  When a bug is detected by testing a quantum program on a quantum computer, we
want to determine its location to fix it. To locate the bug, the quantum
program is divided into several segments, and each segment is tested. However,
to prepare a quantum state that is input to a segment, it is necessary to
execute all the segments ahead of that segment in a quantum computer. This
means that the cost of testing each segment depends on its location. We can
also locate a buggy segment only if it is confirmed that there are no bugs in
all segments ahead of that buggy segment. Since a quantum program is tested
statistically on the basis of measurement results, there is a tradeoff between
testing accuracy and cost. Although these characteristics are unique to quantum
programs and complicate locating bugs, they have not been investigated. We
suggest for the first time that these characteristics should be considered to
efficiently locate bugs. We are also the first to propose a bug-locating method
that takes these characteristics into account. The results from experiments
indicate that the bug-locating cost, represented as the number of executed
quantum gates, can be reduced with the proposed method compared with naive
methods.
