Suppose we are given two identical copies of an unknown quantum state and we wish to delete one copy from among the given two copies. The quantum no-deletion principle restricts us from perfectly deleting a copy but it does not prohibit us from deleting a copy approximately. Here we construct two types of a " universal quantum deletion machine" which approximately deletes a copy such that the fidelity of deletion does not depend on the input state. The two types of universal quantum deletion machines are (1) a conventional deletion machine described by one unitary operator and (2) a modified deletion machine described by two unitary operators. Here it is shown that modified deletion machine deletes a qubit with fidelity 4 3 , which is the maximum limit for deleting an unknown quantum state. In addition to this we also show that the modified deletion machine retains the qubit in the first mode with average fidelity 0.77 (approx.)which is slightly greater than the fidelity of measurement for two given identical state,showing how precisely one can determine its state [13]. We also show that the deletion machine itself is input state independent i.e. the information is not hidden in the deleting machine, and hence we can delete the information completely from the deletion machine.

We present a scheme for broadcasting of continuous variable entanglement. We show how an initial two-mode squeezed state of the electromagnetic field shared by two distant parties can be broadcasted into two nonlocal bipartite entangled states. Our protocol uses a local linear amplifier and a beam splitter at each end. We compute the fidelity of the output entangled states and show that the broadcasting can be implemented for a variety of input squeezed states and amplifier phases.PACS numbers: 03.67. Mn,42.50.Dv Quantum entanglement is now recognized as a powerful resource in communication and computation protocols [1]. The first nontrivial consequence of entanglement on quantum ontology was noticed many years ago within the context of continuous variable systems [2]. In recent times there has been a rapid development of the theory of entanglement pertaining to infinite dimensional Hilbert spaces [3]. Many well-known results of discrete variable systems relating to classification and manipulation of entanglement take novel forms for the case of continuous variables [4]. There still remains a lot to be understood in the information theory for continuous variables which has potentially vast practical ramifications.An interesting issue is that of broadcasting of quantum entanglement, viz., whether the entanglement shared by a single pair can be transmitted to two less entangled pairs by local operations at both ends. Unlike classical correlations, quantum entanglement cannot always be broadcasted, as has been proved for general mixed states in finite dimensions [5]. Since broadcasting involves copying of local information, and the exact cloning of an unknown quantum state is impossible, the no-cloning theorem[6] and its consequences imply limitations on this procedure. For the case of pure states in finite dimensions, implementation of broadcasting imposes restrictions on the initial state[7] and conditions on the fidelity of the cloning process [8]. No scheme has yet been proposed, however, for the broadcasting of continuous variable entanglement.The cloning of continuous quantum variables has nonetheless, been studied by several authors. Various schemes for duplication of coherent states with optimal fidelity and economical means have been suggested [9]. Operations of cloning machines with networks of linear amplifiers and beam splitters have been proposed [10,11]. It is thus relevant to investigate whether such ideas of copying local information can be elaborated for broadcasting entangled states of continuous variables. To this end we extend the procedure of cloning of a single-mode squeezed state of the electromagnetic field proposed by Braunstein et al.[10] to the case of a bipartite entangled two-mode squeezed state. By applying a linear amplifier and a beam splitter available locally with each party, we show using the covariance matrix approach[4] how the initial entangled state can be broadcasted into two nonlocal and bipartite entangled states.Before describing our scheme for broadcasting in detail, i...

The ability of entangled states to act as resource for teleportation is linked to a property of the fully entangled fraction. We show that the set of states with their fully entangled fraction bounded by a threshold value required for performing teleportation is both convex and compact. This feature enables for the existence of hermitian witness operators the measurement of which could distinguish unknown states useful for performing teleportation. We present an example of such a witness operator illustrating it for different classes of states.PACS numbers: 03.67.-a, 03.67.Mn A. Introduction.-Quantum information processing is now widely recognized as a powerful tool for implementing tasks that cannot be performed using classical means [1]. A large number of algorithms for various information processing tasks such as super dense coding[2], teleportation [3] and key generation [4] have been proposed and experimentally demonstrated. At the practical level information processing is implemented by manipulating states of quantum particles, and it is well known that not all quantum states can be used for such purposes. Hence, given an unknown state, one of the most relevant issues here is to determine whether it is useful for quantum information processing.The key ingredient for performing many information processing tasks is provided by quantum entanglement. The experimental detection of entanglement is facilitated by the existence of entanglement witnesses [5,6] which are hermitian operators with at least one negative eigenvalue. The existence of entanglement witnesses is a consequence of the Hahn-Banach theorem in functional analysis [7,8] providing a necessary and sufficient condition to detect entanglement. Motivated by the nature of different classes of entangled states, various methods have been suggested to construct entanglement witnesses [9][10][11][12]. Study of entanglement witnesses [13] has proceeded in directions such as the construction of optimal witnesses [9,11], Schmidt number witnesses [14], and common witnesses [15]. The possibility of experimental detection of entanglement through the measurement of expectation values of witness operators for unknown states is facilitated by the decomposition of witnesses in terms of Pauli spin matrices for qubits [16] and Gell-Mann matrices in higher dimensions [17]. For macroscopic systems the properties of thermodynamic quantities provide a useful avenue for detection of entanglement [18].Teleportation [3] is a typical information processing task where at present there is intense activity in extending the experimental frontiers [19]. However, it is well known that not all entangled states are useful for teleportation. For example, while the entangled Werner state [20] in 2 ⊗ 2 dimensions is a useful resource [21], another class of maximally entangled mixed states [22], as well as other non-maximally entangled mixed states achieve a fidelity higher than the classical limit only when their magnitude of entanglement exceeds a certain value [23]. The problem of det...

Abstract:In this work we study the quantum deletion machine with two transformers and show that the deletion machine with single transformer performs better than the deletion machine with more than two transformers. We also observe that the fidelity of deletion depends on the blank state used in the deleter and so for different blank state the fidelity is different. Further, we study the PatiBraunsein deleter with transformer.

Quantum discord is a prominent measure of quantum correlations, playing an important role in expanding its horizon beyond entanglement. Here we provide an operational meaning of (geometric) discord, which quantifies the amount of non-classical correlation of an arbitrary quantum system in terms of its minimal distance from the set of classical states, in terms of teleportation fidelity for general two qubit and d ⊗ d dimensional isotropic and Werner states. A critical value of the discord is found beyond which the two qubit state must violate the Bell inequality. This is illustrated by an open system model of a dissipative two qubit. For the d ⊗ d dimensional states the lower bound of discord is shown to be obtainable from an experimentally measurable witness operator.Introduction.-Quantum correlations occupy a central position in the quest for understanding and harvesting the power of quantum mechanics. This point of view has been highlighted in recent times by numerous developments in the field of quantum information. Entanglement [1], till about a decade back, was considered synonymous with quantum correlations. This was a natural outcome of the quest to understand the role of nonlocality in quantum mechanics, having a historical lineage from Einstein-Podolsky-Rosen [2], to Bell's inequality [3], leading to refinements resulting in the Bell-CHSH (Clauser-Horn-Shimony-Holt) inequalities [4]. With the advent of quantum discord [5,6], the difference between the quantum generalizations of two classically equivalent formulations of mutual information, realization dawned that quantum correlations are bigger than entanglement. Thus, for example, separable states, having by definition zero entanglement, could have non-zero discord. Also, in the DQC1 model [7], entanglement is negligible but there is sufficient amount of quantum discord for a speed up over the best known classical algorithms.