NVIDIA / NVIDIA/cuda-quantum

unitary operator for x.ctrl uses different ordering of qubits in the`register_operation` and the unitary matrix listed in docs is incorrect, also uses inconsistent ordering

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Description

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Describe the bug

CUDA-Q uses the ordering 00, 10, 01, 11 when printing out the amplitudes using the get_state command. Therefore the unitary matrix for the x.ctrl should be:
$$\begin{pmatrix} 1 & 0 & 0 & 0 \ 0 & 0 & 0 & 1 \ 0 & 0 & 1 & 0 \ 0 & 1 & 0 & 0 \end{pmatrix}$$

The ordering is displayed in docs instead (see screenshot).
Screenshot 2024-08-23 at 12 53 25 PM

The CNOT matrix as shown in the screenshot applied to | + 0 > = [ 1 0 0 1] would output [1 0 1 0] which is | 0 0 > + | 0 1 > which is incorrect for CNOT.
(amplitudes suppressed)

Steps to reproduce the bug

view webpage and carry out the matrix multiplication to see that the unitary is incorrect when applied to the state ()

Expected behavior

ordering of qubits preserved throughout the exposition

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment
  • CUDA Quantum version: any
  • Python version:
  • C++ compiler:
  • Operating system:
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First steps

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  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the default_ops.html page and compare its x.ctrl matrix with the ordering used by get_state and register_operation. Check the CNOT example against the stated convention, then make the relevant documentation or operation definition consistent and verify the matrix multiplication.

Written by the indexing model from the issue text.

Assessment

Tech stack
cpp
Domain
documentation, quantum-computing
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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