Correlations in quantum information

Top PDF Correlations in quantum information:

A study of the dynamics of quantum correlations

A study of the dynamics of quantum correlations

One of the purposes of this work was to provide an introduction to the field of quantum information theory. We began studying the classical theory of informa- tion in Ch. 2. We have introduced the Shannon entropy and seen two different interpretations for it. From one perspective, the Shannon entropy measures the amount of information (uncertainty) of a message (random variable) and, from the other, it tells us the minimum amount of binary digits needed to encode faithfully a given message. Then, we discussed two-messages entropies, the mutual informa- tion, which measures the correlations between two messages or the information they share in common; the conditional entropy, which gives the amount of information we have about some message when we have a previous knowledge of another; the joint entropy, which measures the non-redundant information of two messages; and the relative entropy, which measures the loss of information when we use an ap- proximate message instead of the original. Then, in Ch. 3 we reviewed some basic concepts in quantum mechanics as well as introduced some more advanced ones such as the dynamics of open quantum systems and generalized measurement dynamics. In Ch. 4 we begin our exploration of quantum information. We began considering the von Neumann entropy and saw that it is the analogous of Shannon entropy in the quantum realm. In other words, it gives the amount of qubits necessary to transmit (transpose) faithfully a quantum signal (density operator). Then, we studied the concept of correlations in quantum theory as well as developed some measures of quantum correlations. For pure bipartite states, we can measure the entanglement with the entropy of entanglement, the purity or the Schmidt rank. For mixed states, we measure the amount of correlations with the entanglement of formation, the concurrence, the quantum mutual information and the quantum
Mostrar mais

153 Ler mais

Quantum correlations and quantum coherence in open quantum systems

Quantum correlations and quantum coherence in open quantum systems

However, recent developments in the field of new materials have led to observe and control quantum systems at different times, length scales and energy ranges (BREUER et al., 2016; VEGA; ALONSO, 2017). In many of these scenarios, a large separation between system and environment time scales can no longer be assumed, leading to non- Markovian behavior and eventually a back-flow of information from the environment into the system. Indeed, the Markovian behavior is always an idealization in the de- scription of the quantum dynamics, with non-Markovianity being non-negligible in a number of different scenarios, such as biological (ISHIZAKI; FLEMING, 2009; REBEN- TROST; CHAKRABORTY; ASPURU-GUZIK, 2009; LIANG, 2010; CHEN et al., 2015) or condensed matter systems (WOLF et al., 2008; APOLLARO et al., 2011; HAIKKA; JOHNSON; MANISCALCO, 2013). A considerable amount of literature has been published on rigorously define non-Markovian dynamics in the quantum case, different approaches have been followed and several methods have been proposed (see, e. g., (RIVAS; HUELGA; PLENIO, 2014; POLLOCK et al., 2015; POLLOCK et al., 2018; BREUER et al., 2016; VEGA; ALONSO, 2017)). From an applied point of view, non-Markovian dynamics may be a resource for quantum tasks through an increase in the capacities of quantum channels (BYLICKA; CHRUŚCIŃSKI; MANISCALCO, 2014). Moreover, it also exhibits applications in fault-tolerant quantum computation (AHARONOV; KITAEV; PRESKILL, 2006). Basically, two main questions need to be discussed in this context (RIVAS; HUELGA; PLENIO, 2014):
Mostrar mais

143 Ler mais

Extracting Information from Qubit-Environment Correlations

Extracting Information from Qubit-Environment Correlations

Most works on open quantum systems generally focus on the reduced physical system by tracing out the environment degrees of freedom. Here we show that the qubit distributions with the environment are essential for a thorough analysis, and demonstrate that the way that quantum correlations are distributed in a quantum register is constrained by the way in which each subsystem gets correlated with the environment. For a two-qubit system coupled to a common dissipative environment E, we show how to optimise interqubit correlations and entanglement via a quantification of the qubit-environment information flow, in a process that, perhaps surprisingly, does not rely on the knowledge of the state of the environment. To illustrate our findings, we consider an optically-driven bipartite interacting qubit AB system under the action of E . By tailoring the light-matter interaction, a relationship between the qubits early stage
Mostrar mais

