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I tried to do qubit mapping called 'SABRE' and execute transpiled quantum circuit. I use FakeTokyo backend which has some noise because it is mock version of IBMQ 20-Tokyo. I know using this backend makes some noise but will not change final result when I measure my circuit. But, when I transpile my quantum circuit like below code, the result is always different. I wonder why this happens.

Source code:

from qiskit.test.mock import FakeTokyo
from qiskit.circuit.measure import measure
from qiskit.execute_function import execute
from qiskit.circuit.quantumcircuit import QuantumCircuit
from qiskit.compiler import transpile

backend = FakeTokyo()

final_circuit = transpile(circuits=circuit,
                          backend=backend,
                          routing_method='sabre',
                          layout_method='sabre',
                          basis_gates=['u1', 'u2', 'u3', 'cx', 'id'])

final_circuit.barrier()
final_circuit.measure_all()

results = execute(final_circuit, backend).result()
counts = results.get_counts()
print("result : ", counts)

circuits = circuit means I input list of quantum circuits into transpile function. I really don't know why the results always different.

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Set the seed_transpiler. See the documentation for transpiler() here.

from qiskit.test.mock import FakeTokyo
from qiskit.circuit.measure import measure
from qiskit.execute_function import execute
from qiskit.circuit.quantumcircuit import QuantumCircuit
from qiskit.compiler import transpile

backend = FakeTokyo()

final_circuit = transpile(circuits=circuit,
                          backend=backend,
                          routing_method='sabre',
                          layout_method='sabre',  
                          seed_transpiler= 1000,
                          basis_gates=['u1', 'u2', 'u3', 'cx', 'id'])

final_circuit.barrier()
final_circuit.measure_all()

results = execute(final_circuit, backend).result()
counts = results.get_counts()
print("result : ", counts)
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  • $\begingroup$ What is seed_transpiler? Please show me the example of seed_transpiler $\endgroup$
    – 김동민
    Jun 22 at 2:15
  • $\begingroup$ It sets random seed for the stochastic parts of the transpiler. The transpiler/compiler uses a heuristic approach at decomposing the circuit. If you set the seed to a specific value then the transpiled circuit will be the same each time. If not, then each execution, the transpiler will pick a random seed hence lead to different results. Look at the code I put down... which is the same as the code you had, I just set the seed to a fixed value of 1000. $\endgroup$
    – KAJ226
    Jun 22 at 3:05
  • $\begingroup$ Thank you, I found the seed what I need. But I wonder two things. $\endgroup$
    – 김동민
    Jun 23 at 0:18
  • $\begingroup$ 1. Should I need to set specific seed to get the result that I need? 2. What is seed exactly? $\endgroup$
    – 김동민
    Jun 23 at 3:53
  • $\begingroup$ 1) You can set it to any specific value you want. But keep it the same through out. 2) See this: en.wikipedia.org/wiki/Random_seed $\endgroup$
    – KAJ226
    Jun 23 at 4:27

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