Moments of the Hilbert-Schmidt probability distributions over determinants of real two-qubit density matrices and of their partial transposes

TL;DR

This paper computes exact moments of the probability distribution of the determinant of the partial transpose of real two-qubit density matrices, providing insights into their separability probabilities under the Hilbert-Schmidt measure.

Contribution

It derives explicit formulas for the moments of the distribution over the entire range, including the first nine moments, and extends results to complex and quaternionic cases.

Findings

Exact moments of the distribution are calculated up to the ninth order.

An upper bound of approximately 0.875 is established for the separability probability.

General formulas for the m-th moment across different cases are provided.

Abstract

The nonnegativity of the determinant of the partial transpose of a two-qubit (4 x 4) density matrix is both a necessary and sufficient condition for its separability. While the determinant is restricted to the interval [0,1/256], the determinant of the partial transpose can range over [-1/16,1/256], with negative values corresponding to entangled states. We report here the exact values of the first nine moments of the probability distribution of the partial transpose over this interval, with respect to the Hilbert-Schmidt (metric volume element) measure on the nine-dimensional convex set of real two-qubit density matrices. Rational functions C_{2 j}(m), yielding the coefficients of the 2j-th power of even polynomials occurring at intermediate steps in our derivation of the m-th moment, emerge. These functions possess poles at finite series of consecutive half-integers (m=-3/2,-1/2,...,(2j-1)/2), and certain (trivial) roots at finite series of consecutive natural numbers (m=0, 1,...). Additionally, the (nontrivial) dominant roots of C_{2 j}(m) approach the same half-integer values (m = (2 j-1)/2, (2 j-3)/2,...), as j increases. The first two moments (mean and variance) found--when employed in the one-sided Chebyshev inequality--give an upper bound of 30397/34749 = 0.874759 on the separability probability of real two-qubit density matrices. We are able to report general formulas for the m-th moment of the Hilbert-Schmidt probability distribution of the density matrix determinant over [0,1/256], in the real, complex and quaternionic two-qubit cases.