It is vital to minimise the impact of errors for near-future quantum devicesthat will lack the resources for full fault tolerance. Two quantum errormitigation (QEM) techniques have been introduced recently, namely errorextrapolation [Li 2017,Temme 2017] and quasi-probability decomposition [Temme2017]. To enable practical implementation of these ideas, here we account forthe inevitable imperfections in the experimentalist's knowledge of the errormodel itself. We describe a protocol for systematically measuring the effect oferrors so as to design efficient QEM circuits. We find that the effect oflocalised Markovian errors can be fully eliminated by inserting or replacingsome gates with certain single-qubit Clifford gates and measurements. Finally,having introduced an exponential variant of the extrapolation method wecontrast the QEM techniques using exact numerical simulation of up to 19 qubitsin the context of a 'SWAP test' circuit. Our optimised methods dramaticallyreduce the circuit's output error without increasing the qubit count or timerequirements.
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