My question is a bit broad, but my concern is mainly on understand the ratio between the number of possible linear combinations that can be decomposed in a direct product of states and the number of possible entangled states in that system. The system is a just an $n$-qubits set, as an usual vector space, with the assumption that all the states in a certain superposition can be expected as equally probable. As an example to clarify what I mean take:

$$ \left| \psi \right\rangle = \frac{\sqrt{2}}{2} (\left| 00 \right\rangle + \left| 11 \right\rangle) $$

I know that the ratio in a 2-qubits system should be 50%, but what about $n=100$?

To improve the clarity of the question I would ask, how do you interpret this kind of graph?Product = a state that can be decomposed uniquely by a tensorr product. Entangled = it's own defition.

Where Product = a state that can be decomposed uniquely by a tensor product. Entangled = it's own defition. Is this a true picture of what's going on in an $n$-qubit system or not?

  • $\begingroup$ As far as I am aware you can not import the physics package (which includes the bra-ket commands) (or any other package, for that matter). However, the commands \rangle and \langle will work as a substitute. $\endgroup$
    – JSdJ
    Jan 13, 2020 at 15:48
  • $\begingroup$ Here's the tutorial. $\endgroup$
    – Nat
    Jan 14, 2020 at 5:35
  • $\begingroup$ I don't understand the question. Are you asking what is the probability of finding an entangled state when sampling random pure $n$-qubit pure states? Or the probability of finding an entangled state when sampling among the $n$-qubit pure balanced states? Or something else? $\endgroup$
    – glS
    Jan 14, 2020 at 19:57
  • $\begingroup$ Thanks JSdJ for your help. $\endgroup$
    – Kamui9610
    Jan 14, 2020 at 20:15
  • 1
    $\begingroup$ then the answer should be that the probability is $1$: almost all states are entangled. See this answer. See also this paper $\endgroup$
    – glS
    Jan 14, 2020 at 20:41

1 Answer 1


The probability of finding an entangled state while randomly sampling (from the uniform distribution) over pure states is one: almost all pure states are entangled.

This is shown in this answer on physics.SE. This paper might also be of interest.

The gist is that separable states correspond to rank-1 matrices, which have zero measure in the set of all matrices. This is also discussed in this post on math.SE.

Another less rigorous argument could be to observe that separable states are always infinitesimally close to entangled out: given an arbitrary separable pure state $|\Psi\rangle\equiv|\psi\rangle\otimes|\phi\rangle$, take $|u\rangle,|v\rangle$ such that $\langle u|\psi\rangle=\langle v|\phi\rangle=0$. Then, the state $$\frac{1}{\sqrt{1+\epsilon^2}}\left[|\Psi\rangle+\epsilon(|u\rangle\otimes|v\rangle)\right]$$ is entangled for all $\epsilon>0$ and can be made to be infinitesimally close to $|\Psi\rangle$. This hints towards the set of separable states having empty interior (although technically, having empty interior is not in itself enough to prove having zero measure, see e.g. this blog post).


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