  There are considerable obstacles against realizing practical quantum
computing: a significant amount of computation errors and the restricted qubit
count. As a unified method of noise-agnostic quantum error mitigation (QEM)
methods, i.e., the quantum subspace expansion and virtual purification, a
generalized quantum subspace expansion (GSE) has recently been proposed that is
significantly robust against stochastic and coherent errors. However, GSE
requires entangled measurements between copies of the quantum states, which is
a significant drawback under the current situation of the restricted number of
qubits and their connectivity. In this work, we propose a resource-efficient
implementation of GSE, which we name "Dual-GSE", circumventing significant
overheads of state copies by constructing an ansatz of error-mitigated quantum
states via dual-state purification. Remarkably, Dual-GSE can further simulate
larger quantum systems beyond the size of available quantum hardware with a
suitable ansatz construction inspired by divide-and-conquer methods that forge
entanglement classically. This also significantly reduces the measurement
overhead because we only need to measure subsystems' Pauli operators. The
proposed method is demonstrated by a numerical simulation of the eight-qubit
transverse-field Ising model, showing that our method estimates the ground
state energy with high precision under gate noise with low mitigation overhead
and practical sampling cost.
