Add a default max_step to processors using the smallest gate time (qu…

…tip#2040)

* Add a default max_step using the smallest gate time

* Increase the maximal number of steps allowed in the test

doc/changes/2040.bugfix

@@ -0,0 +1 @@
+Add a default max_step to processors

qutip/qip/device/processor.py

@@ -12,6 +12,7 @@
 from qutip.tensor import tensor
 from qutip.mesolve import mesolve
 from qutip.mcsolve import mcsolve
+from qutip import Options
 from qutip.qip.circuit import QubitCircuit
 from qutip.qip.noise import (
     Noise, RelaxationNoise, DecoherenceNoise,
@@ -738,6 +739,11 @@ def run_state(self, init_state=None, analytical=False, states=None,
         else:
             kwargs["c_ops"] = sys_c_ops
 
+        # set the max step size as 1/2 of the smallest gate time.
+        options = kwargs.get("options", Options())
+        if options.max_step == 0.:
+            options.max_step = np.min(np.diff(self.get_full_tlist())) / 2
+        kwargs["options"] = options
         # choose solver:
         if solver == "mesolve":
             evo_result = mesolve(

qutip/tests/test_device.py

@@ -91,7 +91,7 @@ def test_numerical_evolution(
         init_state = qutip.tensor(extra, state)
     else:
         init_state = state
-    options = qutip.Options(store_final_state=True, nsteps=50_000)
+    options = qutip.Options(store_final_state=True, nsteps=100_000)
     result = device.run_state(init_state=init_state,
                               analytical=False,
                               options=options)
@@ -138,7 +138,7 @@ def test_numerical_circuit(circuit, device_class, kwargs, schedule_mode):
         init_state = qutip.tensor(extra, state)
     else:
         init_state = state
-    options = qutip.Options(store_final_state=True, nsteps=50_000)
+    options = qutip.Options(store_final_state=True, nsteps=100_000)
     result = device.run_state(init_state=init_state,
                               analytical=False,
                               options=options)

qutip/tests/test_processor.py

@@ -4,7 +4,7 @@
     assert_, run_module_suite, assert_allclose, assert_equal)
 import numpy as np
 
-from qutip.qip.device.processor import Processor
+from qutip.qip.device import Processor, LinearSpinChain
 from qutip.states import basis
 from qutip.operators import sigmaz, sigmax, sigmay, identity, destroy
 from qutip.qip.operations.gates import hadamard_transform
@@ -16,6 +16,9 @@
 from qutip.qip.qubits import qubit_states
 from qutip.metrics import fidelity
 from qutip.qip.pulse import Pulse
+from qutip.qip.circuit import QubitCircuit
+
+import pytest
 
 
 class TestCircuitProcessor:
@@ -322,7 +325,35 @@ def testChooseSolver(self):
         result = proc.run_state(init_state=init_state, solver="mcsolve")
         assert_allclose(
             fidelity(result.states[-1], qubit_states(2, [0, 1, 0, 0])),
-            1, rtol=1.e-7) 
+            1, rtol=1.e-7)
+
+    def test_max_step_size(self):
+        num_qubits = 2
+        init_state = tensor([basis(2, 1), basis(2, 1)])
+        qc = QubitCircuit(2)
+
+        # ISWAP acts trivially on the initial states.
+        # If no max_step are defined,
+        # the solver will choose a step size too large
+        # such that the X gate will be skipped.
+        qc.add_gate("ISWAP", targets=[0, 1])
+        qc.add_gate("ISWAP", targets=[0, 1])
+        qc.add_gate("X", targets=[0])
+        processor = LinearSpinChain(num_qubits)
+        processor.load_circuit(qc)
+
+        # No max_step
+        final_state = processor.run_state(
+            init_state, options=Options(max_step=10000) # too large max_step
+        ).states[-1]
+        expected_state = tensor([basis(2, 0), basis(2, 1)])
+        assert pytest.approx(fidelity(final_state, expected_state), 0.001) == 0
+
+        # With default max_step
+        final_state = processor.run_state(init_state).states[-1]
+        expected_state = tensor([basis(2, 0), basis(2, 1)])
+        assert pytest.approx(fidelity(final_state, expected_state), 0.001) == 1
+
 
 if __name__ == "__main__":
     run_module_suite()