I am working on implementing shor algorithm for finding prime factors of N = 15. I am trying to understand a code of qiskit, but i am confused how and why the code editor is using swap codes to define mode function. Here is the code:
def c_amod15(a, power):
"""Controlled multiplication by a mod 15"""
if a not in [2,4,7,8,11,13]:
raise ValueError("'a' must be 2,4,7,8,11 or 13")
U = QuantumCircuit(4)
for _iteration in range(power):
if a in [2,13]:
U.swap(2,3)
U.swap(1,2)
U.swap(0,1)
if a in [7,8]:
U.swap(0,1)
U.swap(1,2)
U.swap(2,3)
if a in [4, 11]:
U.swap(1,3)
U.swap(0,2)
if a in [7,11,13]:
for q in range(4):
U.x(q)
U = U.to_gate()
U.name = f"{a}^{power} mod 15"
c_U = U.control()
return c_U
