Quantum randomness is an essential key to understanding the dynamics of complex many-body systems and also a powerful tool for quantum engineering. However, exact realizations of quantum randomness take an extremely long time and are infeasible in many-body systems, leading to the notion of quantum pseudorandomness, also known as unitary designs. Here, to explore microscopic dynamics of generating quantum pseudorandomness in many-body systems, we provide new efficient constructions of unitary designs and propose a design Hamiltonian, a random Hamiltonian of which dynamics always forms a unitary design after a threshold time. The new constructions are based on the alternate applications of random potentials in the generalized position and momentum spaces, and we provide explicit quantum circuits generating quantum pseudorandomness significantly more efficient than previous ones. We then provide a design Hamiltonian in disordered systems with periodically changing spin-glass-type interactions. The design Hamiltonian generates quantum pseudorandomness in a constant time even in the system composed of a large number of spins. We also point out the close relationship between the design Hamiltonian and quantum chaos.

VL - 7 ER - TY - JOUR T1 - Enhancement of low-temperature thermometry by strong coupling JF - PHYSICAL REVIEW A Y1 - 2017 A1 - Correa, Luis A. A1 - Perarnau-Llobet, Marti A1 - Hovhannisyan, Karen V. A1 - Hernandez-Santana, Senaida A1 - Mehboudi, Mohammad A1 - Sanpera, Anna AB - We consider the problem of estimating the temperature T of a very cold equilibrium sample. The temperature estimates are drawn from measurements performed on a quantum Brownian probe strongly coupled to it. We model this scenario by resorting to the canonical Caldeira-Leggett Hamiltonian and find analytically the exact stationary state of the probe for arbitrary coupling strength. In general, the probe does not reach thermal equilibrium with the sample, due to their nonperturbative interaction. We argue that this is advantageous for low-temperature thermometry, as we show in our model that (i) the thermometric precision at low T can be significantly enhanced by strengthening the probe-sampling coupling, (ii) the variance of a suitable quadrature of our Brownian thermometer can yield temperature estimates with nearly minimal statistical uncertainty, and (iii) the spectral density of the probe-sample coupling may be engineered to further improve thermometric performance. These observations may find applications in practical nanoscale thermometry at low temperatures-a regime which is particularly relevant to quantum technologies. VL - 96 ER - TY - JOUR T1 - Entanglement and nonlocality in diagonal symmetric states of JF - Physical Review A Y1 - 2017 A1 - Quesada, Ruben A1 - Rana, Swapan A1 - Sanpera, Anna VL - 95 UR - http://link.aps.org/doi/10.1103/PhysRevA.95.042128http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.95.042128/fulltexthttp://link.aps.org/article/10.1103/PhysRevA.95.042128 JO - Phys. Rev. A ER - TY - JOUR T1 - Equilibration Time Scales of Physically Relevant Observables JF - PHYSICAL REVIEW X Y1 - 2017 A1 - Garcia-Pintos, Luis Pedro A1 - Linden, Noah A1 - Malabarba, Artur S. L. A1 - Short, Anthony J. A1 - Winter, Andreas AB -We address the problem of understanding, from first principles, the conditions under which a quantum system equilibrates rapidly with respect to a concrete observable. On the one hand, previously known general upper bounds on the time scales of equilibration were unrealistically long, with times scaling linearly with the dimension of the Hilbert space. These bounds proved to be tight since particular constructions of observables scaling in this way were found. On the other hand, the computed equilibration time scales for certain classes of typical measurements, or under the evolution of typical Hamiltonians, are unrealistically short. However, most physically relevant situations fall outside these two classes. In this paper, we provide a new upper bound on the equilibration time scales which, under some physically reasonable conditions, give much more realistic results than previously known. In particular, we apply this result to the paradigmatic case of a system interacting with a thermal bath, where we obtain an upper bound for the equilibration time scale independent of the size of the bath. In this way, we find general conditions that single out observables with realistic equilibration times within a physically relevant setup.

VL - 7 ER - TY - JOUR T1 - Erratum: Resource theory of coherence: Beyond states [Phys. Rev. AThe detection of change points is a pivotal task in statistical analysis. In the quantum realm, it is a new primitive where one aims at identifying the point where a source that supposedly prepares a sequence of particles in identical quantum states starts preparing a mutated one. We obtain the optimal procedure to identify the change point with certainty-naturally at the price of having a certain probability of getting an inconclusive answer. We obtain the analytical form of the optimal probability of successful identification for any length of the particle sequence. We show that the conditional success probabilities of identifying each possible change point show an unexpected oscillatory behavior. We also discuss local (online) protocols and compare them with the optimal procedure.

