def bravyi_kitaev(operator, n_qubits=None):
"""Apply the Bravyi-Kitaev transform.
Implementation from arXiv:quant-ph/0003137 and
"A New Data Structure for Cumulative Frequency Tables" by Peter M. Fenwick.
Note that this implementation is equivalent to the one described in
arXiv:1208.5986, and is different from the one described in
arXiv:1701.07072. The one described in arXiv:1701.07072 is implemented
in OpenFermion as `bravyi_kitaev_tree`.
Args:
operator (openfermion.ops.FermionOperator):
A FermionOperator to transform.
n_qubits (int|None):
Can force the number of qubits in the resulting operator above the
number that appear in the input operator.
Returns:
transformed_operator: An instance of the QubitOperator class.
Raises:
ValueError: Invalid number of qubits specified.
"""
# Compute the number of qubits.
if n_qubits is None:
n_qubits = count_qubits(operator)
if n_qubits < count_qubits(operator):
raise ValueError('Invalid number of qubits specified.')
# Compute transformed operator.
transformed_terms = (_transform_operator_term(
term=term, coefficient=operator.terms[term], n_qubits=n_qubits)
for term in operator.terms)
return inline_sum(summands=transformed_terms, seed=QubitOperator())
def _bravyi_kitaev_majorana_operator(operator, n_qubits):
# Compute the number of qubits.
N = count_qubits(operator)
if n_qubits is None:
n_qubits = N
if n_qubits < N:
raise ValueError('Invalid number of qubits specified.')
# Compute transformed operator.
transformed_terms = (
_transform_majorana_term(term=term,
coefficient=coeff,
n_qubits=n_qubits)
for term, coeff in operator.terms.items()
)
return inline_sum(summands=transformed_terms, seed=QubitOperator())
def bravyi_kitaev_tree(operator, n_qubits=None):
"""Apply the "tree" Bravyi-Kitaev transform.
Implementation from arxiv:1701.07072
Note that this implementation is different from the one described in
arXiv:quant-ph/0003137. In particular, it gives different results
when the total number of modes is not a power of 2. The one described
in arXiv:quant-ph/0003137 is the same as the one described in
arXiv:1208.5986, and it is implemented in OpenFermion under the name
`bravyi_kitaev`.
Args:
operator (openfermion.ops.FermionOperator):
A FermionOperator to transform.
n_qubits (int|None):
Can force the number of qubits in the resulting operator above the
number that appear in the input operator.
Returns:
transformed_operator: An instance of the QubitOperator class.
Raises:
ValueError: Invalid number of qubits specified.
"""
# Compute the number of qubits.
from openfermion.utils import count_qubits
if n_qubits is None:
n_qubits = count_qubits(operator)
if n_qubits < count_qubits(operator):
raise ValueError('Invalid number of qubits specified.')
# Build the Fenwick tree.
fenwick_tree = FenwickTree(n_qubits)
# Compute transformed operator.
transformed_terms = (_transform_operator_term(
term=term, coefficient=operator.terms[term], fenwick_tree=fenwick_tree)
for term in operator.terms)
return inline_sum(summands=transformed_terms, seed=QubitOperator())