Struggling to understand Identical particles

Question

I'm struggling with the concept of identical particles in QM. Say I have two electrons, with one trapped on my left $\left|\uparrow\right\rangle_1$ and one trapped on my right $\left|\downarrow\right\rangle_2$. The 2-particle wavefunction is then clearly $\left|\uparrow\right\rangle_1 \otimes \left|\downarrow\right\rangle_2$. If an exchange happens, clearly the wavefunction would be $\left|\downarrow\right\rangle_1 \otimes \left|\uparrow\right\rangle_2$, which is totally different and I can detect this exchange by measuring in the appropriate basis. The 2-particle system does not seem to violate any law of QM to me, and I can do all kind of operations on them including entanglement.

Yet, why is it that when I open QM textbook like Griffiths and Sakurai etc. in the Identical Particles chapters, they always say that this is impossible, that in order to construct a many-particle wavefunction, one must know that

God doesn't know which is which, because there is no such thing as "this" electron, or "that" electron

and

we (can't) follow the trajectory because that would entail a position measurement at each instant of time, which necessarily disturbs the system

I mean, am I tracking the electrons if I trap them on 2 sides of my table? Even if I am tracking the electron, what is the bad thing about it that makes my many-particle state $\left|\uparrow\right\rangle_1 \otimes \left|\downarrow\right\rangle_2$ invalid to QM? What am I misunderstanding?

I think what you're looking for is the concept of "Effectively distinguishable particles". — Tobias Fünke, Dec 27 '22 at 19:07
Are the 1 and 2 subscripts supposed to denote position, or are they meaningless labels? This is a crucial point. If they are positions, then several current answers are wrong. — benrg, Dec 28 '22 at 19:43
I think I got the point. The 1 and 2 are Hilbert spaces, and the exchange operators switch these Hilbert spaces (or the order of the tensor product), meaning effectively it'd switch all properties (left/right, up/down) were the particles have an identity associated to these label/Hilbert spaces. — Kim Dong, Dec 29 '22 at 21:27
I am unsure but > No-cloning_theorem at wikipedia (maybe can help someone here) ""In physics, the no-cloning theorem states that it is impossible to create an independent and identical copy of an arbitrary unknown quantum state,"" (..) — William Martens, Jan 01 '23 at 23:34

Andrew · Accepted Answer · 2022-12-28T19:47:08.327

12

The state of the electrons in the situation you describe is actually not $|\uparrow, {\rm left}\rangle_1 \otimes |\downarrow, \rm{right}\rangle_2$. It is instead \begin{equation} |\Psi\rangle = \frac{1}{\sqrt{2}}\Big(|\uparrow, {\rm left}\rangle_1 \otimes |\downarrow, {\rm right} \rangle_2 - |\downarrow, {\rm right} \rangle_1 \otimes |\uparrow, {\rm left} \rangle_2 \Big) \end{equation} where the label "left" or "right" refers to the spatial location of the particle, and the indices $1$ and $2$ refer to the "identity" of the particle. In particular, you cannot say whether electron 1 or electron 2 is the one that is trapped at your left, with spin up.

This may feel surprising and counterintuitive. However, if you think through the experimental setup carefully, you will find there is no way for you to say whether electron 1 or 2 is "really" the one in the left trap. There is no way for you to "mark" either of the two electrons.

If it helps, you can view indistinguishable particles as a mathematical possibility that quantum mechanics allows for. Maybe you don't intuitively believe electrons must be indistinguishable, but you have to accept that quantum mechanics logically allows indistinguishable particles. Indistinguishable particles live on a subset of Hilbert space where the state is fully symmetric or fully antisymmetric under exchange of particles.

It then becomes an experimental question about whether real-world electrons are described by the quantum mechanics distinguishable or indistinguishable particles. Electrons exhibit Fermi-Dirac statistics, a fact that forms the basis of modern condensed-matter theory, and therefore in some sense is experimentally tested every time you use a conductor or insulator, or every time a material scientist designs electronic properties of a material. So as an experimental question, it is settled that electrons behave as indistinguishable fermions.

