Lecture 3: Tensor Product Ansatz

Transcription

1 Lecture 3: Tensor Product nsatz Graduate Lectures Dr Gunnar Möller Cavendish Laboratory, University of Cambridge slide credits: Philippe Corboz (ETH / msterdam) January 2014 Cavendish Laboratory

2 Part I: Introduction on the blackboard...

3 Reduced density matrix MX i = s kk u k i v k i k Reduced density matrix of left side: describes system on the left side B =tr B [ ] =tr B [ ih ] = X k k u k ihu k k = s 2 kk probability DMRG: Keeping the basis states on the left (right) side with largest probabilities gives the best approximation to the exact wave function Entanglement entropy: S() = tr[ log ]= X k k log k Product state:... Maximally entangled state: S() = 1 log 1 = 0 S() = X k 1 M log 1 M = logm # relevant states exp(s) How large is S in a physical state? How does it scale with system size?

4 How many relevant singular values? s kk i = MX k s kk u k i v k i how many relevant singular values? bond dimension D = U l s V r physical state i i = X k s kk u k i v k i all singular values important keeping the largest singular values minimizes the error relevant singular values truncate this part M k i i KEY IDE OF DMRG!

5 Examples for Entanglement spectra e.g. fractional quantum Hall states (on the sphere) n = ln x n Laughlin state (bosons, N=12) ν=1/2 Coulomb, bosons, N= L z n = ln x n L z Universal part provides signature of chiral boson edge theory State is already wellapproximated by eigenvalues of low entanglement energy

6 rea law of the entanglement entropy 1D... E E L... 2D E L Entanglement entropy S() = tr[ log ]= i i log i # relevant states exp(s) General (random) state Physical state (local Hamiltonian) S(L) L d (volume) S(L) L d 1 (area law) Note: Some (critical) ground states have a logarithmic correction to the area law 1D S(L) =const = const 2D S(L) L exp( L)

7 Entanglement entropy & area law: Proofs (incomplete list) Gapped 1D systems have an area law! S(L) <S max = const( ) Hastings D critical system: S(L) = c 3 log(l) Vidal, Latorre, Rico, Kitaev 2003 Calabrese & Cardy 2004 rea law for (quadratic) gapped bosonic systems Plenio, Eisert, Dreißig, Cramer 2005 Cramer & Eisert 2006 there are gapless systems in 2D with area law (without logarithmic correction) Verstraete, Wolf, Perez-Garcia, Cirac D free fermion system: S(L) L log(l) rea law holds at finite temperature (mutual information) for all local Hamiltonians Wolf 2006, Gioev & Klich 2006 Barthel, Chung, Schollwoeck 2006 Cramer, Eisert, Plenio 2007 Wolf, Verstraete, Hastings, Cirac,

8 Examples of tensor networks = some number slide credits: Philippe Corboz (ETH / msterdam)

9 MPS & PEPS 1D MPS Matrix-product state Bond dimension D Physical indices (lattices sites) S. R. White, PRL 69, 2863 (1992) Fannes et al., CMP 144, 443 (1992) Östlund, Rommer, PRL 75, 3537 (1995) Reproduces area-law in 1D S(L) =const

10 MPS & PEPS 1D MPS Matrix-product state Bond dimension D L E rank( ) apple D S() apple log(d) =const Reproduces area-law in 1D S(L) =const

11 MPS & PEPS 1D MPS Matrix-product state Bond dimension D 2D can we use an MPS? Physical indices (lattices sites) L S. R. White, PRL 69, 2863 (1992) Fannes et al., CMP 144, 443 (1992) Östlund, Rommer, PRL 75, 3537 (1995)!!! rea-law in 2D!!! Reproduces area-law in 1D S(L) L S(L) =const D exp(l)

12 MPS & PEPS 1D MPS Matrix-product state Bond dimension D 2D PEPS (TPS) projected entangled-pair state (tensor product state) Bond dimension D Physical indices (lattices sites) S. R. White, PRL 69, 2863 (1992) Fannes et al., CMP 144, 443 (1992) Östlund, Rommer, PRL 75, 3537 (1995) F. Verstraete, J. I. Cirac, cond-mat/ Reproduces area-law in 1D S(L) =const Reproduces area-law in 2D S(L) L

13 PEPS: rea law one thick bond of dimension D L D L B L S() apple L log D L each cut auxiliary bond can contribute (at most) log D to the entanglement entropy The number of cuts scales with the cut length Reproduces area-law in 2D S(L) L

