
64 L. Kong et al. / Nuclear Physics B 922 (2017) 62–76
we show that there is a canonical morphism ρ : P
n
(Z
n
(C
n
))
C
n
→ C
n
, where
denotes
the stacking of two topological orders of the same dimension.
3. We show that the pair (P
n
(Z
n
(C
n
)), ρ), satisfies the universal property of the center of an
algebra in mathematics (see Theorem 5.1). This implies that bulk
= center.
This result is independent of how we describe the boundary/bulk phase mathematically. It is a
non-trivial result that leads to concrete physical predictions (see Remark 5.4).
We denote n+1D topological orders by A
n+1
, B
n+1
, C
n+1
, D
n+1
, etc. If the spacetime di-
mension n+1is clear from the context, we abbreviate C
n+1
as C. We denote the trivial n+1D
topological order by 1
n+1
. In physics, the trivial topological order 1
n+1
corresponds to the equiv-
alence class of the product states [10].
This paper is written for wo
rking condensed matter physicists, especially for those working
in the field of topological phases of matters. In order to convey the simply idea, we try to keep
the categorical language to the minimum. In particular, we collect all mathematically technical
parts in Remarks and Examples. The main te
xt should be readable to those who do not have any
extra background in category theory beyond those basics that have already been widely used in
condensed matter physics (see [24]).
Acknowledgments: X-G.W is supported by NSF Grant No. DMR-1506475 and NSFC
11274192. HZ is supported by NSFC under Grant No. 11131008.
2. Basics of topological orders
In this section, we recall some basic facts about topological orders, state our key assumptions
and set our notations.
A potentially anomalous nD topological order C
n
can always be realized as a gapped boundary
of an anomaly-free n+1D topological order E
n+1
[28]. In physics, it is generally believed that the
bulk anomaly-free topological order E
n+1
is uniquely determined by its gapped boundary phase
C
n
. One way to see this is to note that there is no preferred length scale. In order to define the
boundary phase C
n
, we need define the equivalence class of quantum states by allowing proper
deformation of the state in an arbitrary large neighborhood of the boundary (without closing the
gap). As a consequence, E
n+1
should be unique (see [28] for details). This uniqueness is the key
assumption of this paper. We highlight it here.
Unique-bulk h
ypothesis: for any given nD potentially anomalous topological order C
n
, there
is a unique anomaly-free n+1D topological order, denoted by Z
n
(C
n
), such that C
n
can be
realized as a gapped boundary of Z
n
(C
n
).
We denote E
n+1
by Z
n
(C
n
) and refer to it as the bulk of C
n
. It is clear that Z
n
(1
n
) = 1
n+1
. In this
work, we often use the following picture:
to illustrate the geometric relation between the boundary phase C
n
and the bulk phase Z
n
(C
n
).
More precisely, the n+1D bulk
phase Z
n
(C
n
) (defined on an open n-disk as the space manifold)
is depicted by an open interval and the nD boundary phase C
n
(on an open n−1-disk) is depicted
by one of the end point of the open interval.