# Rigorous proof of Bohr-Sommerfeld quantization

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Bohr-Sommerfeld quantization provides an approximate recipe for recovering the spectrum of a quantum integrable system. Is there a mathematically rigorous explanation why this recipe works? In particular, I suppose it gives an exact description of the large quantum number asymptotics, which should be a theorem.

Also, is there a way to make the recipe more precise by adding corrections of some sort?

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recategorized Apr 6, 2014
You know about the WKB approximation, right?

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Yes, it is called WKB and it has its own expansion series (I heard - a rather converging one).

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I have no idea how, but this question magically lost its subcategory... I have added it back now... Strange.

I would love to find time to read Part V of the ChaosBook.org on this topic to be able to understand how does all this semi-classical quantization business work in 1) larger number of degrees of freedom (quasiperiodic motion) and 2) chaotic motion.

@Void: Please give a link to ''Part V of the ChaosBook.org on this topic'', and ask your question as an independent question in a new thread, as semiclassical approximation is a far wider subject than Bohr-Sommerfled quantization (which works only for integrable systems, where it is exact).

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Yes, it can be made precise and corresponds to the leading order of the semiclassical expansion (WKB approximation) in $\hbar$. See Faddeev-Yakubovsky's "Lectures on quantum mechanics for mathematics students" (§20, formula (13)). An approach inspired by geometric quantization is explained in chapter 4 in Bates-Weinstein's Lectures on the Geometry of Quantization.

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answered Nov 19, 2011 by (1,120 points)
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This answer addresses the geometrical origin of the Bohr-Sommerfeld condition. In geometric quantization, the additional structure required beyond the symplectic data of the phase space is a polarization. The quantization spaces are constructed as spaces of polarized sections with respect a polarization. The most "obvious" type of a polarization is the Kahler polarization, where the quantization spaces are spaces of holomorphic sections of a pre-quantum line bundle. Simple examples of systems which can be quantized by means of a Kahler polarization are the Harmonic oscillator and the spin. Another type of polarization is the real polarization (Please see for example the Blau's lecttures), which is locally equivalent to the polarization of a cotangent bundle. A real polarizations foilates the phase space (symplectic manifold) into Lagrangian submanifolds. When the leaves are compact, the quantum Hilbert space consists of sections with support only on certain leaves, which are exactly those which satisfy the Bohr-Sommerfeld condition. In this case, the quantum phase space is generated by distributional sections supported solely on the Bohr-Sommerfeld leaves (This result is due to Snyatycki). For example in the case of the spin, the Bohr-Sommerfeld leaves are small circles at half integer values of the $z$ coordinate in the two dimensional sphere. A more sophisticated example of Bohr-Sommerfeld leaves is the Gelfand-Cetlin system on flag manifolds.

Many classical phase admit both Kahler and real polarizations. It is interesting that in many cases the quantization Hilbert spaces are unitarily equivalent (i.e. the quantization is polarization independent). Please see for example Nohara's exposition.

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answered Nov 20, 2011 by (4,095 points)
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Contrarily to what is generally believed, a semiclassical approximation is achieved through two different series: One is WKB series and the other is the Wigner-Kirkwood series, the latter being a gradient expansion. In both cases, eigenvalues are obtained by the Bohr-Sommerfeld rule but just at the leading order. I have proved this here (this paper appeared in Proceedings of Royal Society A). This proof is rigorous and quite different from what one finds on standard textbooks. Besides, it produces the full series for the exact eigenvalues with at leading order the ordinary Bohr-Sommerfeld rule.

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answered Nov 20, 2011 by (345 points)

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