the dmrg and matrix product states
DESCRIPTION
The DMRG and Matrix Product States. Adrian Feiguin. The DMRG transformation. When we add a site to the block we obtain the wave function for the larger block as:. Let’s change the notation…. We can repeat this transformation for each l , and recursively we find. - PowerPoint PPT PresentationTRANSCRIPT
The DMRG and Matrix Product States
Adrian Feiguin
The DMRG transformationWhen we add a site to the block we obtain the wave function for the larger block as:
11
,1,
llll
sllll ssA
Let’s change the notation…
llll ssAll 1, 1
1,
11lls
lllllLll ssU
We can repeat this transformation for each l, and recursively we find
}{1,21 ...|][...][][|
1211s
lll sssAsAsAll
Notice the single index. The matrix corresponding to the open end is actually a vector!
Some properties the A matricesRecall that the matrices A in our case come from the rotation matrices U
A= 2m
m
AtA= X =1
This is not necessarily the case for arbitrary MPS’s, and normalization is usually a big issue!
The DMRG wave-function in more detail…
}{1,2,11,21
,11
...|][...][][][...][][
||
32211211s
LLlllll
llll
sssBsBsBsAsAsALllllll
ll
}{,2, ...|][...][][|
321s
LlLlll sssBsBsBLllll
We can repeat the previous recursion from left to right…
At a given point we may have
Without loss of generality, we can rewrite it:
}{1,2,21 ...|][][...][][
1211s
LL sssMsMsMsMLLL
MPS wave-function for open boundary conditions
Diagrammatic representation of MPS
The matrices can be represented diagrammatically as
][sA s
s
][sA
The dimension D of the left and right indices is called the “bond dimension”
And the contractions, as:
1 2 3s1 s2
MPS for open boundary conditions
}{1
1
}{121
}{1,2,21
...|][
...|][]...[][
...|][][...][][1211
sL
L
ll
sLL
sLL
sssM
sssMsMsM
sssMsMsMsMLLL
1 2 L
s1 s2 s3 s4 … sL
MPS for periodic boundary conditions
}{1
1
}{121
}{1,,2,21
...|][Tr
...|][]...[][Tr
...|][][...][][11211
sL
L
ll
sLL
sLL
sssM
sssMsMsM
sssMsMsMsMLLLL
1 2 3 L 1
s1 s2 s3 s4 … sL
Properties of Matrix Product States
Inner product:
1 2 Ls1 s2 s3 s4 … sL
1' 2' L'
Addition:
L
LL
sLL
sLL
sLL
MMMMMM
NNN
MM
N
ssNNN
ssMMMssMMM
~...~~...
...
~00
with
...|...
...|~...~~;...|...
21
2121
}{121
}{121
}{121
Gauge Transformation
= X X-1
There are more than one way to write the same MPS.This gives you a tool to othonormalize the MPS basis
Operators
O
The operator acts on the spin index only
' elementsh matrix wit a is sOsO
Matrix product basis
}{
1,21 ...|][...][][|1211
slll sssAsAsA
ll
1 2 ls1 s2 s3 s4 sl
}{,2, ...|][...][][|
321s
LlLlll sssBsBsBLllll
1l 2llsl+1 sl+2 sl+3 sl+4 sL
L
ll '|As we saw before, in the dmrg basis we get:
1 2L
1' 2'L'
ll ',
The DMRG w.f. in diagrams
}{1,2,1,21
}{1,2,11,21
...|][...][][][...][][
...|][...][][][...][][
322111211
32211211
sLLlll
sLLlllll
sssBsBsBsAsAsA
sssBsBsBsAsAsA
Lllllllll
Lllllll
1 2 ls1 s2 s3 s4 sl
2l 3l1lsl+1 sl+2 sl+3 sL
Ll 1l
(It’s a just little more complicated if we add the two sites in the center)
The AKLT State 1 with
31 2
11 SSSSSHi
iiiiAKLT
We replace the spins S=1 by a pair of spins S=1/2 that are completely symmetrized
ibiai
ibiaibiai
ibiai
210
… and the spins on different sites are forming a singlet
biaibiai ,1,,1,2
1
a b
The AKLT as a MPS
The local projection operators onto the physical S=1 states are
1000
;0
21
210
;0001 0
ababab MMM
The mapping on the spin S=1 chain then reads
}{ ,
,,, },{}{...2
22
1
11s ba
sba
sba
sba basMMM L
LL
111
2
32
1
21
132
2
2221
1
11
,,,}{
,,,
}{,,,,,,
with }{...
}{...
ll
l
ll
l
ll
L
L
L
L
LL
abs
bas
aas
saa
saa
saaAKLT
sab
sbaab
sbaab
sbaAKLT
MAsAAA
sMMMP
Projecting the singlet wave-function we obtain
The singlet wave function with singlet on all bonds is
with },{...}{ ,
,,, 13221 s ba
ababab baL
02
12
10ab
What are PEPS?“Projected Entangled Pair States” are a generalization of MPS to “tensor networks” (also referred to as “tensor renormalization group”)
Variational MPSWe can postulate a variational principle, starting from the assumption that the MPS is a good way to represent a state. Each matrix A has DxD elements and we can consider each of them as a variational parameter. Thus, we have to minimize the energy with respect to these coefficients, leading to the following optimization problem:
HA
min
DMRG does something very close to this…
MPS representation of the time-evolution
A MPS wave-function is written as
}{ }{212211
212211}{
,...,][...][][
,...,][]...[][Tr
14321
i j
N
i
sNNN
NNNs
ssssAsAsA
ssssAsAsA
The matrices can be represented diagramaticaly as
][sA s
And the contractions (coefficients), as:
1 2 3 N 1s1 s2 s3 s4 sN
s1 s2 s3 sN
MPS representation of the time-evolutionThe two-site time-evolution operator will act as:
1 2 3 N 1
U
s4 s5
Which translates as:
65
54
54
5454]'[]'[ 55
','
,','44 sAUsA
ss
ssss
s1 s2 s3
1 2 3
U
N 14 4 5 6 6
s4 s5
s6 sN
Swap gatesIn the MPS representation is easy to exchange the states of two sites by applying a “swap gate” si sj
s’i s’j
E.M Stoudenmire and S.R. White, NJP (2010)
],[ jiS
And we can apply the evolution operator between sites far apart as:
s1 s2 s3 sN
1 2 3 N 1
U
Matrix product basis
}{1,21 ...|][...][][|
1211s
lll sssAsAsAll
1 2 ls1 s2 s3 s4 sl
}{,2, ...|][...][][|
321s
LlLlll sssBsBsBLllll
1l 2llsl+1 sl+2 sl+3 sl+4 sL
L
(a)
(b)