Energy as witness of multipartite entanglement in spin clusters

We derive energy minima for biseparable states in three- and four-spin systems, with Heisenberg Hamiltonian and s <= 5/2. These provide lower bounds for tripartite and quadripartite entanglement in chains and rings with larger spin number N. We demonstrate that the ground state of an $N$-spin Heisenberg chain is $N$-partite entangled, and compute the energy gap with respect to biseparable states for N <= 8

Towards the chemical tuning of entanglement in molecular nanomagnets

Antiferromagnetic spin rings represent prototypical realizations of highly correlated, low-dimensional systems. Here we theoretically show how the introduction of magnetic defects by controlled chemical substitutions results in a strong spatial modulation of spin-pair entanglement within each ring. Entanglement between local degrees of freedom (individual spins) and collective ones (total ring spins) are shown to coexist in exchange-coupled ring dimers, as can be deduced from general symmetry arguments. We verify the persistence of these features at finite temperatures, and discuss them in terms of experimentally accessible observables.Comment: 5 pages, 4 figure

Noisy quantum walks of two indistinguishable interacting particles

We investigate the dynamics of continuous-time two-particle quantum walks on a one-dimensional noisy lattice. Depending on the initial condition, we show how the interplay between particle indistinguishability and interaction determines distinct propagation regimes. A realistic model for the environment is considered by introducing non-Gaussian noise as time-dependent fluctuations of the tunneling amplitudes between adjacent sites. We observe that the combined effect of particle interaction and fast noise (weak coupling with the environment) provides a faster propagation compared to the noiseless case. This effect can be understood in terms of the band structure of the Hubbard model, and a detailed analysis as a function of both noise and system parameters is presented

Lattice quantum magnetometry

We put forward the idea of lattice quantum magnetometry, i.e., quantum sensing of magnetic fields by a charged (spinless) particle placed on a finite two-dimensional lattice. In particular, we focus on the detection of a locally static transverse magnetic field, either homogeneous or inhomogeneous, by performing ground-state measurements. The system turns out to be of interest as a quantum magnetometer, since it provides non-negligible quantum Fisher information (QFI) in a large range of configurations. Moreover, the QFI shows some relevant peaks, determined by the spectral properties of the Hamiltonian, suggesting that certain values of the magnetic fields may be estimated better than others, depending on the value of other tunable parameters. We also assess the performance of coarse-grained position measurement, showing that it may be employed to realize nearly optimal estimation strategies

Back and forth from Fock space to Hilbert space: a guide for commuters

Quantum states of systems made of many identical particles, e.g. those described by Fermi-Hubbard and Bose-Hubbard models, are conveniently depicted in the Fock space. However, in order to evaluate some specific observables or to study system dynamics, it is often more effective to employ the Hilbert space description. Moving effectively from one description to the other is thus a desirable feature, especially when a numerical approach is needed. Here we recall the construction of the Fock space for systems of indistinguishable particles, and then present a set of recipes and advice for students and researchers with the need to commute back and forth from one description to the other. The two-particle case is discussed in some detail, and a few guidelines for numerical implementations are given