Source code for qutip.partial_transpose

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#    Significant parts of this code was contributed by Jonas Neergaard-Nielsen
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__all__ = ['partial_transpose']

import numpy as np
import scipy.sparse as sp

from qutip.qobj import Qobj
from qutip.states import (state_index_number, state_number_index,
                          state_number_enumerate)


[docs]def partial_transpose(rho, mask, method='dense'): """ Return the partial transpose of a Qobj instance `rho`, where `mask` is an array/list with length that equals the number of components of `rho` (that is, the length of `rho.dims[0]`), and the values in `mask` indicates whether or not the corresponding subsystem is to be transposed. The elements in `mask` can be boolean or integers `0` or `1`, where `True`/`1` indicates that the corresponding subsystem should be tranposed. Parameters ---------- rho : :class:`qutip.qobj` A density matrix. mask : *list* / *array* A mask that selects which subsystems should be transposed. method : str choice of method, `dense` or `sparse`. The default method is `dense`. The `sparse` implementation can be faster for large and sparse systems (hundreds of quantum states). Returns ------- rho_pr: :class:`qutip.qobj` A density matrix with the selected subsystems transposed. """ if method == 'sparse': return _partial_transpose_sparse(rho, mask) else: return _partial_transpose_dense(rho, mask)
def _partial_transpose_dense(rho, mask): """ Based on Jonas' implementation using numpy. Very fast for dense problems. """ nsys = len(mask) pt_dims = np.arange(2 * nsys).reshape(2, nsys).T pt_idx = np.concatenate([[pt_dims[n, mask[n]] for n in range(nsys)], [pt_dims[n, 1 - mask[n]] for n in range(nsys)]]) data = rho.data.toarray().reshape( np.array(rho.dims).flatten()).transpose(pt_idx).reshape(rho.shape) return Qobj(data, dims=rho.dims) def _partial_transpose_sparse(rho, mask): """ Implement the partial transpose using the CSR sparse matrix. """ data = sp.lil_matrix((rho.shape[0], rho.shape[1]), dtype=complex) for m in range(len(rho.data.indptr) - 1): n1 = rho.data.indptr[m] n2 = rho.data.indptr[m + 1] psi_A = state_index_number(rho.dims[0], m) for idx, n in enumerate(rho.data.indices[n1:n2]): psi_B = state_index_number(rho.dims[1], n) m_pt = state_number_index( rho.dims[1], np.choose(mask, [psi_A, psi_B])) n_pt = state_number_index( rho.dims[0], np.choose(mask, [psi_B, psi_A])) data[m_pt, n_pt] = rho.data.data[n1 + idx] return Qobj(data.tocsr(), dims=rho.dims) def _partial_transpose_reference(rho, mask): """ This is a reference implementation that explicitly loops over all states and performs the transpose. It's slow but easy to understand and useful for testing. """ A_pt = np.zeros(rho.shape, dtype=complex) for psi_A in state_number_enumerate(rho.dims[0]): m = state_number_index(rho.dims[0], psi_A) for psi_B in state_number_enumerate(rho.dims[1]): n = state_number_index(rho.dims[1], psi_B) m_pt = state_number_index( rho.dims[1], np.choose(mask, [psi_A, psi_B])) n_pt = state_number_index( rho.dims[0], np.choose(mask, [psi_B, psi_A])) A_pt[m_pt, n_pt] = rho.data[m, n] return Qobj(A_pt, dims=rho.dims)