Hilbert–Schmidt quantum coherence in multi-qudit systems

Abstract

Using Bloch's parametrization for qudits ($d$-level quantum systems), we write the Hilbert-Schmidt distance (HSD) between two generic $n$-qudit states as an Euclidean distance between two vectors of observables mean values in $\mathbb{R}^{\Pi_{s=1}^{n}d_{s}^{2}-1}$, where $d_{s}$ is the dimension for qudit $s$. Then, applying the generalized Gell Mann's matrices to generate $SU(d_{s})$, we use that result to obtain the Hilbert-Schmidt quantum coherence (HSC) of $n$-qudit systems. As examples, we consider in details one-qubit, one-qutrit, two-qubit, and two copies of one-qubit states. In this last case, the possibility for controlling local and non-local coherences by tuning local populations is studied and the contrasting behaviors of HSC, $l_{1}$-norm coherence, and relative entropy of coherence in this regard are noticed. We also investigate the decoherent dynamics of these coherence functions under the action of qutrit dephasing and dissipation channels. At last, we analyze the non-monotonicity of HSD under tensor products and report the first instance of a consequence (for coherence quantification) of this kind of property of a quantum distance measure.