nLab trace

Idea

Given an endomorphism $f:A\to A$ in a traced monoidal category $\mathscr{C}$ there is a natural notation of a trace $\tr(f):1\to 1$, where $1$ is the tensor unit. When $\mathscr{C}$ is a $\mathbb{C}$-linear category for which the tensor unit (e.g. a fusion categories), we can canonically identity $\tr(f)$ with a complex number. That is, the trace of $f$ is the unique scalar $\lambda$ such that

$$\tr(f)=\lambda \cdot \text{id}_A.$$

In the case that $f$ is a linear endomorphism of a finite dimensional vector space, we recover the usual notion of trace from linear algebra.

Definition

The idea of the trace operation is easily seen in string diagram notation: essentially one takes an endomorphism $f: A\xrightarrow{}A$, and “closes the loop”.

$$\array{ 1 \\ \;\;\;\downarrow^{tr(f)} \\ 1 } \;\;\; := \;\;\; \array{ & 1 \\ & \downarrow \\ a^* &\otimes& a \\ \downarrow^{\mathrlap{Id_{a^*}}} && \;\;\downarrow^f \\ a^* &\otimes& a \\ & \downarrow^{\mathrlap{b_{a^*, a}}} \\ a &\otimes& a^* \\ & \downarrow \\ & 1 }$$

This definition makes sense in any braided monoidal category, but often in non-symmetric cases one wants instead a slightly modified version which requires the extra structure of a balancing.

The trace of the identity $1_a:a \to a$ is called the dimension or Euler characteristic of $a$.

Examples

• $C = Vect$ with its standard monoidal structure (tensor product of vector spaces): in this case tr(f) is the usual trace of a linear map;

• $C = SuperVect = (Vect_{\mathbb{Z}_2}, \otimes, b)$, the category of $\mathbb{Z}_2$-graded vector spaces with the nontrivial symmetric braiding which is $-1$ on two odd graded vector spaces: in this case the above is the supertrace on supervectorspaces, $str(V) = tr(V_{even}) - tr(V_odd)$.

• $C = Span(Top^{op})$: here the trace is the co-span co-trace which can be seen as describing the gluing of in/out boundaries of cobordisms

• $C = Span(Grpd)$: this reproduces the notion of trace of a linear map within the interpretation of spans of groupoids as linear maps in the context of groupoidification and geometric function theory, made explicit at span trace

• In the symmetric monoidal (infinity,1)-category of spectra, the trace on the identity on a suspension spectrum of a manifold $X$ is the Euler characteristic of $X$ (see there).

Generalizations

Vertical categorification

See trace of a category.

Horizontal categorification

See trace in a bicategory?.

Partial trace

Let $V \in C$ be a dualizable object, and $W$ any object. From an endomorphism $f$ of $V \otimes W$, one can produce an endomorphism $tr_V(f)$ of $W$, by applying the duality data of $V$. In terms of string diagrams, this is “bending along” the strand representing $V$.

TO DO: Draw the diagram just described.

Matrix representation

Suppose $V$, $W$ are finite-dimensional vector spaces over a field, with dimensions $m$ and $n$, respectively. For any space $A$ let $L(A)$ denote the space of linear operators on $A$. The partial trace over $W$, Tr$_{W}$, is a mapping

$$T \in L(V \otimes W) \mapsto Tr_{W}(T) \in L(V).$$

Definition

Let $e_{1}, \ldots, e_{m}$ and $f_{1}, \ldots, f_{n}$ be bases for $V$ and $W$ respectively. Then $T$ has a matrix representation $\{a_{k l,i j}\}$ where $1 \le k,i \le m$ and $1 \le l,j \le n$ relative to the basis of the space $V \otimes W$ given by $e_{k} \otimes f_{l}$. Consider the sum

$$b_{k,i} = \sum_{j=1}^{n}a_{k j,i j}$$

for $k,i$ over $1, \ldots, m$. This gives the matrix $b_{k,i}$. The associated linear operator on $V$ is independent of the choice of bases and is defined as the partial trace.

Example

Consider a quantum system, $\rho$, in the presence of an environment, $\rho_{env}$. Consider what is known in quantum information theory as the CNOT gate:

$$U={|00\rangle}{\langle 00|} + {|01\rangle}{\langle 01|} + {|11\rangle}{\langle 10|} + {|10\rangle}{\langle 11|}.$$

Suppose our system has the simple state ${|1\rangle}{\langle 1|}$ and the environment has the simple state ${|0\rangle}{\langle 0|}$. Then $\rho \otimes \rho_{env} = {|10\rangle}{\langle 10|}$. In the quantum operation formalism we have

$$T(\rho) = \frac{1}{2}Tr_{env}U(\rho \otimes \rho_{env})U^{\dagger} = \frac{1}{2}Tr_{env}({|10\rangle}{\langle 10|} + {|11\rangle}{\langle 11|}) = \frac{{|1\rangle}{\langle 1|}{\langle 0|0\rangle} + {|1\rangle}{\langle 1|}{\langle 1|1\rangle}}{2} = {|1\rangle}{\langle 1|}$$

where we inserted the normalization factor $\frac{1}{2}$.

Related concepts

• relation between trace and determinant

• traced monoidal category

• Euler characteristic

• bicategorical trace,

  • Reidemeister trace

• higher trace

• Dennis trace, cyclotomic trace

• trace of horizontal to round chord diagrams

References

The categorical notion of trace in a monoidal category is due to

• Albrecht Dold, and Dieter Puppe, Duality, trace, and transfer In Proceedings of the Inter-

  national Conference on Geometric Topology (Warsaw, 1978), pages 81{102, Warsaw, 1980. PWN.

and

• Max Kelly M. L. Laplaza, Coherence for compact closed categories J. Pure Appl. Algebra, 19:193{213, 1980.

Surveys include

• Peter Selinger, A survey of graphical languages for monoidal categories (pdf), Section 5

• Kate Ponto, Mike Shulman, Traces in symmetric monoidal categories (pdf).

Generalization of this to indexed monoidal categories is in

• Kate Ponto, Mike Shulman, Duality and traces for indexed monoidal categories, Theory and Applications of Categories, Vol. 26, 2012, No. 23, pp 582-659 (arXiv:1211.1555)

and to bicategories in

• Kate Ponto, Mike Shulman, Shadows and traces in bicategories, JHRS, (arXiv:0910.1306)

Further developments are in

• Andre Joyal, Ross Street, and Dominic Verity, Traced Monoidal Categories

• David Ben-Zvi, David Nadler, Nonlinear traces (arXiv:1305.7175)

For the notion of partial trace, particularly its application to quantum mechanics, see:

• Nielsen and Chuang, Quantum Computation and Quantum Information