An $ n $ qudit code always has distance bounded above by $ d \leq \frac{n+1}{2} $ (Edit: the correct bound appears to be $ d \leq \frac{n}{2}+1 $ but the $ [[6,0,4]] $ hexacode and the $ [[2,0,2]] $ Bell state are the only qubit codes that violates the tighter bound $ d \leq \frac{n+1}{2} $. Indeed even the even tighter bound $ d \leq \frac{n}{2} $ appears to be only violated by a few codes namely $ [[6,0,4]], [[5,1,3]], [[3,0,2]],[[2,0,2]] $). The $ 5 $ qudit generalization of the Knill-Laflamme code with parameters $ [[5,1,3]]_q $ ( $ q $ denoting the size of the qudit) has stabilizer generators \begin{align*} & XZZ^\dagger X^\dagger I \\ & IXZZ^\dagger X^\dagger \\ & X^\dagger IXZZ^\dagger \\ & Z^\dagger X^\dagger IXZ \end{align*} and for any $ q $ this code saturates the bound $ d \leq \frac{n+1}{2} $ since $$ 3 = \frac{5+1}{2} $$ The three qutrit code with parameters $ [[3,1,2]]_3 $ also saturates this bound $$ 2 = \frac{3+1}{2} $$ Are there any other qudit codes that saturate this bound? Or are the $ [[5,1,3]]_q $ and $ [[3,1,2]]_3 $ codes essentially unique in this respect?
2 Answers
The bound you are referring to is a special case of the quantum Singleton bound. This bound says that given a code $((n,K,d))_q$ we have $$ K \leq q^{n-2(d-1)}. $$ In the special case that we encode $k$-qu$q$its into $n$-qu$q$its (i.e., $K = q^k$) we have $$ d \leq \frac{n-k+2}{2}. $$ Thus the bound you provided is a special case of the quantum singleton bound when $k = 1$.
A code that satisfies the quantum Singleton bound with equality is called a quantum maximum-distance-separable code (or quantum MDS code). So your question is basically asking which MDS codes exist.
Qubit $q=2$ MDS codes are completely classified and there aren't that many of them. They are $[[6,0,4]]_2$, $[[5,1,3]]_2$, and the even length codes $[[2m,2m-2,2]]_2$ (also the trivial codes $[[n,n,1]]_2$ but those are sort of fake). For higher $q$, there are many more MDS codes, for example see Quantum MDS Codes over Small Fields .
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Thinking about your first question regarding qubit codes, another bound that we know must apply (at least for non-degenerate codes) is the Quantum Hamming Bound, $$ 2^{n-k}\geq\sum_{w=0}^{t}3^w\binom{n}{w}, $$ assuming that $d=2t+1$ is odd. We make this as loose as possible by setting $k=1$, and we take $d=(n+1)/2$ (and therefore $n=4t+1$). So now we can ask for what values of $n$ is the Hamming bound satisfied? We can explicitly check for small $n$: only $n=5,9$ work. For large enough $n$ (I didn't do this carefully, but $n\geq 21$ seemed to be about right), you can inductively prove that if the Hamming bound isn't satisfied for $n$, then it isn't for $n+4$ either.
Now, we know that $n=5$ is a valid solution, so what about $n=9$? According to codetables.de, the best code $[[9,1,d]]$ has $d=3$, not $d=5$. Thus, the 5-qubit code is the only odd distance non-degenerate qubit code satisfying that bound.
I've emphasised that I've put in a non-degenerate assumption. I don't think that's too big an assumption - it's tough to find examples of degenerate codes that beat the Hamming bound and, even then, the only ones I'm aware of do so by a very small amount. So, I suspect this conclusion still holds when you include degenerate codes.
I assume (without having tried) that you could construct something similar for $q>2$.
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