  One of the major challenges for erroneous quantum computers is undoubtedly
the control over the effect of noise. Considering the rapid growth of available
quantum resources that are not fully fault-tolerant, it is crucial to develop
practical hardware-friendly quantum error mitigation (QEM) techniques to
suppress unwanted errors. Here, we propose a novel generalized quantum subspace
expansion method which can handle stochastic, coherent, and algorithmic errors
in quantum computers. By fully exploiting the substantially extended subspace,
we can efficiently mitigate the noise present in the spectra of a given
Hamiltonian, without relying on any information of noise. The performance of
our method is discussed under two highly practical setups: the quantum
subspaces are mainly spanned by powers of the noisy state $\rho^m$ and a set of
error-boosted states, respectively. We numerically demonstrate in both
situations that we can suppress errors by orders of magnitude, and show that
out protocol inherits the advantages of previous error-agnostic QEM techniques
as well as overcoming their drawbacks.
