  The inherent irreversibility of quantum dynamics for open systems poses a
significant barrier to the inversion of unknown quantum processes. To tackle
this challenge, we propose the framework of virtual combs that exploit the
unknown process iteratively with additional classical post-processing to
simulate the process inverse. Notably, we demonstrate that an $n$-slot virtual
comb can exactly reverse a depolarizing channel with one unknown noise
parameter out of $n+1$ potential candidates, and a 1-slot virtual comb can
exactly reverse an arbitrary pair of quantum channels. We further explore the
approximate inversion of an unknown channel within a given channel set. A
worst-case error decay of $\mathcal{O}(n^{-1})$ is unveiled for depolarizing
channels within a specified noise region. Moreover, we show that virtual combs
can universally reverse unitary operations and investigate the trade-off
between the slot number and the sampling overhead.