VL - 119 ER - TY - JOUR T1 - Entanglement and Coherence in Quantum State Merging JF - Physical Review Letters Y1 - 2016 A1 - Streltsov, A. A1 - Chitambar, E. A1 - Rana, S. A1 - Bera, N. A1 - Winter, A. A1 - Lewenstein, M. VL - 116 UR - http://link.aps.org/doi/10.1103/PhysRevLett.116.240405http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.116.240405/fulltexthttp://link.aps.org/accepted/10.1103/PhysRevLett.116.240405http://link.aps.org/article/10.1103/PhysRevLett.116.240405 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Entanglement-saving channels JF - Journal of Mathematical Physics Y1 - 2016 A1 - Lami, L. A1 - Giovannetti, V. VL - 57 IS - 3 JO - J. Math. Phys. ER - TY - JOUR T1 - Estimating quantum chromatic numbers JF - Journal of Functional Analysis Y1 - 2016 A1 - Paulsen, Vern I. A1 - Severini, Simone A1 - Stahlke, Daniel A1 - Todorov, Ivan G. A1 - Winter, Andreas VL - 270 IS - 6 JO - Journal of Functional Analysis ER - TY - JOUR T1 - Entanglement–breaking indices JF - Journal of Mathematical Physics Y1 - 2015 A1 - Lami, L. A1 - Giovannetti, V. VL - 56 IS - 9 JO - J. Math. Phys. ER - TY - JOUR T1 - Experimental access to higher-dimensional entangled quantum systems using integrated optics JF - Optica Y1 - 2015 A1 - Christoph Schaeff A1 - Robert Polster A1 - Huber, Marcus A1 - Ramelow, Sven A1 - Anton Zeilinger KW - Integrated optics KW - quantum optics AB - Integrated optics allows for the generation and control of increasingly complex photonic states on chip-based architectures. Here, we implement two entangled qutrits&\#x2014;a nine-dimensional quantum system&\#x2014;and demonstrate an exceptionally high degree of experimental control. The approach, which is conceptually different to common bulk optical implementations, is heavily based on methods of integrated in-fiber and on-chip technologies and further motivated by methods commonly used in today&\#x2019;s telecommunications industry. The system is composed of an in-fiber source creating entangled qutrit states of any amplitude and phase, and an on-chip integrated general Multiport enabling the realization of any desired local unitary transformation within the two qutrit nine-dimensional Hilbert space. The complete design is readily extendible toward higher dimensions with moderate increase in complexity. Ultimately, our scheme allows for complete on-chip integration. We demonstrate the flexibility and generality of our system by realizing a complete characterization of the two-qutrit space of higher-order Einstein&\#x2013;Podolsky&\#x2013;Rosen correlations. PB - OSA VL - 2 UR - http://www.osapublishing.org/optica/abstract.cfm?URI=optica-2-6-523 ER - TY - JOUR T1 - Extractable Work from Correlations JF - Physical Review X Y1 - 2015 A1 - Martí Perarnau-Llobet A1 - Karen V. Hovhannisyan A1 - Huber, Marcus A1 - Paul Skrzypczyk A1 - Nicolas Brunner A1 - Acín, Antonio VL - 5 UR - http://link.aps.org/doi/10.1103/PhysRevX.5.041011 ER - TY - JOUR T1 - Entanglement enhances cooling in microscopic quantum refrigerators JF - Physical Review E Y1 - 2014 A1 - Nicolas Brunner A1 - Huber, Marcus A1 - Noah Linden A1 - Sandu Popescu A1 - Ralph Silva A1 - Paul Skrzypczyk AB - Small self-contained quantum thermal machines function without external source of work or control, but using only incoherent interactions with thermal baths. Here we investigate the role of entanglement in a small self-contained quantum refrigerator. We first show that entanglement is detrimental as far as efficiency is concerned---fridges operating at efficiencies close to the Carnot limit do not feature any entanglement. Moving away from the Carnot regime, we show that entanglement can enhance cooling and energy transport. Hence a truly quantum refrigerator can outperform a classical one. Furthermore, the amount of entanglement alone quantifies the enhancement in cooling. VL - 89 IS - 3 JO - Phys. Rev. E ER - TY - JOUR T1 - Entanglement properties of spin models in triangular lattices JF - Journal of Statistical Mechanics: Theory and Experiment Y1 - 2014 A1 - Moreno-Cardoner, M. A1 - Paganelli, S. A1 - De Chiara, G. A1 - Sanpera, A. AB - The 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. VL - 2014 IS - 10 JO - J. Stat. Mech. ER - TY - JOUR T1 - Everything You Always Wanted to Know About LOCC (But Were Afraid to Ask) JF - Communications in Mathematical Physics Y1 - 2014 A1 - Eric Chitambar A1 - Debbie Leung A1 - Mančinska, Laura A1 - Maris Ozols A1 - Winter, Andreas VL - 328 IS - 1 JO - Commun. Math. Phys. ER - TY - JOUR T1 - Entangled singularity patterns of photons in {Ince}-{Gauss} modes JF - Physical Review A Y1 - 2013 A1 - Krenn, Mario A1 - Robert Fickler A1 - Huber, Marcus A1 - Radek Lapkiewicz A1 - Plick, William A1 - Ramelow, Sven A1 - Anton Zeilinger VL - 87 UR - http://link.aps.org/doi/10.1103/PhysRevA.87.012326 ER - TY - JOUR T1 - Entanglement Generation is Not Necessary for Optimal Work Extraction JF - Physical Review Letters Y1 - 2013 A1 - Hovhannisyan, Karen A1 - Martí Perarnau-Llobet A1 - Huber, Marcus A1 - Acín, Antonio AB - We consider reversible work extraction from identical quantum batteries. From an ensemble of individually passive states, work can be produced only via global unitary (and thus entangling) operations. However, we show here that there always exists a method to extract all possible work without creating any entanglement, at the price of generically requiring more operations (i.e. additional time). We then study faster methods to extract work and provide a quantitative relation between the amount of generated multipartite entanglement and extractable work. Our results suggest a general relation between entanglement generation and the power of work extraction. VL - 111 IS - 24 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Entropy vector formalism and the structure of multidimensional entanglement in multipartite systems JF - Physical Review A Y1 - 2013 A1 - Huber, Marcus A1 - Martí Perarnau-Llobet A1 - de Vicente, Julio I. VL - 88 UR - http://arxiv.org/abs/1307.3541 IS - 4 JO - Phys. Rev. A ER - TY - JOUR T1 - Examining the dimensionality of genuine multipartite entanglement JF - Quantum Information Processing Y1 - 2013 A1 - Spengler, Christoph A1 - Huber, Marcus A1 - Gabriel, Andreas A1 - Hiesmayr, Beatrix C. VL - 12 UR - http://link.springer.com/10.1007/s11128-012-0369-8 ER - TY - THES T1 - Entanglement distribution in quantum complex networks T2 - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics Y1 - 2012 A1 - Cuquet, Martí AB - This thesis deals with the study of quantum networks with a complex structure, the implications this structure has in the distribution of entanglement and how their functioning can be enhanced by operating in the quantum regime. We first consider a complex network of bipartite states, both pure and mixed, and study the distribution of long-distance entanglement. Then, we move to a network with noisy channels and study the creation and distribution of large, multipartite states. The work contained in this thesis is primarily motivated by the idea that the interplay between quantum information and complex networks may give rise to a new understanding and characterization of natural systems. Complex networks are of particular importance in communication infrastructures, as most present telecommunication networks have a complex structure. In the case of quantum networks, which are the necessary framework for distributed quantum processing and for quantum communication, it is very plausible that in the future they acquire a complex topology resembling that of existing networks, or even that methods will be developed to use current infrastructures in the quantum regime. A central task in quantum networks is to devise strategies to distribute entanglement among its nodes. In the first part of this thesis, we consider the distribution of bipartite entanglement as an entanglement percolation process in a complex network. Within this approach, perfect entanglement is established probabilistically between two arbitrary nodes. We see that for large networks, the probability of doing so is a constant strictly greater than zero (and independent of the size of the network) if the initial amount of entanglement is above a certain critical value. Quantum mechanics offer here the possibility to change the structure of the network without need to establish new, "physical" channels. By a proper local transformation of the network, the critical entanglement can be decreased and the probability increased. We apply this transformation to complex network models with arbitrary degree distribution. In the case of a noisy network of mixed states, we see that for some classes of states, the same approach of entanglement percolation can be used. For general mixed states, we consider a limited-path-length