# Optimizing a Tensor Network using Tensorflow¶

In this example we show how a general machine learning strategy can be used to optimize arbitrary tensor networks with respect to any target loss function.

We’ll take the example of maximizing the overlap of some matrix product state with periodic boundary conditions with a densely represented state, since this does not have a simple, deterministic alternative.

quimb makes use of opt_einsum which can contract tensors with a variety of backends. Here we’ll use tensorflow.

[1]:

%matplotlib inline
import quimb as qu
import quimb.tensor as qtn
from quimb.tensor.optimize_tensorflow import TNOptimizer

# create a session to explicitly not use eager-mode
import tensorflow as tf
sess = tf.InteractiveSession()


First, find a (dense) PBC groundstate, $$| gs \rangle$$:

[2]:

L = 16
H = qu.ham_heis(L, sparse=True, cyclic=True)
gs = qu.groundstate(H)


Then we convert it to a dense 1D ‘tensor network’:

[3]:

# this converts the dense vector to an effective 1D tensor network (with only one tensor)
target = qtn.Dense1D(gs)
print(target)

Dense1D([
Tensor(shape=(2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2), inds=('k0', 'k1', 'k2', 'k3', 'k4', 'k5', 'k6', 'k7', 'k8', 'k9', 'k10', 'k11', 'k12', 'k13', 'k14', 'k15'), tags={'I7', 'I1', 'I11', 'I8', 'I0', 'I14', 'I9', 'I6', 'I12', 'I5', 'I15', 'I4', 'I3', 'I13', 'I10', 'I2'}),
], structure='I{}', nsites=16)


Next we create an initial guess random MPS, $$|\psi\rangle$$:

[4]:

bond_dim = 32
mps = qtn.MPS_rand_state(L, bond_dim, cyclic=True)
mps.graph()


We now need to set-up the function that ‘prepares’ our tensor network. In the current example this involves making sure the state is always normalized.

[5]:

def normalize_state(psi):
return psi / (psi.H @ psi) ** 0.5


Then we need to set-up our ‘loss’ function, the function that returns the scalar quantity we want to minimize.

[6]:

def negative_overlap(psi, target):
return - (psi.H @ target) ** 2  # minus so as to minimize

[7]:

optmzr = TNOptimizer(
mps,  # our initial input, the tensors of which to optimize
loss_fn=negative_overlap,
norm_fn=normalize_state,
loss_constants={'target': target},  # this is a constant TN to supply to loss_fn
)


Now we are ready to optimize our tensor network! Note how we supplied the constant tensor network target - its tensors will not be changed.

[8]:

mps_opt = optmzr.optimize(100)  # perform 100 gradient descent steps

-0.9963936206854458: 100%|██████████| 100/100 [00:10<00:00, 10.56it/s]


The output optimized (and normalize) tensor netwwork has already been converted back to numpy:

[9]:

type(mps_opt[0].data)

[9]:

numpy.ndarray


And we can explicitly check the returned state indeed matches the loss shown above:

[10]:

((mps_opt.H & target) ^ all) ** 2

[10]:

0.9964696823881


Other things to think about might be:

• playing with the optimizer type (here ADAM) and settings (e.g. learning rate). 'scipy' is another good choice which defaults to the L-BFGS-B algorithm.

• using single precision data for better GPU acceleration

• try torch instead using from quimb.tensor.optimize_pytorch import TNOptimizer, though you won’t be able to optimize non-real tensor networks