In this work, we show that the zero field excitation spectra in the quantum spin ice candidate pyrochlore compound Yb2Ti2O7 is a continuum characterized by a very broad and almost flat dynamical response which extends up to 1 − 1.5 meV, coexisting or not with a quasi-elastic response depending on the wave-vector. The spectra do not evolve between 50 mK and 2 K, indicating that the spin dynamics is only little affected by the temperature in both the short-range correlated and ordered regimes. Although classical spin dynamics simulations qualitatively capture some of the experimental observations, we show that they fail to reproduce this broad continuum. In particular, the simulations predict an energy scale twice smaller than the experimental observations. This analysis is based on a careful determination of the exchange couplings, able to reproduce both the zero field diffuse scattering and the spin wave spectrum rising in the field polarized state. According to this analysis, Yb2Ti2O7 lies at the border between a ferro and an antiferromagnetic phase. These results suggest that the unconventional ground state of Yb2Ti2O7 is governed by strong quantum fluctuations arising from the competition between those phases. The observed spectra may correspond to a continuum of deconfined spinons as expected in quantum spin liquids. arXiv:1506.01729v1 [cond-mat.str-el]

Entanglement is a concept that has defied common sense since the discovery of quantum mechanics. Two particles are said to be entangled when the quantum state of each particle cannot be described independently, no matter how far apart in space and time the two particles are. We demonstrate experimentally that unpaired spins separated by several hundred ångström entangle through a collection of spin singlets made up of antiferromagnetic spin-1/2 chains in a bulk material. Low-temperature magnetization and specific heat studies as a function of magnetic field reveal the occurrence of very dilute spin dimers and at least two quantum phase transitions related to the breaking of excited local triplets. The mechanism at the origin of the unpaired spins inside the quantum chains is the inter-modulation potential between two sublattices, and may be replicated using well-designed synthetic multilayers. E ntanglement is an essential feature of the macroscopic world 1 , despite the fact that human perception has difficulty accepting such strange predictions. A very fruitful area of research today is quantum computing technology, where entanglement is being harnessed for eventual use in quantum computation and communications 2,3 . Scientists are also developing new areas of research where entanglement could lie at the origin of the interactions in ever larger objects, ranging from photons to clusters of atoms to molecules-and even to cosmological objects.Most of the protocols for quantum communication rely on entangled photons 4 , as they are weakly interacting and can be easily manipulated using existing optical fibre technology. However, it is not always convenient to use photons to exchange information as, in particular, it is not straightforward to convert information between physically different qubits. Another approach described in seminal papers by Bose 5,6 suggested the use of spin chains as quantum channels for short-or mid-range communication, showing that, by means of the magnetic interaction between the spins that compose the chain, the transfer of information arises naturally as the system evolves dynamically, without the requirement for any external control. Transferring quantum information between distant qubits through spin chains would be highly desirable. However, this procedure often requires the repeated application of swapping gates and is, consequently, very demanding experimentally. Moreover, the features that characterize the transmission, such as teleportation, fidelity, transfer quality and speed, depend on the properties of the underlying quantum system 2,3 .Other developments have explored the conditions for longdistance entanglement without the need to perform operations and measurements. Among the many possible spin configurations it seems that antiferromagnetic spin arrays characterized by spin gaps above the ground state can exhibit true entanglement over long distances 7 . Furthermore, the entanglement is a slowly decreasing function of distance, allowing robust teleportation across finite di...