Initial version of a quantum circuit editor.

Overview

Here’s the result so far:

The key detail is the two \( f(x) \) blocks. This circuit performs classical computing on the measured qubits mid-circuit to allow for additional insertion of mid-circuit gates.

Why care?

Quantum Error Correction! I’ll go more into detail later, but essentially to do QEC, we need a way to make mid circuit measurements and update the circuit for errors.

Why care about Quantum Error Correction? Quantum computers are super sensitive to noise ( see the density operator post for more info. We need Quantum Error Correction to build usable Quantum Computers.

What went well?

• The circuit is built entirely out of HTML!
  • This is a little overkill… I probably should have gone with canvas, but it’s neat that you can build something like that with just HTML. I didn’t even use z-index. Everything is explicitly checked and added. (Lots of flexbox). Technically the LaTeX might be using canvas, not sure.
• Mid-circuit measurements works! This is something my last quantum simulator didn’t have. The ability to measure a single qubit during mid execution is pretty cool (and it was easier than I thought. Just tensor some projection state between a bunch of identity matrixes)
• For building gates like CCCNOTCCC, I used to have some pretty complex algorithm, but now that I’m one quantum smarter, it turns out I can do it a little simpler by being smart about the projection operators for the conditionals.
• Adding classical functions “works”:

c1.appendFunction((classicalBits: number[]): Moment[] => {
  let out = [];
  if (classicalBits[0]) {
    out.push(new Moment(XGate, [0], [], [], 2, 2));
  }
  if (classicalBits[1]) {
    out.push(new Moment(XGate, [1], [], [], 2, 2));
  }

  return out;
});

It’s a little ugly, but I’ll clean that up in the future. Each moment is a slice in time, with the gate, the gateIndexes, controlIndexes, classicalIndexes, and then circuit size.

What didn’t go well?

• Well, it went fine. But building circuits in HTML is a little bit of a pain. I’m holding off on adding barriers and muli-wire gates because I don’t want to touch it. But I will one day. I think overall I prefer HTML to canvas (for portability and interoperability)

Future

I have an ever increasing list of todo’s. But I think the main ones are:

• Start implementing different QEC schemes! I think I have all the pieces.
• Allow the user to turn on the noise models as seen in the RB playground to simulate errors
• Implement drag and drop (another thing I know how to do, I’m just dragging my feet on since it’s a pain)
• More than like 8 qubits is pretty slow. Probably need to use one of the GPU matrix libraries instead of rolling my own matrix math.
• Deploy!

Hopefully within a week or two people can use the circuit build to play with different error correction schemes!