entanglement percolation constrained by the amount of noise in the connections. We see how complex networks still offer a great advantage in the probability of connecting two nodes. In the second part, we move to the multipartite scenario. We study the creation and distribution of graph states with a structure that mimic the underlying communication network. In this case, we use an arbitrary complex network of noisy channels, and consider that operations and measurements are also noisy. We propose an efficient scheme to distribute and purify small subgraphs, which are then merged to reproduce the desired state. We compare this approach with two bipartite protocols that rely on a central station and full knowledge of the network structure. We show that the fidelity of the generated graphs can be written as the partition function of a classical disordered spin system (a spin glass), and its decay rate is the analog of the free energy. Applying the three protocols to a one-dimensional network and to complex networks, we see that they are all comparable, and in some cases the proposed subgraph protocol, which needs only local information of the network, performs even better. JF - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics PB - Universitat Autònoma de Barcelona CY - Barcelona VL - Doctorat en Física ER - TY - THES T1 - Entanglement percolation with graph-states' T2 - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics Y1 - 2012 A1 - Franquet, Abert JF - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics PB - Universitat Autònoma de Barcelona CY - Barcelona VL - Grau en Física ER - TY - JOUR T1 - Entanglement Spectrum, Critical Exponents, and Order Parameters in Quantum Spin Chains JF - Physical Review Letters Y1 - 2012 A1 - De Chiara, G. A1 - Lepori, L. A1 - Lewenstein, M. A1 - Sanpera, A. AB - We 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. VL - 109 IS - 23 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Entanglement detection in hybrid optomechanical systems JF - Phys. Rev. A Y1 - 2011 A1 - De Chiara, Gabriele A1 - Paternostro, Mauro A1 - Palma, G. Massimo AB - We 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. VL - 83 UR - http://link.aps.org/doi/10.1103/PhysRevA.83.052324 ER - TY - JOUR T1 - Entangling two distant oscillators with a quantum reservoir JF - EPL (Europhysics Letters) Y1 - 2011 A1 - Wolf, A A1 - De Chiara, G. A1 - Kajari, E A1 - Lutz, E A1 - G. Morigi AB - The 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. VL - 95 UR - http://epljournal.edpsciences.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/epl/abs/2011/18/epl13816/epl13816.html IS - 6 JO - EPL ER - TY - JOUR T1 - Estimation of quantum finite mixtures JF - Physical Review A Y1 - 2010 A1 - de Vicente, J. I. A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emilio AB - We consider the problem of determining the weights of a quantum ensemble. That is to say, given a quantum system that is in a set of possible known states according to an unknown probability law, we give strategies to estimate the individual probabilities, weights, or mixing proportions. Such strategies can be used to estimate the frequencies at which different independent signals are emitted by a source. They can also be used to estimate the weights of particular terms in a canonical decomposition of a quantum channel. The quality of these strategies is quantified by a covariance-type error matrix. According with this cost function, we give optimal strategies in both the single-shot and multiple-copy scenarios. The latter is also analyzed in the asymptotic limit of large number of copies. We give closed expressions of the error matrix for two-component quantum mixtures of qubit systems. The Fisher information plays an unusual role in the problem at hand, providing exact expressions of the minimum covariance matrix for any number of copies. VL - 81 UR - http://link.aps.org/doi/10.1103/PhysRevA.81.012332 ER - TY - JOUR T1 - Entanglement Percolation in Quantum Complex Networks JF - Physical Review Letters Y1 - 2009 A1 - Cuquet, Martí A1 - Calsamiglia, John KW - complex networks KW - entanglement distribution KW - entanglement percolation KW - erdos-renyi KW - generating function KW - quantum complex networks KW - quantum networks KW - scale-free KW - small-world AB - Quantum networks are essential to quantum information distributed applications, and communicatingover