print(moment) is confusing for crossing gates

[tanujkhattar]

Description of the issue When using line qubits and crossing gates in a single moment; the output on printing the moment is confusing and does not show the overlap correctly, unlike the output obtained on printing the whole circuit.

How to reproduce the issue

a, b, c, d = cirq.LineQubit.range(4)
c1 = cirq.Circuit([cirq.CNOT(a, d), cirq.CNOT(b, c)])
c2 = cirq.Circuit([cirq.CNOT(a, c), cirq.CNOT(b, d)])
In [22]: print(c1, '\n', c2)
      ┌──┐
0: ────@─────
       │
1: ────┼@────
       ││
2: ────┼X────
       │
3: ────X─────
      └──┘
       ┌──┐
0: ────@─────
       │
1: ────┼@────
       ││
2: ────X┼────
        │
3: ─────X────
      └──┘
In [23]: print(c1[0], '\n', c2[0]) # <-- Confusing: The output should be different similar to circuit diagram above. 
  ╷ 0 1 2 3
╶─┼─────────
0 │ @─@─X─X
  │
   ╷ 0 1 2 3
╶─┼─────────
0 │ @─@─X─X
  │

Another Example:

In [18]: print(cirq.Circuit(cirq.ControlledGate(cirq.X, num_controls = 3).on(a, b, c, d))[0])
    ...: print(cirq.Circuit([cirq.CNOT(a, d), cirq.CZ(b, c)])[0])
  ╷ 0 1 2 3
╶─┼─────────
0 │ @─@─@─X
  │
  ╷ 0 1 2 3
╶─┼─────────
0 │ @─@─@─X
  │

Cirq version

0.11.0.dev

[balopat]

Agreed, this is a case that was not covered seemingly in https://github.com/quantumlib/Cirq/pull/3403

[MichaelBroughton]

@tanujkhattar Is this still relevant ?

[cduck]

@MichaelBroughton This issue is still relevant as of Cirq 1.1.0. I recently discovered this confusing print output while debugging.

[cduck]

The behavior is similar but transposed if you use NamedQubit.

q0, q1, q2, q3 = cirq.LineQubit.range(4)
print(cirq.Moment(cirq.CZ(q0, q1), cirq.CZ(q2, q3)))
#   ╷ 0 1 2 3
# ╶─┼─────────
# 0 │ @─@ @─@
#   │

q0, q1, q2, q3 = cirq.LineQubit.range(4)
print(cirq.Moment(cirq.CZ(q0, q2), cirq.CZ(q1, q3)))  # q1<->q2
#  ╷ 0 1 2 3
#╶─┼─────────
#0 │ @─@─@─@
#  │

q0, q1, q2, q3 = cirq.NamedQubit.range(4, prefix='q')
print(cirq.Moment(cirq.CZ(q0, q1), cirq.CZ(q2, q3)))
#   ╷ None
#╶──┼──────
#q0 │ @
#   │ │
#q1 │ @
#   │
#q2 │ @
#   │ │
#q3 │ @
#   │

q0, q1, q2, q3 = cirq.NamedQubit.range(4, prefix='q')
print(cirq.Moment(cirq.CZ(q0, q2), cirq.CZ(q1, q3)))  # q1<->q2
#   ╷ None
#╶──┼──────
#q0 │ @
#   │ │
#q1 │ @
#   │ │
#q2 │ @
#   │ │
#q3 │ @
#   │