The fact that electrons are indistinguishable fermions is also understood theoretically using relativistic quantum field theory.

edited Dec 28 '22 at 19:47

answered Dec 27 '22 at 19:08

Andrew

48,573

1

I am very much struggling with this... Isn't the $S_z$ operator commute with $H$, therefore if I measure it once it should technically stay the same unless if I evolve or measure it in some noncommuting basis? How is it that after I measure and make sure that it is $|\uparrow\rangle_1 \otimes |\downarrow\rangle_2$, it actually isn't that anymore but the one you given above? – Kim Dong Dec 27 '22 at 19:13
1

@KimDong You can measure the spin of the left particle, but not of particle #1. In the state $\Psi\rangle$ in my answer, there are a superposition of two terms. In the first term, $|\uparrow\rangle_1 \otimes | \downarrow\rangle_2$, particle #1 is measured on the left with spin up. In the second term, $|\downarrow\rangle_1\otimes | \uparrow\rangle_2$, particle #2 is measured on the left with spin up. That is an eigenstate of the $S_z$ operator since there is definite spin for the particle on the left. You just can't say whether particle 1 or 2 is the one with spin up. – Andrew Dec 27 '22 at 19:23
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@Andrew Put differently, it makes no sense, a priori, to speak of "particle 1" and "particle 2". However, if you can effectively distinguish them, then this can make sense - and whether you call the particle "left particle" or "particle 1" does not make a difference then: You can, effectively, label it. See also the references I commented under the question (and the references therein). – Tobias Fünke Dec 27 '22 at 19:26
Hmm, I think I'm improving, but not quite there yet. Then what is the "qubit 0" and "qubit 1" that we encounter in quantum computing, which is often made up of electron or photon? I mean, clearly, I can go to the IBMQ or Xanadu, and prepare q0 at 1, and q1 at 0, and they say it'd be $|01\rangle$. But as you said, isn't it actually $|01\rangle-|10\rangle$ if it's made of ions or electron, and something else if it's photon? – Kim Dong Dec 27 '22 at 19:33
2

@KimDong Ah that's interesting. I am not an expert with quantum computing, but I think it is possible that they suppress this subtlety to simplify descriptions of quantum algorithms. It is true that in some situations, you can treat electrons as if they were distinguishable and get away with it. (For example, we don't antisymmetrize the wavefunction over every electron in the Universe even though technically we should). However, that would only be an approximation that would work in some cases; in general you need to account for the Fermi-Dirac statistics to describe electron behavior. – Andrew Dec 27 '22 at 19:41
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@KimDong Consider two qubits, each made of two boxes plus one electron in either box. This is more or less the question situation: you can say "the electron in qubit 0 is in the 0 box" but not "electron 0 is in qubit 0." You encode the computational states into the physical states: e.g. $|10\rangle\mapsto\frac1{\sqrt2}(|0_11_0\rangle-|1_00_1\rangle)$ "qubit 0 is 1, qubit 1 is 0" is a superposition of "electron 0 in qubit 0 box 1, electron 1 in qubit 1 box 0" and "electron 0 in qubit 1 box 0, electron 1 in qubit 0 box 1". It's not even an "approximation", just a different choice of basis. – HTNW Dec 28 '22 at 08:44
My impression is that OP meant the 1 and 2 subscripts to refer to the location of the particle, in which case the correct wave function is different from what you wrote. In any case, I think you should clarify that the correct answer depends on the meaning of the subscripts, since that could well be the cause of many people's confusion. – benrg Dec 28 '22 at 19:41
@benrg Thanks, I modified the notation to clarify. – Andrew Dec 28 '22 at 19:47
Is this not simply a matter of the uncertainty principle regarding the position? That is, if we have two electrons far away from each other, it makes sense to treat them as separate entities; that does not make them "different" from each other except for their properties, but we would not expect to measure the properties of one in the location of the other, and in that sense location establishes identity, which can be understood as persistence of properties. This is never strictly true (the probability is always non-zero), but almost. It becomes wrong at small distances: – Peter - Reinstate Monica Dec 29 '22 at 02:48
We cannot know what we we'll measure, and identity is lost, because there is none beyond the properties. Even the separate identities of formerly distinguishable electrons are lost (which may be taken as "they never had any"): They are now, in a sense, one system (like hybridized electrons in a benzene ring). – Peter - Reinstate Monica Dec 29 '22 at 02:48
@Peter-ReinstateMonica I'm not sure I followed your comment, but there is more to the indistinguishability of identical particles than the uncertainty principle relating position and momentum. I say this because distinguishable particles obey the uncertainty principle, but not Bose or Fermi statistics. – Andrew Dec 29 '22 at 02:53
@Andrew Oh. You mean we can distinguish other kinds of particles? That would be an interesting part of your answer. – Peter - Reinstate Monica Dec 29 '22 at 02:54
@Peter-ReinstateMonica An electron and a proton are distinguishable. – Andrew Dec 29 '22 at 02:54
@Andrew Ah, OK, yes. So we do have properties that establish identity (however collective: what constitutes the particle kind, e.g. electrons vs. protons); but among the same kind we have properties that don't establish identity (when close enough we cannot tell which one it is, or rather: The question doesn't make sense). – Peter - Reinstate Monica Dec 29 '22 at 02:58
@Peter-ReinstateMonica Right, it doesn't make sense to ask "which" electron is associated with a given hydrogen atom, since all electrons are identical. – Andrew Dec 29 '22 at 03:00

