Quantum Gauge Networks A New Kind of Tensor Network

2025-04-29 0 0 1.57MB 21 页 10玖币
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Quantum Gauge Networks: A New Kind of Tensor Network
Kevin Slagle1,2,3
1Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005 USA
2Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
3Institute for Quantum Information and Matter and Walter Burke Institute for Theoretical Physics, California Institute of Technology,
Pasadena, California 91125, USA
Although tensor networks are powerful
tools for simulating low-dimensional quantum
physics, tensor network algorithms are very
computationally costly in higher spatial
dimensions. We introduce quantum gauge
networks: a different kind of tensor network
ansatz for which the computation cost of
simulations does not explicitly increase for
larger spatial dimensions. We take inspiration
from the gauge picture of quantum dynamics
[1], which consists of a local wavefunction
for each patch of space, with neighboring
patches related by unitary connections. A
quantum gauge network (QGN) has a similar
structure, except the Hilbert space dimensions
of the local wavefunctions and connections
are truncated. We describe how a QGN
can be obtained from a generic wavefunction
or matrix product state (MPS). All 2k-point
correlation functions of any wavefunction for
Mmany operators can be encoded exactly
by a QGN with bond dimension O(Mk). In
comparison, for just k= 1, an exponentially
larger bond dimension of 2M/6is generically
required for an MPS of qubits. We provide
a simple QGN algorithm for approximate
simulations of quantum dynamics in any
spatial dimension. The approximate dynamics
can achieve exact energy conservation for
time-independent Hamiltonians, and spatial
symmetries can also be maintained exactly.
We benchmark the algorithm by simulating
the quantum quench of fermionic Hamiltonians
in up to three spatial dimensions.
Contents
1 Introduction 1
2 Quantum Gauge Networks 3
2.1 QGN from Truncation Maps . . . . . . 4
2.2 Truncation Map Construction . . . . . 5
2.2.1 Warmup Example . . . . . . . 5
2.2.2 Generic Case . . . . . . . . . . 6
2.2.3 Long-Range Correlation Func-
tions............... 6
3 Time Evolution Algorithm 7
3.1 Fermion Quench . . . . . . . . . . . . 8
4 Outlook 11
Acknowledgments 12
References 12
A Higgsed Lattice Gauge Theory 15
B Matrix Product State Mapping 15
B.1 MPS Review . . . . . . . . . . . . . . 15
B.2 Quantum Gauge Network from MPS . 16
C QGN Examples 16
C.1 Mixed State Example . . . . . . . . . 16
C.1.1 Kronecker Product Operators . 17
C.2 Cat State Example . . . . . . . . . . . 17
C.3 Bosonic Coherent States . . . . . . . . 18
C.4 Fermion Slater Determinants . . . . . 18
C.5 Rainbow State . . . . . . . . . . . . . 19
D Energy Conservation 19
E Ising Model Quench 19
F Modified Runge-Kutta 20
1 Introduction
Tensor network algorithms [2,3,4,5,6,7] are very
useful for simulating strongly-correlated quantum
physics. In one spatial dimension, matrix product
state (MPS) algorithms [3,4,5] are often the best
available tool for this task. Although still useful,
tensor network algorithms in higher dimensions [3,
5,8,9,10,11,12,13,14,15,16] typically suffer
from computational costs that scale as a high power
of the bond dimension. Therefore, we are motivated
to study a different kind of tensor network ansatz
that is more computationally efficient in many spatial
dimensions.
We take inspiration from the gauge picture of
quantum dynamics [1], which adds “gauge fields” to
Schr¨odinger’s picture in order to make spatial locality
explicit in the equations of motion. In the gauge
Accepted in Quantum 2023-09-04, click title to verify. Published under CC-BY 4.0. 1
arXiv:2210.12151v5 [quant-ph] 11 Sep 2023
picture, one first chooses a collection of possibly-
overlapping patches of space that cover space. For
example, one could choose the patches to be pairs of
nearest-neighbor sites on a lattice. We use capital
letters, I,J, or K, to denote a spatial patch.
