add truncation and expansion with qubit state

src/qutip_qip/qubits.py

@@ -1,7 +1,10 @@
 """Generating qubit states."""
-__all__ = ["qubit_states"]
 
+__all__ = ["qubit_states", "truncate_to_qubit_state", "expand_qubit_state"]
+
+import qutip
 from qutip import tensor, basis
+import numpy as np
 from numpy import sqrt
 
 
@@ -35,3 +38,141 @@ def qubit_states(N=1, states=[0]):
             for alpha in state_list
         ]
     )
+
+
+def _find_reduced_indices(dims):
+    """Find the forresponding indices for a given qu"""
+    if len(dims) == 1:
+        return np.array([0, 1], dtype=np.intp)
+    else:
+        rest_reduced_indices = _find_reduced_indices(dims[1:])
+        rest_full_dims = np.product(dims[1:])
+        #  [indices + 0, indices + a]
+        reduced_indices = np.array(
+            [[0], [rest_full_dims]], dtype=np.intp
+        ) + np.array(
+            [rest_reduced_indices, rest_reduced_indices], dtype=np.intp
+        )
+        return reduced_indices.reshape(reduced_indices.size)
+
+
+def truncate_to_qubit_state(state):
+    """
+    Truncate a given quantum state into the computational subspace,
+    i.e., each subspace is a 2-level system such as
+    ``dims=[[2, 2, 2...],[2, 2, 2...]]``.
+    The non-computational state will be discarded.
+    Notice that the trace truncated state is in general small than 1.
+
+    Parameters
+    ----------
+    state : :obj:`qutip.Qobj`
+        The input quantum state, either a ket, a bra or a square operator.
+
+    Returns
+    -------
+    truncated_state : :obj:`qutip.Qobj`
+        The truncated state.
+
+    Examples
+    --------
+    >>> import qutip
+    >>> from qutip_qip.qubits import truncate_to_qubit_state
+    >>> state = qutip.rand_ket(6, dims=[[2, 3], [1, 1]], seed=0)
+    >>> state  # doctest: +NORMALIZE_WHITESPACE
+        Quantum object: dims = [[2, 3], [1, 1]], shape = (6, 1), type = ket
+        Qobj data =
+        [[-0.10369056+0.02570495j]
+        [ 0.34852171+0.06053252j]
+        [-0.05552249+0.37861125j]
+        [ 0.41247492-0.38160681j]
+        [ 0.25951892-0.36729024j]
+        [ 0.12978757-0.42681426j]]
+    >>> truncate_to_qubit_state(state)  # doctest: +NORMALIZE_WHITESPACE
+        Quantum object: dims = [[2, 2], [1, 1]], shape = (4, 1), type = ket
+        Qobj data =
+        [[-0.10369056+0.02570495j]
+        [ 0.34852171+0.06053252j]
+        [ 0.41247492-0.38160681j]
+        [ 0.25951892-0.36729024j]]
+    """
+
+    if state.isbra:
+        dims = state.dims[1]
+        reduced_dims = [[1] * len(dims), [2] * len(dims)]
+    elif state.isket:
+        dims = state.dims[0]
+        reduced_dims = [[2] * len(dims), [1] * len(dims)]
+    elif state.isoper and state.dims[0] == state.dims[1]:
+        dims = state.dims[0]
+        reduced_dims = [[2] * len(dims), [2] * len(dims)]
+    else:
+        raise ValueError("Can only truncate bra, ket or square operator.")
+    reduced_indices = _find_reduced_indices(dims)
+    # See NumPy Advanced Indexing. Choose the corresponding rows and columns.
+    zero_slice = np.array([0], dtype=np.intp)
+    reduced_indices1 = reduced_indices if not state.isbra else zero_slice
+    reduced_indices2 = reduced_indices if not state.isket else zero_slice
+    output = state[reduced_indices1[:, np.newaxis], reduced_indices2]
+    return qutip.Qobj(output, dims=reduced_dims)
+
+
+def expand_qubit_state(state, dims):
+    """
+    Expand a given qubit state into the a quantum state into a
+    multi-dimensional quantum state.
+    The additional matrix entries are filled with zeros.
+
+    Parameters
+    ----------
+    state : :obj:`qutip.Qobj`
+        The input quantum state, either a ket, a bra or a square operator.
+
+    Returns
+    -------
+    expanded_state : :obj:`qutip.Qobj`
+        The expanded state.
+
+    Examples
+    --------
+    >>> import qutip
+    >>> from qutip_qip.qubits import expand_qubit_state
+    >>> state = qutip.rand_ket(4, dims=[[2, 2], [1, 1]], seed=0)
+    >>> state  # doctest: +NORMALIZE_WHITESPACE
+        Quantum object: dims = [[2, 2], [1, 1]], shape = (4, 1), type = ket
+        Qobj data =
+        [[ 0.22362331-0.09566496j]
+        [-0.11702026+0.60050111j]
+        [ 0.15751346+0.71069216j]
+        [ 0.06879596-0.1786583j ]]
+    >>> expand_qubit_state(state, [3, 2])  # doctest: +NORMALIZE_WHITESPACE
+        Quantum object: dims = [[3, 2], [1, 1]], shape = (6, 1), type = ket
+        Qobj data =
+        [[ 0.22362331-0.09566496j]
+        [-0.11702026+0.60050111j]
+        [ 0.15751346+0.71069216j]
+        [ 0.06879596-0.1786583j ]
+        [ 0.        +0.j        ]
+        [ 0.        +0.j        ]]
+    """
+    if state.isbra:
+        reduced_dims = state.dims[1]
+        full_dims = [[1] * len(dims), dims]
+    elif state.isket:
+        reduced_dims = state.dims[0]
+        full_dims = [dims, [1] * len(dims)]
+    elif state.isoper and state.dims[0] == state.dims[1]:
+        reduced_dims = state.dims[0]
+        full_dims = [dims, dims]
+    else:
+        raise ValueError("Can only expand bra, ket or square operator.")
+    if not all([d == 2 for d in reduced_dims]):
+        raise ValueError("The input state is not a qubit state.")
+    reduced_indices = _find_reduced_indices(dims)
+
+    zero_slice = np.array([0], dtype=np.intp)
+    output = np.zeros([np.product(d) for d in full_dims], dtype=complex)
+    reduced_indices1 = reduced_indices if not state.isbra else zero_slice
+    reduced_indices2 = reduced_indices if not state.isket else zero_slice
+    output[reduced_indices1[:, np.newaxis], reduced_indices2] = state
+    return qutip.Qobj(output, dims=full_dims)

