Topological quantum field theory

A topological quantum field theory (or topological field theory or TQFT) is a quantum field theory which computes topological invariants.

Although TQFTs were invented by physicists, they are also of mathematical interest, being related to, among other things, knot theory and the theory of four-manifolds in algebraic topology, and to the theory of moduli spaces in algebraic geometry. Donaldson, Jones, Witten, and Kontsevich have all won Fields Medals for work related to topological field theory.

In condensed matter physics, topological quantum field theories are the low energy effective theories of topologically ordered states, such as fractional quantum Hall states, string-net condensed states, and other strongly correlated quantum liquid states.

Overview

In a topological field theory, the correlation functions do not depend on the metric of spacetime. This means that the theory is not sensitive to changes in the shape of spacetime; if the spacetime warps or contracts, the correlation functions do not change. Consequently, they are topological invariants.

Topological field theories are not very interesting on the flat Minkowski spacetime used in particle physics. Minkowski space can be contracted to a point, so a TQFT on Minkowski space computes only trivial topological invariants. Consequently, TQFTs are usually studied on curved spacetimes, such as, for example, Riemann surfaces. Most of the known topological field theories are defined on spacetimes of dimension less than five. It seems that a few higher-dimensional theories exist, but they are not very well understood.

Quantum gravity is believed to be background-independent (in some suitable sense), and TQFTs provide examples of background independent quantum field theories. This has prompted ongoing theoretical investigation of this class of models.

(Caveat: It is often said that TQFTs have only finitely many degrees of freedom. This is not a fundamental property. It happens to be true in most of the examples that physicists and mathematicians study, but it is not necessary. A topological sigma model with target infinite-dimensional projective space, if such a thing could be defined, would have countably infinitely many degrees of freedom.)

Specific models

The known topological field theories fall into two general classes: Schwarz-type TQFTs and Witten-type TQFTs. Witten TQFTs are also sometimes referred to as cohomological field theories.

Schwarz-type TQFTs

In Schwarz-type TQFTs, the correlation functions or partition functions of the system are computed by the path integral of metric independent action functionals. For instance, in the BF model, the spacetime is a two-dimensional manifold M, the observables are constructed from a two-form F, an auxiliary scalar B, and their derivatives. The action (which determines the path integral) is

$S=\int_M B F\,$

The spacetime metric does not appear anywhere in the theory, so the theory is explicitly topologically invariant. The first example appeared in 1977 and is due to A. Schwarz, its action functional is:

$\int_M A\wedge dA.$

Another more famous example is Chern–Simons theory, which can be used to compute knot invariants. In general partition functions depend on metric but the above examples are shown to be independent on metric.

Witten-type TQFTs

The first example of the topological field theories of Witten-type appeared in Witten's paper in 1988 (Witten 1988a), i.e. topological Yang–Mills theory in four dimensions. Though its action functional contains metric gαβ, after topological twist it turns out to be metric independent. The independence of Stress-energy tensor Tαβ of the system on metric depends on whether BRST-operator is closed. Following Witten's example a lot of examples are found in string theory.

Mathematical formulations

The original Atiyah-Segal axioms

Atiyah suggested a set of axioms for topological quantum field theory (Atiyah 1988a) which was inspired by Segal's proposed axioms for conformal field theory, and Witten's idea of the geometric meaning of supersymmetry, (Witten 1982). Atiyah's axioms are constructed on gluing the boundary with differentiable (topological or continuous) transformation, while Segal's are with conformal transformation. These axioms have been relatively useful for mathematical treatments of Schwarz-type QFTs, although it isn't clear that they capture the whole structure of Witten-type QFTs. The basic idea is that a TQFT is a functor from a certain category of cobordisms to the category of vector spaces.

There are in fact two different sets of axioms which could reasonably be called the Atiyah axioms. These axioms differ basically in whether or not they study a TQFT defined on a single fixed n-dimensional Riemannian / Lorentzian spacetime M or a TQFT defined on all n-dimensional spacetimes at once.

Let Λ be a commutative ring with 1 (for almost all real-world purposes we will have Λ = Z, R or C). Atiyah originally proposed the axioms of a topological quantum field theory (TQFT) in dimension d defined over a ground ring Λ as following:

• A finitely generated Λ-module Z(Σ) associated to each oriented closed smooth d-dimensional manifold Σ (corresponding to the homotopy axiom),
• An element Z(M) ∈ Z(∂M) associated to each oriented smooth (d+1)-dimensional manifold (with boundary) M (corresponding to an additive axiom).