7 Ler mais

Characterization of quantum correlations in low-dimensional molecular magnetic systems

Characterization of quantum correlations in low-dimensional molecular magnetic systems

In summary, our main result was to provide to the literature analytical expressions for quantum information quanti¯ers, such as the entanglement witness, entangle- ment of formation, Bell's inequality violation and geometric quantum discord as a function of quantities typically obtained in neutron scattering via scalar structure factor. We provide one path towards identifying the presence of quantum correla- tions and quantum nonlocality in a two-qubit system such as a Heisenberg spin- 1=2 dimer, using di®ractive properties without making any assumption about its mac- roscopic properties or the external conditions under which the neutrons are scattered. We present an alternative way to describe the quantum properties of a sample ma- terial via neutron scattering experiments. Our results open doors for the detection and manipulation of quantum correlations through neutron scattering experiments in magnetic systems, such as the molecular magnets ruled by Heisenberg Hamiltonians; leading to promising applications in quantum information science, since these materials can be promising platforms in quantum information processing.
Mostrar mais

186 Ler mais

Information–theoretic implications of quantum causal structures

Information–theoretic implications of quantum causal structures

It is a relatively new insight of classical statistics that empirical data can contain information about causation rather than mere correlation. First algorithms have been proposed that are capable of testing whether a presumed causal relationship is compatible with an observed distribution. However, no systematic method is known for treating such problems in a way that generalizes to quantum systems. Here, we describe a general algorithm for computing information–theoretic constraints on the correlations that can arise from a given causal structure, where we allow for quantum systems as well as classical random variables. The general technique is applied to two relevant cases: first, we show that the principle of information causality appears naturally in our framework and go on to generalize and strengthen it. Second, we derive bounds on the correlations that can occur in a networked architecture, where a set of few-body quantum systems is distributed among some parties.
Mostrar mais

8 Ler mais

Bounding the sets of classical and quantum correlations in networks

Bounding the sets of classical and quantum correlations in networks

The authors are grateful to Elie Wolfe, Peter Wittek, Flavio Baccari, and Marco Túlio Quintino for insightful discussions. This work is supported by the ERC CoG QITBOX, the AXA Chair in Quantum Information Science, Fundacio Obra Social “la Caixa” (LCF/BQ/ ES15/10360001), the Spanish MINECO (QIBEQI FIS2016-80773-P and Severo Ochoa SEV-2015-0522), Fundacio Cellex, Generalitat de Catalunya (CERCA Program and SGR1381), the John Templeton Foundation via the Grant Q-CAUSAL No. 61084, the Serrapilheira Institute (Grant No. Serra-1708-15763), the São Paulo Research Foundation FAPESP (Grant No. 2018/07258-7), the Brazilian National Council for Scientific and Technological Development (CNPq) via the National Institute for Science and Technology on Quantum Information (INCT-IQ) and Grants No. 307172/ 2017-1 and No. 406574/2018-9, the Brazilian agencies MCTIC and MEC, the Austrian Science Fund (FWF) standalone Project No. P 30947, and the Foundation for Polish Science (IRAP project, ICTQT, Contract No. 2018/ MAB/5, cofinanced by EU within Smart Growth Operational Programme).
Mostrar mais

6 Ler mais

Sudden change in quantum and classical correlations and the Unruh effect

Sudden change in quantum and classical correlations and the Unruh effect

Parallel to these developments, theoretical studies an- alyzing the role played by relativity in the behavior of quantum systems when (i) the relative motion and (ii) proper acceleration of the communicating partners is large have attracted much attention (see Ref. [17] for a recent review). This is not only interesting in its own right but may also have practical importance because of new trends of implementing quantum-information protocols at global scales through the use of satellite systems [18–22]. Concerning condition (i), the remarkable fact was shown in Ref. [23] that the von Neumann entropy associated with the reduced spin-density matrix of a single particle is not Lorentz invariant. This is so because, in general, the spin is Wigner rotated under Lorentz boosts in a direction which depends on the particle momentum, thus entangling both degrees of freedom. Similarly, it was shown that the entanglement for a two-particle spin system is not Lorentz invariant either [24]. The Lorentz invariance of the entanglement distillability of a bipartite mixed spin state was investigated in Ref. [25]. The degree of violation of the Clauser-Horne-Shimony-Holt inequality as seen by
Mostrar mais

