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I am working on trellis structures of stabilizer codes, where I construct a trellis for a given set of stabilizer generators and a given syndrome. I want to find any arbitrary error out of all the possible n-bit Pauli errors that correspond to the given syndrome. What's the computationally fastest/most efficient method for doing this?

Example:
Let's say we have the five-qubit code with generators $S=\{XZZXI,ZYYZI,IXZZX,IZYYZ\}$ and the error syndrome $s=1000$. We can tell by observation that $ZIII$ is a possible error for this syndrome. I want to implement a method to find such an error for any given stablizer set and syndrome (any error works, it does not have to be of least weight)

I am trying to implement this method from scratch (I am using Python), but I would also love to know if there are any existing modules/package that do this. Thanks.

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For an $n$ qubit system with $k$ linearly independent stabilizers, you can write out the $k\times 2n$ binary matrix that specifies the stabilizers: each row is one of the stabilizers. The first $n$ entries specify where $X$s are applied. The second $n$ entries specify where the $Z$s are applied. So, in your example case, you have $$ H=\begin{bmatrix} 1 & 0 & 0 & 1 & 0 & 0 & 1 & 1 & 0 & 0 \\ 0 & 1 & 1 & 0 & 0 & 1 & 1 & 1 & 1 & 0 \\ 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 & 1 & 0 \\ 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 1 & 1 \end{bmatrix}. $$ You're trying to find en error $e$ that satisfies $$ He=s. $$ So, what you want to do is use row operations (modulo 2) to cast $H$ into a standard form such that you can identify an entire $m\times m$ identity block (they don't have to be consecutive columns). You can always do this because the rows are linearly independent. You'll want to keep track of these row operations in a matrix $M$, i.e. $MH$ is cast into the standard form. Thus, now you're trying to solve $$ MHe=Ms. $$ The trick now is that the columns of $H$ corresponding to that identity block, you can just set the corresponding bits of $e$ equal to $Ms$.

In fact, in your example, you don't need an $M$; it's already in standard form. You're just looking at columns 1, 6, 5, 10. So, for any $s$, you're just going to set $$ \begin{bmatrix} e_1 \\ e_6 \\ e_5 \\ e_{10} \end{bmatrix}=s. $$ Thus, your error operator will look like $$ Z_1^{s_1}Z_5^{s_3}X_1^{s_2}X_5^{s_4} $$ (because in the error term, $e$, you've swapped the $X$ and $Z$ terms because $X$ stabilizers detect $Z$ errors).

In terms of coding, I often use Mathematica for this stuff because many of its functions have a built in Modulus 2 option. In python, however, you might consider the ldpc package. This has a mod2 sub-package that can do the inversion etc for you. Or, when it performs row reduction for you, it will also output the matrix $M$ that does the row reduction, which many packages don't.

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  • $\begingroup$ Thanks! I am guessing the identity block gives errors which are non-commuting with only one stabilizer, while the row reduction puts the stabilizers in a form to make that possible. Is my assessment correct? $\endgroup$
    – Enigma
    Commented Nov 7 at 10:33
  • $\begingroup$ There is also the galois Python library that makes a lot of numpy functions directly work over finite field. It is not focused on linear codes but might be interesting if you are used to numpy. $\endgroup$
    – AG47
    Commented Nov 7 at 11:02
  • $\begingroup$ @Enigma Pretty much, yes. We're redefining the choice of generators (by row reduction) so that there are single-qubit errors which each only anti-commute with one of the new generators $\endgroup$
    – DaftWullie
    Commented Nov 7 at 11:23

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