Enhancements to the Rust circuit drawer

[eliarbel]

The ability to draw quantum circuits natively from Rust (i.e. from CircuitData) as text was added in #15357. The initial implementation of this feature was done with the C API usage in mind, hence included support for drawing circuit elements which can be constructed from C API, which were standard and unitary gates and standard instructions during the development period of #15357. While the Rust circuit drawer produces output which is very similar to the one produced by the Python drawer hence already usable, there are still known gaps in the current implementation which should be closed to improve usability. In addition, the Rust model has evolved since the initial submission of #15357 and includes operation types which are not supported currently by the Rust circuit drawer.
This issue describes the known gaps in the drawer which are worth handling for improved usability.

• 1) Add support for PauliProductMeasurement and PauliProductRotation gates. These are now part of the Rust model and should be rendered as BoxedElement::Multi visualization element.
  Add Pauli product gates support to the Rust circuit drawer #15948

• 2) Print $pi$ fractions and multiples nicely. E.g., render this:

   ┌───────┐┌────────┐┌─────────┐┌──────────┐
q: ┤ Rx(π) ├┤ Rx(2π) ├┤ Rx(π/3) ├┤ Rx(3π/2) ├
   └───────┘└────────┘└─────────┘└──────────┘

instead of this:

     ┌───────────────────────┐┌───────────────────────┐┌────────────────────────┐┌──────────────────────┐
q_0: ┤ Rx(3.141592653589793) ├┤ Rx(6.283185307179586) ├┤ Rx(1.0471975511965976) ├┤ Rx(4.71238898038469) ├
     └───────────────────────┘└───────────────────────┘└────────────────────────┘└──────────────────────┘

We should have something in Rust similar to

qiskit/qiskit/circuit/tools/pi_check.py

Line 27 in 919eaf6

def pi_check(inpt, eps=1e-9, output="text", ndigits=None):

• fix: PI format for rust Circuit display #15917

• 3) Truncate long decimal values, e.g. render the box label Rx(1.1235) instead of Rx(1.123456789).
  Format floating-point values in the Rust circuit drawer #15899

• 4) This is a minor aesthetic improvement: fix label rendering of multi-wire boxes. There are two issues: a) labels should not be left-padded with space and (handled in Add Pauli product gates support to the Rust circuit drawer #15948) b) vertical alignment should be fixed for boxes which span even number of wires. For example, render this:

     ┌───────────┐┌──────┐
q_0: ┤0          ├┤0     ├
     │  Rxx(1.2) ││      │
q_1: ┤1          ├┤      ├
     └───────────┘│  Ecr │
q_2: ─────────────┤      ├
                  │      │
q_3: ─────────────┤1     ├
                  └──────┘

instead of this:

     ┌────────────┐┌───────┐
q_0: ┤0  Rxx(1.2) ├┤0      ├
     │            ││       │
q_1: ┤1           ├┤   Ecr ├
     └────────────┘│       │
q_2: ──────────────┤       ├
                   │       │
q_3: ──────────────┤1      ├
                   └───────┘

The vertical alignment issue is being handled here: #16207

• 5) Draw measurement box like this:

┌─┐         
┤M├
└╥┘

instead of:

┌───┐
┤ M ├
└─╥─┘

Like the previous bullet, this is mostly an aesthetic improvement, but worthwhile to further visually highlight the difference between measurement instructions and e.g. standard gates.

• Make Measure box thinner in the Rust circuit drawer #16174

• 6) Add visualization of control flow elements. Although control flow construction is not exposed in the C API yet, they are already native part of the Rust model so can be natively supported in the Rust circuit drawer. It would be good to start adding control flow support in the drawer even before it is supported in the C API.
  Add control flow op support to the Rust circuit drawer #16063

---

Other known differences between the Rust circuit drawer and the Python one, are included here for completeness. These would be nice to close, but arguably they are really minor from usability perspective:

• 7) Draw CPhase gate like this:

q_0: ─■───────
      │P(0.5) 
q_1: ─■───────

instead of:

q_0: ─────■─────
     ┌────┴────┐
q_1: ┤ P(0.5)  ├
     └─────────┘

• Update the CPhase gate visualization #16074
• 8) Avoid adding auto-indices numbers for single bit registers. For example, render this:

   ┌───┐┌─┐
q: ┤ H ├┤M├
   └───┘└╥┘
c: ══════╩═

instead of this:

     ┌───┐┌───┐
q_0: ┤ H ├┤ M ├
     └───┘└─╥─┘
c_0: ═══════╩══

Note that for classical bits the above is relevant only if cregbundle=False (which is the case in this example).

• Avoid adding auto-indices numbers for single bit registers #15966

• 9) There was an issue with vertical line alignment of controlled gates whose label length are even, which was fixed in 9a00b5a. The fix was hacky in the sense that it added an extra padding space to the label string, making its length even. A better fix, i.e. without adding an extra space, would require changing the way an entire visual element padding is computed in final drawing stage, once the text width of each circuit layer is already known.

---

• Add a short description about the major visualization updates in the release notes of 2.5 (will be handled by the package maintainers).
  Add a release note for the Rust circuit drawer #16463

[OnyxBrumeSky]

Hello, I am willing to help on the fix nb 2 as it seems to be an easy one. I already sent my pr. I am available if anything needs to be changed especially on where to write my functions and my tests.

[eliarbel]

We already have 3 PRs addressing some of the points above, but they don't have release notes yet. Since the work here is not about API changes or functionality updates of any sort, I don't think we need standard reno for them. On the other hand, those PR update the way the drawer is communicating with the user so it would still be nice to at least have a short summary of the major updates in the release notes. To this end, I've added another bullet point in the task list above.

[mtreinish]

Eli Arbel (@eliarbel) do you think after closing the feature gaps outlined here we can replace the Python drawer code with the rust drawer? Or is there more work to track on that?

[eliarbel]

Matthew Treinish (@mtreinish) while the goal of this issue is to make the Rust circuit drawer more complete with respect to our Rust model representation and to improve some low‑hanging fruit usability aspects, it will not bring us to feature parity between the Rust and the Python text circuit drawers. We’re still missing things like removing idle wires, barrier/expression label limits, wire reversing/reordering and more. I think we should support more features before migrating completely to the Rust drawer. If you like to make such a migration a goal, I can open an issue to list the features we're still missing from the Python drawer so we can discuss which ones to deprecate (if any) and then track the remaining ones. Let me know.

[mtreinish]

I wasn't thinking about it until reading through the issue just now. But if we could unify the drawer implementation I think that would make things easier in the long term just from a maintenance perspective. Obviously not a high priority goal, but something we can work towards over time.