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Bug fix in the C++ simulator: density matrix and probabilities are sometimes erroneous #518

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Merged
atilag merged 2 commits into from
May 30, 2018
Merged

Bug fix in the C++ simulator: density matrix and probabilities are sometimes erroneous #518

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08a2fa4
Bug fix in the C++ simulator: wrong density matrix and probabilities …
yaelbh May 24, 2018
5dd00f1
Remove tab and add spaces for style consitency
atilag May 30, 2018
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10 changes: 7 additions & 3 deletions src/qasm-simulator-cpp/src/engines/vector_engine.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -172,10 +172,14 @@ void VectorEngine::execute(const Circuit &prog, BaseBackend<QubitVector> *be,
// Check to see if circuit is ideal and allows for measurement optimization
if (prog.opt_meas && prog.noise.ideal) {

// This optimization replaces the shots by a single shot + sampling
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@chriseclectic chriseclectic May 29, 2018

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Could you add a second comment line here: // We need to subtract the additional shots added by BaseEngine class

// We need to subtract the additional shots added by BaseEngine class
total_shots -= (nshots-1);

// Find position of first measurement operation
uint_t pos = 0;
while (prog.operations[pos].id != gate_t::Measure &&
pos < prog.operations.size()) {
while (pos < prog.operations.size() &&
prog.operations[pos].id != gate_t::Measure) {
pos++;
}
// Execute operations before measurements
Expand Down Expand Up @@ -492,4 +496,4 @@ inline void from_json(const json_t &js, VectorEngine &eng) {
//------------------------------------------------------------------------------
} // end namespace QISKIT

#endif
#endif
3 changes: 2 additions & 1 deletion test/python/qobj/cpp_measure_opt.json
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Expand Up @@ -3,7 +3,8 @@
"config": {
"shots": 2000,
"seed": 1,
"backend_name": "local_qasm_simulator_cpp"
"backend_name": "local_qasm_simulator_cpp",
"data" : ["density_matrix", "probabilities"]
},
"circuits": [
{
Expand Down
9 changes: 7 additions & 2 deletions test/python/test_qasm_simulator_cpp.py
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Expand Up @@ -17,6 +17,8 @@

# =============================================================================

from test.python.common import QiskitTestCase

import json
import unittest
import numpy as np
Expand All @@ -31,7 +33,6 @@
from qiskit.backends.local.qasm_simulator_cpp import (QasmSimulatorCpp,
cx_error_matrix,
x90_error_matrix)
from .common import QiskitTestCase


class TestLocalQasmSimulatorCpp(QiskitTestCase):
Expand Down Expand Up @@ -205,13 +206,17 @@ def test_qobj_measure_opt(self):
snapshots = result.get_snapshots(name)
self.assertEqual(set(snapshots), {'0'},
msg=name + ' snapshot keys')
self.assertEqual(len(snapshots['0']), 1,
self.assertEqual(len(snapshots['0']), 3,
msg=name + ' snapshot length')
state = snapshots['0']['statevector'][0]
expected_state = expected_data[name]['statevector']
fidelity = np.abs(expected_state.dot(state.conj())) ** 2
self.assertAlmostEqual(fidelity, 1.0, places=10,
msg=name + ' snapshot fidelity')
rho = snapshots['0']['density_matrix']
self.assertAlmostEqual(np.trace(rho), 1)
prob = snapshots['0']['probabilities']
self.assertAlmostEqual(np.sum(prob), 1)

def test_qobj_measure_opt_flag(self):
filename = self._get_resource_path('qobj/cpp_measure_opt_flag.json')
Expand Down