Print subcircuits out-of-line

[95-martin-orion]

Is your feature request related to a use case or problem? Please describe.

In #3580, a circuit containing CircuitOperations will print those operations inline. This can make the diagram much larger in the best case, and impossible to read in more complex situations.

Describe the solution you'd like

CircuitOperations should be printed outside the circuit, with an identifier replacing them inline. Example:

a, b, c = cirq.LineQubit.range(2)
op = cirq.CircuitOperation(cirq.FrozenCircuit(
  cirq.H(a), cirq.CZ(a, b), cirq.H(a)
))
circuit = cirq.Circuit(
  cirq.X(a), cirq.H(b), cirq.X(c),
  op.with_qubits(c, a),
)
print(circuit)

# output
circuit body:
0 --X--[ (1) subcircuit ]--
       |
1 --H--+-------------------
       |
2 --X--[ (0) subcircuit ]--

subcircuit:
0 --H--@--H--
       |
1 -----@-----

Some way to explicitly name FrozenCircuits would be useful for this.

What is the urgency from your perspective for this issue? Is it blocking important work?

P2 - we should do it in the next couple of quarters

[balopat]

I don't think I agree with always putting CircuitOperation out of line. I could see how the inline diagram can be useful in smaller cases. Maybe we could control the behavior by a flag on CircuitOperation itself?

[95-martin-orion]

Child of #3235

[thanacles]

Do you think this would benefit from something like the fix for: https://github.com/quantumlib/Cirq/issues/3184

[95-martin-orion]

It could, but without an out-of-line copy of the subcircuit the value is limited compared to MatrixGate:

• The contents of a CircuitOperation is just a Circuit: no exponential explosion like MatrixGate.
• At the same time, a CircuitOperation can be much more complex than a MatrixGate; omitting the subcircuit potentially obscures many moments of a circuit in its diagram.