Exploring weak value arguments and Bargmann invariants in $N$-level quantum systems through the Majorana symmetric representation

TL;DR

This paper develops a geometric framework using the Majorana symmetric representation to analyze the argument of weak values and Bargmann invariants in N-level quantum systems, connecting complex projective space and Bloch sphere visualizations.

Contribution

It introduces a novel geometric description of weak value arguments for general observables in N-level systems using the Majorana representation, linking higher-dimensional quantum states to Bloch sphere solid angles.

Findings

Weak value argument corresponds to symplectic area in complex projective space.

The argument of any observable's weak value can be represented as a sum of solid angles on the Bloch sphere.

The approach applies to Bargmann invariants and Kirkwood-Dirac distributions, especially for spin-1 systems.

Abstract

This work examines the argument of weak values for general observables and develops a geometric description 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 is constructed from the application of the observable over the initial state, after normalization by 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 $(\text{CP}^{N-1})$. This symplectic area cannot be visualized directly but it can be represented geometrically with a sum of $N-1$ solid angles on the Bloch sphere using the Majorana stellar representation. By combining these two ideas, we show that the argument of the weak value of any observable (i.e., not just projectors) can be described with the Majorana representation, as the sum of $N-1$ solid angles on the Bloch sphere. These two approaches provide two geometrical descriptions, a first one in the complex projective space $\text{CP}^{N-1}$ and a second one on the Bloch sphere, after mapping the problem from the original $N$-dimensional quantum state space $(\text{CP}^{N-1})$ to a multi-qubit description in three-dimensional space 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, as well as to the argument of the Kirkwood-Dirac quasi-probability distribution. Finally, we focus on the argument of the weak value of a general spin-1 operator when its modulus diverges towards infinity.