Finding Traps in Non-linear Spin Arrays

Precise knowledge of the Hamiltonian of a system is a key to many of its applications. Tasks such state transfer or quantum computation have been well studied with a linear chain, but hardly with systems, which do not possess a linear structure. While this difference does not disturb the end-to-end dynamics of a single excitation, the evolution is significantly changed in other subspaces. Here we quantify the difference between a linear chain and a pseudo-chain, which have more than one spin at some site (block). We show how to estimate a number of all spins in the system and the intra-block coupling constants. We also suggest how it is possible to eliminate excitations trapped in such blocks, which may disturb the state transfer. Importantly, one uses only at-ends data and needs to be able to put the system to either the maximally magnetized or the maximally mixed state. This can obtained by controlling a global decoherence parameter, such as temperature.Comment: 5 pages, 1 figur

Lower Bounds on the Communication Complexity of Binary Local Quantum Measurement Simulation

We consider the problem of the classical simulation of quantum measurements in the scenario of communication complexity. Regev and Toner (2007) have presented a 2-bit protocol which simulates one particular correlation function arising from binary projective quantum measurements on arbitrary state, and in particular does not preserve local averages. The question of simulating other correlation functions using a protocol with bounded communication, or preserving local averages, has been posed as an open one. Within this paper we resolve it in the negative: we show that any such protocol must have unbounded communication for some subset of executions. In particular, we show that for any protocol, there exist inputs for which the random variable describing the number of communicated bits has arbitrarily large variance

Entanglement witnesses with variable number of local measurements

We present a class of entanglement identifiers which has the following experimentally friendly feature: once the expectation value of the identifier exceeds some definite limit, we can conclude the state is entangled, even if not all measurements defining the identifier have been performed. These identifiers are in the form of sums of non-negative functions of correlations in a quantum state, mostly squares of correlations, and we illustrate their use and strengths on various examples.Comment: 6 pages, 1 figur

Detecting genuine multipartite entanglement of pure states with bipartite correlations

Monogamy of bipartite correlations leads, for arbitrary pure multi-qubit states, to simple conditions able to indicate various types of multipartite entanglement by being capable to exclude the possibility of k-separability.Comment: journal versio

Nonlocality activation in entanglement swapping chains

We consider multiple entanglement swappings performed on a chain of bipartite states. Each state does not violate CHSH inequality. We show that before some critical number of entanglement swappings is achieved the output state does not violate this inequality either. However, if this number is achieved then for some results of Bell measurements obtained in the protocol of entanglement swapping the output state violates CHSH inequality. Moreover, we show that for different states we have different critical numbers for which CHSH inequality is activated.Comment: 4 page