Setting noise for local qasm simulator (c++)

[chunfuchen]

Informations
local_qasm_simulator_cpp fails when setting particular noise params (the one copied from simulator readme.)
https://github.com/QISKit/qiskit-core/tree/master/src/qasm-simulator-cpp#example

• QISKit (Python SDK) version: 0.5.3 and build from the master branch
• Operating system: mac OS 10.13.3
• Python version: 3.6.3

What is the current behavior?
c++ simulator fails with particular noise params setting. (Note that python simulator works well, and if I deleted U_error field, it works well, too.)

case 1 (installed from pip), trackback

Traceback (most recent call last):
  File "/Users/rchen/.pythonVirtualEnv/quantum/lib/python3.6/site-packages/qiskit/backends/local/localjob.py", line 66, in result
    return self._future.result(timeout=timeout)
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/Users/rchen/.pythonVirtualEnv/quantum/lib/python3.6/site-packages/qiskit/backends/local/qasm_simulator_cpp.py", line 90, in _run_job
    result = run(qobj, self._configuration['exe'])
  File "/Users/rchen/.pythonVirtualEnv/quantum/lib/python3.6/site-packages/qiskit/backends/local/qasm_simulator_cpp.py", line 235, in run
    return json.loads(sim_output, cls=QASMSimulatorDecoder)
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/__init__.py", line 367, in loads
    return cls(**kw).decode(s)
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/decoder.py", line 357, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

case 2 (installed qiskit from the master branch)

Traceback (most recent call last):
  File "/Users/rchen/Developer/Quantum/qiskit-sdk-py/qiskit/_result.py", line 259, in get_counts
    return self.get_data(circuit_name)['counts']
  File "/Users/rchen/Developer/Quantum/qiskit-sdk-py/qiskit/_result.py", line 239, in get_data
    raise QISKitError('No data for circuit "{0}"'.format(circuit_name))
qiskit._qiskiterror.QISKitError: 'No data for circuit "circuit1"'

Steps to reproduce the problem

Here is the codes:

from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister
from qiskit.wrapper import available_backends, get_backend
from qiskit.wrapper import execute as q_execute

q = QuantumRegister(2, name='q')
c = ClassicalRegister(2, name='c')
qc = QuantumCircuit(q,c)

qc.h(q[0])
qc.h(q[1])
qc.cx(q[0], q[1])
qc.measure(q, c)
noise_params = {'U':
                        {'gate_time': 1,
                        'p_depol': 0.001,
                        'p_pauli': [0, 0, 0.01],
                        'U_error': [
                                [[1, 0], [0, 0]],
                                [[0, 0], [0.995004165, 0.099833417]]
                                ]
                        }
                    }
config = {"noise_params": noise_params}
ret = q_execute(qc, 'local_qasm_simulator_cpp', shots=1024, config=config)
ret = ret.result()
print(ret.get_counts())

What is the expected behavior?

Run without error.

Suggested solutions

No idea.

[chriseclectic]

This is because the simulator checks if the U_error matrix is unitary and (should) raise an error if it isn't. In your example z = 0.995004165 + 1j * 0.099833417, and abs(z) = 0.99999999975862153, not 1.

If I change the noise params in your code to the following it runs:

z = 0.995004165 + 1j * 0.099833417
z = z / abs(z)
u_error = np.array([[1, 0], [0, z]])
noise_params = {'U':
                        {'gate_time': 1,
                        'p_depol': 0.001,
                        'p_pauli': [0, 0, 0.01],
                        'U_error': u_error
                        }
                    }

However there still seems to be a bug that the error that should be raised by the simulator isn't being caught and passed through to the results: If I run the compiled qojb directly from the command line it just appears to abort.

[chunfuchen]

okay, I do not double check whether or not the U_error is a unitary matrix, I just copy and paste the example in the README.

Thanks.

[ajavadia]

So it seems like to close this issue, the following should be done:
a) the README should be updated to include the normalization.
b) the error raised by the simulator should be passed through to the result and available to the user.

[jaygambetta]

closing as this is not part of terra anymore and will be documented in aer.