  In ideal quantum circuits, qubits are tacitly assumed to be uniformly
fabricated and operated by prescribed signals. In reality, however, we must
cope with different control signals to adjust individual qubits, which requires
a large overhead of control circuits. Here, we theoretically investigate how
random local fields affect cluster states and surface code states which
constitute the key highly entangled states in quantum computation. We find
similar behavior of temporal degradation of the fidelity for both cluster
states and surface code states for the number of qubits up to ten. We find that
the effect of local field fluctuations is greatly mitigated if the magnitude of
fluctuations can be suppressed below 10 % of the energy gap $\Delta$ for both
cluster states and surface code states. If the magnitude of fluctuations
exceeds $\Delta/2$, the fidelity for both states deteriorates dramatically. A
simple estimation based on the average fidelity up to $t\sim 2\hbar/\Delta$
shows that the maximum number of qubits that can be corrected with the 1 %
error threshold is less than 31 for surface code states and 27 for cluster
states. This means that the error correction should be carried out during a
time shorter than $2\hbar/\Delta$.
