379500aInequalities for the ranks of multipartite quantum states0 aInequalities for the ranks of multipartite quantum states c07/2014 a153 - 1710 v4523 aWe investigate relations between the ranks of marginals of multipartite quantum states. These are the Schmidt ranks across all possible bipartitions and constitute a natural quantification of multipartite entanglement dimensionality. We show that there exist inequalities constraining the possible distribution of ranks. This is analogous to the case of von Neumann entropy (\alpha-R\'enyi entropy for \alpha=1), where nontrivial inequalities constraining the distribution of entropies (such as e.g. strong subadditivity) are known. It was also recently discovered that all other \alpha-R\'enyi entropies for α∈(0,1)∪(1,∞) satisfy only one trivial linear inequality (non-negativity) and the distribution of entropies for α∈(0,1) is completely unconstrained beyond non-negativity. Our result resolves an important open question by showing that also the case of \alpha=0 (logarithm of the rank) is restricted by nontrivial linear relations and thus the cases of von Neumann entropy (i.e., \alpha=1) and 0-R\'enyi entropy are exceptionally interesting measures of entanglement in the multipartite setting.1 aCadney, Josh1 aHuber, Marcus1 aLinden, Noah1 aWinter, Andreas uhttp://grupsderecerca.uab.cat/giq/publications/inequalities-ranks-multipartite-quantum-states01718nas a2200181 4500008003900000245011900039210006900158260001100227490000600238520103200244100002001276700002201296700001801318700002401336700002201360700002101382856013301403 2014 d00aInterface between path and orbital angular momentum entanglement for high-dimensional photonic quantum information0 aInterface between path and orbital angular momentum entanglement c7/20140 v53 aPhotonics has become a mature field of quantum information science, where integrated optical circuits offer a way to scale the complexity of the setup as well as the dimensionality of the quantum state. On photonic chips, paths are the natural way to encode information. To distribute those high-dimensional quantum states over large distances, transverse spatial modes, like orbital angular momentum (OAM) possessing Laguerre Gauss modes, are favourable as flying information carriers. Here we demonstrate a quantum interface between these two vibrant photonic fields. We create three-dimensional path entanglement between two photons in a non-linear crystal and use a mode sorter as the quantum interface to transfer the entanglement to the OAM degree of freedom. Thus our results show a novel, flexible way to create high-dimensional spatial mode entanglement. Moreover, they pave the way to implement broad complex quantum networks where high-dimensionally entangled states could be distributed over distant photonic chips.1 aFickler, Robert1 aLapkiewicz, Radek1 aHuber, Marcus1 aLavery, Martin, P J1 aPadgett, Miles, J1 aZeilinger, Anton uhttp://grupsderecerca.uab.cat/giq/publications/interface-between-path-and-orbital-angular-momentum-entanglement-high-dimensional00586nas a2200181 4500008003900000020002200039022001400061245004000075210004000115260005100155300001400206490001400220100002000234700002100254700002500275700002100300856008300321 2013 d a978-3-642-36899-8 a1611-334900aIdentification via Quantum Channels0 aIdentification via Quantum Channels aBerlin, HeidelbergbSpringer Berlin Heidelberg a217 - 2330 vLNCS 77771 aWinter, Andreas1 aAydinian, Harout1 aCicalese, Ferdinando1 aDeppe, Christian uhttp://grupsderecerca.uab.cat/giq/publications/identification-quantum-channels01671nas a2200169 4500008004100000245011200041210006900153260000900222300001100231490000700242520102900249653002001278100002201298700002201320700002001342856013901362 2010 eng d00aIon-trap simulation of the quantum phase transition in an exactly solvable model of spins coupled to bosons0 aIontrap simulation of the quantum phase transition in an exactly c2010 a0521180 v813 aIt is known that arrays of trapped ions can be used to efficiently simulate a variety of many-body quantum systems. Here we show how it is possible to build a model representing a spin chain interacting with bosons that is exactly solvable. The exact spectrum of the model at zero temperature and the ground-state properties are studied. We show that a quantum phase transition occurs when the coupling between spins and bosons reaches a critical value, which corresponds to a level crossing in the energy spectrum. Once the critical point is reached, the number of bosonic excitations in the ground state, which can be assumed as an order parameter, starts to be different from zero. The population of the bosonic mode is accompanied by a macroscopic magnetization of the spins. This double effect could represent a useful resource for phase transition detection since a measure of the phonon can give information about the phase of the spin system. A finite-temperature phase diagram is also given in the adiabatic regime.10aQuantum Physics1 aGiorgi, Gian Luca1 aPaganelli, Simone1 aGalve, Fernando uhttp://grupsderecerca.uab.cat/giq/publications/ion-trap-simulation-quantum-phase-transition-exactly-solvable-model-spins-coupled-boson