# A question of separability and entanglement

• A
• andresB
andresB
I was reading a review on single-particle entanglement https://onlinelibrary.wiley.com/doi/10.1002/qute.202000014 and the author says this

It's probably a simple/well known result, but I'm not seeing any trivial proof at the moment. Moreover, I'm interested in the general case of arbitrary number of subsystems where the Hilbert space of the subsystems can be of any dimension (including a continuous degrees of freedom).

So, it's the result true in general for any composite system?

andresB said:
t's the result true in general for any composite system?
Yes. It follows immediately from the fact that an entangled state, by definition, cannot be expressed as a single term of the form ##\psi_1 \otimes \psi_2##, where ##\psi_1## and ##\psi_2## are states in the Hilbert spaces of subsystem 1 and subsystem 2.

Here's a proof of the result - it actually seems to require that eigenstates of the observable are non-degenerate.

Let ##\mathcal{H}_A## and ##\mathcal{H}_B## be two Hilbert spaces, and let ##\hat{\mathcal O}_A## be a Hermitian operator on ##\mathcal{H}_A## representing the observable.

By the spectral theorem, let ##\{ \ket{ \varphi_A^{(i)} } \}_i## be a basis for ##\mathcal{H}_A## which has eigenvalues ##\hat{\mathcal O}_A \ket{ \varphi_A^{(i)} } = \lambda_A^{(i)} \ket{ \varphi_A^{(i)} }##. Also let ##\{ \ket{ \varphi_B^{(j)} } \}_j## be a basis for ##\mathcal{H}_B##. Then, a possible basis for the Hilbert space ##\mathcal{H} = \mathcal{H}_A \otimes \mathcal{H}_B## is ##\{ \ket{ \varphi_A^{(i)} }\ket{ \varphi_B^{(j)} } \}_{ij}##.

To prove the theorem, consider a general state ##\ket{\psi} \in \mathcal{H} ##. We will show that if ##\ket{\psi}## has a definite eigenstate under ##\hat{\mathcal O}_A##, then it must be possible to factorize ##\ket{\psi}## across the two Hilbert spaces, meaning that it is not entangled.

Using the basis for ##\mathcal H##, we can represent ##\ket\psi## as$$\ket\psi = \sum_{ij} c_{ij} \ket{ \varphi_A^{(i)} }\ket{ \varphi_B^{(j)} }$$ Then, suppose that ##\hat{\mathcal O}_A \otimes \hat{I}_B \ket\psi = \mu \ket\psi##. Substituting into the previous equation yields$$\hat{\mathcal O}_A \otimes \hat{I}_B \sum_{ij} c_{ij} \ket{ \varphi_A^{(i)} }\ket{ \varphi_B^{(j)} } = \mu \sum_{ij} c_{ij} \ket{ \varphi_A^{(i)} }\ket{ \varphi_B^{(j)} }$$Applying the operators to the LHS results in$$\sum_{ij} c_{ij} \lambda_A^{(i)} \ket{ \varphi_A^{(i)} }\ket{ \varphi_B^{(j)} } = \mu \sum_{ij} c_{ij} \ket{ \varphi_A^{(i)} }\ket{ \varphi_B^{(j)} }$$ Premultiplying by ##\bra{ \varphi_A^{(i')} } \bra{ \varphi_B^{(j')} }## on both sides yields
$$c_{i'j'} \lambda_A^{(i')} = c_{i'j'} \mu$$ If ##c_{i'j'} \neq 0##, then we conclude that ##\lambda_A^{(i')} = \mu##. But hang on - we could say this is for any value of ##i'##. This means we have a contradiction if ##c_{i'j'}## and ##c_{i''j'}## are both nonzero for distinct eigenvalues ##\lambda_A^{(i')}## and ##\lambda_A^{(i'')}##.

So, if we further suppose that all the eigenvalues ##\lambda_A^{(i)}## are distinct, then we can only have one value of ##i## for which ##c_{ij}## is nonzero, proving that the state is not entangled.

(To be honest, that proof makes the result look more complicated than it really is... I think the easiest way to see the result is just to consider the case for 2-dimensional Hilbert spaces.)

andresB
Jonomyster said:
it actually seems to require that eigenstates of the observable are non-degenerate.
Yes, I believe that's true. I forgot to mention that qualifier in post #2.\

I think there's an analogous result for the degenerate case, but its proof is more complicated.

Jonomyster said:
Here's a proof of the result - it actually seems to require that eigenstates of the observable are non-degenerate.

Ok, got it. But what about wavefunctions?

For example, given a wavefunction ##\psi(x,y)## that can be separated by change of variables
##X =X(x,y)## and ##Y =Y(x,y)## so that
$$\psi(X,Y)=\psi_{1}(X)\psi_{2}(Y),$$
can we say that this imply that there exist self-adjoint operators with ##\psi_{1}## and ##\psi_{2}## as eigenfunctions?

The above is the kind of things we see in, for example, the coupled Harmonic oscillators.

andresB said:
Wave functions are just one particular representation of quantum states. The proof that was given does not assume any particular representation; it just uses general properties of quantum states as vectors in a Hilbert space.

andresB said:
continuous degrees of freedom?
It looks to me like a similar proof using integrals instead of sums in order to be applicable to continuous degrees of freedom should be straightforward. Note that you can write kets for states of quantum systems with continuous degrees of freedom; they're still vectors in a Hilbert space and kets are notation for vectors in a Hilbert space. The difference is that the index on the basis kets now needs to be continuous and you need to do integrals instead of sums.