Each patch is assigned a local wavefunction |ΨI
(with the same Hilbert space dimension as the usual
wavefunction), and the Hilbert spaces of neighboring
patches are related by unitary connections ˆ
UIJ (which
act on the entire Hilbert space), as depicted in Fig. 1.
In the simplest setting, the Hamiltonian is written as
a sum over terms ˆ
HIthat act within a single patch I:
ˆ
H=X
I
ˆ
HI(1)
The local wavefunctions and connections time-evolve
according to
t|ΨI=iˆ
HI|ΨI
tˆ
UIJ =iˆ
HIˆ
UIJ +iˆ
UIJ ˆ
HJ
(2)
where
ˆ
HI=JI̸=
X
J
ˆ
UIJ ˆ
HJˆ
UJI (3)
is the sum of local Hamiltonian terms supported on
patches that overlap with patch I. Typically, we
initialize ˆ
UIJ (0) = ˆ
1and |ΨI(0)=|Ψ(0)at time
t= 0, where ˆ
1is the identity operator and |Ψ(t)
is the usual wavefunction in the Schr¨odinger picture.
The expectation value Ψ|ˆ
AI|Ψof a local operator
ˆ
AIthat only acts within the patch Ican be evaluated
in the gauge picture as ΨI|ˆ
AI|ΨI. To calculate
an expectation value Ψ|ˆ
AIˆ
BJ|Ψfor a product of
operators acting on different patches, a connection
ˆ
UIJ must be inserted in the gauge picture, as in
ΨI|ˆ
AIˆ
UIJ ˆ
BJ|ΨJ. Such correlation functions can
be used to calculate the density matrix from the
gauge picture local wavefunctions and connections.
For example, for a system of nqubits, the density
matrix is
ˆρ= 2nX
µ1···µn
ˆσµ1
1· · · ˆσµn
n(4)
Ψ1|ˆσµ1
1ˆ
U1,2ˆσµ2
2· · · ˆ
Un1,n ˆσµn
n|Ψn
where ˆσµ
iare Pauli operators, and here we take each
patch to consist of just a single qubit (for simplicity)
so that I= 1, . . . , n indexes the qubits/patches along
some path. The local wavefunction |ΨIis local in
the sense that its dynamics are local and connections
are required to extract information about operators
outside the patch I. Time evolution preserves the
following network of relations:
ˆ
UIJ |ΨJ=|ΨI
ˆ
UIJ ˆ
UJK =ˆ
UIK
(5)
along with ˆ
U
IJ =ˆ
UJI and ˆ
UII =ˆ
1.
V
I
J
ψIψJ
Figure 1: An example of a chain of qubits (black dots)
and spatial patches (colored ovals) consisting of pairs
of neighboring qubits. In the gauge picture, a local
wavefunction |ΨIis associated with each patch I, and the
Hilbert spaces of neighboring patches are related by unitary
connections ˆ
UIJ . A quantum gauge network analogously
consists of local wavefunctions |ψIin truncated Hilbert
spaces that are related by non-unitary connections VIJ .
In this work, we truncate the Hilbert spaces of the
local wavefunctions and connections in the gauage
picture so that approximate quantum dynamics
simulations can be performed on a computer. The
utility of the gauge picture for approximating
quantum mechanics is that locality is explicit in both
the time dynamics and the structure of the local
wavefunctions and connections. Furthermore, the
connections allow us to utilize different truncated
Hilbert spaces for different patches of space. We call
the resulting network of truncated local wavefunctions
and connections a quantum gauge network (QGN).
