Bipartite Case Research Articles

Exact value for the average optimal cost of the bipartite traveling salesman and two-factor problems in two dimensions

We show that the average cost for the traveling-salesman problem in two dimensions, which is the archetypal problem in combinatorial optimization, in the bipartite case, is simply related to the average cost of the assignment problem with the same Euclidean, increasing, convex weights. In this way we extend a result already known in one dimension where exact solutions are avalaible. The recently determined average cost for the assignment when the cost function is the square of the distance between the points provides therefore an exact prediction $$\overline{E_N} = \frac{1}{\pi}\, \log N$$ for large number of points $2N$. As a byproduct of our analysis also the loop covering problem has the same optimal average cost. We also explain why this result cannot be extended at higher dimensions. We numerically check the exact predictions.

Genuine correlated coherence

We introduce a notion of genuine correlated coherence. Such a notion is based on the possibility of concentrating on individual systems the coherence present in a distributed system, by making use of incoherent unitary transformations. We define an entropic quantifier of genuine correlated multipartite coherence for generic mixed states, and we focus on the bipartite pure-state case. In the latter case we derive necessary and sufficient conditions for the possibility of fully localizing the coherence, hence identifying the conditions for genuine correlated bipartite coherence. We analyze in detail the quantitative problem for the case of two-qubit pure states, identifying the states with the largest amount of genuine correlated coherence. Interestingly, such states do not have maximal global coherence nor maximal coherence rank.

Self-testing multipartite entangled states through projections onto two systems

Finding ways to test the behaviour of quantum devices is a timely enterprise, especially in light of the rapid development of quantum technologies. Device-independent self-testing is one desirable approach, as it makes minimal assumptions on the devices being tested. In this work, we address the question of which states can be self-tested. This has been answered recently in the bipartite case (Coladangelo et al 2017 Nat. Commun. 8 15485), while it is largely unexplored in the multipartite case, with only a few scattered results, using a variety of different methods: maximal violation of a Bell inequality, numerical SWAP method, stabiliser self-testing etc. In this work, we investigate a simple, and potentially unifying, approach: combining projections onto two-qubit spaces (projecting parties or degrees of freedom) and then using maximal violation of the tilted CHSH inequalities. This allows one to obtain self-testing of Dicke states and partially entangled GHZ states with two measurements per party, and also to recover self-testing of graph states (previously known only through stabiliser methods). Finally, we give the first self-test of a class of multipartite qudit states: we generalise the self-testing of partially entangled GHZ states by adapting techniques from (Coladangelo et al 2017 Nat. Commun. 8 15485), and show that all multipartite states which admit a Schmidt decomposition can be self-tested with few measurements.

Plastic number and possible optimal solutions for an Euclidean 2-matching in one dimension

In this work we consider the problem of finding the minimum-weight loop cover of an undirected graph. This combinatorial optimization problem is called 2-matching and can be seen as a relaxation of the traveling salesman problem since one does not have a unique loop condition. We consider this problem both on the complete bipartite and complete graph embedded in a one dimensional (1D) interval, the weights are chosen as a convex function of the Euclidean distance between each couple of points. Randomness is introduced throwing the points in space independently and uniformly. We derive the average optimal cost in the limit of a large number of points. We prove that the possible solutions are characterized by the presence of ‘shoelace’ loops containing two or three points of each type in the complete bipartite case, and three, four or five points in the complete one. This gives rise to an exponential number of possible solutions scaling as , where is the plastic constant. This is at variance to what happens in the previously studied 1D models, such as the matching and the traveling salesman problem, where for every instance of the disorder there is only one possible solution.

Tripartite quantum correlations in the renormalized space of Heisenberg-Ising spin−1∕2 chain

The dynamics of tripartite entanglement and tripartite quantum discord in the superposition of the two degenerate ground states of the Heisenberg-Ising spin −1∕2 chain with the increasing size of the system are studied through quantum renormalization group (QRG) technique. It is shown that although, both the correlations exhibit quantum phase transition in the thermodynamics limit, however, quantum discord is more sensitive to the size of the system. Unlike the bipartite case, the tripartite correlations are maximum when the Ising coupling between the spins dominates the Heisenberg coupling. Interestingly enough, it is found that the regions of entangled phase and product phase in the tripartite case are swapped with respect to the ones in the bipartite case.

