quicophy · meandmytram · Apr 6, 2022 · Mar 23, 2021 · Mar 23, 2021 · Mar 23, 2021
diff --git a/experiments/decoding_classical.py b/experiments/decoding_classical.py
@@ -2,6 +2,7 @@
 In this experiment, we decode a classical error correction code.
 First, we build the codeword MPS and test it against the dense form.
 Then, we demostrate simple decoding of a classical LDPC code with DMRG.
+The script should be launched from the root of the project directory.
 """
 
 import sys
@@ -117,7 +118,7 @@ def get_mpo(self):
 # Simply speaking, it flips the bit with probability `p` and leaves it
 # the same with probability `1-p`.
 
-
+# TODO this exists in qecstruct (find out if there's a square root)
 def binary_symmetric_channel(prob):
     """
     This function returns a single-site MPO,
@@ -135,7 +136,7 @@ def binary_symmetric_channel(prob):
 # Below, we define some utility functions to operate with error correction codes from qecstruct.
 # Co-authored by Stefanos Kourtis.
 
-
+# TODO exists in qecstruct
 def linear_code_checks(code):
     """
     Given a linear code, returns a list of its checks, where each check
@@ -160,6 +161,7 @@ def linear_code_checks(code):
     return checks
 
 
+# TODO exists in qecstruct
 def get_codewords(code):
     """
     Return the list of codewords of a linear code. Codewords are returned
@@ -335,7 +337,8 @@ def get_codewords(code):
     print("Retreiving a perturbed codeword: ")
 
     # Building an initial and a perturbed codeword.
-    # TODO
+    # TODO start from random codeword
+    # TODO the error model exists in qecstruct
     INITIAL_STRING = "0" * NUM_BITS
     PERTURBED_STRING = ""
     for i in range(NUM_BITS):
@@ -378,7 +381,7 @@ def get_codewords(code):
     print("DMRG running:")
     mps_dmrg_start = create_product_state(len(density_mpo), which="0")
     # TODO start with random product state
-    engine = dmrg(mps_dmrg_start.copy(), density_mpo, chi_max=128, cut=1e-14, mode="LA")
+    engine = dmrg(mps_dmrg_start, density_mpo, chi_max=128, cut=1e-14, mode="LA")
     engine.run(20)
     mps_dmrg_final = engine.mps.to_right_canonical()
 

diff --git a/experiments/ising.py b/experiments/ising.py
@@ -339,7 +339,7 @@ def average_chain_x_magnetization(self, mps):
         ham_mpo = ising_mpo.hamiltonian_mpo()
         ham_exact = ising_exact.hamiltonian_dense()
         mps_start = create_product_state(NUMBER_OF_SITES, which="0")
-        engine = dmrg(mps_start.copy(), ham_mpo, chi_max=64, cut=1e-14, mode="SA")
+        engine = dmrg(mps_start, ham_mpo, chi_max=64, cut=1e-14, mode="SA")
         engine.run(10)
         ground_state_mps = engine.mps
         ground_state_exact = eigsh(ham_exact, k=6)[1][:, 0]

diff --git a/mpopt/mps/canonical.py b/mpopt/mps/canonical.py
@@ -26,7 +26,6 @@
 """
 
 from functools import reduce
-from copy import deepcopy
 from opt_einsum import contract
 import numpy as np
 from mpopt.mps.explicit import ExplicitMPS
@@ -44,7 +43,6 @@ def find_orth_centre(mps):
             Note that the MPS is not given as `ExplicitMPS` instance here.
     """
 
-    mps = deepcopy(mps)
     mps = _add_ghost_dimensions(mps)
 
     length = len(mps)
@@ -115,7 +113,6 @@ def move_orth_centre(mps, init_pos, final_pos):
             If inital_pos or final_pos does not match the MPS length.
     """
 
-    mps = deepcopy(mps)
     mps = _add_ghost_dimensions(mps)
 
     length = len(mps)
@@ -179,7 +176,6 @@ def _move_orth_centre_singular_values(mps, init_pos, final_pos):
             If inital_pos or final_pos does not match the MPS length.
     """
 
-    mps = deepcopy(mps)
     length = len(mps)
 
     mps = _add_ghost_dimensions(mps)
@@ -224,7 +220,6 @@ def is_canonical(mps):
     not the `ExplicitMPS` class instance.
     """
 
-    mps = deepcopy(mps)
     mps = _add_ghost_dimensions(mps)
 
     # Check if the form is left- or right- canonical
@@ -264,7 +259,6 @@ def is_canonical(mps):
 
 def _move_orth_centre_to_border(mps, init_orth_centre_index):
 
-    mps = deepcopy(mps)
     mps = _add_ghost_dimensions(mps)
     length = len(mps)
 
@@ -293,7 +287,6 @@ def to_explicit(mps):
             Minimum singular values to keep.
     """
 