An advantage of quantum gauge networks is that
unlike traditional tensor networks (e.g. PEPS [3]),
a QGN only involves matrices and vectors (rather
than tensors with many indices) regardless of the
spatial dimension. As such, it is natural for a
QGN algorithm to only require a computation time
(e.g. CPU time) that scales as O(χ3), regardless
of the number of spatial dimensions, where χis the
dimension of the truncated Hilbert spaces. χcan be
viewed as the bond dimension of the QGN. Thus, the
natural O(χ3)computation time for a QGN algorithm
is the same as for MPS algorithms, which are very
efficient in one spatial dimension (1D). In three spatial
dimensions (3D), the most computationally efficient
tensor network in previous literature may be the
isometric tensor network [8,9,10], for which the
computation time of the TEBD3algorithm is O(χ12)
[11].1Thus, QGN algorithms can require significantly
less computation time for fixed bond dimension. This
is useful since larger bond dimensions allow more
correlations to be transported through the tensor
network.
The computation time per variational parameter
also scales favorable for QGN algorithms. For
a QGN, the number of parameters scales as χ2
(due to the matrix-valued connections). Therefore,
the computation time per parameter scales as
1More generally, the TEBD3computation time is
O(D10χ2) + O(Dχ6) when the bond dimension χof the central
bonds is different from the bond dimension Dof the other
bonds. [11] In 2D, the TEBD2computation time scales as
χ7. [8]
Accepted in Quantum 2023-09-04, click title to verify. Published under CC-BY 4.0. 2
χ32=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 χ126=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 |ψIfor 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 |ΨIand 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 |ψIand connections
VIJ , which have truncated dimensions χIand χI×χJ,
respectively, where typically χIN. We use capital
and lower-case Greek letters (e.g. |ΨIvs |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=1σµ, 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
ˆρ2nX
µ1···µn
ˆσµ1
1· · · ˆσµn
n(7)
ψ1|σµ1
1V1,2σµ2
2· · · Vn1,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
(a) (b) (c)
Figure 2: Three different paths that cover all lattice sites.
Also similar to the gauge picture, quantum gauge
networks exhibit a local gauge symmetry:
|ψI⟩ → ΛI|ΨI
VIJ ΛIVIJ Λ
J
AIΛIAIΛ
I
(8)
where ΛIis a unitary matrix. Expectation values
must be invariant under this symmetry.
Since local expectation values Ψ|ˆ
AI|Ψ⟩ ≈
ψI|AI|ψIare encoded in the local wavefunctions
|ψI, we can think of the local wavefunction as a
purified reduced density matrix for a spatial patch.
The connections VIJ encode long-range correlations
between the spatial patches.
It is desirable for a QGN to at least approximately
obey the following consistency conditions
VIJ |ψJ⟩ ≈ |ψI
VIJ VJ K VIK
(9)
Although it is easy to make the first relation exact,
the second will typically only hold approximately.
Typically, a QGN will only possess a VIJ for nearby
patches Iand J(and not for far away patches).
Thus, the second relation only applies if all three
connections (VIJ ,VJ K , and VIK ) are contained in
the QGN. The connections VIJ should have singular
values less than or equal to 1 (to ensure that
expectation values are never larger than the largest
eigenvalue of the measured operator).
When VIJ |ψJ=|ψIholds exactly, QGN
connected correlation functions obey the usual
identity (with a VIJ inserted):
ψIAI− ⟨AIVIJ BJ− ⟨BJψJ
=ψI|AIVIJ BJ|ψJ⟩−⟨AI⟩ ⟨BJ(10)
where we abbreviate AI=ψI|AI|ψIand
BJ=ψJ|BJ|ψJ. Therefore, if the QGN
connected correlation function [Eq. (10)] is small, we
are guaranteed that ψI|AIVIJ BJ|ψJ⟩ ≈ ⟨AI⟩ ⟨BJ,
as one should expect. The equations in this paragraph
also hold for longer chains of connections; e.g. they
also hold if we replace VIJ with VI K VKLVLI .
original
Hilbert space
I
QI
J
K
QK
truncated Hilbert spaces
Figure 3: Each truncation map QImaps a subspace of
the original Hilbert space on to the truncated Hilbert space
associated with patch I.
