Fix bug for circuit with selected measurement result (#167)

* Fix bug for circuit with selected measurement result

The parameter measure_results was not correctly processed in QubitCircuit.run because the Simulator is initialized twice. This commit removes the parameters `state`, `cbits` and `measure_results` from the `__init__` of CircuitSimulator and only initializes the simulator before it is run.

The CircuitSimulator class should only include information about the circuit while the information about the state is only provided when running the simulation.

doc/source/qip-simulator.rst

@@ -156,8 +156,8 @@ and computation proceeds. To demonstrate, we continue with our previous circuit:
 .. testcode::
 
   from qutip_qip.circuit import CircuitSimulator
-
-  sim = CircuitSimulator(qc, state=zero_state)
+  sim = CircuitSimulator(qc)
+  sim.initialize(zero_state)
 
 This initializes the simulator object and carries out any pre-computation
 required. There are two ways to carry out state evolution with the simulator.

src/qutip_qip/circuit/circuit.py

@@ -490,30 +490,19 @@ def run(
         final_state : Qobj
                 output state of the circuit run.
         """
-
         if state.isket:
-            sim = CircuitSimulator(
-                self,
-                state,
-                cbits,
-                U_list,
-                measure_results,
-                "state_vector_simulator",
-                precompute_unitary,
-            )
+            mode = "state_vector_simulator"
         elif state.isoper:
-            sim = CircuitSimulator(
-                self,
-                state,
-                cbits,
-                U_list,
-                measure_results,
-                "density_matrix_simulator",
-                precompute_unitary,
-            )
+            mode = "density_matrix_simulator"
         else:
             raise TypeError("State is not a ket or a density matrix.")
-        return sim.run(state, cbits).get_final_states(0)
+        sim = CircuitSimulator(
+            self,
+            U_list,
+            mode,
+            precompute_unitary,
+        )
+        return sim.run(state, cbits, measure_results).get_final_states(0)
 
     def run_statistics(
         self, state, U_list=None, cbits=None, precompute_unitary=False
@@ -530,11 +519,6 @@ def run_statistics(
                 initialization of the classical bits.
         U_list: list of Qobj, optional
             list of predefined unitaries corresponding to circuit.
-        measure_results : tuple of ints, optional
-            optional specification of each measurement result to enable
-            post-selection. If specified, the measurement results are
-            set to the tuple of bits (sequentially) instead of being
-            chosen at random.
         precompute_unitary: Boolean, optional
             Specify if computation is done by pre-computing and aggregating
             gate unitaries. Possibly a faster method in the case of
@@ -546,27 +530,18 @@ def run_statistics(
             Return a CircuitResult object containing
             output states and and their probabilities.
         """
-
         if state.isket:
-            sim = CircuitSimulator(
-                self,
-                state,
-                cbits,
-                U_list,
-                mode="state_vector_simulator",
-                precompute_unitary=precompute_unitary,
-            )
+            mode = "state_vector_simulator"
         elif state.isoper:
-            sim = CircuitSimulator(
-                self,
-                state,
-                cbits,
-                U_list,
-                mode="density_matrix_simulator",
-                precompute_unitary=precompute_unitary,
-            )
+            mode = "density_matrix_simulator"
         else:
             raise TypeError("State is not a ket or a density matrix.")
+        sim = CircuitSimulator(
+            self,
+            U_list,
+            mode,
+            precompute_unitary,
+        )
         return sim.run_statistics(state, cbits)
 
     def resolve_gates(self, basis=["CNOT", "RX", "RY", "RZ"]):

src/qutip_qip/circuit/circuitsimulator.py

@@ -11,6 +11,7 @@
     gate_sequence_product,
 )
 from qutip import basis, ket2dm, Qobj, tensor
+import warnings
 
 
 __all__ = ["CircuitSimulator", "CircuitResult"]
@@ -253,12 +254,12 @@ class CircuitSimulator:
     def __init__(
         self,
         qc,
-        state=None,
-        cbits=None,
         U_list=None,
-        measure_results=None,
         mode="state_vector_simulator",
         precompute_unitary=False,
+        state=None,
+        cbits=None,
+        measure_results=None,
     ):
         """
         Simulate state evolution for Quantum Circuits.
@@ -268,21 +269,9 @@ def __init__(
         qc : :class:`.QubitCircuit`
             Quantum Circuit to be simulated.
 
-        state: ket or oper
-            ket or density matrix
-
-        cbits: list of int, optional
-            initial value of classical bits
-
         U_list: list of Qobj, optional
             list of predefined unitaries corresponding to circuit.
 
-        measure_results : tuple of ints, optional
-            optional specification of each measurement result to enable
-            post-selection. If specified, the measurement results are
-            set to the tuple of bits (sequentially) instead of being
-            chosen at random.
-
         mode: string, optional
             Specify if input state (and therefore computation) is in
             state-vector mode or in density matrix mode.
@@ -321,7 +310,13 @@ def __init__(
         else:
             self._process_ops()
 
-        self.initialize(state, cbits, measure_results)
+        if any(p is not None for p in (state, cbits, measure_results)):
+            warnings.warn(
+                "Initializing the quantum state, cbits and measure_results "
+                "when initializing the simulator is deprecated. "
+                "The inputs are ignored. "
+                "They should, instead, be provided when running the simulation."
+            )
 
     def _process_ops(self):
         """

tests/test_circuit.py

@@ -598,6 +598,23 @@ def test_measurement_circuit(self):
                 else:
                     assert simulator.cbits[0] != simulator.cbits[1]
 
+    def test_circuit_with_selected_measurement_result(self):
+        qc = QubitCircuit(N=1, num_cbits=1)
+        qc.add_gate("SNOT", targets=0)
+        qc.add_measurement("M0", targets=0, classical_store=0)
+
+        # We reset the random seed so that
+        # if we don's select the measurement result,
+        # the two circuit should return the same value.
+        np.random.seed(0)
+        final_state = qc.run(qp.basis(2, 0), cbits=[0], measure_results=[0])
+        fid = pytest.approx(qp.fidelity(final_state, basis(2, 0)))
+        assert fid == 1.0
+        np.random.seed(0)
+        final_state = qc.run(qp.basis(2, 0), cbits=[0], measure_results=[1])
+        fid = pytest.approx(qp.fidelity(final_state, basis(2, 1)))
+        assert fid == 1.0
+
     def test_gate_product(self):
 
         filename = "qft.qasm"