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. uhttps://grupsderecerca.uab.cat/giq/node/85101523nas a2200145 4500008003900000022001400039245004500053210004500098490000800143520111500151100001901266700002401285700002001309856004801329 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 uhttps://grupsderecerca.uab.cat/giq/node/84700795nas a2200157 4500008003900000245007000039210006900109260000800178300000700186490000600193520031700199100002600516700001900542700002000561856005600581 2017 d00aMacroscopic superpositions require tremendous measurement devices0 aMacroscopic superpositions require tremendous measurement device cnov a340 v13 aMichalis Skotiniotis, Wolfgang Dür, and Pavel Sekatski, Quantum 1, 34 (2017). https://doi.org/10.22331/q-2017-11-21-34 We consider fundamental limits on the detectable size of macroscopic quantum superpositions. We argue that a full quantum mechanical treatment of system plus measurement device is required,…1 aSkotiniotis, Michalis1 aDür, Wolfgang1 aSekatski, Pavel uhttps://quantum-journal.org/papers/q-2017-11-21-34/01377nas a2200181 4500008003900000022001400039245006900053210006800122260000800190490000700198520083300205100002601038700002201064700001901086700001801105700002401123856004801147 2017 d a2469-995000aMagnetic phases of spin-1 lattice gases with random interactions0 aMagnetic phases of spin1 lattice gases with random interactions cjun0 v953 aA spin-1 atomic gas in an optical lattice, in the unit-filling Mott insulator (MI) phase and in the presence of disordered spin-dependent interaction, is considered. In this regime, at zero temperature, the system is well described by a disordered rotationally invariant spin-1 bilinear-biquadratic model. We study, via the density matrix renormalization group algorithm, a bounded disorder model such that the spin interactions can be locally either ferromagnetic or antiferromagnetic. Random interactions induce the appearance of a disordered ferromagnetic phase characterized by a nonvanishing value of the spin glass order parameter across the boundary between a ferromagnetic phase and a dimer phase exhibiting random singlet order. We also study the distribution of the block entanglement entropy in the different regions.1 aMcAlpine, Kenneth, D.1 aPaganelli, Simone1 aCiuchi, Sergio1 aSanpera, Anna1 aDe Chiara, Gabriele uhttps://grupsderecerca.uab.cat/giq/node/84900827nas a2200169 4500008003900000245005700039210005700096260000800153300000700161490000600168520033800174100002000512700002600532700002400558700001900582856005600601 2017 d00aQuantum metrology with full and fast quantum control0 aQuantum metrology with full and fast quantum control csep a270 v13 aPavel Sekatski, Michalis Skotiniotis, Janek Kołodyński, and Wolfgang Dür, Quantum 1, 27 (2017). https://doi.org/10.22331/q-2017-09-06-27 We establish general limits on how precise a parameter, e.g. frequency or the strength of a magnetic field, can be estimated with the aid of full and fast quantum control. We consider uncorr…1 aSekatski, Pavel1 aSkotiniotis, Michalis1 aKołodyński, Janek1 aDür, Wolfgang uhttps://quantum-journal.org/papers/q-2017-09-06-27/01611nas a2200157 4500008003900000022001400039245004800053210004600101490000700147520116500154100002101319700002401340700002101364700002001385856004801405 2017 d a2469-992600aResource theory of coherence: Beyon} states0 aResource theory of coherence Beyon states0 v953 aWe generalize the recently proposed resource theory of coherence (or superposition) [T. Baumgratz et al., Phys. Rev. Lett. 113, 140401 (2014); A. Winter and D. Yang, Phys. Rev. Lett. 116, 120404 ( 2016)] to the setting where not just the free (”incoherent”) resources, but also the manipulated objects, are quantum operations rather than states. In particular, we discuss an information theoretic notion of the coherence capacity of a quantum channel and prove a single-letter formula for it in the case of unitaries. Then we move to the coherence cost of simulating a channel and prove achievability results for unitaries and general channels acting on a d-dimensional system; we show that a maximally coherent state of rank d is always sufficient as a resource if incoherent operations are allowed, and one of rank d(2) for “strictly incoherent” operations. We also show lower bounds on the simulation cost of channels that allow us to conclude that there exists bound coherence in operations, i.e., maps with nonzero cost of implementing them but zero coherence capacity; this is in contrast to states, which do not exhibit bound coherence.