8 Ler mais

Carboxylate-based molecular magnet: one path toward achieving stable quantum correlations at room temperature

Carboxylate-based molecular magnet: one path toward achieving stable quantum correlations at room temperature

The control of quantum correlations in solid state systems by means of material engineering is a broad avenue to be explored, since it makes possible steps toward to the limits of quantum mechanics and design of novel materials with applications on emerging quantum technologies. In this context, this Letter explores the potential of molecular magnets to be prototypes of materials for quantum information technology. More precisely, we engineered a material and from its geometric quantum discord we found significant quantum correlations up to 9.540 K (even without entanglement); and, in addition, a pure singlet state occupied up to 83 K (above liquid nitrogen temperature). These results could only be achieved due to the carboxylate group promoting a metal-to-metal huge magnetic interaction.
Mostrar mais

5 Ler mais

Quantum correlations and coherence in spin-1 Heisenberg chains

Quantum correlations and coherence in spin-1 Heisenberg chains

constituents. Therefore, in addition to the traditional ways of witnessing quantum phase transitions, it has been recently suggested that the tools of quantum information theory [ 3 ] can also be exploited to characterize the transition points (TPs) of quantum phase transitions. Especially, in quantum spin models, the behavior of entanglement [ 4 ], quantum discord [ 5 ], and many other correlation measures have been investigated, and their performance in detecting the TPs of the QPTs have been discussed [ 6 , 7 ]. Recently, a new line of research has emerged that concerns itself with the characterization and quantification of quantum coherence contained in a quantum state [ 8 – 12 ]. Based on these new quantum coherence measures, similar analysis have been done in the ground states of several spin chains [ 7 ]. However, many of these studies focusing on quantum correlations in spin chains have been done for spin-1/2 systems [ 6 , 7 ], where analytical solutions are available in many cases. On the other hand, spin-1 models have richer phase diagrams and show more complex physical phenomena, yet methods for obtaining the ground state of such systems are rather more involved [ 13 – 30 ]. For instance, a very important distinctive property of the integer-spin quantum systems as compared to the half-integer ones is the Haldane conjecture, which states that the system has a gapped ground state, giving rise to the so-called Haldane phase [ 31 ].
Mostrar mais

9 Ler mais

Coherent measurements in quantum metrology

Coherent measurements in quantum metrology

Quantum mechanical systems can be used to outperform classical ones in information processing [ 1 ]. Quantum correlations can be employed to beat the shot-noise (standard) limit in metrology protocols. Such parameter estimation methods are crucial for both theoretical advances and the development of technologies. However, almost all quantum technologies operate with some level of noise and how quantum enhancement fares in the presence of noise is still unclear. Another intrinsic quantum feature is the measurement process, which, in general, disturbs the system being measured and the outcome depends on a basis choice. Here, we explore this distinction to uncover a difference between coherent and adaptive readout strategies in a quantum metrology protocol in the presence of noise.
Mostrar mais

13 Ler mais

Quantum Spin Transport in Mesoscopic Interferometer

Quantum Spin Transport in Mesoscopic Interferometer

The parameter ', Eq. (3) represents the phase difference of electrons passing through the upper and the lower arms of the ring. In Eq. (1), V (r) is the effective potential for trans- mission of electrons through the quantum dot which depends, mainly, on the tunnel barrier between the quantum dot and the lead. Applying external magnetic field, B, normal to the plane of the device, then the Aharonov-Bohm phase picked up by an electron encircling this magnetic flux is given by