These data are subject to the following axioms (4 and 5 were added by Atiyah):

1. Z is functorial with respect to orientation preserving diffeomorphisms of Σ and M,
2. Z is involutory, i.e. Z(Σ*) = Z(Σ)* where Σ* is Σ with opposite orientation and Z(Σ)* denotes the dual module,
3. Z is multiplicative.
4. Z(φ) = Λ for the d-dimensional empty manifold and Z(φ) = 1 for the (d+1)-dimensional empty manifold.
5. Z(M*) = Z(M) (the hermitian axiom). Equivalently, Z(M*) is the disjoint of Z(M)

Remark. If for a closed manifold M we view Z(M) as a numerical invariant, then for a manifold with boundary we should think of Z(M) ∈ Z(∂M) as a "relative" invariant. Let f : Σ × I → Σ × I be an orientation preserving diffeomorphism, and identify opposite ends of Σ × I by f. This gives a manifold Σf and our axioms imply

$Z(\Sigma_f)=\text{Trace}\ \Sigma(f)$

where Σ(f) is the induced automorphism of Z(Σ).

Remark. For a manifold M with boundary Σ we can always form the double $M\cup_\Sigma M^*$ which is a closed manifold. The fifth shows that

$Z(M\cup_\Sigma M^*)=|Z(M)|^2$

where on the right we compute the norm in the hermitian (possibly indefinite) metric.

The relation to physics

Physically (2)+(4) is related to relativistic invariance while (3)+(5) is indicative of the quantum nature of the theory.

Σ is meant to indicate the physical space (usually, d = 3 for standard physics) and the extra dimension in Σ × I is "imaginary" time. The space Z(M) is the Hilbert space of the quantum theory and a physical theory, with a Hamiltonian H, will have an time evolution operator eitH or an "imaginary time" operator e−tH. The main feature of topological QFTs is that H = 0, which implies that there is no real dynamics or propagation, along the cylinder Σ × I. However, there can be non-trivial "propagation" (or tunneling amplitudes) from Σ0 to Σ1 through an intervening manifold M with $\partial M=\Sigma^*_0\cup\Sigma_1$; this reflects the topology of M.

If ∂M = Σ, then the distinguished vector Z(M) in the Hilbert space Z(Σ) is thought of as the vacuum state defined by M. For a closed manifold M the number Z(M) is the vacuum expectation value. In analogy with statistical mechanics it is also called the partition function.

The reason why a theory with zero Hamiltonian can be sensibly formulated in the Feynman path integral approach to QFT. This incorporates relativistic invariance (which caters for general (d+1)-dimensional "spacetimes") and the theory is formally defined by writing down a suitable Lagrangian - a functional of the classical fields of the theory. A Lagrangian which involves only first derivatives in time formally leads to a zero Hamiltonian, but the Lagrangian itself may have non-trivial features which relate it to the topology of M.

Atiyah's examples

In 1988, M. Atiyah published a paper in which he described many new examples of topological quantum field theory that were considered at that time. (Atiyah 1988) It contains some new topological invariants and the new ideas, which are Casson invariant, Donaldson invariant, Gromov's theory, Floer homology and Jones-Witten's theory.

d = 0

In this case Σ consists of finitely many points. To single point we associate a vector space V = Z(point) and to n-points the n-fold tensor product: Vn = V ⊗ ... ⊗ V. The symmetric group Sn acts on Vn. A standard way to get the quantum Hilbert space is to give a classical symplectic manifold (or phase space) and then quantize it. Let us extend Sn to compact Lie group G and consider "integrable" orbits for which the symplectic structure comes from a line bundle then quantization leads to the irreducible representations V of G. This is the physical interpretation of the Borel-Weil theorem or the Borel-Weil-Bott theorem. The Lagrangian of these theories is the classical action (holonomy of the line bundle). Thus topological QFT's with d = 0 relate naturally to the classical representation theory of Lie groups and symmetric groups.

d = 1

We should consider periodic boundary conditions given by closed loops in a compact symplectic manifold X. Along to (Witten 1982) holonomy round such loops used in the case of d = 0 as a Lagrangian is used to modify the Hamiltonian. For a closed surface M the invariant Z(M) of the theory is the number of pseudo holomorphic maps f : MX in the sense of Gromov (they are ordinary holomorphic maps if X is a Kaehler manifold). If this number becomes to infinite i.e. if there are "moduli", then we must fix further data on M. This can be done by picking some points Pi and then looking at holomorphic maps f : MX with f(Pi) constrained to lie on a fixed hyperplane. (Witten 1988b) has written down the relevant Lagrangian for this theory. Floer has given a rigorous treatment, i.e. Floer homology, based on (Witten 1982)'s Morse theory ideas, for the case when the boundary conditions are the interval instead of periodic, the initial and end-points of paths lie on two fixed Lagrangian submanifolds. This theory has been developed as Gromov–Witten invariant theory.