1 aBen Dana, Khaled1 aDiaz, Maria, Garcia1 aMejatty, Mohamed1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/node/84800606nas a2200145 4500008003900000022001400039245013300053210006900186260001100255300001200266490000700278100001700285700002000302856013800322 2016 d a1557-965400aNo-Signalling-Assisted Zero-Error Capacity of Quantum Channels and an Information Theoretic Interpretation of the Lovász Number0 aNoSignallingAssisted ZeroError Capacity of Quantum Channels and c2/2016 a891-9140 v621 aDuan, Runyao1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/publications/no-signalling-assisted-zero-error-capacity-quantum-channels-and-information-theoretic00598nas a2200181 4500008003900000022001400039245006000053210005900113260001100172490000800183100002200191700002100213700001800234700001700252700002000269700002000289856010700309 2016 d a1079-711400aShould Entanglement Measures be Monogamous or Faithful?0 aShould Entanglement Measures be Monogamous or Faithful c8/20160 v1171 aLancien, Cécilia1 aDi Martino, Sara1 aHuber, Marcus1 aPiani, Marco1 aAdesso, Gerardo1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/publications/should-entanglement-measures-be-monogamous-or-faithful03324nas a2200445 4500008003900000022001400039245010700053210006900160260000800229300001600237490000700253520193200260653003302192653002002225653002202245653002502267653001902292653003202311653002002343653001302363653003502376653001302411653003102424653004202455653002302497653003102520653002102551653002602572653002502598653002402623653001402647653003902661653003102700653003102731653002402762100001302786700001702799700001402816856004802830 2016 d a0018-944800aOn {Zero}-{Error} {Communication} via {Quantum} {Channels} in the {Presence} of {Noiseless} {Feedback}0 aZero Error Communication via Quantum Channels in the Presence of csep a5260–52770 v623 aWe initiate the study of zero-error communication via quantum channels when the receiver and the sender have at their disposal a noiseless feedback channel of unlimited quantum capacity, generalizing Shannon's zero-error communication theory with instantaneous feedback. We first show that this capacity is only a function of the linear span of Choi-Kraus operators of the channel, which generalizes the bipartite equivocation graph of a classical channel, and which we dub non-commutative bipartite graph. Then, we go on to show that the feedback-assisted capacity is non-zero (allowing for a constant amount of activating noiseless communication) if and only if the non-commutative bipartite graph is non-trivial, and give a number of equivalent characterizations. This result involves a far-reaching extension of the conclusive exclusion of quantum states. We then present an upper bound on the feedback-assisted zero-error capacity, motivated by a conjecture originally made by Shannon and proved later by Ahlswede. We demonstrate that this bound to have many good properties, including being additive and given by a minimax formula. We also prove a coding theorem showing that this quantity is the entanglement-assisted capacity against an adversarially chosen channel from the set of all channels with the same Choi-Kraus span, which can also be interpreted as the feedback-assisted unambiguous capacity. The proof relies on a generalization of the Postselection Lemma (de Finetti reduction) that allows to reflect additional constraints, and which we believe to be of independent interest. This capacity is a relaxation of the feedback-assisted zero-error capacity; however, we have to leave open the question of whether they coincide in general. We illustrate our ideas with a number of examples, including classical-quantum channels and Weyl diagonal channels, and close with an extensive discussion of open questions.10abipartite equivocation graph10aBipartite graph10aCapacity planning10aChoi-Kraus operators10acoding theorem10acommutative bipartite graph10aElectronic mail10aencoding10aentanglement-assisted capacity10afeedback10afeedback-assisted capacity10afeedback-assisted zero-error capacity10ainformation theory10anoiseless feedback channel10aquantum channels10aquantum communication10aquantum entanglement10aQuantum information10aReceivers10aShannon's zero-error communication10atelecommunication channels10aunlimited quantum capacity10azero-error capacity1 aDuan, R.1 aSeverini, S.1 aWinter, A uhttps://grupsderecerca.uab.cat/giq/node/87001352nas a2200181 4500008003900000022001400039245011200053210006900165260001200234490000700246520069100253100002300944700001400967700001700981700001700998700001501015856014001030 2014 d a1550-235X00aCase study of the uniaxial anisotropic spin-1 bilinear-biquadratic Heisenberg model on a