On the Significance of the Quantum Mechanical Covariance Matrix.

The characterization of quantum correlations, being stronger than classical, yet weaker than those appearing in non-signaling models, still poses many riddles. In this work, we show that the extent of binary correlations in a general class of nonlocal theories can be characterized by the existence of a certain covariance matrix. The set of quantum realizable two-point correlators in the bipartite case then arises from a subtle restriction on the structure of this general covariance matrix. We also identify a class of theories whose covariance has neither a quantum nor an “almost quantum” origin, but which nevertheless produce the accessible two-point quantum mechanical correlators. Our approach leads to richer Bell-type inequalities in which the extent of nonlocality is intimately related to a non-additive entropic measure. In particular, it suggests that the Tsallis entropy with parameter is a natural operational measure of non-classicality. Moreover, when generalizing this covariance matrix, we find novel characterizations of the quantum mechanical set of correlators in multipartite scenarios. All these predictions might be experimentally validated when adding weak measurements to the conventional Bell test (without adding postselection).

Enhancing the absorption and energy transfer process via quantum entanglement

The quantum network model is widely used to describe the dynamics of excitation energy transfer in photosynthesis complexes. Different from the previous schemes, we explore a specific network model, which includes both light-harvesting and energy transfer process. Here, we define a rescaled measure to manifest the energy transfer efficiency from external driving to the sink, and the external driving fields are used to simulate the energy absorption process. To study the role of initial state in the light-harvesting and energy transfer process, we assume the initial state of the donors to be two-qubit and three-qubit entangled states, respectively. In the two-qubit initial state case, we find that the initial entanglement between the donors can help to improve the absorption and energy transfer process for both the near-resonant and large-detuning cases. For the case of three-qubit initial state, we can see that the transfer efficiency will reach a larger value faster in the tripartite entanglement case compared to the bipartite entanglement case.

On the difference between the (revised) Szeged index and the Wiener index of cacti

A connected graph is said to be a cactus if each of its blocks is either a cycle or an edge. Let Cn be the set of all n-vertex cacti with circumference at least 4, and let Cn,k be the set of all n-vertex cacti containing exactly k⩾1 cycles where n⩾3k+1. In this paper, lower bounds on the difference between the (revised) Szeged index and Wiener index of graphs in Cn (resp. Cn,k) are proved. The minimum and the second minimum values on the difference between the Szeged index and Wiener index of graphs among Cn are determined. The bound on the minimum value is strengthened in the bipartite case. A lower bound on the difference between the revised Szeged index and Wiener index of graphs among Cn,k is also established. Along the way the corresponding extremal graphs are identified.

Supersaturated Sparse Graphs and Hypergraphs

AbstractA central problem in extremal graph theory is to estimate, for a given graph H, the number of H-free graphs on a given set of n vertices. In the case when H is not bipartite, Erd̋s, Frankl, and Rödl proved that there are 2(1+o(1))ex(n, H) such graphs. In the bipartite case, however, bounds of the form 2O(ex(n, H)) have been proven only for relatively few special graphs H. As a 1st attempt at addressing this problem in full generality, we show that such a bound follows merely from a rather natural assumption on the growth rate of n ↦ ex(n, H); an analogous statement remains true when H is a uniform hypergraph. Subsequently, we derive several new results, along with most previously known estimates, as simple corollaries of our theorem. At the heart of our proof lies a general supersaturation statement that extends the seminal work of Erd̋s and Simonovits. The bounds on the number of H-free hypergraphs are derived from it using the method of hypergraph containers.

Scaling limits of random bipartite planar maps with a prescribed degree sequence

We study the asymptotic behavior of uniform random maps with a prescribed face‐degree sequence, in the bipartite case, as the number of faces tends to infinity. Under mild assumptions, we show that, properly rescaled, such maps converge in distribution toward the Brownian map in the Gromov–Hausdorff sense. This result encompasses a previous one of Le Gall for uniform random q‐angulations where q is an even integer. It applies also to random maps sampled from a Boltzmann distribution, under a second moment assumption only, conditioned to be large in either of the sense of the number of edges, vertices, or faces. The proof relies on the convergence of so‐called “discrete snakes” obtained by adding spatial positions to the nodes of uniform random plane trees with a prescribed child sequence recently studied by Broutin and Marckert. This paper can alternatively be seen as a contribution to the study of the geometry of such trees.

