IdentityGate Visualization

[glanzz]

Description of the issue
IdentityGate constructor accepts a single parameter which is the number of qubits it acts on.
When the parameter value is set to 0, the circuit works fine until the circuit is printed. Program crashes with an ValueError.

How to reproduce the issue

import cirq as c

idgate = c.IdentityGate(0)


qubit = c.LineQubit(0)
circuit = c.Circuit()
print(idgate())
circuit.append(idgate())

print(circuit)
circuit += c.inverse(circuit)
circuit.append(c.measure_each(qubit))

print(c.Simulator().run(circuit, repetitions=100))

File "/Users/xxx/identity_gate_cirq.py", line 11, in <module>
    print(circuit)
  File "/Users/xxx/cirqenv/lib/python3.11/site-packages/cirq/circuits/circuit.py", line 285, in __str__
    return self.to_text_diagram()
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/xxx/cirqenv/lib/python3.11/site-packages/cirq/circuits/circuit.py", line 1197, in to_text_diagram
    diagram = self.to_text_diagram_drawer(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/xxx/cirqenv/lib/python3.11/site-packages/cirq/circuits/circuit.py", line 1276, in to_text_diagram_drawer
    _draw_moment_in_diagram(
  File "/Users/xxx/cirqenv/lib/python3.11/site-packages/cirq/circuits/circuit.py", line 2612, in _draw_moment_in_diagram
    _draw_moment_annotations(
  File "/Users/xxx/cirqenv/lib/python3.11/site-packages/cirq/circuits/circuit.py", line 2543, in _draw_moment_annotations
    info = get_circuit_diagram_info(annotation, args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/xxx/cirqenv/lib/python3.11/site-packages/cirq/protocols/circuit_diagram_info_protocol.py", line 345, in _op_info_with_fallback
    raise ValueError(f'Wanted diagram info from {op!r} for {rows!r}) but got {info!r}')
ValueError: Wanted diagram info from cirq.IdentityGate(0)() for []) but got cirq.CircuitDiagramInfo(wire_symbols=(), exponent=1, connected=True, exponent_qubit_index=None, auto_exponent_parens=True)

Cirq version
You can get the cirq version by printing cirq.__version__. From the command line:

Cirq: 1.4.1

[mhucka]

Discussed during bit.ly/cirq-cynq today (2024-10-16): probably the solution is to disallow 0 qubit inputs and add validation to IdentityGate, because this is a situation that doesn't make sense physically.

[daxfohl]

I feel like we've seen situations where these are created in decomposition or transformers or whatever. Even if they're just temporary and eventually removed from the circuit, disallowing them from even being constructed could break things in ways that may be painful to work around.

In the case of diagrams, I know that PauliString returns NotImplemented from _circuit_diagram_info_ if it's empty, which causes the gate diagram to be placed by itself under all the qubits, similar to a GlobalPhaseGate. So, perhaps do the same for the I gate. Or perhaps it's possible to generalize the behavior of zero-qubit gates from whatever calls into _circuit_diagram_info_, so that not every varadic gate has to explicitly implement this.