# How to generate all stabilizer states numerically?

I would like to obtain a list of all stabilizer states in the given dimension (not necessarily qubit systems). What is an efficient way of generating this list numerically?

• There is a question about how to generate 5-qubit stabilizer code, follow the link(quantumcomputing.stackexchange.com/questions/14264/…). – Yitian Wang Nov 27 '20 at 1:33
• As Hilbert space vectors or projectors? – Markus Heinrich Nov 27 '20 at 12:09
• @MarkusHeinrich Either is fine, whichever is simpler would probably be best! – Ver Nov 27 '20 at 12:12

First of all, keep in mind that there is no efficient way of generating them, simply because the number of stabiliser states increases super-exponentially with the number of qudits $$n$$ (like $$p^{O(n^2)}$$).

Second, I can only give you hints how to do it here, since a detailed treatise would take too long. Sorry. As far as I know, there is also no literature with details on implementation where I can point you to. I'm currently writing a tutorial on the stabiliser formalism which will contain more details and possibly even pseudo-code. But the pre-print will not be out before beginning of next year.

We consider stabiliser states in prime dimension $$p$$ (if you want prime-power, like e.g. for MUBs, that's similar). Define $$\omega:=\exp(2\pi i /p)$$ to be the standard choice of a primitive $$p$$-th root of unity.

## State vector representation

For $$p\neq 2$$, the Hilbert space vectors representing stabiliser states can be chosen to be of the following form (Refs. [1,2]) $$|K, q, b\rangle := \frac{1}{|K|} \sum_{x\in K} \omega^{q(x)+b\cdot x} |x\rangle,$$ where $$K\subset\mathbb F_p^n$$ is an affine subspace, $$q$$ is a quadratic form on $$K$$, and $$b\in\mathbb F_p^n$$.

For $$p=2$$, this is $$|K, q, b \rangle := \frac{1}{|K|} \sum_{x\in K} i^{b\cdot x}\omega^{q(x)} |x\rangle,$$ i.e. the linear part is computed mod 4 instead of mod 2.

Now you can enumerate all stabiliser state by iterating over all possible affine subspaces $$K$$, and then over quadratic and linear forms on $$K$$. The later part is easy once you have fixed a basis of $$K$$.

## Density matrix representation

The projectors of stabiliser states are determined by their stabiliser group $$S$$ as follows $$P(S) = \frac{1}{p^n} \sum_{g\in S} g.$$ The stabiliser groups modulo signs correspond to so-called Lagrangian subspaces $$L\subset \mathbb F_p^{2n}$$ (see my answer here). These are $$n$$-dimensional linear subspaces such that the symplectic form of any two elements vanishes (Ref. [3]): $$0 = [v,w] := \sum_{i=1}^{n} v_i w_{n+i} - w_i v_{n+i}, \quad \forall v,w\in L.$$ Any vector $$v\in L$$ correspond to a Pauli/Weyl operator $$W(v)$$ and the above equation expresses that those should commute.

Now, for $$p\neq 2$$, we can rewrite the projector as $$P(S) = P(L,h) = \frac{1}{p^n} \sum_{v\in L} \omega^{[h,v]} W(v).$$ From this representation, we can see that the coefficients of $$P(L,h)$$ in the Weyl basis (i.e. its Bloch vector) are given by $$\omega^{[h,v]}$$ if $$v\in L$$ and 0 otherwise. Hence, one can again iterate over Lagrangian subspaces $$L$$, and then over the vectors $$h$$. The latter part is again simplest in a basis of $$L$$. In the case $$p\neq 2$$, one can alternatively use the representation of stabiliser states by their Wigner functions.

For $$p= 2$$, this is a bit uglier. The coefficients here have the form $$P(S) = P(L,h) = \frac{1}{2^n} \sum_{v\in L} (-1)^{\alpha(v)} W(v).$$ For a suitable function $$\alpha$$. To compute $$\alpha$$, one can fix it arbitrarily on a basis of $$L$$ (i.e. choose signs for the generators of $$S$$) and then compute the coefficients of all other elements from that using $$\alpha(v+w) = \beta(v,w) - \alpha(v) - \alpha(w)$$ where $$\beta$$ is some horrible function determining whether you get an additional sign when you multiply two Pauli operators. Its explicit form is e.g. given in a paper of mine (Ref. [4, App. A])), but also elsewhere.

Literature

1. Dehaene, Jeroen, and Bart De Moor. “The Clifford Group, Stabilizer States, and Linear and Quadratic Operations over GF(2).” Physical Review A 68, no. 4 (2003). https://doi.org/10.1103/PhysRevA.68.042318.
2. Gross, D., and M. Van den Nest. “The LU-LC Conjecture, Diagonal Local Operations and Quadratic Forms over GF(2),” (2007). https://arxiv.org/abs/0707.4000v2.
3. Gross, D. “Hudson’s Theorem for Finite-Dimensional Quantum Systems.” Journal of Mathematical Physics 47, no. 12 (2006): 122107. https://doi.org/10.1063/1.2393152.
4. Heimendahl, Arne, Markus Heinrich, and David Gross. “The Axiomatic and the Operational Approaches to Resource Theories of Magic Do Not Coincide.” arXiv:2011.11651 (2020). http://arxiv.org/abs/2011.11651.