Controlled empty subroutine causes ValueError on simulator #6730
Description
Benny and I found this bug
Description of the issue
Controlled empty subroutine causes ValueError on simulator
How to reproduce the issue
from cirq import (Circuit, CNOT, CZ, Y, Simulator,
NamedQubit, InsertStrategy, CircuitOperation, measure)
from pathlib import Path
from pyqir import qubit
from helpers.cirq_helpers import compare_circuits, individual_pass
from cirq.transformers import *
# Adding qubits
qubits = NamedQubit.range(4, prefix="q")
subcirc1 = Circuit()
subcirc1.append(CZ(qubits[0],qubits[3]), strategy = InsertStrategy.INLINE)
subcirc1.append(CZ(qubits[3],qubits[0]), strategy = InsertStrategy.INLINE)
subcirc1.append(CNOT(qubits[3],qubits[0]), strategy = InsertStrategy.EARLIEST)
subcirc1_op = CircuitOperation(subcirc1.freeze())
subcirc2 = Circuit()
#subcirc2.append(Y(qubits[0]))
subcirc2_op = CircuitOperation(subcirc2.freeze())
main_circ = Circuit()
main_circ.append(measure(qubits[0], key="cbit0"))
main_circ.append(subcirc1_op.with_classical_controls('cbit0'))
main_circ.append(measure(qubits[1], key="cbit1"))
main_circ.append(subcirc2_op.with_classical_controls('cbit1'))
print(main_circ)
main_circ.append(measure(qubits, key="results"))
simulator = Simulator()
circ = simulator.run(main_circ, repetitions=10)Uncommenting the Y gate within the subroutine makes this work. Although users may not do something like this, it would probably be nice if this was treated like an identity instead.
┌──┐
[ q0: ───@───@───X─── ]
q0: ───────M─────[ │ │ │ ]──────────────────────────────────
║ [ q3: ───@───@───@─── ].with_classical_controls(cbit0)
║ ║
q1: ───────╫M────╫────────────────────────────────────────────────────────
║║ ║
q3: ───────╫╫────#2───────────────────────────────────────────────────────
║║ ║
cbit0: ════@╬════╩════════════════════════════════════════════════════════
║
cbit1: ═════@═════════════════════════════════════════════════════════════
[ ].with_classical_controls(cbit1)
└──┘
Traceback (most recent call last):
File "quantum_circuits/circuit1.py", line 32, in <module>
circ = simulator.run(main_circ, repetitions=10)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".venv/lib/python3.11/site-packages/cirq/work/sampler.py", line 63, in run
return self.run_sweep(program, param_resolver, repetitions)[0]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".venv/lib/python3.11/site-packages/cirq/sim/simulator.py", line 72, in run_sweep
return list(self.run_sweep_iter(program, params, repetitions))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".venv/lib/python3.11/site-packages/cirq/sim/simulator.py", line 103, in run_sweep_iter
records = self._run(
^^^^^^^^^^
File ".venv/lib/python3.11/site-packages/cirq/sim/simulator_base.py", line 245, in _run
for step_result in self._core_iterator(circuit=prefix, sim_state=sim_state):
File ".venv/lib/python3.11/site-packages/cirq/sim/simulator_base.py", line 220, in _core_iterator
protocols.act_on(op, sim_state)
File ".venv/lib/python3.11/site-packages/cirq/protocols/act_on_protocol.py", line 141, in act_on
result = action_act_on(sim_state) if is_op else action_act_on(sim_state, qubits)
^^^^^^^^^^^^^^^^^^^^^^^^
File ".venv/lib/python3.11/site-packages/cirq/ops/classically_controlled_operation.py", line 187, in _act_on_
if all(c.resolve(sim_state.classical_data) for c in self._conditions):
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".venv/lib/python3.11/site-packages/cirq/ops/classically_controlled_operation.py", line 187, in <genexpr>
if all(c.resolve(sim_state.classical_data) for c in self._conditions):
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".venv/lib/python3.11/site-packages/cirq/value/condition.py", line 104, in resolve
raise ValueError(f'Measurement key {self.key} missing when testing classical control')
ValueError: Measurement key cbit1 missing when testing classical control
Cirq version
You can get the cirq version by printing cirq.__version__. From the command line:
1.4.1