tests/test_qubits.py

@@ -1,12 +1,20 @@
 from numpy.testing import assert_, run_module_suite
-from qutip_qip.qubits import qubit_states
-from qutip import (tensor,  basis) 
+import numpy as np
+import pytest
+import qutip
+from qutip import tensor, basis
+from qutip_qip.qubits import (
+    qubit_states,
+    truncate_to_qubit_state,
+    expand_qubit_state,
+)
 
 
 class TestQubits:
     """
     A test class for the QuTiP functions for qubits.
     """
+
     def testQubitStates(self):
         """
         Tests the qubit_states function.
@@ -23,6 +31,72 @@ def testQubitStates(self):
         psi01_b = qubit_states(N=2, states=[0, 1])
         assert_(psi01_a == psi01_b)
 
+    @pytest.mark.parametrize(
+        "state, full_dims",
+        [
+            (qutip.rand_dm(18, dims=[[3, 2, 3], [3, 2, 3]]), [3, 2, 3]),
+            (qutip.rand_ket(18, dims=[[2, 3, 3], [1, 1, 1]]), [2, 3, 3]),
+            (
+                qutip.Qobj(
+                    qutip.rand_ket(18).full().transpose(),
+                    dims=[[1, 1, 1], [3, 2, 3]],
+                ),
+                [3, 2, 3],
+            ),
+        ],
+    )
+    def test_state_truncation(self, state, full_dims):
+        reduced_state = truncate_to_qubit_state(state)
+        for _ in range(5):
+            ind1 = np.random.choice([0, 1], len(full_dims))
+            ind2 = np.random.choice([0, 1], len(full_dims))
+            ind_red1 = np.ravel_multi_index(ind1, [2] * len(full_dims))
+            ind_red2 = np.ravel_multi_index(ind2, [2] * len(full_dims))
+            ind_full1 = np.ravel_multi_index(ind1, full_dims)
+            ind_full2 = np.ravel_multi_index(ind2, full_dims)
+
+            if state.isoper:
+                assert (
+                    reduced_state[ind_red1, ind_red2]
+                    == state[ind_full1, ind_full2]
+                )
+            elif state.isket:
+                assert reduced_state[ind_red1, 0] == state[ind_full1, 0]
+            elif state.isbra:
+                assert reduced_state[0, ind_red2] == state[0, ind_full2]
+
+    @pytest.mark.parametrize(
+        "state, full_dims",
+        [
+            (qutip.rand_dm(8, dims=[[2, 2, 2], [2, 2, 2]]), [3, 2, 3]),
+            (qutip.rand_ket(8, dims=[[2, 2, 2], [1, 1, 1]]), [2, 3, 3]),
+            (
+                qutip.Qobj(
+                    qutip.rand_ket(8).full().transpose(),
+                    dims=[[1, 1, 1], [2, 2, 2]],
+                ),
+                [3, 2, 3],
+            ),
+        ],
+    )
+    def test_state_expansion(self, state, full_dims):
+        full_state = expand_qubit_state(state, full_dims)
+        for _ in range(5):
+            ind_red1, ind_red2 = np.random.randint(2 ** len(full_dims), size=2)
+            reduced_dims = [2] * len(full_dims)
+            ind_full1 = np.ravel_multi_index(
+                np.unravel_index(ind_red1, reduced_dims), full_dims
+            )
+            ind_full2 = np.ravel_multi_index(
+                np.unravel_index(ind_red2, reduced_dims), full_dims
+            )
 
-if __name__ == "__main__":
-    run_module_suite()
+            if state.isoper:
+                assert (
+                    state[ind_red1, ind_red2]
+                    == full_state[ind_full1, ind_full2]
+                )
+            elif state.isket:
+                assert state[ind_red1, 0] == full_state[ind_full1, 0]
+            elif state.isbra:
+                assert state[0, ind_red2] == full_state[0, ind_full2]