Types of quantum field thories
symmetric monoidal (∞,1)-category of spectra
Deformation quantization focuses on the algebras of observables of a physical system (hence on the Heisenberg picture): it provides rules for how to deform the commutative algebra of classical observables to a non-commutative algebra of quantum observables. (This is in contrast to geometric quantization, which focuses on the spaces of states and hence on the Schrödinger picture.)
Usually and traditionally, deformation quantization refers to (just) formal deformations, in the sense that it produces formal power series expansions in a formal parameter (physically: Planck's constant) of the product in the deformed algebra of observables.
|classical mechanics||semiclassical approximation||…||formal deformation quantization||quantum mechanics|
But there are refinements of this to C-star algebraic deformation quantization which studies the proper deformation to a genuine C-star algebra of observables. (This in turn is related to genuine geometric quantization via the notion of geometric quantization of symplectic groupoids.)
Generally speaking, physics is based on strict quantization, rather than formal deformation quantization, although conventional quantization sometimes leads to problems that can be treated by deformation quantization.
As other methods of quantization, deformation quantization has as input a description of a classical mechanical system, which is in this case most often a smooth Poisson manifold. The deformation quantization replaces the algebra of smooth functions on the Poisson manifold with the same vector space, but equipped with new noncommutative associative unital product whose commutator agrees, up to order , with the underlying Poisson bracket. Of course the proper study of quantization of Poisson manifolds studied the appropriate notion at the level of sheaves of algebras. Gluing local solutions to the quantization problem furthermore involves stacks and specifically gerbes.
If the result of deformation quantization is an algebra over the power series ring of a formal parameter (thought of as Planck's constant) such that the limit reproduces the starting point of the deformation, then one speaks of
In much of the literature this is regarded as the default meaning of “deformation quantization”. But this is really the case corresponding to perturbation theory in quantum field theory. A “genuine” or “strict” deformation quantization
is supposed to result in a non-formal deformation, which in terms of the above formal power series at least means that one can set such that all expressions in converge, but which in general is taken to mean something stronger, such as that there is a continuous field of C-star algebraic deformation quantization.
We first give the traditional
In this formulation one sees that for genral the construction applies to -dimensional quantum field theory (with quantum mechanics for be 1-dimensional quantum field theory, for instance the sigma-model “on the worldline” of a particle). A formulation of deformation quantization to local quantum field theory formulated in terms of factorization algebras of observables over spacetime/worldvolume is then discussed in
where are bilinear maps on such that .
|dimension||classical field theory||Lagrangian BV quantum field theory||factorization algebra of observables|
|general||P-n algebra||BD-n algebra?||E-n algebra|
|Poisson 0-algebra||BD-0 algebra? = BD algebra||E-0 algebra? = pointed space|
|P-1 algebra = Poisson algebra||BD-1 algebra?||E-1 algebra? = A-∞ algebra|
Vladimir Drinfel'd has sketched a proof (and gave main ingredients) to show that every Poisson Lie group can be deformation quantized to a Hopf algebra; this proof has been completed by Etingof and Kazhdan. Maxim Kontsevich proved a certain formality theorem (formality is here in the sense of formal dg-algebra in rational homotopy theory) whose main corollary (and motivation) was the statement that every Poisson manifold has a deformation quantization (Kontsevich 97).
For symplectic manifolds and those Poisson manifolds that have a regular foliation by symplectic leafs, the theory of deformation quantization is much simpler; Boris Fedosov gave a construction of star products on symplectic manifolds using symplectic connections on smooth manifolds (Fedosov 94). An analogous argument was given by Roman Bezrukavnikov and Dmitry Kaledin in the context of an algebraic symplectic form (BK 04).
Caution: the following are rough notes from a talk by J.D.S. Jones (Cambridge, 8.1.2013); there are probably many typos and sign errors.
(Kontsevich). Every Poisson manifold has a (formal) deformation quantization.
This was shown in (Kontsevich 97). There the deformed product is constructed by a kind of Feynman diagram perturbation series. Later this was identified as the perturbation series of the Poisson sigma-model for the given Poisson manifold. See there for more details.
Let be a -vector space and consider . We define a “circle operator” as follows: for and , we define as the map
For , let . (This is graded symmetric.) It follows that the commutator of is given by
where when . This defines a graded Lie bracket of degree -1.