-    mps = deepcopy(mps)
     length = len(mps)
 
     centres = find_orth_centre(mps)
@@ -332,7 +325,6 @@ def _add_ghost_dimensions(mps):
     This is a helper function.
     """
 
-    mps = deepcopy(mps)
     for i in [0, -1]:
         if len(mps[i].shape) == 2:
             mps[i] = np.expand_dims(mps[i], i)  # convention, see the MPS class
@@ -341,13 +333,11 @@ def _add_ghost_dimensions(mps):
 
 def inner_product(mps_1, mps_2):
     """
-    # TODO docstring
     Returns an inner product between 2 Matrix Product States.
+    Note, that this function takes an MPS as a list of tensors,
+    not the `ExplicitMPS` class instance.
     """
 
-    mps_1 = deepcopy(mps_1)
-    mps_2 = deepcopy(mps_2)
-
     if len(mps_1) != len(mps_2):
         raise ValueError(
             f"The number of sites in the first MPS is ({len(mps_1)}), while "
@@ -390,7 +380,6 @@ def to_dense(mps, flatten=True):
             Whether to merge all the physical indices to form a vector.
     """
 
-    mps = deepcopy(mps)
     dense = reduce(lambda a, b: np.tensordot(a, b, (-1, 0)), mps)
 
     if flatten:

diff --git a/mpopt/mps/explicit.py b/mpopt/mps/explicit.py
@@ -82,7 +82,7 @@ def __iter__(self):
 
     def copy(self):
         """
-        Returns a copy of the current MPS
+        Returns a copy of the current MPS.
         """
         return ExplicitMPS(self.tensors.copy(), self.singular_values.copy())
 

diff --git a/mpopt/optimizer/dmrg.py b/mpopt/optimizer/dmrg.py
@@ -111,14 +111,16 @@ class DMRG:
             .--dL            dR--.
     """
 
-    def __init__(self, mps, mpo, chi_max, cut, mode):
+    def __init__(self, mps, mpo, chi_max, cut, mode, copy=True):
         if len(mps) != len(mpo):
             raise ValueError(
                 f"The MPS has length ({len(mps)}), "
                 f"the MPO has length ({len(mpo)}), "
                 "but the lengths should be equal."
             )
-        self.mps = deepcopy(mps)
+        self.mps = mps
+        if copy:
+            self.mps = deepcopy(mps)
         self.left_environments = [None] * len(mps)
         self.right_environments = [None] * len(mps)
         self.mpo = mpo

diff --git a/mpopt/utils/utils.py b/mpopt/utils/utils.py
@@ -29,7 +29,6 @@ def svd(mat, cut=1e-14, chi_max=1e5, renormalise=False):
             Unitary matrix having right singular vectors as rows.
     """
 
-    mat = np.copy(mat)
     u_l, singular_values, v_r = np.linalg.svd(mat, full_matrices=False)
 
     max_num = min(chi_max, np.sum(singular_values > cut))
@@ -73,9 +72,6 @@ def kron_tensors(tensor_1, tensor_2, conjugate_second=False, merge_physicals=Tru
             Whether to merge physical indices.
     """
 
-    tensor_1 = np.copy(tensor_1)
-    tensor_2 = np.copy(tensor_2)
-
     if len(tensor_1.shape) != len(tensor_2.shape):
         raise ValueError("The tensors must have equal numbers of dimensions.")
 
@@ -127,8 +123,7 @@ def split_two_site_tensor(theta, chi_max=1e5, cut=1e-14, renormalise=False):
             Right isometry on site j, with legs vC, j, vR.
     """
 
-    # merge the legs to form a matrix to feed into svd
-    theta = np.copy(theta)
+    # merge the legs to form a matrix to feed into the svd
     chi_v_l, d_l, d_r, chi_v_r = theta.shape
     theta = theta.reshape((chi_v_l * d_l, d_r * chi_v_r))
 
@@ -318,7 +313,7 @@ def mpo_from_matrix(matrix, num_sites, interlaced=True, phys_dim=2, chi_max=1e5)
             f"or {tuple(phys_dims)}, instead, the matrix given has shape {matrix.shape}."
         )
 
-    # Copying the matrix not to change the origonal one inplace
+    # Copying the matrix not to change the original one inplace
     mat = matrix.copy()
 
     # Reshaping the matrix

diff --git a/tests/optimizer/test_dmrg.py b/tests/optimizer/test_dmrg.py
@@ -27,7 +27,7 @@ def test_ground_states():
 
         mps_start = create_product_state(number_of_sites, which="0")
 
-        engine = dmrg(mps_start.copy(), ham_mpo, chi_max=64, cut=1e-14, mode="SA")
+        engine = dmrg(mps_start, ham_mpo, chi_max=64, cut=1e-14, mode="SA")
         engine.run(10)
         ground_state_mps = engine.mps
         ground_state_exact = eigsh(ham_exact, k=6)[1][:, 0]