Mapping Error in BasicAer

[dcmckayibm]

'OPENQASM 2.0;\ninclude "qelib1.inc";\nqreg q0[5];\ncreg c0[3];\nx q0[4];\nmeasure q0[1] -> c0[0];\nmeasure q0[3] -> c0[1];\nmeasure q0[4] -> c0[2];\n'

The above qasm should give all shots in 100, but they are all in 000. Only happens in BasicAer, not in Aer.

Informations

• Qiskit Terra version: 0.7
• Python version: 3.6
• Operating system:

What is the current behavior?

Steps to reproduce the problem

What is the expected behavior?

Suggested solutions

[jaygambetta]

I confirm there is a bug

qr = QuantumRegister(5)
cr = ClassicalRegister(3)
circuit = QuantumCircuit(qr, cr)
circuit.x(qr[4])
circuit.measure(qr[1], cr[0])
circuit.measure(qr[3], cr[1])
circuit.measure(qr[4], cr[2])

backend = BasicAer.get_backend('qasm_simulator')
job = execute(circuit, backend)
result = job.result()
counts1 = result.get_counts()
print(counts1)

backend = Aer.get_backend('qasm_simulator')
job = execute(circuit, backend)
result = job.result()
counts2 = result.get_counts()
print(counts2)

gives the outcome david is saying.

{'000': 1024}
{'100': 1024}

[jaygambetta]

@chriseclectic can you have a look at this.

[ajavadia]

The bug is in the BasicAer python simulator. Specifically it is occurring in the _add_sample_measure function, which is responsible for finding a probability distribution from the statevector and sampling from that. This function cannot handle the case where only a subset of the total qubits are measured.