Jonomyster
PeterDonis said:
It looks to me like a similar proof using integrals instead of sums in order to be applicable to continuous degrees of freedom should be straightforward.
I think it's worth seeing this written out for a simple example. Consider a single particle which can move along the real number line. The corresponding Hilbert space has a basis ##\{ \ket{x} : x \in \mathbb{R} \} ##. Here, the elements ##\ket x##, ##\ket y##, etc. represent the states "particle at ##x##", "particle at ##y##", etc. For continuous systems, we can use ##\delta##-function normalisation:
$$\braket{x | y} = \delta(x-y)$$Then, a general state ##\ket\psi## can be represented as a superposition of these position states. That is,
$$\ket\psi = \int_{\mathbb{R}} \psi(x') \ket{x'} dx'$$ Notice what happens when we take the inner product between ##\ket\psi## and another state ##\ket x##:
\begin{aligned} \braket{x | \psi} &= \int_{\mathbb{R}} \psi(x') \braket{x | x'} dx' \\[.8em] &= \int_{\mathbb{R}} \psi(x') \delta(x-x') dx' \\[.8em] &= \psi(x) \end{aligned} So from this perspective, the wavefunction ##\psi(x)## can be thought of as the amount of amplitude the state ##\ket\psi## has at position ##x##.

Jonomyster said:
The corresponding Hilbert space has a basis ##\{ \ket{x} : x \in \mathbb{R} \} ##.
Actually, no, those kets are not in the Hilbert space. In wave function terms, they are delta functions, and delta functions are not square integrable. To do the proof you are looking for using position eigenstates (or momentum eigenstates, for that matter) as a basis, you would need to use something like the rigged Hilbert space formalism that is described in, e.g., Ballentine.

PeterDonis said:
Actually, no, those kets are not in the Hilbert space. In wave function terms, they are delta functions, and delta functions are not square integrable. To do the proof you are looking for using position eigenstates (or momentum eigenstates, for that matter) as a basis, you would need to use something like the rigged Hilbert space formalism that is described in, e.g., Ballentine.
Didn't realise that, thanks!

PeterDonis said:
Wave functions are just one particular representation of quantum states. The proof that was given does not assume any particular representation; it just uses general properties of quantum states as vectors in a Hilbert space.

It looks to me like a similar proof using integrals instead of sums in order to be applicable to continuous degrees of freedom should be straightforward. Note that you can write kets for states of quantum systems with continuous degrees of freedom; they're still vectors in a Hilbert space and kets are notation for vectors in a Hilbert space. The difference is that the index on the basis kets now needs to be continuous and you need to do integrals instead of sums.

I don't think is that simple since self-adjointness is a much more tricky concept in the infinite-diemsional case.

For example, the eigenstates of the hydrogen atom separates as ##\psi=\psi_{CM}\psi_{Rel}## for the center of mass and the relative motion of the proton-electron. ##\psi_{CM}## is given by plane waves that are not square-integrable so they do not belong to a Hilbert space. In this case, though, we can find a self-adjoint operator, the momentum of the center of mass, that have ##\psi_{CM}## as eigenfunctions.

But how to be sure in general that we can always find suitable self-adjoint operators for any decomposition of a wavefunction?

andresB said:
how to be sure in general that we can always find suitable self-adjoint operators for any decomposition of a wavefunction?
If we can't, then the question in the OP can't even be posed. The question assumes that a self-adjoint operator exists representing an observable on a particular subsystem (tensored with the identity on other subsystems), and asks whether an entangled state of the system can be an eigenstate of that operator. If there is no such operator, the question isn't even well-defined.

To put this another way: the question can be phrased as, is there a self-adjoint operator representing an observable on a subsystem, for which an entangled state of the overall system is an eigenstate? The answer will always be "no", but it could be for one of two reasons: either there are self-adjoint operators representing observables on subsystems, but an entangled state of the system is not an eigenstate of any of them (that's the case addressed by the proofs discussed in this thread); or there are no self-adjoint operators at all on any of the subsystems in the particular decomposition of the system we are looking at. In the latter case the "no" answer is trivial: if we can't even find any self-adjoint operators in the first place, we can't even get to the point of asking whether an entangled state can be an eigenstate of such an operator.

andresB said:
I don't think is that simple since self-adjointness is a much more tricky concept in the infinite-diemsional case.

For example, the eigenstates of the hydrogen atom separates as ##\psi=\psi_{CM}\psi_{Rel}## for the center of mass and the relative motion of the proton-electron. ##\psi_{CM}## is given by plane waves that are not square-integrable so they do not belong to a Hilbert space. In this case, though, we can find a self-adjoint operator, the momentum of the center of mass, that have ##\psi_{CM}## as eigenfunctions.

But how to be sure in general that we can always find suitable self-adjoint operators for any decomposition of a wavefunction?
I haven't thought about your question deeply enough to answer it, but have you looked at Hall's book Quantum Theory For Mathematicians? He explores the nature and existence of self-adjoint operators is infinite dimensional spaces quite thoroughly.

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