support cu1 fusion

[hhorii]

Summary

Support fusion of cu1

Details and comments

cu1 is a basic operator since #258, but has not been supported in fusion.

[atilag]

How is the improvement using this optimization?
Do we have some benchmarks?

[atilag]

You might have unintentionally left this commented line I guess.. :)

[atilag]

it would be great if we can add a comment in this method to explain what is trying to do, something like:

// This method will seek for this pattern of gates in the circuit:
// ---[u1]---|---[u1]---|---
// --------[cx]-------[cx]--
// And optimize it by blablabla...

[atilag]

Thanks @hhorii !

[hhorii]

@atilag With this PR, I confirmed 37% reduction of elapsed time of 25-qubit qft on MacBook Pro (2.6 GHz Intel Core i7, 16GB DDR4) .

import time
import math
from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
from qiskit.compiler import assemble, transpile
from qiskit.providers.aer import QasmSimulator

def qft(qreg, circuit):
    n = len(qreg)
    for i in range(n):
        circuit.h(qreg[i])
    for i in range(n):
        for j in range(i):
            circuit.cu1(math.pi/float(2**(i-j)), qreg[i], qreg[j])
        circuit.h(qreg[i])
    return circuit

qubit = 25
backend = QasmSimulator()

print ('app,qubit,time')

backend_options = {}
backend_options['fusion_enable'] = True

q = QuantumRegister(qubit,"q")
c = ClassicalRegister(qubit, "c")
circ = qft(q, QuantumCircuit(q, c, name="qft"))
circ.barrier()
for i in range(qubit):
    circ.measure(q[i], c[i])

qobj = assemble(circ)

start_simulation = time.time()
backend.run(qobj, backend_options=backend_options).result()
end_simulation = time.time()

print ('qft,{qubit},{simulation}'.format(
    qubit=qubit, 
    simulation=(end_simulation - start_simulation)))

before:
app,qubit,time
qft,25,7.1155619621276855

after:
app,qubit,time
qft,25,4.471642017364502