# How many $n$-qubit states are entangled?

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? 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?

• 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.
– JSdJ
Jan 13 '20 at 15:48
• – Nat
Jan 14 '20 at 5:35
• 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?
– glS
Jan 14 '20 at 19:57
• Thanks JSdJ for your help. Jan 14 '20 at 20:15
• then the answer should be that the probability is $1$: almost all states are entangled. See this answer. See also this paper
– glS
Jan 14 '20 at 20:41

Another less rigorous argument could be to observe that separable states are always infinitesimally close to entangled out: given an arbitrary separable pure state $$|\psi\rangle\otimes|\phi\rangle$$, take $$|u\rangle,|v\rangle$$ such that $$|\Psi\rangle\equiv\langle u|\psi\rangle=\langle v|\phi\rangle=0$$. 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).