Cost-Effective Realization of n-Bit Toffoli Gates for IBM Quantum
  Computers Using the Bloch Sphere Approach and IBM Native Gates
A cost-effective n-bit Toffoli gate is proposed to be realized (or
transpiled) based on the layouts (linear, T-like, and I-like) and the number of
n physical qubits for IBM quantum computers. This proposed gate is termed the
"layout-aware n-bit Toffoli gate". The layout-aware n-bit Toffoli gate is
designed using the visual approach of the Bloch sphere, from the visual
representations of the rotational quantum operations for IBM native gates. In
this paper, we also proposed a new formula for the quantum cost, which
calculates the total number of native gates, the crossing connections, and the
depth of the final transpiled quantum circuit. This formula is termed the
"transpilation quantum cost". After transpilation, our proposed layout-aware
n-bit Toffoli gate always has a much lower transpilation quantum cost than that
of the conventional n-bit Toffoli gate, where 3 <= n <= 7 qubits, for different
IBM quantum computers.
Ali Al-Bayaty Marek Perkowski