Apply this example to the construction of deformation quantization. The star product is uniquely determined by given by . What we want is that
write this out and we get the equation
or ; this is the Maurer-Cartan equation. Hence we are looking for solutions of the M-C equation but in the Hochschild complex . One should note that is actually a derivation of the Lie bracket, hence we have a dg-Lie algebra.
(HKR theorem). .
(Note that should be interpreted as .) Under this isomorphism the Poisson bracket is mapped to the Poisson tensor:
For vector fields and , the Schouten bracket satisfies (1) (the Lie bracket), and (2) ; note that this completely determines it (everything is locally given by wedges…).
In the Hochschild cohomology of , and , so we have a solution to M-C in .
Let and be differential graded Lie algebras (dgL). A quasi-isomorphism is a homomorphism of dgLs that induces an isomorphism on homology. and are quasi-isomorphic if there exists with quasi-isomorphisms . It can be verified that this is an equivalence relation.
(Kontsevich). If is quasi-isomorphic to then there is a solution to the M-C equation in iff there is a solution to the M-C equation in .
(Kontsevich formality). is quasi-isomorphic to . ()
Hence there is a solution to M-C in , and hence there is a deformation quantization (!).
for , ; this satisfies also .
The Deligne conjecture gives a relationship between these things.
In , we have:
Such a thing is called a Gerstenhaber algebra. Note that we do not have these relations in , they are only true modulo boundaries.
(Deligne conjecture). is a -algebra, which is a Gerstenhaber algebra up to coherent homotopy.
In (Kontsevich 99) it was indicated that a quotient group of the motivic Galois group apparently equivalent to the Grothendieck-Teichmüller group naturally acts on the space of formal deformation quantizations of a finite dimensional manifold. See also at cosmic Galois group.
This has been formalized as follows. The formal deformation quantization of (Kontsevich 97) is all induced by the Kontsevich formality theorem, which states that ober suitable manifolds/varieties there is an equivalence of L-∞ algebras
identifying the multivector fields on with the Hochschild cohomology complex (of its function algebra). Every choice of such an equivalence induces one formal deformation quantization of a Poisson manifold , and the two quantizations induced by two equivalent (homotopic) equivalences are in turn equivalent.
Therefore one may regard the ∞-groupoid as the “space of formal deformation quantizations” of .
In (Dolgushev 1109, theorem 6.2, Dolgushev 1111, theorem 3.1) it is shown that the set of connected components of this space is, up to a choice of basepoint, the Grothendieck-Teichmüller group, hence is a torsor over that group.
(This is based on identifications of the GRT Lie algebra with the degree-0 chain cohomology of the graph complex, due to Thomas Willwacher. See at Grothendieck-Teichmüller group – relation to the graph complex.
Aspects of the generalization of this statement to more general spaces then are discussed in Dolgushev-Rogers-Willwacher 12.
|Poisson algebra||Poisson manifold|
|deformation quantization||geometric quantization|
|algebra of observables||space of states|
|Heisenberg picture||Schrödinger picture|
|higher algebra||higher geometry|
|Poisson n-algebra||n-plectic manifold|
|En-algebras||higher symplectic geometry|
|BD-BV quantization||higher geometric quantization|
|factorization algebra of observables||extended quantum field theory|
|factorization homology||cobordism representation|
Examples of sequences of infinitesimal and local structures
|first order infinitesimal||formal = arbitrary order infinitesimal||local = stalkwise||finite|
|derivative||Taylor series||germ||smooth function|
|tangent vector||jet||germ of curve||curve|
|square-0 ring extension||nilpotent ring extension||ring extension|
|Lie algebra||formal group||local Lie group||Lie group|
|Poisson manifold||formal deformation quantization||local strict deformation quantization||strict deformation quantization|
Boris Fedosov, Formal quantization, Some Topics of Modern Mathematics and their Applications to Problems of Mathematical Physics (in Russian), Moscow (1985), 129- 136.
Boris Fedosov, Index theorem in the algebra of quantum observables, Sov. Phys. Dokl. 34 (1989), 318-321.
For algebraic forms this is discussed in
More discussion of this approach is in
A direct and general formula for the deformation quantization of any Poisson manifold was given in
The classification of the space of such formal deformation quantization is discussed in
The relation geometric quantization is discussed in
Christoph Nölle, Geometric and deformation quantization (arXiv:0903.5336)
and some remarks on the relation are also in section 1.4 of
which is about quantization via the A-model.
On the stack of deformation quantizations:
See also at motives in physics.