Another example is Holomorphic Conformal Field Theory. This might not be strictly topological quantum field theory at that time because Hilbert spaces are infinite dimensional. The conformal field theories are also related to compact Lie group G in which the classical phase consists of a central extension of the loop group LG. Quantizing these produces the Hilbert spaces of the theory of irreducible (projective) representations of LG. The group Diff+(S1) now substitutes for the symmetric group and play an important role. The partition function in such theories depends on complex structure: it is not purely topological.

d = 2

Jones-Witten theory is the most important theory in this case. Here the classical phase space, associated to a closed surface Σ is the moduli space of flat G-bundle over Σ. The Lagrangian is an integer multiple of the Chern–Simons function of a G-connection on a 3-manifold (which has to be "framed"). The integer multiple k, called the level, is a parameter of the theory and k → ∞ gives the classical limit. This theory can be naturally coupled with the d = 0 theory to produce a "relative" theory. The details have been described by Witten who shows that partition function for a (framed) link in the 3-sphere is just the value of the Jones polynomial for a suitable root of unity. The theory can be defined over the relevant cyclotomic field. By considering Riemann surface with boundary, we can couple it to the d = 1 conformal theory instead of coupling d = 2 theory to d = 0. This theory has been developed as the Jones-Witten theory and turned out to be the trigger binding the knot theory and the quantum theory.

d = 3

Donaldson has defined integer invariant of smooth 4-manifolds by using moduli spaces of SU(2)-instantons. These invariants are polynomials on the second homology. Thus 4-manifolds should have extra data consisting of the symmetric algebra of H2. (Witten 1988a) has produced a super-symmetric Lagrangian which formally reproduces the Donaldson theory. Witten's formula might be understood as an infinite-dimensional analogue of the Gauss–Bonnet theorem. At a later date, this theory is further developed and become the Seiberg–Witten gauge theory which reduces SU(2) to U(1) in N = 2, d = 4 gauge theory. The Hamiltonian version of the theory has been developed by Floer in terms of the space of connections on a 3-manifold. Floer uses the Chern–Simons function, which is the Lagrangian of the Jones-Witten theory to modify the Hamiltonian. In detail see (Atiyah 1988). (Witten 1988a) has also shown how one can couple the d = 3 and d = 1 theories together: this is quite analogous to the coupling between d = 2 and d = 0 in the Jones-Witten theory.

Now, it isn't considered on a fixed dimension but on all the dimensions at the same time, namely, topological field theory is viewed as a functor.

The case of a fixed spacetime

Let BordM be the category whose morphisms are n-dimensional submanifolds of M and whose objects are connected components of the boundaries of such submanifolds. Regard two morphisms as equivalent if they are homotopic via submanifolds of M, and so form the quotient category hBordM: The objects in hBordM are the objects of BordM, and the morphisms of hBordM are homotopy equivalence classes of morphisms in BordM. A TQFT on M is a symmetric monoidal functor from hBordM to the category of vector spaces.

Note that cobordisms can, if their boundaries match up, be sewn together to form a new bordism. This is the composition law for morphisms in the cobordism category. Since functors are required to preserve composition, this says that the linear map corresponding to a sewn together morphism is just the composition of the linear map for each piece.

There is an equivalence of categories between the category of 2-dimensional topological quantum field theories and the category of commutative Frobenius algebras.

All n-dimensional spacetimes at once

The pair of pants is a (1+1)-dimensional bordism, which corresponds to a product or coproduct in a 2-dimensional TQFT.

To consider all spacetimes at once, it is necessary to replace hBordM by a larger category. So let Bordn be the category of bordisms, i.e. the category whose morphisms are n-dimensional manifolds with boundary, and whose objects are the connected components of the boundaries of n-dimensional manifolds. (Note that any (n−1)-dimensional manifold may appear as an object in Bordn.) As above, regard two morphisms in Bordn as equivalent if they are homotopic, and form the quotient category hBordn. Bordn is a monoidal category under the operation which takes two bordisms to the bordism made from their disjoint union. A TQFT on n-dimensional manifolds is then a functor from hBordn to the category of vector spaces, which takes disjoint unions of bordisms to the tensor product of them.

For example, for (1+1)-dimensional bordisms (2-dimensional bordisms between 1-dimensional manifolds), the map associated with a pair of pants gives a product or coproduct, depending on how the boundary components are grouped – which is commutative or cocommutative, while the map associated with a disk gives a counit (trace) or unit (scalars), depending on grouping of boundary, and thus (1+1)-dimension TQFTs correspond to Frobenius algebras.

Furthermore, we consider simultaneously 4-dimensional, 3-dimensional and 2-dimensional manifolds that are related by the above bordisms, then obtain ample and important examples.

Development at a later time

Looking at the development of topological quantum field theory we should consider that it has many applications to Seiberg–Witten gauge theory, topological string theory, the relationship between knot theory and quantum theory, and quantum knot invariants. Furthermore it has provided objects of great interest to both mathematics and physics. Also of important recent interest is non-local operators in TQFT.(Gukov & Kapustin (2013)) If string theory is viewed as the fundamental, then non-local TQFTs can be viewed as non-physical models that provide a computationally efficient approximation to local string theory.