triangular lattice0 aCase study of the uniaxial anisotropic spin1 bilinearbiquadratic c10/20140 v903 aWe study the spin-1 model on a triangular lattice in the presence of a uniaxial anisotropy field using a cluster mean-field (CMF) approach. The interplay among antiferromagnetic exchange, lattice geometry, and anisotropy forces Gutzwiller mean-field approaches to fail in a certain region of the phase diagram. There, the CMF method yields two supersolid phases compatible with those present in the spin−1/2 XXZ model onto which the spin-1 system maps. Between these two supersolid phases, the three-sublattice order is broken and the results of the CMF approach depend heavily on the geometry and size of the cluster. We discuss the possible presence of a spin liquid in this region.1 aMoreno-Cardoner, M1 aPerrin, H1 aPaganelli, S1 aDe Chiara, G1 aSanpera, A uhttps://grupsderecerca.uab.cat/giq/publications/case-study-uniaxial-anisotropic-spin-1-bilinear-biquadratic-heisenberg-model-triangular00523nas a2200121 4500008003900000245008500039210006900124260003700193300001200230100001700242700002000259856012200279 2014 d00aConstant compositions in the sphere packing bound for classical-quantum channels0 aConstant compositions in the sphere packing bound for classicalq aHonolulu, HI, USAbIEEEc07/2014 a151-1551 aDalai, Marco1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/publications/constant-compositions-sphere-packing-bound-classical-quantum-channels01365nas a2200169 4500008003900000245006600039210006600105260001200171300001100183490000900194520081200203100002301015700001701038700001701055700001501072856010801087 2014 d00aEntanglement properties of spin models in triangular lattices0 aEntanglement properties of spin models in triangular lattices c10/2014 aP100080 v20143 aThe different quantum phases appearing in strongly correlated systems as well as their transitions are closely related to the entanglement shared between their constituents. In 1D systems, it is well established that the entanglement spectrum is linked to the symmetries that protect the different quantum phases. This relation extends even further at the phase transitions where a direct link associates the entanglement spectrum to the conformal field theory describing the former. For 2D systems much less is known. The lattice geometry becomes a crucial aspect to consider when studying entanglement and phase transitions. Here, we analyze the entanglement properties of triangular spin lattice models by also considering concepts borrowed from quantum information theory such as geometric entanglement.1 aMoreno-Cardoner, M1 aPaganelli, S1 aDe Chiara, G1 aSanpera, A uhttps://grupsderecerca.uab.cat/giq/publications/entanglement-properties-spin-models-triangular-lattices01541nas a2200181 4500008003900000022001400039245008900053210006900142260001100211490000700222520091500229100001801144700001401162700001901176700001701195700001501212856013201227 2014 d a1094-162200aLong-range multipartite entanglement close to a first-order quantum phase transition0 aLongrange multipartite entanglement close to a firstorder quantu c3/20140 v893 aWe provide insight into the quantum correlations structure present in strongly correlated systems beyond the standard framework of bipartite entanglement. To this aim we first exploit rotationally invariant states as a test bed to detect genuine tripartite entanglement beyond the nearest neighbor in spin-1/2 models. Then we construct in a closed analytical form a family of entanglement witnesses which provides a sufficient condition to determine if a state of a many-body system formed by an arbitrary number of spin-1/2 particles possesses genuine tripartite entanglement, independently of the details of the model. We illustrate our method by analyzing in detail the anisotropic XXZ spin chain close to its phase transitions, where we demonstrate the presence of long-range multipartite entanglement near the critical point and the breaking of the symmetries associated with the quantum phase transition.1 aStasińska, J1 aRogers, B1 aPaternostro, M1 aDe Chiara, G1 aSanpera, A uhttps://grupsderecerca.uab.cat/giq/publications/long-range-multipartite-entanglement-close-first-order-quantum-phase-transition00667nas a2200181 4500008003900000022001400039245009500053210006900148260001100217300001400228490000700242100002400249700001800273700002000291700001900311700002000330856013500350 2014 d a1557-965400aThe Quantum Reverse Shannon Theorem and Resource Tradeoffs for Simulating Quantum Channels0 aQuantum Reverse Shannon Theorem and Resource Tradeoffs for Simul c5/2014 a2926-29590 v601 