cirq.Circuit(cirq.I(cirq.LineQubit(0))) segfaults when trying to execute on qsim

[vtomole]

Trying to run a circuit that contains an identity operation on the simulator:

import cirq
import qsimcirq

circuit = cirq.Circuit(cirq.I(cirq.LineQubit(0)))

qsim_simulator = qsimcirq.QSimSimulator()

print("Running simulation for the following circuit:")
print(circuit)

qsim_results = qsim_simulator.compute_amplitudes(
    circuit, bitstrings=[0b00, 0b01])

print("qsim results:")
print(qsim_results)

Outputs:

Running simulation for the following circuit:
0: ───I───
Segmentation fault (core dumped)

[95-martin-orion]

Further testing suggests that the circuit must contain only identity gates to trigger the segfault - for example, X(q0), I(q1) will run successfully.

qsim_circuit is conspicuously lacking in translators for identity gates - it's possible they got lost in recent optimizations. I'll take a closer look.

[95-martin-orion]

It looks like the translation is fine - cirq.decompose converts to an empty circuit - but compute_amplitudes is misbehaving due to all qubits having been abstracted away. qsim ought to preserve the identity qubits for use in amplitude retrieval.

Some similar tests for comparison:

>>> qsim_results = qsim_simulator.compute_amplitudes(cirq.Circuit(), bitstrings=[0b00, 0b01])
# wrong bitstring length in bitstrings_str: got 1; should be 0.
# Unable to parse provided bit strings.

>>> qsim_results = qsim_simulator.compute_amplitudes(cirq.Circuit(), bitstrings=[])
# Segmentation fault