2.1 QGN from Truncation Maps
In this subsection, we study a concrete construction
to obtain a quantum gauge network. The input for
this QGN construction is a wavefunction |Ψ, along
with a truncation map QIfor each patch of space. If
we want to construct a QGN from a density matrix
instead, then |Ψshould be chosen to be a purification
of the density matrix. The truncation maps are χI×N
matrices (where Nis the dimension of the full Hilbert
space) that satisfy:
QIQ
I=ˆ
1
Q
IQI|Ψ=|Ψ(11)
Therefore Q
Iis an isometry matrix whose image
includes the wavefunction. (A matrix Mis isometric
if MM=1.) Intuitively, each QImaps a select
subspace of states into a truncated Hilbert space, as
depicted in Fig. 3. In the next subsection, we will
explain how one can obtain useful truncation maps.
With this data, we can construct the following
QGN:
|ψI=QI|Ψ
VIJ =QIQ
J
(12)
Note that VII =1due to Eq. (11). Local operators ˆ
AI
with support on a spatial patch Iare also truncated:
AI=QIˆ
AIQ
I(13)
Note that this equation can be used for both bosonic
and fermionic operators. In the limit that the bond
dimension χINapproaches the full Hilbert space
dimension N, this truncation mapping will result in a
QGN that encodes correlation functions [e.g. Eq. (6)]
exactly.
The operator norm of VIJ is bounded by
||VIJ ||op ≤ ||QI||op||Q
J||op = 1. Thus, the resulting
connections VIJ have singular values that are less than
or equal to 1. One can verify that
VIJ |ψJ=|ψI(14)
from Eq. (9)holds exactly.
Accepted in Quantum 2023-09-04, click title to verify. Published under CC-BY 4.0. 4
In practice, the truncation mapping can only be
directly applied for wavefunctions that are simple
enough such that the truncation can be efficiently
computed. However, the construction is also useful for
theoretically understanding how a QGN can encode a
wavefunction.
A trivial example of a QGN can be obtained from
a product state wavefunction |Ψ=|ψ1⟩ ⊗ |ψ2⟩ ⊗
· · · |ψn. The truncation maps can be chosen to be
QI=|ψI⟩ ⟨Ψ|, with I= 1, . . . , n. This results in a
QGN with local wavefunctions |ψIand connections
VIJ =|ψI⟩⊗⟨ψJ|. The truncated operators simply
act within the on-site Hilbert space. Additional
examples of quantum gauge networks can be found
in Appendix C.
Note that not all quantum gauge networks can be
obtained from the truncation mapping. For example,
generating a QGN by sampling random numbers for
|ψIand VIJ will result in a very unphysical QGN
that is not consistent with any wavefunction. This
is in contrast to MPS or PEPS tensor networks, for
which a random number initialization still returns a
physical (although unnormalized) wavefunction.
In Appendix B, we show how to obtain truncation
maps from the canonical form of a matrix product
state (MPS) [3,4,5]. If the MPS has bond dimension
χ, then the QGN will have bond dimensions equal
to 2, where dis the Hilbert space dimension at
each site. (d= 2 for qubits.) The idea of the
mapping is that the canonical form of an MPS can
consist of a center tensor at Ithat is surrounded by
isometric tensors. The isometric tensors are then used
to construct a truncation map QI, while the center
tensor is a local wavefunction |ψI.
2.2 Truncation Map Construction
The accuracy of the truncation critically depends on a
good choice of truncation maps QI. Suppose we want
to choose truncation maps such that the quantum
gauge network exactly encodes the expectation values
of a chosen collection of operator strings. For
example, the expectation value of ˆ
AIˆ
BJˆ
CKis encoded
exactly if
ψI|AIVIJ BJVJ K CK|ψK=Ψ|ˆ
AIˆ
BJˆ
CK|Ψ(15)
Below, we show that this exact encoding can be
achieved by choosing truncation maps such that the
image of each Q
Iis the span of certain strings of
operators involved in the expectation values.