4 Ler mais

Automated quantum operations in photonic qutrits

Automated quantum operations in photonic qutrits

We report an experimental implementation of automated state transformations on spatial photonic qutrits following the theoretical proposal made by Baldijão et al. [ Phys. Rev. A 96 , 032329 ( 2017 )]. A qutrit state is simulated by using three Gaussian beams, and after some state operations, the transformed state is available in the end in terms of the basis state. The state transformation setup uses a spatial light modulator and a calcite-based interferometer. The results reveal the usefulness of the operation method. The experimental data show a good agreement with theoretical predictions, opening possibilities for explorations in higher dimensions and in a wide range of applications. This is a necessary step in qualifying spatial photonic qudits as a competitive setup for experimental research in the implementation of quantum algorithms which demand a large number of steps. DOI: 10.1103/PhysRevA.97.022301
Mostrar mais

7 Ler mais

Creative licenses for information use in information society

Creative licenses for information use in information society

Neste artigo, discutem-se os conceitos de copyleft e de licenças criativas de uso de informação e outros bens imateriais, como forma inovadora de relação contratual entre produtores [r]

8 Ler mais

Enhancement Of Carrier Lifetimes In Type-ii Quantum Dot/quantum Well Hybrid Structures

Enhancement Of Carrier Lifetimes In Type-ii Quantum Dot/quantum Well Hybrid Structures

In this work, we analyse the optical properties of GaSb QDs coupled to GaAs QWs via a variable width AlGaAs spacer. Using continuous wave (CW) photoluminescence (PL) and photoluminescence excitation (PLE) measure- ments, we demonstrate that, for barrier thicknesses below 10 nm, the optical emissions from GaSb QDs and wetting layer (WL) dominate and the QW emission is drastically reduced due to the escape of holes from the QW to the GaSb layer. In contrary to what is usually assumed, time-resolved (TR) PL measurements show that recombination rates are not proportional to the carrier density, thus leading to non- exponential time-transients which change with barrier thickness and are strongly dependent on the initial carrier concentration pumped into the system. By varying laser power for more than two orders of magnitude, we show that the carrier dynamics can only be satisfactorily described by taking into account all terms in the BR approximation for recombination rates, i.e., both linear and quadratic terms of the photocreated carrier density. Using this model, we obtain carrier lifetimes of up to 5 ls for GaSb QDs, which are con- siderably longer than the reported values for GaSb/GaAs QDs. 13 , 15 , 17 , 19 , 31 Our results keep alive the discussion about the BR approximation validity reported by Hodgson et al. 21
Mostrar mais

8 Ler mais

Coin state properties in quantum walks

Coin state properties in quantum walks

Numerical consequences. Numerical values of the state vector Y(x,n) have been obtained for a large number of initial conditions g, different choices of coin operator angle h, and system size 2L 1 1. For the sake of definitiveness, we always consider x g [2L,L]. The asymptotic magnetization depends on the values of both g and h, as illustrated by the curves  M|g and  M|h in Fig. 1. It is possible to see that the numerical values corroborate the analytical expressions given above. For arbitrary initial combinations of up and down components, the typical patterns change continuously from one of the extreme cases to the other. The extreme values of  M, which depend on the initial conditions, lie in the interval 1{ p ffiffiffi 2 2,1 . In Fig. 1b we illustrate the dependence of  M|g. It is amazing to see that, by an adequate choice of the coin operator (through the selected value of h), it is
Mostrar mais

6 Ler mais

Quantum deformation of quantum cosmology

Quantum deformation of quantum cosmology

Quantum groups and algebras emerged from studies on quantum integrable models using the quantum inverse scattering method [17] and led to certain deformations of classical matrix groups and the corresponding Lie algebras. The original main reason for the great significance of quantum groups was that they are related to the so called quantum Yang-Baxter equation [18] which plays a major role in quantum integrable systems, conformal field theory [19], solvable lattice models [20], knot theory [21], topological quantum compu- tation [22]. Phenomenological applications of quantum groups in nuclear [23] and molecular spectroscopy [24] lead to significant re- sults showing that the vibrational-rotational spectra of nuclei and molecules can be fit into schemes in which the number of phenomeno- logical deformation parameters required are very much fewer than the number of traditional phenomenological parameters.
Mostrar mais