aBennett, Charles, H1 aDevetak, Igor1 aHarrow, Aram, W1 aShor, Peter, W1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/publications/quantum-reverse-shannon-theorem-and-resource-tradeoffs-simulating-quantum-channels00537nas a2200169 4500008003900000022001400039245006000053210006000113260001200173300001100185490000700196100001500203700001400218700001800232700001700250856010000267 2013 d a1751-812100aDecomposition of any quantum measurement into extremals0 aDecomposition of any quantum measurement into extremals c09/2013 a3753020 v461 aSentís, G1 aGendra, B1 aBartlett, S D1 aDoherty, A C uhttps://grupsderecerca.uab.cat/giq/publications/decomposition-any-quantum-measurement-extremals00587nas a2200181 4500008003900000020002200039022001400061245004000075210004000115260005100155300001400206490001400220100002000234700002100254700002500275700002100300856008400321 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 uhttps://grupsderecerca.uab.cat/giq/publications/identification-quantum-channels00560nas a2200169 4500008003900000022001400039245006000053210005900113260001200172300001600184490000700200100001900207700002100226700002000247700002000267856010300287 2013 d a1557-965400aQuantum Rate-Distortion Coding With Auxiliary Resources0 aQuantum RateDistortion Coding With Auxiliary Resources c10/2013 a6755 - 67730 v591 aWilde, Mark, M1 aDatta, Nilanjana1 aHsieh, Min-Hsiu1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/publications/quantum-rate-distortion-coding-auxiliary-resources00516nas a2200169 4500008003900000022001300039245004800052210004600100260000900146300001100155490000700166100002100173700002000194700001900214700002000233856009300253 2013 d a0022248800aQuantum-to-classical rate distortion coding0 aQuantumtoclassical rate distortion coding c2013 a0422010 v541 aDatta, Nilanjana1 aHsieh, Min-Hsiu1 aWilde, Mark, M1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/publications/quantum-classical-rate-distortion-coding00517nas a2200145 4500008003900000022001400039245007200053210006900125260001100194490000700205100001400212700001700226700001500243856011300258 2013 d a1550-235X00aScaling of the entanglement spectrum near quantum phase transitions0 aScaling of the entanglement spectrum near quantum phase transiti c6/20130 v871 aLepori, L1 aDe Chiara, G1 aSanpera, A uhttps://grupsderecerca.uab.cat/giq/publications/scaling-entanglement-spectrum-near-quantum-phase-transitions00616nas a2200157 4500008003900000022001400039245010300053210006900156260001200225300001600237490000700253100001700260700002100277700002000298856014000318 2013 d a1557-965400aZero-Error Communication via Quantum Channels, Noncommutative Graphs, and a Quantum Lovász Number0 aZeroError Communication via Quantum Channels Noncommutative Grap c02/2013 a1164 - 11740 v591 aDuan, Runyao1 aSeverini, Simone1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/publications/zero-error-communication-quantum-channels-noncommutative-graphs-and-quantum-lov%C3%A1sz01182nas a2200169 4500008003900000022001400039245009100053210006900144260001200213490000800225520058100233100001700814700001400831700001800845700001500863856013400878 2012 d a1079-711400aEntanglement Spectrum, Critical Exponents, and Order Parameters in Quantum Spin Chains0 aEntanglement Spectrum Critical Exponents and Order Parameters in c12/20120 v1093 aWe investigate the entanglement spectrum near criticality in finite quantum spin chains. Using finite size scaling we show that when approaching a quantum phase transition, the Schmidt gap, i.e., the difference between the two largest eigenvalues of the reduced density matrix λ1, λ2, signals the critical point and scales with universal critical exponents related to the relevant operators of the corresponding perturbed conformal field theory describing the critical point. Such scaling behavior allows us to identify explicitly the Schmidt gap as a local order parameter.1 aDe Chiara, G1 aLepori, L1 aLewenstein, M1 aSanpera, A uhttps://grupsderecerca.uab.cat/giq/publications/entanglement-spectrum-critical-exponents-and-order-parameters-quantum-spin-chains01227nas a2200157 4500008003900000022001400039245007300053210006900126260001100195490000700206520075500213100001700968700001800985700001501003856005101018 2011 d a1550-235X00aBilinear-biquadratic spin-1 chain undergoing quadratic Zeeman effect0 aBilinearbiquadratic spin1 chain undergoing quadratic Zeeman effe c8/20110 v843 aThe Heisenberg model for spin-1 bosons in one dimension presents many different quantum phases, including the famous topological Haldane phase. Here we study the robustness of such phases in front of a SU(2) symmetry-breaking