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In text diagrams, writing a classical bit is shown as a control while using it as a control is not shown as a control #5688

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Strilanc opened this issue Jul 8, 2022 · 5 comments · May be fixed by #7233
Open

In text diagrams, writing a classical bit is shown as a control while using it as a control is not shown as a control #5688

Strilanc opened this issue Jul 8, 2022 · 5 comments · May be fixed by #7233
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area/circuits area/visualization kind/bug-report Something doesn't seem to work. triage/accepted A consensus emerged that this bug report, feature request, or other action should be worked on
Milestone
2025-T4: Improve ...

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@Strilanc
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Strilanc commented Jul 8, 2022 •
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import cirq

print(cirq.Circuit(
    cirq.measure(cirq.LineQubit(0), key="abc"),
    cirq.X(cirq.LineQubit(0)).with_classical_controls("abc"),
))

0: ─────M───X───
        ║   ║
abc: ═══@═══^═══

The @ means control elsewhere in diagrams. The character below the M isn't a control it's an output; it should probably be an X. And the ^ for the control on the NOT gate should be an @ since that means control.
@Strilanc Strilanc added the kind/bug-report Something doesn't seem to work. label Jul 8, 2022
@Strilanc Strilanc changed the title In text diagrams, writing a classical bit is shown as a control and using it is not shown as a control In text diagrams, writing a classical bit is shown as a control while using it as a control is not shown as a control Jul 8, 2022
@dabacon
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dabacon commented Jul 12, 2022

Would making the second one an "@" would be confusing, since it might make someone think it actually is a CNOT? Similarly for the first making it an "X" would also be slightly misleading in that people could think this means a measurement is a someone an "X" gate.

@Strilanc
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Strilanc commented Jul 16, 2022 •
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Would making the second one an "@" would be confusing, since it might make someone think it actually is a CNOT?

But it is a controlled not. It's a classically controlled not. They wouldn't be confused about that, they would be right. In any case, the double lines on the vertical bar is a dead giveaway that it's classically controlled.

people could think this means a measurement is a someone an "X" gate.

A measurement kinda is a NOT gate on a classical bit, controlled by a quantum bit. I don't think there's any physical experiment you can do that distinguishes "measurement" from "CNOT from qubit to bit where bit is promised to be initially OFF".

@daxfohl
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daxfohl commented Jul 16, 2022

I agree with changing the control to @. For changing the target to X, my main concern is that now that we allow multiple writes to the same measurement key, X looks like an XOR, when it's actually just a write. Otherwise I agree here as well.

(I had also been thinking that we might want to add an "op" param to measure, allowing things like cirq.measure(q, key=m, op=XOR), which would XOR the current measurement with the previous one, and I kind of wanted to reserve X for that.)

@viathor viathor added triage/discuss Needs decision / discussion, bring these up during Cirq Cynque area/circuits area/visualization labels Jul 26, 2022
@verult
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verult commented Aug 10, 2022

From Cirq sync - biggest question is whether changing text diagram output would be considered a breaking change.

@dstrain115 dstrain115 added triage/accepted A consensus emerged that this bug report, feature request, or other action should be worked on and removed triage/discuss Needs decision / discussion, bring these up during Cirq Cynque labels Aug 17, 2022
@seunomonije
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From Cirq sync:

  • Maybe the choice that we made to display classical control is not the best design choice, may not really be a bug
  • More of a behavior change to something that is obscure. Current behavior is awkward and questionable whether or not it’s intended.
  • Maybe we should introduce a new tag for issues like this

Consensus: We should accept this as a bug and get fixed

@daxfohl daxfohl linked a pull request Apr 6, 2025 that will close this issue
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area/circuits area/visualization kind/bug-report Something doesn't seem to work. triage/accepted A consensus emerged that this bug report, feature request, or other action should be worked on
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2025-T4: Improve capabilities for cir...
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Fix classical control diagram symbols daxfohl/Cirq
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@Strilanc @dabacon @daxfohl @mhucka @verult @seunomonije @dstrain115 @viathor