them is a key challenge. Complex networks have richand intriguing properties, which are as yet unexplored in thequantum setting. Here, we study the effect of entanglement percolationas a means to establish long-distance entanglement between arbitrary nodesof quantum complex networks. We develop a theory to analyticallystudy random graphs with arbitrary degree distribution and give exactresults for some models. Our findings are in good agreementwith numerical simulations and show that the proposed quantum strategiesenhance the percolation threshold substantially. Simulations also show a clearenhancement in small-world and other real-world networks. PB - APS VL - 103 UR - http://link.aps.org/abstract/PRL/v103/e240503 ER - TY - JOUR T1 - Efficiency in Quantum Key Distribution Protocols with Entangled Gaussian States JF - Open Systems & Information Dynamics Y1 - 2007 A1 - Rodó, Carles A1 - Romero-Isart, Oriol A1 - Eckert, Kai A1 - Sanpera, Anna KW - gaussian states KW - quantum cryptography KW - quantum key distribution AB - Abstract Quantum key distribution (QKD) refers to specific quantum strategies which permit the secure distribution of a secret key between two parties that wish to communicate secretly. Quantum cryptography has proven unconditionally secure in ideal scenarios and has been successfully implemented using quantum states with finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we analyze the efficiency of QKD protocols that use as a resource entangled gaussian states and gaussian operations only. In this framework, it has already been shown that QKD is possible [1] but the issue of its efficiency has not been considered. We propose a figure of merit (the efficiency E) to quantify the number of classical correlated bits that can be used to distill a key from a sample of N entangled states. We relate the efficiency of the protocol to the entanglement and purity of the states shared between the parties. VL - 14 UR - http://dx.doi.org/10.1007/s11080-007-9030-x ER - TY - JOUR T1 - Efficient quantum-state transfer in spin chains via adiabatic passage JF - New Journal of Physics Y1 - 2007 A1 - Eckert, Kai A1 - Romero-Isart, Oriol A1 - Sanpera, Anna KW - spin chain KW - state transfer VL - 9 UR - http://iopscience.iop.org/1367-2630/9/5/155/pdf/1367-2630\_9\_5\_155.pdf ER - TY - JOUR T1 - Entanglement-assisted alignment of reference frames using a dense covariant coding JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2004 A1 - Bagan, Emili A1 - Baig, Marià A1 - Muñoz-Tapia, Ramon AB - We present a procedure inspired by dense coding, which enables a highly efficient transmission of information of a continuous nature. The procedure requires the sender and the recipient to share a maximally entangled state. We deal with the concrete problem of aligning reference frames or trihedra by means of a quantum system. We find the optimal covariant measurement and compute the corresponding average error, which has a remarkably simple close form. The connection of this procedure with that of estimating unitary transformations on qubits is briefly discussed. PB - American Physical Society VL - 69 UR - http://link.aps.org/doi/10.1103/PhysRevA.69.050303 ER - TY - JOUR T1 - Experimental Detection of Multipartite Entanglement using Witness Operators JF - Physical Review Letters Y1 - 2004 A1 - Bourennane, Mohamed A1 - Eibl, Manfred A1 - Kurtsiefer, Christian A1 - Gaertner, Sascha A1 - Weinfurter, Harald A1 - Gühne, Otfried A1 - Hyllus, Philipp A1 - Bruß, Dagmar A1 - Lewenstein, Maciej A1 - Sanpera, Anna VL - 92 UR - http://prl.aps.org/abstract/PRL/v92/i8/e087902 ER - TY - CHAP T1 - Entanglement properties of composite quantum systems T2 - Quantum Information Processing Y1 - 2003 A1 - Eckert, Kai A1 - Guhne, O. A1 - Hulpke, Florian A1 - Hyllus, Philipp A1 - Korbicz, J. A1 - Mompart, Jordi A1 - Bruss, Dagmar A1 - Lewenstein, Maciej A1 - Sanpera, Anna ED - Leuchs, G. ED - Beth, T. JF - Quantum Information Processing PB - Wilew-VCH CY - Berlin SN - 352740371X ER - TY - JOUR T1 - Experimental detection of entanglement via witness operators and local measurements JF - Journal of Modern Optics Y1 - 2003 A1 - Gühne, Otfried A1 - Hyllus, Philipp A1 - Bruss, Dagmar A1 - Ekert, Artur K. A1 - Lewenstein, Maciej A1 - Macchiavello, Chiara A1 - Sanpera, Anna VL - 50 UR - http://www.informaworld.com/smpp/36433790-99697256/content\~{}db=all\~{}content=a713826204 ER -