field as well as the emergence of unique phases. Previous studies have analyzed the effect of such uniaxial anisotropy in some restricted relevant points of the phase diagram. Here we extend those studies and present the complete phase diagram of the spin-1 chain with uniaxial anisotropy. To this aim, we employ the density-matrix renormalization group together with analytical approaches. The complete phase diagram can be realized using ultracold spinor gases in the Mott insulator regime under a quadratic Zeeman effect.1 aDe Chiara, G1 aLewenstein, M1 aSanpera, A uhttp://prb.aps.org/abstract/PRB/v84/i5/e05445101005nas a2200157 4500008003900000024002000039245006000059210006000119300001100179490000700190520052600197100002400723700002300747700002200770856005500792 2011 d aarXiv:1105.051300aEntanglement detection in hybrid optomechanical systems0 aEntanglement detection in hybrid optomechanical systems a0523240 v833 aWe study a device formed by a Bose Einstein condensate (BEC) coupled to the field of a cavity with a moving end-mirror and find a working point such that the mirror-light entanglement is reproduced by the BEC-light quantum correlations. This provides an experimentally viable tool for inferring mirror-light entanglement with only a limited set of assumptions. We prove the existence of tripartite entanglement in the hybrid device, persisting up to temperatures of a few milli-Kelvin, and discuss a scheme to detect it. 1 aDe Chiara, Gabriele1 aPaternostro, Mauro1 aPalma, Massimo, G uhttp://link.aps.org/doi/10.1103/PhysRevA.83.05232401403nas a2200193 4500008003900000022001400039245006400053210006400117260001200181300001000193490000700203520078300210100001200993700001701005700001401022700001201036700001401048856014701062 2011 d a1286-485400aEntangling two distant oscillators with a quantum reservoir0 aEntangling two distant oscillators with a quantum reservoir c09/2011 a600080 v953 aThe generation of entanglement between two oscillators that interact via a common reservoir is theoretically studied. The reservoir is modeled by a one-dimensional harmonic crystal initially in thermal equilibrium. Starting from a separable state, the oscillators can become entangled after a transient time, that is of the order of the thermalization time scale. This behaviour is observed at finite temperature even when the oscillators are at a distance significantly larger than the crystal's interparticle spacing. The underlying physical mechanisms can be explained by the dynamical properties of the collective variables of the two oscillators which may decouple from or be squeezed by the reservoir. Our predictions can be tested with an ion chain in a linear Paul trap.1 aWolf, A1 aDe Chiara, G1 aKajari, E1 aLutz, E1 aMorigi, G uhttp://epljournal.edpsciences.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/epl/abs/2011/18/epl13816/epl13816.html01162nas a2200145 4500008003900000245005400039210005400093300001100147490000700158520074100165100002400906700002000930700001500950856005100965 2011 d00aProbing magnetic order in ultracold lattice gases0 aProbing magnetic order in ultracold lattice gases a0216040 v833 aA forthcoming challenge in ultracold lattice gases is the simulation of quantum magnetism. That involves both the preparation of the lattice atomic gas in the desired spin state and the probing of the state. Here we demonstrate how a probing scheme based on atom-light interfaces gives access to the order parameters of nontrivial quantum magnetic phases, allowing us to characterize univocally strongly correlated magnetic systems produced in ultracold gases. This method, which is also nondemolishing, yields spatially resolved spin correlations and can be applied to bosons or fermions. As a proof of principle, we apply this method to detect the complete phase diagram displayed by a chain of (rotationally invariant) spin-1 bosons.1 aDe Chiara, Gabriele1 aRomero-Isart, O1 aSanpera, A uhttp://pra.aps.org/abstract/PRA/v83/i2/e02160401272nas a2200181 4500008003900000022001400039245009200053210006900145260001100214490000700225520072800232100001400960700001500974700001700989700001501006700001801021856005101039 2011 d a1094-162200aQuantifying, characterizing, and controlling information flow in ultracold atomic gases0 aQuantifying characterizing and controlling information flow in u c9/20110 v843 aWe study quantum information flow in a model comprised of a trapped impurity qubit immersed in a Bose-Einstein-condensed reservoir. We demonstrate how information flux between the qubit and the condensate can be manipulated by engineering the ultracold reservoir within experimentally