  As the hardware technology for quantum computing advances, its possible
applications are actively searched and developed. However, such applications
still suffer from the noise on quantum devices, in particular when using
two-qubit gates whose fidelity is relatively low. One way to overcome this
difficulty is to substitute such non-local operations by local ones. Such
substitution can be performed by decomposing a non-local channel into a linear
combination of local channels and simulating the original channel with a
quasiprobability-based method. In this work, we first define a quantity that we
call channel robustness of non-locality, which quantifies the cost for the
decomposition. While this quantity is challenging to calculate for a general
non-local channel, we give an upper bound for a general two-qubit unitary
channel by providing an explicit decomposition. The decomposition is obtained
by generalizing our previous work whose application has been restricted to a
certain form of two-qubit unitary. This work develops a framework for a
resource reduction suitable for first-generation quantum devices.