score 6 · Answer 2 · answered Dec 27 '22 at 19:12

6

Tracking the two electrons you're talking about would requiere knowledge of both position and momentum, which contradicts Heisenberg uncertainty principle. Since you cannot track them you don't know which electron is (spin) up $|\uparrow\rangle$ and which one is down $|\downarrow\rangle$. As a consequence, your state for the system must be a combination of both situation, i.e.:

$$ |{\rm state\ of\ 2\ electrons}\rangle = C_1|\uparrow\rangle_1\otimes |\downarrow\rangle_2 + C_2 |\downarrow\rangle_1\otimes |\uparrow\rangle_2 $$

Because of normalization and extracting a global phase (that according to QM is irrelevant) we get

$$ |{\rm state\ of\ 2\ electrons}\rangle = \frac{1}{\sqrt{2}}(|\uparrow\rangle_1\otimes |\downarrow\rangle_2 + e^{i\theta} |\downarrow\rangle_1\otimes |\uparrow\rangle_2) $$

And now, as an empirical fact, nature tells us that $e^{i\theta} = -1$ for fermions (odd half-integer spin: electrons and similar particles) and $+1$ for bosons (integer spin particles).

From the theory point of view, this empiral fact is proven in quantum field theory and it's related to commutation and anticommutation rules of the fields describing such particles.

answered Dec 27 '22 at 19:12

Vicky

1,577

Hmm, I think I'm improving, but not quite there yet. Then what is the "qubit 0" and "qubit 1" that we encounter in quantum computing, which is often made up of electron or photon? I mean, clearly, I can go to the IBMQ or Xanadu, and prepare q0 at 1, and q1 at 0, and they say it'd be $|01\rangle$. But as you said, isn't it actually $|01\rangle-|10\rangle$ if it's made of ions or electron, and something else if it's photon? – Kim Dong Dec 27 '22 at 19:34
1

My knowledge in quantum computing is not very advanced, but it is not needed to answer your question. Learn step by step, quantum computing is a very advanced and complicated subject – Vicky Dec 27 '22 at 22:06
Can you explain how your first sentence is relevant? That's something that's also true for distinguishable particles, so it doesn't seem like it should be able to say anything about the difference in behavior between distinguishable and indistinguishable particles. In addition, the transition from equation (1), in which the $C$'s are arbitrary, to equation (2), where they have equal magnitude is not really justified by either the requirement of normalization or the irrelevance of a global phase. – march Dec 27 '22 at 22:45
Regarding the 1st sentence: it's motivation for eqs. (1) and (2) that apply for both distinguisable and undistinguisable particles. Now if they're distinguisable both states on RHS are equal and the phase is +1. Antisymmetry or symmetry is a fact coming from experiments and from QFT.

Vicky

Dec 27 '22 at 22:57

Regarding going from (1) to (2). I think normalization and global phase are enough for this purpose. Normalization gives you $|C_1|^2 + |C_2|^2 = 1$ so that $C_i = (\sqrt{2})^{-1}\times e^{i\theta_i}$ is a solution. Are there maybe other solutions? Mathetically it is possible, but physically there's no reason a priori to assume that one state or another in RHS of eq(1) is more likely, so the modules of the $C$s must be equal and hence the solution above for $C_i$. Maybe, it is true that this last reasoning should be included, I let it here. Thanks for the feedback

– Vicky Dec 27 '22 at 23:02

score 4 · Answer 3 · answered Dec 27 '22 at 19:05

4

Your mistake is that you assume that you may put a number according to the positions. But actually the identical electron state would look like, \begin{equation} \frac{1}{2}\Big(|\uparrow, \text{left}\rangle_1\otimes|\downarrow, \text{right}\rangle_2-|\downarrow, \text{right}\rangle_1\otimes|\uparrow, \text{left}\rangle_2\Big) \end{equation} I.e. exchange operation exchanges all properties between two particles. Or equivalently it simply exchanges the number you associate with particles between them.

