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I have a CSS code defined through binary matrices $H_X$ and $H_Z$. I also have the logicals $L_X$ and $L_Z$ as binary matrices. I decode $H_X$ and $H_Z$ independently as classical codes. The decoder outputs the corrected codewords (from which I can calculate the residual errors). How do I calculate whether the residual error results in a logical error or not? It should be a (simple?) operation involving $L_X$ and $L_Z$ but I'm not clear on the details.

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  • $\begingroup$ Do you mean $L_X$ and $L_Z$ as binary vectors? $\endgroup$
    – ChrisD
    Commented Sep 28, 2023 at 4:20
  • $\begingroup$ @ChrisD they would be vectors if the code has k=1; otherwise each is a $k \times n$ matrix for an $[[n,k,d]]$ code. $\endgroup$
    – unknown
    Commented Sep 28, 2023 at 15:26
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    $\begingroup$ Ok. A logical error is just when the residual error applies a logical operation. So you could just check whether it is a non-trivial linear combination of the $L_X$ ($L_Z$). Or alternatively whether it anti-commutes with any of the $L_Z$ ($L_X$), for a CSS code this is equivalent to whether $l_Z \cdot r = 1$ for any row $l_Z$ of $L_Z$. $\endgroup$
    – ChrisD
    Commented Sep 29, 2023 at 0:07
  • $\begingroup$ @ChrisD I tried this before posting the question but the simulation results didn't seem right...now it's possible there's a simulation bug somewhere. At any rate if you have a reference for the the above then please post it as an answer; if things checkout I'll be happy to accept. $\endgroup$
    – unknown
    Commented Sep 29, 2023 at 2:55

1 Answer 1

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We have a stabilizer code $C(S)$, formed from stabilizer group $S$, with a set of stabilizer generators $\{S_i\}_i$, and a set of logical operators representatives $\{\bar{X}_i\}_i \cup \{\bar{Z}_i\}_i$.

We know that the logical operators all commute with the stabilizers. We can check this by confirming that $$ [\bar{X}_i, S_j] = 0, \quad\text{for all } i,j, \\ [\bar{Z}_i, S_j] = 0, \quad\text{for all } i,j. $$

At the same time not all logical operators commute with each other (see this Q&A). We have the conditions that $$ [\bar{X}_i, \bar{Z}_i] \ne 0, \quad\text{for all } i,\\ [\bar{X}_i, \bar{X}_j] = 0, \quad\text{for all } i,j,\\ [\bar{Z}_i, \bar{Z}_j] = 0, \quad\text{for all } i,j. $$

Now, our encoded code state $\psi\rangle$ encountered an error $E$, and the decoder provided recovery operation $R$, which leads to what you call the residual error $RE$. Assume that $RE$ brings the state back inside the code, i.e. $RE|\bar\psi\rangle \in C$. Then there are two possibilities,

  • Either $RE \in S$ (successful recovery), or
  • $RE = \bar{A}_i$ for some $A \in \{X,Z\}$ and $i$ (logical error).

To check which case occurred, we can use the second set of commutation relation above. We evaluate $$ [RE, \bar{A}_i] \text{ for all } A \in \{X, Y\}, i. $$

If $[RE, \bar{X}_j] \ne 0$, then that means that $RE = \bar{Z}_j$. Similarly, if $[RE, \bar{Z}_j] \ne 0$, then $RE = \bar{X}_i$.

Checks in the binary vector representation

Suppose the code in question is a $[[n,k,d]]$ code with $m=n-k$ stabilizer generators. Let there be $m_X$ pure-$X$ generators and $m_Z$ pure-$Z$ generators, such that $m_X + m_Z = m$.

We will represent elements of the Pauli group $\mathcal{P}_n$ by binary vectors of length $2n$, with the first $n$ bits acting as the $X$-part and the rest for the $Z$-part. Let us use small letters to label them.

In this picture, the $k$ logical $X$ operators $\{\bar{X}_i\}$ are represented by a $k \times 2n$ matrix $L^X$. The $i$th operator will the $i$th row of the matrix $L^X_i$. Similarly, for the logical $Z$ operators. You have not mentioned in your question if the logical operators of your code are pure $X$ or $Z$. So, I will not assume this to be so.

The error is vector $e$ and recovery operation (provided by the decoder) is vector $r$. The residual error is $f = r+e$, where the addition is modulo 2.

The check $[RE, \bar{X}_j]$, in the binary vector picture translates to the symplectic inner product $f \cdot L^X_j$.

Similarly, the check $[RE, \bar{Z}_j]$ translates to $f\cdot L^Z_j$.

That is all. For checking if the recovery made a logical error, you only need to evaluate these $2k$ symplectic inner products. If your logical operators are pure $X$ and pure $Z$ types, then the symplectic inner products will naturally simplify to normal inner products.

What if the decoder is faulty

The decoder could return a recovery operation $R$ such that $RE|\bar\psi\rangle \not\in C$. To check this, you should verify that $$ [RE,S_i] = 0 \quad \text{ for all } i. $$

In the binary vector picture you will first have to expand $H^X$ to a $m_X \times 2n$ matrix. Similarly $H^Z$. Then $[RE,S_i]$ translates to computing the symplectic inner product $f\cdot s_i$.

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  • $\begingroup$ All this looks reasonable but can you translate the process in terms of $H_X,H_Z,L_X,L_Z$. To simplify things you can consider $H_X$ and $L_Z$ only since I'm decoding the two classical codes independently. Are you saying that just calculate the "syndrome" of the residual error $r$ and $L_Z$? that is a logical error is when $r \cdot L_Z \neq 0$? Is $r \cdot H_X$ relevant? either way a reference would make things complete especially if it works with matrices, dot products, ... $\endgroup$
    – unknown
    Commented Sep 29, 2023 at 18:49
  • $\begingroup$ I have added more details. Hopefully this clarifies more. I have not particularly discussed how things simplify if you are decoding the two classical codes separately. Need too much notation for it. But roughly, if your logical operators are pure $X$ or $Z$, you can determine the residual $X$ error from the physical $X$ errors and the $X$ part of the recovery. If that is $f_X$, then indeed $f_X \cdot L^Z_i \ne 0$ will tell you if a logical error has occurred. $\endgroup$ Commented Sep 30, 2023 at 4:58
  • $\begingroup$ I appreciate the added details, but the case of the "decoder is faulty" isn't clear. If $f \cdot s_i \neq 0$ then this is not considered a logical error?. To keep things simple you can just work with $H_X, L_Z$ and assume the error is a $Z$ error; no need to go to Pauli operators or symplectic products $\endgroup$
    – unknown
    Commented Oct 2, 2023 at 19:55
  • $\begingroup$ I call a decoder-faulty error one in which the recovery operation fails to return the state back to the Code. I call a logical error one in which the state is returned back to the Code, but perturbed by some logical operation on the qubits. Though I think that papers call the union of the two the logical error rate. $\endgroup$ Commented Oct 2, 2023 at 21:53
  • $\begingroup$ this ambiguity is what motivated the question. I see "logical error rate" plots everywhere in papers but I can't find a definitive reference where it's exactly defined $\endgroup$
    – unknown
    Commented Oct 3, 2023 at 0:08

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