14 MPS & PEPS 1D MPS Matrix-product state Bond dimension D 2D PEPS (TPS) projected entangled-pair state (tensor product state) Bond dimension D Physical indices (lattices sites) S. R. White, PRL 69, 2863 (1992) Fannes et al., CMP 144, 443 (1992) Östlund, Rommer, PRL 75, 3537 (1995) F. Verstraete, J. I. Cirac, cond-mat/ Reproduces area-law in 1D S(L) =const Reproduces area-law in 2D S(L) L

15 ipeps 1D MPS Matrix-product state Bond dimension D 2D ipeps infinite projected entangled-pair state Physical indices (lattices sites) Jordan, Orus, Vidal, Verstraete, Cirac, PRL (2008)

16 ipeps with arbitrary unit cells 1D MPS Matrix-product state 2D ipeps with arbitrary unit cell of tensors Bond dimension D D B C D H E F G H E D B C D Physical indices (lattices sites) D H E B F C G D H E H E F G H E here: 4x2 unit cell Corboz, White, Vidal, Troyer, PRB 84 (2011) Run simulations with different unit cell sizes and compare variational energies

17 Hierarchical tensor networks (TTN/MER) MER MPS flat i 1 i 2 i 3 i 4 i 5 i 6 i 7 i 8 i 9 i 10 i 11 i 12 i 13 i 14 i 15 i 16 i 17 i 18 tensors at different length scales

18 Real-space renormalization group transformation effective model H 2 L 2 coarsegrained lattice effective model H 1 L 1 coarsegrained lattice microscopic model H 0 L 0 microscopic lattice

19 Tree Tensor Network (1D) 1D systems (non-critical) S(L) =const = const isometry L 2 coarsegrained lattice L 1 coarsegrained lattice 0 = d # relevant local states L 0 microscopic lattice

20 Tree Tensor Network (1D) 1D critical systems S(L) log(l) poly(l) 2 isometry L 2 coarsegrained lattice 1 L 1 coarsegrained lattice 0 = d # relevant local states L 0 microscopic lattice

21 The MER (The multi-scale entanglement renormalization ansatz) 1D systems (critical) S(L) log(l) = const G. Vidal, PRL 99, (2007) G. Vidal, PRL 101, (2008) KEY: disentanglers reduce the amount of short-range entanglement disentangler isometry L 2 coarsegrained lattice L 1 coarsegrained lattice 0 = d L 0 # relevant local states microscopic lattice

22 MER: Properties Let s compute O O: two-site operator O two-site operator

23 MER: Contraction Causal cone Isometries are isometric O w w = I Disentanglers are unitary u u = I

24 MER: Contraction Causal cone Isometries are isometric O w w = I Disentanglers are unitary u u = I Efficient computation of expectation values of observables!

25 2D MER (top view) Evenbly, Vidal. PRL 102, (2009) Original lattice pply disentanglers ccounts for arealaw in 2D systems S(L) L = const Coarse-grained lattice pply isometries

26 Summary: Tensor network ansatz 2D MER MPS PEPS 1D MER Structure Variational ansatz tensor network ansatz is an efficient variational ansatz for physical states (GS of local H) where the accuracy can be systematically controlled with the bond dimension Reminder: variational wave-function h H i E 0 the lower the energy the better! Different tensor networks can reproduce different entanglement entropy scaling: MPS: area law in 1D MER: log L scaling in 1D (critical systems) PEPS/iPEPS: area law in 2D 2D MER: area law in 2D branching MER: beyond area law in 2D (e.g. L log L scaling) (see Evenbly&Vidal)

27 Computational cost Leading cost: O(D k ) MPS: k =3 PEPS: k 10 N var D 4 cost (N var ) 2.5 polynomial scaling but large exponent! How large does have to be? D It depends on the amount of entanglement in the system! MPS Bond dimension D (i)peps Bond dimension D MPS for 2D system: D~exp(W) accurate for cylinders up to a width W~10 Typical size (2D): D=3000 O(10 7 ) params. D=10 O(10 4 ) params. 3 orders of magnitude smaller!

28 Summary: Tensor network algorithms 2D MER MPS PEPS 1D MER Structure Variational ansatz Find the best (ground) state Compute observables O iterative optimization of individual tensors (energy minimization) imaginary time evolution Contraction of the tensor network exact / approximate