14 Ler mais

Quantum Key Distribution Post Processing - A study on the Information Reconciliation Cascade Protocol

Quantum Key Distribution Post Processing - A study on the Information Reconciliation Cascade Protocol

Although no pratical implementation or experiment was made, in the same paper, the authors introduce the concept of Block Reuse 2 . They argue that it is possible to optimize the protocol by maintaining a record of all subblocks exchanged during BINARY executions and also using them for the cascade effect. That is, after the first iteration, upon correcting one error, instead of adding to the set of blocks to correct just the block from each iteration containing the corrected index, every subblock containing that index that was exchanged during the BINARY protocol should also be added to the set. As these blocks will be smaller, fewer parity exchanges will be needed to correct an error.
Mostrar mais

100 Ler mais

Quantum realism and quantum surrealism

Quantum realism and quantum surrealism

This theorem already hints of a theme that shall be recurrent in the search for ontological models: we can in fact make ontological models for quantum theory, and in fact we can make them almost in any way that we like, but there’s a price to pay: the various aspects of the model become more and more intertwined. We can’t really talk of independent quantum systems, separation between state and experiment, nor even (as we shall see in the next section) talk about a measurement outcome without talking about the whole experiment. Of course, this bodes very badly for the idea of ontological models: in the extreme limit of this interdependence our ontological model only lists possible experiments and their results, without ever trying to make sense of them in a simpler and more general theory. A model like this wouldn’t be falsifiable by its very nature, but precisely because of this it is a perversion of the scientific method [19], and should therefore be rejected on methodological grounds.
Mostrar mais

71 Ler mais

Information visualisation in information retrieval systems

Information visualisation in information retrieval systems

Skozi predstavitev vizualizacije informacij smo pravzaprav odgovarjali na neka- tera osnovna vprašanja, ki se zastavljajo ob srečanju s tematiko: zakaj, kako in kaj. Pregled študij je pokazal, da odgovor na nobeno izmed zastavljenih vprašanj ni enoznačen ali prav gotovo ne preprost. Rezultati študij in eksperimentov niso naklonjeni vizualizaciji in le malo poskusov je v resnici zaživelo, pa vendar se zdi, da raziskovalci še vedno verjamejo v moč vizualizacije. Song (2000) to do- bro opiše z besedami, da kljub vsemu ve, da vizualizacija lahko pomaga uporab- nikom, le da za zdaj še nismo odkrili pravega načina. Morda pa se čas vizualiza- cije šele začenja, saj se spletna orodja (in s tem tudi ljudje?) vse bolj premikajo k vizualnim podobam. Tudi sami sistemi za poizvedovanje so v zadnjem letu naredili velik preskok v obvladovanju in predstavljanju rezultatov in ne more- mo zanikati potencialne uporabnosti vizualizacije ter dejstva, da so morda sedaj, bolj kot kdajkoli prej, pripravljeni za nadgradnjo v obliki vizualizacije odnosov in povezav med dokumenti.
Mostrar mais

20 Ler mais

How structure determines correlations in neuronal networks.

How structure determines correlations in neuronal networks.

excitation, such that m~0. The estimated mean degree is consistent with many different topologies. Let us consider the case of a uniform random network of 15000 neurons with connection probability 0.1. For comparison we also look at a densely connected subnetwork of just 2500 neurons with a connection probability of 0.6. The first model results in a spectral radius r&1:4 for the connectivity matrix G, hence falling in the linearly unstable regime. In contrast, the second network displays a Figure 9. Patches in separate populations selectively affect high-order common input motifs. A: Connection rules for networks with patches. Output connections of a neuron are restricted to a randomly chosen region. B: Average correlations depending on patch size. Comparison between random networks, patchy networks with randomly distributed neuron type and separate populations, average across 5 networks, error bars from standard deviation. Only separate populations lead to increased correlations. Larger increase occurs for smaller patch-size. Total connection probability p~0:1, other parameters as in Figure 2. C: Contributions of different motifs. Differences to random networks occur only in common input terms of higher order. Patch size S~600.
Mostrar mais

14 Ler mais

Show all 10000 documents...