Regular fuzzy equivalences on two - mode fuzzy Networksi

The notion of social roles is a centerpiece of most sociological theoretical considerations. Regular equivalences were introduced by White and Reitz in [15] as the least restrictive among the most commonly used definitions of equivalence in social network analysis. In this paper we consider a generalization of this notion to a bipartite case. We define a pair of regular equivalences on a two-mode social network and we provide an algorithm for computing the greatest pair of regular equivalences.

Complex martingales and asymptotic enumeration

AbstractMany enumeration problems in combinatorics, including such fundamental questions as the number of regular graphs, can be expressed as high‐dimensional complex integrals. Motivated by the need for a systematic study of the asymptotic behavior of such integrals, we establish explicit bounds on the exponentials of complex martingales. Those bounds applied to the case of truncated normal distributions are precise enough to include and extend many enumerative results of Barvinok, Canfield, Gao, Greenhill, Hartigan, Isaev, McKay, Wang, Wormald, and others. Our method applies to sums as well as integrals. As a first illustration of the power of our theory, we considerably strengthen existing results on the relationship between random graphs or bipartite graphs with specified degrees and the so‐called β‐model of random graphs with independent edges, which is equivalent to the Rasch model in the bipartite case.

A note on the polytope of bipartite TSP

The main result of this paper is that the polytope of the bipartite TSP is significantly different from that of the general TSP. Comb inequalities are known as facet defining ones in the general case. In the bipartite case, however, many of them are satisfied whenever all degree and subtour elimination constraints are satisfied, i.e. these comb inequalities are not facet defining. The inequalities in question belong to the cases where vertices of one of the two classes occur in less than the half of the intersections of the teeth and the hand. Such side conditions are necessary, as simple example shows that the comb inequality can be violated when each class has vertices in more than the half of the intersections.

Predictive Characteristics of Co-authorship Networks: Comparing the Unweighted, Weighted, and Bipartite Cases

Abstract Purpose This study aims to answer the question to what extent different types of networks can be used to predict future co-authorship among authors. Design/methodology/approach We compare three types of networks: unweighted networks, in which a link represents a past collaboration; weighted networks, in which links are weighted by the number of joint publications; and bipartite author-publication networks. The analysis investigates their relation to positive stability, as well as their potential in predicting links in future versions of the co-authorship network. Several hypotheses are tested. Findings Among other results, we find that weighted networks do not automatically lead to better predictions. Bipartite networks, however, outperform unweighted networks in almost all cases. Research limitations Only two relatively small case studies are considered. Practical implications The study suggests that future link prediction studies on co-occurrence networks should consider using the bipartite network as a training network. Originality/value This is the first systematic comparison of unweighted, weighted, and bipartite training networks in link prediction.

Dimer geometry, amoebae and a vortex dimer model

We present a geometrical approach and introduce a connection for dimer problems on bipartite and non-bipartite graphs. In the bipartite case the connection is flat but has non-trivial holonomy round certain curves. This holonomy has the universality property that it does not change as the number of vertices in the fundamental domain of the graph is increased. It is argued that the K-theory of the torus, with or without punctures, is the appropriate underlying invariant. In the non-bipartite case the connection has non-zero curvature as well as non-zero Chern number. The curvature does not require the introduction of a magnetic field. The phase diagram of these models is captured by what is known as an amoeba. We introduce a dimer model with negative edge weights which correspond to vortices. The amoebae for various models are studied with particular emphasis on the case of negative edge weights. Vortices give rise to new kinds of amoebae with certain singular structures which we investigate. On the amoeba of the vortex full hexagonal lattice we find the partition function corresponds to that of a massless Dirac doublet.

