Test-measured R\'enyi divergences

TL;DR

This paper investigates quantum Renyi divergences defined via measurements, revealing that for b1>1 the regularized measured divergence matches the sandwiched divergence, but for b1<1 it does not, even for classical states.

Contribution

It demonstrates that the regularized measured Renyi divergence does not extend the classical divergence for b1<1, contrasting with the b1>1 case where they coincide.

Findings

For b1>1, the regularized measured divergence equals the sandwiched divergence.

For b1<1, the regularized measured divergence is strictly smaller than the classical Renyi divergence.

Two-outcome measurements suffice for b1>1 but not for b1<1.

Abstract

One possibility of defining a quantum R\'enyi $\alpha$-divergence of two quantum states is to optimize the classical R\'enyi $\alpha$-divergence of their post-measurement probability distributions over all possible measurements (measured R\'enyi divergence), and maybe regularize these quantities over multiple copies of the two states (regularized measured R\'enyi $\alpha$-divergence). A key observation behind the theorem for the strong converse exponent of asymptotic binary quantum state discrimination is that the regularized measured R\'enyi $\alpha$-divergence coincides with the sandwiched R\'enyi $\alpha$-divergence when $\alpha>1$. Moreover, it also follows from the same theorem that to achieve this, it is sufficient to consider $2$-outcome measurements (tests) for any number of copies (this is somewhat surprising, as achieving the measured R\'enyi $\alpha$-divergence for $n$ copies might require a number of measurement outcomes that diverges in $n$, in general). In view of this, it seems natural to expect the same when $\alpha<1$; however, we show that this is not the case. In fact, we show that even for commuting states (classical case) the regularized quantity attainable using $2$-outcome measurements is in general strictly smaller than the R\'enyi $\alpha$-divergence (which is unique in the classical case). In the general quantum case this shows that the above "regularized test-measured" R\'enyi $\alpha$-divergence is not even a quantum extension of the classical R\'enyi divergence when $\alpha<1$, in sharp contrast to the $\alpha>1$ case.