We propose two probabilistic entanglement concentration schemes for a single copy of two-mode squeezed vacuum state. The first scheme is based on the off-resonant interaction of a Rydberg atom with the cavity field while the second setup involves the cross Kerr interaction, auxiliary mode prepared in a strong coherent state and a homodyne detection. We show that the continuousvariable entanglement concentration allows us to improve the fidelity of teleportation of coherent states.

We propose a criterion giving a sufficient condition for quantum states of a harmonic oscillator not to be expressible as a convex mixture of Gaussian states. This nontrivial property is inherent to, e.g., a single-photon state and the criterion thus allows one to reveal a signature of the state even in quantum states with a positive Wigner function. The criterion relies on directly measurable photon number probabilities and enables detection of this manifestation of a single-photon state in quantum states produced by solid-state single-photon sources in a weak coupling regime.

We experimentally demonstrate a protocol for entanglement distribution by a separable quantum system. In our experiment, two spatially separated modes of an electromagnetic field get entangled by local operations, classical communication, and transmission of a correlated but separable mode between them. This highlights the utility of quantum correlations beyond entanglement for the establishment of a fundamental quantum information resource and verifies that its distribution by a dual classical and separable quantum communication is possible. [6,7]. Furthermore, it extends our abilities to process information. Here, entanglement is used as a resource which needs to be shared between remote parties. However, entanglement is not the only manifestation of quantum correlations. Notably, separable quantum states can also be used as a shared resource for quantum communication. The experiment presented in this Letter highlights the quantumness of correlations in separable mixed states and the role of classical information in quantum communication by demonstrating entanglement distribution using merely a separable ancilla mode.The role of entanglement in quantum information is nowadays vividly demonstrated in a number of experiments. A pair of entangled quantum systems shared by two observers enables us to teleport [8] quantum states between them with a fidelity beyond the boundary set by classical physics. Concatenated teleportations [9] can further span entanglement over large distances [10] which can be subsequently used for secure communication [11]. An a priori shared entanglement also allows us to double the rate at which information can be sent through a quantum channel [12] or one can fuse bipartite entanglement into larger entangled cluster states that are "hardware" for quantum computing [13]. * contributed equally to this workThe common feature of all entangling methods used so far is that entanglement is either produced by some global operation on the systems that are to be entangled or it results from a direct transmission of entanglement (possibly mediated by a third system) between the systems. Even entanglement swapping [9,14], capable of establishing entanglement between the systems that do not have a common past, is not an exception to the rule because also here entanglement is directly transmitted between the participants.However, quantum mechanics admits conceptually different means of establishing entanglement which are free of transmission of entanglement. Remarkably, the creation of entanglement between two observers can be disassembled into local operations and the communication of a separable quantum system between them [15]. The impossibility of entanglement creation by LOCC is not violated because communication of a quantum system is involved. The corresponding protocol exists only in a mixed-state scenario and obviously utilizes fewer quantum resources in comparison with the previous cases because communication of only a discordant [16][17][18] separable quantum system is required.In this Le...

We report the first experimental realization of an "optimal" quantum device able to perform a Minimal Disturbance Measurement (MDM) on polarization encoded qubits saturating the theoretical boundary established between the classical knowledge acquired of any input state, i.e. the classical "guess", and the fidelity of the same state after disturbance due to measurement . The device has been physically realized by means of a linear optical qubit manipulation, post-selection measurement and a classical feed-forward process.PACS numbers: 03.67.Hk The measurement process represents the most innovative and distinctive aspect of quantum mechanics respect to classical physics. The main result of the quantum measurement theory is the unavoidable disturbance of the quantum state induced by the measuring process itself, as epitomized by the early Heisenberg's X-ray microscope thought experiment [1]. The balance between the information available on an unknown quantum system and the perturbation induced by the measurement process is of utmost relevance when investigating the quantum world [2,3,4,5]. In spite of this relevance, only in the last years and in the context of Quantum Information (QI) for finite dimensional systems, an exact quantum theoretical formulation of this problem has been developed [6]. When measuring an unknown quantum system |φ two main questions arise: A: how good is the estimation of the state obtained by the measuring process? and B: how much the final state is close to the input one? Adopting the tools developed within QI, the previous questions can be answered by introducing suitable quantitative figures of merit to assess the classical information acquired on the state and the resemblance of the final quantum system to the initial one [7]. The classical guess G attained by applying a state estimation strategy is defined as the mean overlap between the unknown state |φ and the state inferred from the measurement ρ G : G = φ| ρ G |φ while the closeness of the output quantum state ρ F to the input one is expressed by the quantum fidelity F = φ| ρ F |φ . The final problem is then to establish which kind of relation connects these two quantities. The higher is the information achieved the higher is the disturbance applied, and vice versa. For instance to carry out an optimal state estimation strategy on |φ we should perform a von Neuman measurement [8], i.e. a "strong disturbance" one, thus leading to the maximal modification of the initial state. In this case the output quantum fidelity is identical to the classical one. This represents an extreme point of the F − G boundary. On contrary if we want to maintain unchanged the quantum state, i.e.F = 1, we can not obtain any previous information about it. This point defines the other extreme of the F − G plot.In the present work we consider the basic element of quantum information, the qubit, which is encoded in a 2-dimensional quantum system and represents the quantum analogue of the classical bit. Let us start from the situation in which no a prior...

We study quantum correlations beyond entanglement in two-mode Gaussian states of continuous variable systems, by means of the measurement-induced disturbance (MID) and its ameliorated version (AMID). In analogy with the recent studies of the Gaussian quantum discord, we define a Gaussian AMID by constraining the optimization to all bi-local Gaussian positive operator valued measurements. We solve the optimization explicitly for relevant families of states, including squeezed thermal states. Remarkably, we find that there is a finite subset of two-mode Gaussian states, comprising pure states, where non-Gaussian measurements such as photon counting are globally optimal for the AMID and realize a strictly smaller state disturbance compared to the best Gaussian measurements. However, for the majority of two-mode Gaussian states the unoptimized MID provides a loose overestimation of the actual content of quantum correlations, as evidenced by its comparison with Gaussian discord. This feature displays strong similarity with the case of two qubits. Upper and lower bounds for the Gaussian AMID at fixed Gaussian discord are identified. We further present a comparison between Gaussian AMID and Gaussian entanglement of formation, and classify families of two-mode states in terms of their Gaussian AMID, Gaussian discord, and Gaussian entanglement of formation. Our findings provide a further confirmation of the genuinely quantum nature of general Gaussian states, yet they reveal that non-Gaussian measurements can play a crucial role for the optimized extraction and potential exploitation of classical and nonclassical correlations in Gaussian states.