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In subsystem codes, it is well-known that we can extract syndromes by measuring some gauge operators, instead of measuring stabilizers. For example, if a stabilizer $s$ is a product of gauge operators $g_1, g_2,$ and $g_3$, i.e. $s=g_1g_2g_3$, we can obtain the eigenvalue of $s$ by measuring $g_1, g_2,$ and $g_3$ and taking the parities of them. I understand this method.

Although the method above works just for extracting syndrome once, if we measure gauge operators, they become stabilizers. It corresponds to gauge fixing. Thus, in the situations where we need to repeat syndrome extraction, it seems that the code changes before we first extract syndrome and after that. If $s$ is the stabilizer generator before we measure it, stabilizer generators becomes $s, g_1, g_2, g_3$ after we extract the syndrome.

The question is that, is my understanding above true? If so, does the way of implementing logical gates change after we extract syndrome? Also, is it true that in the first round of syndrome extraction, we use only the value of $s$ in decoding, but after that, we use the values of the three of $s, g_1, g_2, g_3$ (because three of them are stabilzer generatos)?

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    $\begingroup$ "If we measure gauge operators, they become stabilizers" This is true only if the gauge operators commute. In general, they don't commute. To get $s$, you measure $g_1,g_2,g_3$. Then to get a stabilizer $s'$ you measure $g_1',g_2',g_3'$. These primed gauge operators don't commute with the unprimed gauge operators, so now $g_1,g_2,g_3$ are no longer stabilizers. To measure $s$ again, you measure $g_1,g_2,g_3$ and you have no idea what outcome you get (although you still know they should multiply to whatever you got before, in the absence of errors. $\endgroup$ Commented Jul 31, 2024 at 20:10

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If you take a subsystem code, the stabilizer generators are defined as $$S=C(G)\cap G$$ where $G$ is the gauge group. It's the commuting elements of the gauge group. Now this could also be a product of elements in the group as you stated, $$s_1=g_1 g_2g_3$$ where $s_1$ is some stabilizer and $g_1, g_2$ and $g_3$ are elements of the gauge group that commute with one another. By measuring them, you fix the gauge into a new code $C_1$ that now has as it's set of generators those 3 gauge operators. The stabilizers of $S$ remain as stabilizers.

But if you now make the choice to measure 3 new gauge operators which do not commute with the previous 3, $g_1', g_2'$ and $g_3'$, then those previous operators are removed from the stabilizer group, these 3 are added, with random outcomes as they anticommute, and you are now in a new gauge, with new stabilizers, giving another code $C_2$. $s_1$ is still a stabilizer of this code. You can still measure it by measuring the 3 original operators, and the product of their outcomes is still determinstic as their product is a stabilizer of the code. However the individual outcomes are still non-deterministic, and you've now removed $g_1', g_2'$ and $g_3'$ as stabilizers from the code.

So while you can measure gauge operators to measure stabilizers of the subsystem code, and these outcomes are deterministic when their product is taken, the gauge operators will be removed from the stabilizers if you need to measure another element of the subsystem generators whose constituent gauge operators do not all commute with the previous set you measured.

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