Use appropriate names in circuit drawings of `qml.cond`

[ankit27kh]

Feature details

Drawing a circuit with qml.cond shows this gate as ControlledOperation in the circuit drawing, Instead, use the name of the function in the condition.

Implementation

No response

How important would you say this feature is?

1: Not important. Would be nice to have.

Additional information

No response

[CatalinaAlbornoz]

It would definitely be nice to have @ankit27kh. I'm not sure how easy or hard this is to implement though. Do you have any thoughts on this @albi3ro?

[albi3ro]

@ankit27kh

Taking the example from the qml.cond docs:

dev = qml.device("default.qubit", wires=2)

def qfunc1(x, wire):
    qml.Hadamard(wire)
    qml.RY(x, wire)

def qfunc2(x, y, z, wire):
    qml.Hadamard(wire)
    qml.Rot(x, y, z, wire)

@qml.qnode(dev)
def qnode(a, x, y, z):
    qml.Hadamard(0)
    m_0 = qml.measure(0)
    qml.cond(m_0, lambda: qfunc1(a, wire=1), lambda: qfunc2(x, y, z, wire=1))()
    return qml.expval(qml.PauliZ(1))

par = np.array(0.3, requires_grad=True)
x = np.array(1.2, requires_grad=True)
y = np.array(1.1, requires_grad=True)
z = np.array(0.3, requires_grad=True)

You can use expansion_strategy="device" to turn the ControlledOperation into a more familiar operations:

print(qml.draw(qnode, expansion_strategy='device')(par, x, y, z))

0: ──H─╭●────────╭●────────╭●────────╭●─────────X─╭●────────╭●────────╭●─────────X──X
1: ────╰Rϕ(1.57)─╰RX(1.57)─╰Rϕ(1.57)─╰RY(0.30)────╰Rϕ(1.57)─╰RX(1.57)─╰Rϕ(1.57)──────

──╭●────────────────────X─┤     
──╰Rot(1.20,1.10,0.30)────┤  <Z>

QNode's apply qml.defer_measurements by default, so it turns Conditional operations into standard controlled operations.

ControlledOperation's don't display very well in circuit diagrams, but I'm trying to replace the class anyway, so eventually, generic controlled operations will display better.

[timmysilv]

Hi y'all 👋 I have PR #4228 up for review to add MPL support for the drawing of mid-circuit measurements connected to conditional operations. It comes with some limitations, but it should provide a fair bit of functionality here! Some notes

• it will only work with qml.draw_mpl for now, and not qml.draw
• As @albi3ro has described above, if you wrap your circuit in a QNode, it will probably apply the deferred measurement principle and won't matter anymore. However, you can draw an un-processed/un-decorated qfunc (see examples in the PR changes) if that's the case

[albi3ro]

The example you posted now returns:

0: ──H──┤↗├──────────────────────────────────────┤     
1: ──────║───H──RY(0.30)──H──Rot(1.20,1.10,0.30)─┤  <Z>
         ╚═══╩══╩═════════╩══╝                         

This was added in PR #4930 and #4901