arXiv subjects

Mehdi Mhalla

Publications and source records attributed to Mehdi Mhalla.

42 records · Page 3Linked to original sources

arXiv2004

Complexity of Graph State Preparation

The graph state formalism is a useful abstraction of entanglement. It is used in some multipartite purification schemes and it adequately represents universal resources for measurement-only quantum computation. We focus in this paper on the complexity of graph state preparation. We consider the number of ancillary qubits, the size of the primitive operators, and the duration of preparation. For each lexicographic order over these parameters we give upper and lower bounds for the complexity of graph state preparation. The first part motivates our work and introduces basic notions and notations for the study of graph states. Then we study some graph properties of graph states, characterizing their minimal degree by local unitary transformations, we propose an algorithm to reduce the degree of a graph state, and show the relationship with Sutner sigma-game. These properties are used in the last part, where algorithms and lower bounds for each lexicographic order over the considered parameters are presented.

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arXiv2004

Quantum query complexity of some graph problems

Quantum algorithms for graph problems are considered, both in the adjacency matrix model and in an adjacency list-like array model. We give almost tight lower and upper bounds for the bounded error quantum query complexity of Connectivity, Strong Connectivity, Minimum Spanning Tree, and Single Source Shortest Paths. For example we show that the query complexity of Minimum Spanning Tree is in Theta(n^{3/2}) in the matrix model and in Theta(sqrt{nm}) in the array model, while the complexity of Connectivity is also in Theta(n^{3/2}) in the matrix model, but in Theta(n) in the array model. The upper bounds utilize search procedures for finding minima of functions under various conditions.

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arXiv2003

Let us play with qubits

Quantum game theory offers a lot of interesting questions, and it is relevant to use the quantum information theory to resolve or improve games with lack of information : how to use the power of quantum entanglement to show the superiority of a quantum player that is allowed to use quantum mechanics versus a classical player, how to use quantum communication properties in cooperative games ... But games are also useful to make notions easier to understand, and permit to apprehend easier new ways of reasoning. The objective of this work is to formalize and to study a simple game with qubits using quantum notions of measurement and superposition but keeping a simple formalism so that knowing quantum mechanics is not necessary to play the game. We solve a quantum combinatorial game by giving a winning strategy for it. We also propose a quantisation of a family of combinatorial games.

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arXiv2003

Quantum query complexity of graph connectivity

Harry Buhrman et al gave an Omega(sqrt n) lower bound for monotone graph properties in the adjacency matrix query model. Their proof is based on the polynomial method. However for some properties stronger lower bounds exist. We give an Omega(n^{3/2}) bound for Graph Connectivity using Andris Ambainis' method, and an O(n^{3/2} log n) upper bound based on Grover's search algorithm. In addition we study the adjacency list query model, where we have almost matching lower and upper bounds for Strong Connectivity of directed graphs.

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arXiv2002

Separability of pure n-qubit states: two characterizations

This paper characterizes two forms of separability of pure states of systems of n qubits: (i) into a tensor product of n qubit states, and (ii), into a tensor product of 2 subsystems states of p and q qubits respectively with p+q=n. For both cases, necessary and sufficient conditions are proved in the form of minimal sets of equalities among pair-wise products of amplitudes. These conditions bear some relation with entanglement measures, and a number of more refined questions about separability in n qubit systems can be studied on the basis of these results.

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arXiv2001

Quantum Period Query Proves NP in BQP

Withdrawn by the authors. Since the elements within each period of function hA are not distinct, period finding cannot operate properly. Thanks to all for the comments and sorry for the inconvenience.

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