In general, we will find that the required bond
dimension χIfor each patch Iis bounded by
χI1 + 2pI[Eq. (23)], where pIis the number of
chosen operator strings (which we want to encode)
that act on the patch I. We will also find that
bond dimension O(Mk)[Eq. (32)] is sufficient to
exactly encode all 2k-point correlation functions of
Mdifferent operators.
2.2.1 Warmup Example
Before presenting a generic algorithm for obtaining
the truncation maps, let us first discuss an instructive
example. Suppose we want to ensure that the QGN
encodes the expectation value in Eq. (15)exactly for
a particular choice of local operators ( ˆ
AI,ˆ
BJ, and
ˆ
CK) and spatial patches I̸=J̸=K. Below, we show
that any choice of truncation maps with the following
images is sufficient:
im(Q
I) = span|Ψ,ˆ
A
I|Ψ
im(Q
J) = span|Ψ,ˆ
A
I|Ψ,ˆ
CK|Ψ
im(Q
K) = span|Ψ,ˆ
CK|Ψ(16)
span|Ψ1,...,|Ψmdenotes the vector space
spanned by the vectors |Ψ1,...,|Ψm. We set
the bond dimensions to be equal to the vector
space dimension of the images: χI=dim(im(Q
I)).
Specifying these images determines the truncation
maps QIup to a unitary gauge transformation
QIΛIQI, where ΛIis a unitary matrix. The choice
of gauge does not affect QGN expectation values. For
examples of a quantum gauge networks obtained in
this way, see Appendix C.
On a computer, a Q
Iwith the desired image
can be calculated from the compact singular value
decomposition MI=Q
ISIRI. Here, MIis a matrix
of column vector, which each encode one of the
wavefunctions contained in the span of im(Q
I).SIis
aχI×χIdiagonal matrix of nonzero singular values,
and R
Iis an isometry matrix.
Note that if |Φ⟩ ∈ im(Q
I), then Q
IQI|Φ=|Φ,
which is a useful property. This follows because
|Φ⟩ ∈ im(Q
I)implies that there exists |ϕsuch
that Q
I|ϕ=|Φ, which then implies that
Q
IQI|Φ=Q
IQIQ
I|ϕ=Q
I|ϕ=|Φwhere the
middle equality follows from QIQ
I=ˆ
1[Eq. (11)].
For each patch, the image must contain |Ψso that
Eq. (11)is satisfied. Next, we show that Eq. (16)is
sufficient to exactly encode the expectation value in
Eq. (15):
ψI|AIVIJ BJVJ K CK|ψK
=Ψ|Q
I(QIˆ
AIQ
I)VIJ BJVJ K (QKˆ
CKQ
K)QK|Ψ
=Ψ|ˆ
AIQ
IVIJ BJVJ K QKˆ
CK|Ψ
=Ψ|ˆ
AIQ
I(QIQ
J)BJ(QJQ
K)QKˆ
CK|Ψ
=Ψ|ˆ
AIQ
JBJQJˆ
CK|Ψ
=Ψ|ˆ
AIQ
J(QJˆ
BJQ
J)QJˆ
CK|Ψ(17)
=Ψ|ˆ
AIˆ
BJˆ
CK|Ψ
We used the identities |ψI=QI|Ψand
VIJ =QIQ
J[Eq. (12)], AI=QIˆ
AIQ
I[Eq. (13)], and
Q
IQI|Φ=|Φwhenever |Φ⟩ ∈ im(Q
I)in Eq. (16).
Note that even if all the local operators commute,
the path of the string matters. For example, Eq. (16)
Accepted in Quantum 2023-09-04, click title to verify. Published under CC-BY 4.0. 5
摘要:

QuantumGaugeNetworks:ANewKindofTensorNetworkKevinSlagle1,2,31DepartmentofElectricalandComputerEngineering,RiceUniversity,Houston,Texas77005USA2DepartmentofPhysics,CaliforniaInstituteofTechnology,Pasadena,California91125,USA3InstituteforQuantumInformationandMatterandWalterBurkeInstituteforTheoretical...

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