qat.mps.StateMPO

class qat.mps.StateMPO(tensor_list: list, tensor_types: list = None, external_dims: list = None)

MPO representing a density matrix.

This can be seen as an MPS representation of the vectorized density matrix |rho>>.

Parameters:

• tensor_list (list(array)) – list of tensors for internal MPS representation

• tensor_types (list(array)) – list of tensor types for internal MPS representation

• external_dims (list(pair(int)) – the list of external dims (ket, bra)

contract()

Contract the MPO into a matrix.

Returns:

The matrix.

Return type:

np.ndarray

expectation_value(obs: Observable) → complex

Compute the expectation value of the Observable.

Parameters:

obs (Observable) – The observable to compute.

Returns:

Expectation value for given observable.

Return type:

complex

get_memory_used()

Compute memory used by the tensor network in megabytes (MB).

Returns:

Number of MB used

Return type:

float

classmethod load(filepath: str)

Load the MPO from an NPZ file.

Parameters:

filepath (str) – Path of the NPZ file.

sample(state_index: int) → float

Get probability of input state.

Parameters:

state_index (int) – State index LSB ordered (0 = |00…00>, 1 = |00…01>, …etc)

Returns:

Probability of the realisation of input state index.

Return type:

float

save(filepath: str)

Save the MPO to the disk as NumPy NPZ file.

Parameters:

filepath (str) – Path where the NPZ file will be saved.

trace() → complex

Compute trace of MPO.