As to where this principle comes from, what prohibits the symmetric states for fermions and anti symmetric for bosons, you won't get the answer in QM. It may be understood only from the spin-statistic theorem in QFT.

answered Dec 27 '22 at 19:05

OON

8,324

1

I am very much struggling with this... Isn't the $S_z$ operator commute with $H$, therefore if I measure it once it should technically stay the same unless if I evolve or measure it in some noncommuting basis? How is it that after I measure and make sure that it is $|\uparrow\rangle_1 \otimes |\downarrow\rangle_2$, it actually isn't that anymore but the one you given above? – Kim Dong Dec 27 '22 at 19:17
2

@KimDong The thing is all the observables must commute with the exchange operator. So you may have e.g. $S_{z,\text{left}}=S_{z, \text{left}1}+S_{z, \text{left}2}$. The state I wrote is its eigenstate. So you may in fact talk about "spin of the particle on the left". But if you measure it later you can't say that it was the same particle you measured earlier. In fact this question makes no sense as you don't have any observables that would correspond to the number of the particle as such observable would not commute with exchange operation – OON Dec 27 '22 at 19:34

hft · Answer 4 · 2022-12-28T20:47:19.493

I'm struggling with the concept of identical particles in QM. Say I have two electrons... What am I misunderstanding?

Not every solution to the Schrodinger equation is a "physical state." (Not every solution describes the natural world as it actually is.)

Consider, for example, a solution to the two-particle Schrodinger equation: $$ \Psi(\vec r_1, s_1;\vec r_2, s_2)\;. $$

If the above function is a solution, then so too is $$ \Psi(\vec r_2, s_2;\vec r_1, s_1) $$ a solution of the Schrodinger equation having the same energy, since the Hamiltonian is symmetric with respect to particle index exchange. If the states are different then this degeneracy is called "exchange degeneracy."

It is a fact of nature that the wave functions that actually describe two physical particles always obey either: $$ \Psi(\vec r_2, s_2;\vec r_1, s_1) =\Psi(\vec r_1, s_1;\vec r_2, s_2)\tag{bosons} $$ or $$ \Psi(\vec r_2, s_2;\vec r_1, s_1) =-\Psi(\vec r_1, s_1;\vec r_2, s_2)\tag{fermions}\;. $$

Therefore a state with only spin dependence such as $$ |\Psi\rangle \sim |\uparrow; \downarrow\rangle $$ is not a physical fermion state.

But a state like $$ |\Psi\rangle \sim |\uparrow; \downarrow\rangle - |\downarrow; \uparrow\rangle $$ is a physical fermion state, since exchanging the indices returns the same state (multiplied by negative one).

Here is a quote from Hans Bethe to make you feel better:

"Now it is a fact of nature, established by many observations, that all actual wave functions in physics obey $P_{ij}\Psi = \pm\Psi$, either with the plus or the minus sign. In other words, from the great multitude of mathematical solutions of $H\Psi = E\Psi$, nature has selected only the nondegenerate ones... Which symmetry applies depends on the type of identical particles involved; experiment shows that $\Psi$ is antisymmetric for electrons, protons, neutrons, mu-mesons..."

What does it mean for a state to be a "physical state"? I think the proposed $|\uparrow\rangle_1 \otimes |\downarrow\rangle_2$ is very much measurable? — Kim Dong, Dec 27 '22 at 19:07
It means a state that actually describes the world. This is basic multi-particle quantum mechanics. Every state is either completely symmetric (bosons) or completely anti-symmetric. — hft, Dec 27 '22 at 19:10
But I can measure 1 particle to be up, and the other to be down, and it's also an experimental fact right? Then isn't in that moment, I would have $|\uparrow\rangle_1 \otimes |\downarrow\rangle_2$? Don't they do this in quantum computing often, with a particle up and one is down? — Kim Dong, Dec 27 '22 at 19:20

score 3 · Answer 5 · answered Dec 27 '22 at 19:30

3

I mean, am I tracking the electrons if I trap them on 2 sides of my table?

Not really. Your trap isn't perfect. You can't literally have an infinite square well to ensure the particle stays trapped. Hence, you can't really know if the particles tunneled and exchanged positions, for example. The only way for you to know whether the particle actually stayed in the same trap the whole time is by measuring its position continuously. If they do switch places, you won't know, because there is (by definition) no way to distinguish.

The main point is that if you stop measuring the position of a particle, you have no way of knowing whether that is still the same particle. Traps aren't perfect, so there is a slight probability your particle tunneled out of it and was replaced by a different one. There are no observational differences, but it is not the same particle anymore.