realistic limits. We show that this system undergoes a transition from Markovian to non-Markovian dynamics, which can be controlled by changing key parameters such as the condensate scattering length. In this way, one can realize a quantum simulator of both Markovian and non-Markovian open quantum systems, the latter ones being characterized by a reverse flow of information from the background gas (reservoir) to the impurity (system).1 aHaikka, P1 aMcEndoo, S1 aDe Chiara, G1 aPalma, G M1 aManiscalco, S uhttp://pra.aps.org/abstract/PRA/v84/i3/e03160200904nas a2200169 4500008003900000024002000039245005900059210005700118260001200175300001100187490000800198520041000206100001900616700002400635700001500659856006000674 2010 d aarxiv:1003.042400aCold-atom induced control of an opto-mechanical device0 aColdatom induced control of an optomechanical device c06/2010 a2436020 v1043 aWe consider a cavity with a vibrating end mirror and coupled to a Bose-Einstein condensate. The cavity field mediates the interplay between mirror and collective oscillations of the atomic density. We study the implications of this dynamics and the possibility of an indirect diagnostic. Our predictions can be observed in a realistic setup that is central to the current quest for mesoscopic quantumness.1 aPaternostro, M1 aDe Chiara, Gabriele1 aPalma, G M u http://link.aps.org/doi/10.1103/PhysRevLett.104.24360201503nas a2200181 4500008003900000024002000039245008500059210006900144260001200213300001100225490000700236520094700243100002401190700002401214700001401238700001401252856005501266 2010 d aarXiv:1001.482700aThe quantum ground state of self-organized atomic crystals in optical resonators0 aquantum ground state of selforganized atomic crystals in optical c04/2010 a0434070 v813 aCold atoms, driven by a laser and simultaneously coupled to the quantum field of an optical resonator, may self-organize in periodic structures. These structures are supported by the optical lattice, which emerges from the laser light they scatter into the cavity mode and form when the laser intensity exceeds a threshold value. We study theoretically the quantum ground state of these structures above the pump threshold of self-organization by mapping the atomic dynamics of the self-organized crystal to a Bose-Hubbard model. We find that the quantum ground state of the self-organized structure can be the one of a Mott insulator, depending on the pump strength of the driving laser. For very large pump strengths, where the intracavity-field intensity is maximum and one would expect a Mott-insulator state, we find intervals of parameters where the phase is compressible. These states could be realized in existing experimental setups.1 aFernández-Vidal, S1 aDe Chiara, Gabriele1 aLarson, J1 aMorigi, G uhttp://link.aps.org/doi/10.1103/PhysRevA.81.04340702221nas a2200181 4500008003900000245007700039210006900116260001200185300001100197490000700208520166400215100002401879700001801903700001401921700001601935700001501951856007301966 2010 d00aSpontaneous nucleation of structural defects in inhomogeneous ion chains0 aSpontaneous nucleation of structural defects in inhomogeneous io c12/2010 a1150030 v123 aStructural defects in ion crystals can be formed during a linear quench of the transverse trapping frequency across the mechanical instability from a linear chain to the zigzag structure. The density of defects after the sweep can be conveniently described by the Kibble-Zurek mechanism. In particular, the number of kinks in the zigzag ordering can be derived from a time-dependent Ginzburg-Landau equation for the order parameter, here the zigzag transverse size, under the assumption that the ions are continuously laser cooled. In a linear Paul trap the transition becomes inhomogeneous, being the charge density larger in the center and more rarefied at the edges. During the linear quench the mechanical instability is first crossed in the center of the chain, and a front, at which the mechanical instability is crossed during the quench, is identified which propagates along the chain from the center to the edges. If the velocity of this front is smaller than the sound velocity, the dynamics becomes adiabatic even in the thermodynamic limit and no defect is produced. Otherwise, the nucleation of kinks is reduced with respect to the case in which the charges are homogeneously distributed, leading to a new scaling of the density of kinks with the quenching rate. The analytical predictions are verified numerically by integrating the Langevin equations of motion of the ions, in presence of a time-dependent transverse confinement. We