Added a trotter circuit construction by for loops

from qcor import *

import time

@qjit

def trotter_circ(q : qreg, exp_args: List[PauliOperator], n_steps: int):

  for i in range(n_steps):

    for i, exp_arg in enumerate(exp_args):

      exp_i_theta(q, 1.0, exp_args[i])

def heisenberg_ham(n_qubits):

  Jz = 1.0

  h = 1.0

  H = 0.0 * X(0)

  for i in range(n_qubits):

    H = H - h * X(i)

  for i in range(n_qubits - 1):

    H = H - Jz * (Z(i) * Z(i + 1))

  return H

n_qubits = [10, 20, 50, 100]

nbSteps = 100

for nbQubits in n_qubits:

  ham_op = heisenberg_ham(nbQubits)

  op_terms = ham_op.getNonIdentitySubTerms()

  q = qalloc(nbQubits)

  start = time.time()

  comp = trotter_circ.extract_composite(q, op_terms, nbSteps)

  end = time.time()

  print("n instructions =", comp.nInstructions(), "; Kernel eval time:", end - start, " [secs]")

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