
χ3/χ2=χ1.5for a QGN. This is the same ratio
as MPS algorithms, which are very efficient in 1D.
For the isometric tensor network TEBD3algorithm
in 3D, this ratio scales as χ12/χ6=χ2[11], which is
remarkably efficient but not as good as the ratio for
a QGN or MPS.
Although most tensor networks typically directly
encode a wavefunction or density matrix, quantum
gauge networks depart from this habit. However,
a density matrix can be computed from a QGN
similar to Eq. (4)for the gauge picture. Unlike a
generic PEPS [3] but similar to a matrix product state
(MPS) or isometric tensor network [8,9,10], local
expectation values can be efficiently computed from
a QGN. A disadvantage of quantum gauge networks
is that unphysical states can also be encoded. As
such, using a QGN to variationally optimize a ground
state is not as straight-forward as for tensor networks
that directly encode a wavefunction. We leave QGN
ground state optimization algorithms to future work.
In this work, we focus on QGN fundamentals and time
dynamics alorithms.
In Sec. 2, we discuss basic properties of quantum
gauge networks and how a QGN can be constructed.
We also show that for an arbitrary wavefunction
(including fermionic wavefunctions), all 2k-point
correlation functions of Mmany operators can be
encoded exactly by a QGN with bond dimension
O(Mk)[Eq. (32)], while an MPS of qubits can require
an exponentially larger bond dimension 2M/6for
k= 1. In Sec. 3, we present a QGN algorithm
for approximately simulating quantum dynamics in
the gauge picture. We benchmark the algorithm
using simulations of fermionic Hamiltonians in spatial
dimensions up to three.
2 Quantum Gauge Networks
To define a quantum gauge network (QGN), we first
choose a collection of possibly-overlapping patches
of space that cover space. For example, one could
choose the patches to consist of just a single lattice
site. Another natural choice is to take the patches
to have the same support as the Hamiltonian terms.
That is, we might choose patches that are pairs of
nearest-neighbor sites if the Hamiltonian terms act
on nearest-neighbor sites. A QGN then consists of
(1) a local wavefunction |ψI⟩for each spatial patch;
(2) non-unitary connections VIJ =V†
JI to relate the
Hilbert spaces of nearby patches, as depicted in Fig. 1;
and (3) a collection of truncated operators to act on
the truncated Hilbert space at each patch.
The local wavefunctions and connections are similar
to the those within the gauge picture [1], except the
Hilbert space is truncated. If the full Hilbert space
has dimension N, then |ΨI⟩and ˆ
UIJ in the gauge
picture have dimensions Nand N×N, respectively.
Since Nis exponentially large in system size, it is
useful to truncate the full Hilbert space dimension
for approximate simulations. Therefore, we consider
truncated local wavefunctions |ψI⟩and connections
VIJ , which have truncated dimensions χIand χI×χJ,
respectively, where typically χI≪N. We use capital
and lower-case Greek letters (e.g. |ΨI⟩vs |ψI⟩)
for wavefunctions in the full and truncated Hilbert
spaces, respectively. Similarly, we place hats on
operators that act within the full Hilbert space (e.g.
ˆ
UIJ ), while operators within the truncated Hilbert
space (e.g. VIJ ) do not have hats.
In order to calculate expectation values of local
operators ˆ
AIin the original Hilbert space, we
must also define truncated operators, i.e. χI×χI
matrices AI, that act on the truncated Hilbert space.
Throughout this work, ˆ
AIalways denotes an operator
that acts within a patch I, and similar for ˆ
BJ,
etc. The truncated operators AIare notationally
distinguished from the original operators ˆ
AIby the
lack of a hat. In Sec. 2.1, we present a concrete
mapping to obtain a QGN and truncated operators.
However, in many cases (e.g. Appendix C.1.1 and E)
the truncated operators can be taken to be a simple
Kronecker product, such as σµ
I=1⊗σµ, where 1is
an identity matrix and σµis a 2×2Pauli matrix.
Ideally, we want the quantum gauge network to
accurately encode approximate expectation values.
For example, if the QGN is an approximation for a
wavefunction |Ψ⟩, then we would like the QGN to
accurately encode local expectation values; i.e. we
want ⟨ψI|AI|ψI⟩ ≈ ⟨Ψ|ˆ
AI|Ψ⟩. Similarly, we typically
also want expectation values of string operators to
also approximately match, e.g.
⟨ψI|AIVIJ BJVJ K CK|ψK⟩≈⟨Ψ|ˆ
AIˆ
BJˆ
CK|Ψ⟩(6)
Note that in order to express a string operator that
acts on multiple spatial patches using a QGN, it is
essential to insert connections VIJ between operators
and wavefunctions associated with different spatial
patches.
Similar to the gauge picture of quantum dynamics,
a density matrix can be extracted from a QGN. Thus,
a QGN most generally encodes a mixed state rather
than a pure state. For example, analogous to Eq. (4)
for a system of nqubits, a density matrix can be
approximately extracted from a QGN via
ˆρ≈2−nX
µ1···µn
ˆσµ1
1· · · ˆσµn
n(7)
⟨ψ1|σµ1
1V1,2σµ2
2· · · Vn−1,n σµn
n|ψn⟩
where ˆσµ
iare Pauli operators. Here, we take each
patch to consist of just a single qubit (for simplicity),
and we use I= 1, . . . , n to index the qubits/patches
along a string of nearest-neighbors, e.g. as in Fig. 2.
However, due to the approximations induced by the
QGN, different paths (e.g. those in Fig. 2) can yield
different density matrices.
Accepted in Quantum 2023-09-04, click title to verify. Published under CC-BY 4.0. 3