Classical and Quantum sl(1|2) Superalgebras, Casimir Operators and Quantum Chain Hamiltonians

We examine the two parameter deformed superalgebra $U_{qs}(sl(1|2))$ and use the results in the construction of quantum chain Hamiltonians. This study is done both in the framework of the Serre presentation and in the $R$-matrix scheme of Faddeev, Reshetikhin and Takhtajan (FRT). We show that there exists an infinite number of Casimir operators, indexed by integers $p > 1$ in the undeformed case and by $p \in Z$ in the deformed case, which obey quadratic relations. The construction of the dual superalgebra of functions on $SL_{qs}(1|2)$ is also given and higher tensor product representations are discussed. Finally, we construct quantum chain Hamiltonians based on the Casimir operators. In the deformed case we find two Hamiltonians which describe deformed $t-J$ models.Comment: 27 pages, LaTeX, one reference moved and one formula adde

Lie Superalgebra Stability and Branes

The algebra of the generators of translations in superspace is unstable, in the sense that infinitesimal perturbations of its structure constants lead to non-isomorphic algebras. We show how superspace extensions remedy this situation (after arguing that remedy is indeed needed) and review the benefits reaped in the description of branes of all kinds in the presence of the extra dimensions.Comment: Talk given at the conference ``Brane New World and Non-commutative Geometry'', held in Torino, October 2000. To appear in the proceedings by World Scientific. 10 pages, 1 figur

Operational Geometry on de Sitter Spacetime

Traditional geometry employs idealized concepts like that of a point or a curve, the operational definition of which relies on the availability of classical point particles as probes. Real, physical objects are quantum in nature though, leading us to consider the implications of using realistic probes in defining an effective spacetime geometry. As an example, we consider de Sitter spacetime and employ the centroid of various composite probes to obtain its effective sectional curvature, which is found to depend on the probe's internal energy, spatial extension, and spin. Possible refinements of our approach are pointed out and remarks are made on the relevance of our results to the quest for a quantum theory of gravity.Comment: Replaced to match the published versio

Quantum metrology of rotations with mixed spin states

The efficiency of a quantum metrology protocol can be considerably reduced by the interaction of a quantum system with its environment, resulting in a loss of purity and, consequently, a mixed state for the probing system. In this paper we examine the potential of mixed spin-$j$ states to achieve sensitivity comparable, and even equal, to that of pure states in the measurement of infinitesimal rotations about arbitrary axes. We introduce the concept of mixed optimal quantum rotosensors based on a maximization of the Fisher quantum information and show that it is related to the notion of anticoherence of spin states and its generalization to subspaces. We present several examples of anticoherent subspaces and their associated mixed optimal quantum rotosensors. We also show that the latter maximize negativity for specific bipartitions, reaching the same maximum value as pure states. These results elucidate the interplay between quantum metrology of rotations, anticoherence and entanglement in the framework of mixed spin states

Star Product and Invariant Integration for Lie type Noncommutative Spacetimes

We present a star product for noncommutative spaces of Lie type, including the so called ``canonical'' case by introducing a central generator, which is compatible with translations and admits a simple, manageable definition of an invariant integral. A quasi-cyclicity property for the latter is shown to hold, which reduces to exact cyclicity when the adjoint representation of the underlying Lie algebra is traceless. Several explicit examples illuminate the formalism, dealing with kappa-Minkowski spacetime and the Heisenberg algebra (``canonical'' noncommutative 2-plane).Comment: 21 page