argue that the non-equilibrium dynamics of an ion chain in a Paul trap constitutes an ideal scenario to test the inhomogeneous extension of the Kibble-Zurek mechanism, which lacks experimental evidence to date. 1 aDe Chiara, Gabriele1 aCampo, Adolfo1 aMorigi, G1 aPlenio, M B1 aRetzker, A uhttp://iopscience.iop.org/1367-2630/12/11/115003?fromSearchPage=true00572nas a2200169 4500008003900000245011700039210006900156260001200225300001100237490000800248100001800256700002400274700001400298700001600312700001500328856005900343 2010 d00aStructural defects in ion crystals by quenching the external potential: the inhomogeneous Kibble-Zurek mechanism0 aStructural defects in ion crystals by quenching the external pot c08/2010 a0757010 v1051 aCampo, Adolfo1 aDe Chiara, Gabriele1 aMorigi, G1 aPlenio, M B1 aRetzker, A uhttp://link.aps.org/doi/10.1103/PhysRevLett.105.07570100525nas a2200169 4500008004100000245006700041210006500108260001200173300001500185490000800200100001900208700001600227700002100243700001800264700002300282856005000305 2008 eng d00aFrustration, Area Law, and Interference in Quantum Spin Models0 aFrustration Area Law and Interference in Quantum Spin Models c10/2008 a187202–40 v1011 aSen(De), Aditi1 aSen, Ujjwal1 aDziarmaga, Jacek1 aSanpera, Anna1 aLewenstein, Maciej uhttp://link.aps.org/abstract/PRL/v101/e18720200696nas a2200241 4500008004100000245005000041210004900091260001200140300001600152490000700168653002800175653003100203653001700234653001400251653002500265100001800290700001900308700002300327700001600350700001700366700002200383856004900405 2008 eng d00aQuantum-limited metrology with product states0 aQuantumlimited metrology with product states c01/2008 a012317–150 v7710aangular momentum theory10aBose-Einstein condensation10abound states10aprotocols10aquantum entanglement1 aBoixo, Sergio1 aDatta, Animesh1 aFlammia, Steven, T1 aShaji, Anil1 aBagan, Emili1 aCaves, Carlton, M uhttp://link.aps.org/abstract/PRA/v77/e01231701278nas a2200157 4500008004100000245003500041210003500076300000800111490000600119520082700125100001700952700002200969700002800991700002401019856007701043 2007 eng d00aHow to hide a secret direction0 aHow to hide a secret direction a2440 v93 aWe present a procedure to share a secret spatial direction in the absence of a common reference frame using a multipartite quantum state. The procedure guarantees that the parties can determine the direction if they perform joint measurements on the state, but fail to do so if they restrict themselves to local operations and classical communication (LOCC). We calculate the fidelity for joint measurements, give bounds on the fidelity achievable by LOCC, and prove that there is a non-vanishing gap between the two of them, even in the limit of infinitely many copies. The robustness of the procedure under particle loss is also studied. As a by-product we find bounds on the probability of discriminating by LOCC between the invariant subspaces of total angular momentum N/2 and N/2-1 in a system of N elementary spins.1 aBagan, Emili1 aCalsamiglia, John1 aDemkowicz-Dobrzanski, R1 aMuñoz-Tapia, Ramon uhttp://iopscience.iop.org/1367-2630/9/8/244/pdf/1367-2630\_9\_8\_244.pdf00625nas a2200181 4500008004100000245009400041210006900135300000800204490000700212653002100219100002300240700001800263700002400281700001400305700001900319700001600338856008900354 2007 eng d00aUltracold atomic gases in optical lattices: mimicking condensed matter physics and beyond0 aUltracold atomic gases in optical lattices mimicking condensed m a2430 v5610aoptical lattices1 aLewenstein, Maciej1 aSanpera, Anna1 aAhufinger, Veronica1 aDamski, B1 aSen(De), Aditi1 aSen, Ujjwal uhttp://www.informaworld.com/smpp/ftinterface\~content=a778175433\~fulltext=71324093000480nas a2200145 4500008004100000245007800041210006900119300000700188490000700195100001600202700002200218700001900240700002100259856005400280 2007 eng d00aWeighted graph states and applications to spin chains, lattices and gases0 aWeighted graph states and applications to spin chains lattices a aS10 v401 aHartmann, L1 aCalsamiglia, John1 aDür, Wolfgang1 aBriegel, Hans, J uhttp://www.iop.org/EJ/abstract/0953-4075/40/9/S0101201nas a2200193 4500008004100000245006800041210006700109260001200176300001100188490000700199520063100206653003000837653001300867100002200880700001600902700001900918700002100937856004900958 2005 eng d00aSpin Gases: Quantum Entanglement Driven by Classical Kinematics0 aSpin Gases Quantum Entanglement Driven by Classical Kinematics c10/2005 a1805020 v953 aA spin gas is a natural extension of a classical gas. It consists of a large number of particles whose (random) motion is described classically, but, in addition, have internal (quantum mechanical) degrees of freedom that interact during collisions. For specific types of quantum interactions we determine the entanglement that occurs naturally in such systems. We analyze how the evolution of the quantum state is determined by the underlying classical kinematics of the gas. For the Boltzmann gas, we calculate the rate at which entanglement is produced and characterize the entanglement properties of the equilibrium state.10aentanglement distribution10aspin gas1 aCalsamiglia, John1 aHartmann, L1 aDür, Wolfgang1 aBriegel, Hans, J uhttp://link.aps.org/abstract/PRL/v95/e18050200513nas a2200181 4500008004100000022001400041245005600055210005500111490000700166100001800173700001500191700001800206700001300224700001400237700001300251700001500264856005200279 2004 eng d a0031-900700aCoherence Properties of Guided-Atom Interferometers0 aCoherence Properties of GuidedAtom Interferometers0 v921 aKreutzmann, H1 aPoulsen, U1 aLewenstein, M1 aDumke, R1 aErtmer, W1 aBirkl, G1 aSanpera, A uhttp://prl.aps.org/abstract/PRL/v92/i16/e16320100548nas a2200181 4500008004100000245003900041210003800080260004500118300001400163100001900177700002100196700002500217700001400242700001900256700002700275700002200302856004200324 2004 eng d00aPopper's test of Quantum Mechanics0 aPoppers test of Quantum Mechanics aMadridbAula Documental de Investigacion a201–2071 aBramon, Albert1 aEscribano, Rafel1 aAlvarez-Estrada, R F1 aDobado, A1 aFernandez, L A1 aMart\'ın-Delgado, M A1 a{Munoz Sudupe}, A uhttp://arxiv.org/abs/quant-ph/050113401507nas a2200169 4500008003700000020002400037245009600061210006900157260006000226300001600286520090900302100001801211700002101229700001901250700002001269856004801289 0 d a{978-1-5386-4781-3}00a{Fully Quantum Arbitrarily Varying Channels: Random Coding Capacity and Capacity Dichotomy}0 aFully Quantum Arbitrarily Varying Channels Random Coding Capacit b{IEEE; IEEE Informat Theory Soc; NSF; Huawei; Qualcomm} a{2012-2016}3 a{We consider a model of communication via a fully quantum jammer channel with quantum jammer, quantum sender and quantum receiver, which we dub quantum arbitrarily varying channel (QAVC). Restricting to finite dimensional user and jammer systems, we show, using permutation symmetry and a de Finetti reduction, how the random coding capacity (classical and quantum) of the QAVC is reduced to the capacity of a naturally associated compound channel, which is obtained by restricting the jammer to i.i.d. input states. Furthermore, we demonstrate that the shared randomness required is at most logarithmic in the block length, via a quantum version of the ``elimination of of correlation{''} using a random matrix tail bound. This implies a dichotomy theorem: either the classical capacity of the QAVC is zero, and then also the quantum capacity is zero, or each capacity equals its random coding variant.}1 aBoche, Holger1 aDeppe, Christian1 aNoetzel, Janis1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/node/92401640nas a2200157 4500008003700000020002400037245007000061210006700131260006000198300001600258520110100274100001801375700002101393700002001414856004801434 0 d a{978-1-5386-4781-3}00a{Secure and Robust Identification via Classical-Quantum Channels}0 aSecure and Robust Identification via ClassicalQuantum Channels b{IEEE; IEEE Informat Theory Soc; NSF; Huawei; Qualcomm} a{2674-2678}3 a{We study the identification capacity of classical-quantum channels ({''}cq-channels{''}), under channel uncertainty and privacy constraints. To be precise, we consider first compound memoryless cq-channels and determine their identification capacity; then we add an eavesdropper, considering compound memoryless wiretap cqq-channels, and determine their secret identification capacity. In the first case (without privacy), we find the identification capacity always equal to the transmission capacity. In the second case, we find a dichotomy: either the secrecy capacity (also known as private capacity) of the channel is zero, and then also the secrecy identification capacity is zero, or the secrecy capacity is positive and then the secrecy identification capacity equals the transmission capacity of the main channel without the wiretapper. We perform the same analysis for the case of arbitrarily varying wiretap cqq-channels (cqq-AVWC), with analogous findings, and make several observations regarding the continuity and super-additivity of the identification capacity in the latter case.}1 aBoche, Holger1 aDeppe, Christian1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/node/925