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improve documentation
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Fe-r-oz committed Dec 1, 2024
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Expand Up @@ -22,9 +22,21 @@ time. Vardy [vardy1997intractability](@cite) demonstrated that computing the min
distance is *NP-hard*, and the corresponding decision problem is *NP-complete*,
making polynomial-time algorithms unlikely.
In the case of *quantum* codes, classical intuition does not always apply. For instance,
the [[7, 1, 3]] Steane code has a minimum distance of three, even though all its elements
have weight four [Sabo:2022smk](@cite).
For *quantum* codes, classical intuition does not always apply. The minimum distance
is given by the minimum weight of a non-trivial logical operator. This is generally
unrelated to the minimum distance of the corresponding stabilizer code when viewed as
a classical, additive code. White and Grassl [white2006new](@cite) proposed mapping
quantum codes to higher-dimensional classical linear codes. This mapping allows the
minimum distance of the quantum additive code to be inferred from that of the classical
linear code but increases parameters from `n` to `3n` and `d` to `2d`, adding complexity.
Furthermore, once a minimal weight vector is identified, it is essential to verify
whether it belongs to the Pauli group `𝒫ₙ` over `n` qubits [Sabo:2022smk](@cite).
Additionally, to illustrate how classical intuition can be misleading in this context,
consider that the [[7, 1, 3]] Steane code has a minimum distance of three, despite all
its elements having a weight of four. This discrepancy occurs because stabilizer codes
are defined by parity-check matrices, while their minimum distances are determined by
the dual [Sabo:2022smk](@cite).
```jldoctest example
julia> using QuantumClifford.ECC: Steane7, distance;
Expand All @@ -47,32 +59,6 @@ julia> distance(Steane7())
3
```
Such discrepancies arise because stabilizer codes are defined by parity-check matrices,
but their minimum distances are determined by the dual, specifically the minimum weight
of non-trivial logical operators.
```jldoctest example
julia> lx = stab_to_gf2(logicalzview(canonicalize!(MixedDestabilizer(c))))
1×14 Matrix{Bool}:
0 0 1 0 1 1 0 0 0 0 0 0 0 0
julia> sum(lx)
3
julia> lz = stab_to_gf2(logicalzview(canonicalize!(MixedDestabilizer(c))))
1×14 Matrix{Bool}:
0 0 0 0 0 0 0 0 1 0 1 0 1 0
julia> sum(lz)
3
```
Brute-force methods remain viable but inefficient. White and Grassl [white2006new](@cite)
proposed mapping quantum codes to higher-dimensional classical linear codes. This mapping
allows the minimum distance of the quantum additive code to be inferred from that of the
classical linear code but increases parameters from `n` to `3n` and `d` to `2d`, adding
complexity.
The minimum distance problem for quantum codes is *NP-hard*, and this hardness extends
to multiplicative and additive approximations, even when restricted to stabilizer or
CSS codes, with the result established through a reduction from classical problems in
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