Hence, in QM, it doesn't make sense to speak of a particular particle when you have another identical particle. You are unable to distinguish them, so you can only describe the pair, or perhaps the left particle. But there exist no tags on the particles telling you which is $1$ and which is $2$. Unless you are willing to continuously measure their positions and hence affect the experiment.

answered Dec 27 '22 at 19:30

Níckolas Alves

17,567

1

Hmm, I think I'm improving, but not quite there yet. Then what is the "qubit 0" and "qubit 1" that we encounter in quantum computing, which is often made up of electron or photon? I mean, clearly, I can go to the IBMQ or Xanadu, and prepare q0 at 1, and q1 at 0, and they say it'd be $|01\rangle$. But as you said, isn't it actually $|01\rangle-|10\rangle$ if it's made of ions or electron, and something else if it's photon? – Kim Dong Dec 27 '22 at 19:34
2

@KimDong I don't know much about quantum computing, but I guess they probably refer to the sites rather than the specific particles. For example, I can't talk about electron $1$ (I can't tag it with a $1$, after all), but there surely is an electron on the left and it has a state. The specific electron might change, but I'm concerned only with the state of the left electron. This is perfectly okay. – Níckolas Alves Dec 27 '22 at 19:42
2

Notice this is okay because I'm not distinguishing the particles, but the sites. I might not be able to say whether electron $1$ is up or down because I don't know who is electron $1$, but I can ask whether the electron on my left is up or down – Níckolas Alves Dec 27 '22 at 19:42
@NíckolasAlves Just a question: When you say this : Hence, in QM, it doesn't make sense to speak of a particular particle when you have another identical particle , How do one know if a particular particle (really) is identical to another? – William Martens Dec 27 '22 at 20:20
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@WilliamMartens they're identical if they're of the same kind: one electron is identical to any other electron, one photon is indistinguisable from antoher photon, and so on. From the POV of quantum field theory, every particle of a particular kind is created by the same field, hence identical – Vicky Dec 27 '22 at 22:09
@Vicky Oh I see, I got confused about the wordings probably, Thanks for the clarification, +1 for the last bit on the extra View on QFT, really. – William Martens Dec 27 '22 at 22:25

score 1 · Answer 6 · answered Dec 28 '22 at 20:22

Exchange symmetry doesn't apply to measurable properties like position and spin. It only applies to unphysical labels.

Your wave function is

$$\left|\uparrow\right\rangle_1 \otimes \left|\downarrow\right\rangle_2$$

Each particle in this wave function has three properties: a spin ($\uparrow$ or $\downarrow$), a subscript ($1$ or $2$), and a position in the tensor product (left or right). But your particles have only two physical properties, spin and position. Obviously spin is spin, but it's not clear what you mean by the other two properties in the wave function. The answer to your question depends on what they mean.

Other answers have assumed that they are both unphysical labels, redundantly encoding the same which-particle information. If they are, then the correct antisymmetrized wave function would be $\left|\uparrow\right\rangle_1 \otimes \left|\downarrow\right\rangle_2 - \left|\downarrow\right\rangle_1 \otimes \left|\uparrow\right\rangle_2$ (up to a scalar factor), where I obtained the second term from the first by reversing the product and swapping $1$ and $2$. But this doesn't really make sense, because physical position, which is crucial to your thought-experiment, is not represented in your wave function at all, so you can't model the swapping of the particles.

If only the tensor-product order is meaningless, and the subscripts represent physical position, then the antisymmetrized wave function would be $\left|\uparrow\right\rangle_1 \otimes \left|\downarrow\right\rangle_2 - \left|\downarrow\right\rangle_2 \otimes \left|\uparrow\right\rangle_1$, where I obtained the second term from the first by reversing the product only. If you physically swap the particles, then the wave function becomes $\left|\uparrow\right\rangle_2 \otimes \left|\downarrow\right\rangle_1 - \left|\downarrow\right\rangle_1 \otimes \left|\uparrow\right\rangle_2$, which I obtained from the previous wave function by swapping $1$ and $2$.

score 0 · Answer 7 · answered Dec 31 '22 at 17:35

You seem to be talking about two particles in the same quantum well or in the same atom. If you had two particles trapped in two adjacent quantum wells, they wouldn't necessarily be bound by the Pauli exclusion principle to have opposite up and down spins. I don't know if quantum entanglement changes that situation.

Struggling to understand Identical particles

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