The relation between Löwdin’s spin projection operator and Pratt’s spin operator

Z. Gershgorn

Abstract An inductive proof is given of Löwdin's theorem about the linear independence and completeness of a properly selected set of projected spin eigenfunctions.

Ruben Pauncz

Ken'ichi Takano

A method to construct various s= 1/2 isotropic spin models with exact dimer ground states is proposed. Constructed models are usually of much simpler forms than those produced by the method using Lowdin's projection operators. They include a series of one-dimensional Heisenberg models which consist of linearly decayed exchange interactions and are natural extensions of the Majumdar-Ghosh model. A two-dimensional spin model with four spin interactions is also constructed to have the same spatial symmetry as a simple square lattice.

F Berencz

Ruben Pauncz

Gökhan Torun

Golden states, from which every state can be prepared by free operations, are the units of a resource theory. In the resource theory of superposition relative to linearly independent, nonorthogonal reference states, they were known only for two reference states and for special overlap matrices, all of them maximally coherent in the Löwdin basis. We determine what decides their existence and when this Löwdin pattern holds. For every overlap matrix, the robustness and the geometric measure of superposition are maximized by the same states, which overlap equally with all reference states; the maximal values are set by the ground-state energy of a classical $XY$ model on the dual basis. These states, and only these, are golden under the maximal superposition-free operations, but they need not be maximally coherent in the Löwdin basis. For the Kraus-type free operations, we prove Gram majorization, a necessary and sufficient criterion for converting pure states into targets with nonzero coefficients, which reduces to majorization for orthonormal reference states. It shows that every golden state of these operations is maximally coherent in the Löwdin basis, yet golden states exist only for special overlap matrices: three reference states that are not orthonormal admit one if and only if their overlaps have a common modulus $s$ and their Bargmann phase $Φ$ obeys $s\geq2\cos[(π+|Φ|)/3]$, so that a geometric phase decides whether the theory has a unit. Finally, equal-overlap states yield a complete family of superposition witnesses in the Löwdin basis, which show that white noise is a resource unless the reference states are orthonormal.