[Physics] Wavefunction “single-valued” vs “up to a phase” ( gauge transformation) Existence of global section in comple line bundle

differential-geometryelectromagnetismquantum mechanicswavefunction

I always meet the following sayings: in one case people say that wavefunction must be single valued, in another case people say that wavefunction could be up to a phase in the same point if there is a gauge transformation. I'm so puzzled about these two sayings. I will list few cases I meet these sayings.

First, consider a free particle in a circle $S^1$ with radius $r$. The Hamitonian is
$$H=\frac{1}{2mr^2}(-i\hbar \frac{\partial}{\partial \phi})^2$$
Then the eigenfunction is
$$\psi= \frac{1}{\sqrt{2\pi r}}e^{i n \phi}$$
Because the wavefunction must be single-valued on $S^1$, $n$ must belong to integers, i.e. $n\in\mathbb{Z}$.

Second case, consider a particle in a circle $S^1$ with radius $r$ and put the flux $\Phi$ in the center of the circle. Then the Hamitonian is
$$H=\frac{1}{2mr^2}(-i\hbar \frac{\partial}{\partial \phi}+\frac{e\Phi}{2\pi})^2$$

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The eigenfunction is still
$$\psi= \frac{1}{\sqrt{2\pi r}}e^{i n \phi}$$
Also because the wavefunction must be single-valued on $S^1$, $n$ must belong to integers, i.e. $n\in\mathbb{Z}$. The only difference is that there will be some shift of eigenvalue.

Third case, consider a particle in a Torus $T^2$, with two length $L_x$ and $L_y$. And we put a uniform magnetic field $B$ through the torus' surface. If we choose the Landau's gauge $A_x = 0$, $A_y= B x$. The Hamitonian is now
$$H=\frac{1}{2m}(p_x^2 +(p_y+eBx)^2)$$

We know in this case the symmetry of Hamitonian is called magnetic translation group.
$$T(\mathbf{d})= e^{-i \mathbf{d}\cdot(i\nabla+e \mathbf{A}/\hbar)}$$
that is $[T(\mathbf{d}),H]=0 $
So the eigenfunction $\psi(x,y)$ should be invariant under $T(\mathbf{d})$.
$$T_x \psi(x,y)=\psi(x+L_x,y)=\psi(x,y)$$
$$T_y \psi(x,y)=e^{-ieBL_yx/\hbar}\psi(x,y+L_y)=\psi(x,y)$$
with $T_x =T((L_x,0))$ and $T_y=T((0,L_y))$.
So we see the wavefunction is not single valued in this case,i.e. $\psi(x,y+L_y)\ne \psi(x,y)$.

My question is :
In what case, we admit wavefunction is not single-valued? We see all cases with physical space multiply connected. Both 2nd and 3rd cases are the system with electromagnetic field/ gauge field, why 1st, 2nd is still single-valued but 3rd not? It seems existence of nontrivial topology of physical space or magnetic field/gauge field is not the answer.

PS: Thanks to @David Bar Moshe, I never realized this question may be related to the global section of complex line bundles.

Best Answer

In quantum mechanics the normalization of the wave function is not important since we compute expectations according to: $$\langle O \rangle = \frac{\psi^{\dagger} O \psi}{\psi^{\dagger} \psi}$$ This is the reason that wave functions are identified with sections of complex line bundles. Please see this introduction for physicists by Orlando Alvarez.

When a line bundle is trivial its space of sections can be formed from true functions, which should be single valued.

The equivalence class of line bundles over a manifold $M$ is called the Picard group $\mathrm{Pic}(M)$. Each element (besides the unity) of this group gives rise to a nonequivalent quantization in which the phase factor cannot be removed by a gauge transformation.

Please see Prieto and Vitolo for a brief explanation.

On differentiable manifolds, the Picard group is isomorphic to the second cohomology group over the integers

$$\mathrm{Pic}(M) \cong H^2(M, \mathbb{Z})$$

This is why it is rarely mentioned in quantum mechanics texts which rather refer the corresponding element from $H^2(M, \mathbb{Z})$ representing the first Chern class.

It should be emphasized:

(1) that even when the Picard group is trivial or the quantization corresponds to a trivial element, we can have multiple valued wave functions, but the multiple valuedness can be removed by a gauge transformation.

(2) The first Chern class is not a sufficient classifier of nonequivalent quantizations. It does not detect effects like the Aharonov-Bohm effect. These are detected by an element of the group $ \mathrm{Hom}(\pi_1(M), U(1))$, please see for example, Doebner and Tolar.

(3) The relevant manifold $M$ is the phase space. Since the given example are of point particles, whose phase space is the cotangent bundle of a configuration space, the nontrivial topology lies in the configuration space and we can talk about line bundles over the configuration space.

Returning to your examples: The first two describe motion on the circle $S^1$. By dimensional reasoning $H^2(S^1, \mathbb{Z})=0$, thus wave functions can be chosen to be true functions. The second example refers to the case described in the second remark above since $\pi_1(S^1) = \mathbb{Z}$

In the third example $H^2(T^2, \mathbb{Z})= \mathbb{Z}$, generated by integer multiples of the basic area element, thus for a nonvanishing magnetic field the wave functions cannot be taken as true functions.

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