Geometrical interpretation of the argument of Bargmann invariants and weak values in $N$-level quantum systems applying the Majorana symmetric representation (2211.05692v1)

Abstract: In this paper, we study the argument of weak values of general observables, succeeding to give a geometric description to this argument on the Bloch sphere. We apply the Majorana symmetric representation to reach this goal. The weak value of a general observable is proportional to the weak value of an effective projector: it arises from the normalized application of the observable over the initial state, with a constant of proportionality that is real. The argument of the weak value of a projector on a pure state of an $N$-level system corresponds to a symplectic area in the complex projective space $(\text{CP}^{N-1})$, which can be represented geometrically with a sum of $N-1$ solid angles on the Bloch sphere using the Majorana stellar representation. Here, we show that the argument of the weak value of a general observable can be described, using Majorana representation, as the sum of $N-1$ solid angles on the Bloch sphere, merging both studies. These two approaches provide two geometrical descriptions, a first one in $\text{CP}^{N-1}$ and a second one on the Bloch sphere, after mapping the problem from the original space $(\text{CP}^{N-1})$ by making use of the Majorana representation. These results can also be applied to the argument of the third-order Bargmann invariant, the most fundamental order as the argument of any higher order invariant can be expressed as a sum of the argument of third-order Bargmann invariants. Finally, we focus on the argument of the weak value of a general spin-1 operator when its modulus diverges towards infinity. This divergence amplifies signals with great usefulness in experiments.