Entanglement and the generalized uncertainty principle

We examine quantum gravity effects on entanglement by a straightforward application of the generalized uncertainty principle (GUP) to continuous-variable systems. In particular, we study the following cases: the modified uncertainty relation of two identical entangled particles (Rigolin, 2002), and the inseparability conditions for entangled particles in the bipartite (Duan, Giedke, Cirac and Zoller, 2000) and tripartite (van Loock and Furusawa, 2003) cases. Rigolin showed a decrease in the lower bound of the product of the uncertainties of the position and momentum for two identical entangled particles while Duan and van Loock derived inseparability conditions for EPR-like operators. In all three cases, the GUP correction resulted in a higher value of the bounds: a higher lower bound for the Rigolin's result and a higher upper bound for the inseparability condition in Duan's and van Loock's relations. In Rigolin's case, the GUP correction decreased the disagreement with the Heisenberg uncertainty relation while in Duan's and Loock's cases,the inseparability and entanglement conditions are enhanced. Interestingly, the GUP corrections tend to make quantum mechanical effects more pronounced.

Independent sets, matchings, and occupancy fractions

AbstractWe prove tight upper bounds on the logarithmic derivative of the independence and matching polynomials of ‐regular graphs. For independent sets, this theorem is a strengthening of the results of Kahn, Galvin and Tetali, and Zhao showing that a union of copies of maximizes the number of independent sets and the independence polynomial of a ‐regular graph.For matchings, this shows that the matching polynomial and the total number of matchings of a ‐regular graph are maximized by a union of copies of . Using this we prove the asymptotic upper matching conjecture of Friedland, Krop, Lundow, and Markström.In probabilistic language, our main theorems state that for all ‐regular graphs and all , the occupancy fraction of the hard‐core model and the edge occupancy fraction of the monomer‐dimer model with fugacity are maximized by . Our method involves constrained optimization problems over distributions of random variables and applies to all ‐regular graphs directly, without a reduction to the bipartite case.

Quantum correlations in composite systems

We study emerging notions of quantum correlations in compound systems. Based on different definitions of quantumness in individual subsystems, we investigate how they extend to the joint description of a composite system. Especially, we study the bipartite case and the connection of two typically applied and distinctively different concepts of nonclassicality in quantum optics and quantum information. Our investigation includes the representation of correlated states in terms of quasiprobability matrices, a comparative study of joint and conditional quantum correlations, and an entanglement characterization. It is, for example, shown that our composition approach always includes entanglement as one form of quantum correlations. Yet, other forms of quantum correlations can also occur without entanglement. Finally, we give an outlook towards multimode systems and temporal correlations.

Phase-sensitive cascaded four-wave mixing processes for generating continuous-variable entanglement.

Quantum entanglement shared by different parties enhances their capabilities to communicate, which is the core content of continuous-variable quantum optics and quantum information science. Here, we study an experimentally feasible scheme for generating quantum entanglement of bipartite and tripartite cases based on phase-sensitive cascaded four-wave mixing processes in rubidium vapor. Quantum entanglement of bipartite and tripartite cases in our system, which can be manipulated by the phases and the intensity gains of the input beams, is predicted. We also find a sufficient optimal single-condition criterion to give a valid description for genuine tripartite quantum entanglement in our system. The sufficient optimal single-condition criterion is convenient and can be extended to genuine multipartite entanglement.

Maximum nonlocality in the (3,2,2) scenario

We identify the simplest combinations of entanglement and incompatibility giving the maximum quantum violation for each of the 46 classes of tight Bell inequalities for the (3,2,2) scenario, i.e., three parties, two measurements per party, and two outcomes per measurement. This allows us to classify the maximum quantum nonlocality according to the simplest resources needed to achieve it. We show that entanglement and incompatibility only produce maximum nonlocality when they are combined in specific ways. For each entanglement class there is, in most cases, just one incompatibility class leading to maximum nonlocality. We also identify two interesting cases. We show that the maximum quantum violation of \'Sliwa inequality 23 only occurs when the third party measures the identity, so nonlocality cannot increase when we add a third party to the bipartite case. Almost quantum correlations predict that adding a new party increases nonlocality. This points out that either almost quantum correlations violate a fundamental principle or that there is a form of tripartite entanglement which quantum theory cannot account for. The other interesting case is the maximum quantum violation of \'Sliwa inequality 26, which, like the Mermin inequality, requires maximum incompatibility for all parties. In contrast, it requires a specific entangled state which has the same tripartite negativity as the W state.