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Source code for qiskit.algorithms.amplitude_amplifiers.amplification_problem

# This code is part of Qiskit.
#
# (C) Copyright IBM 2021.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.

"""The Amplification problem class."""

from typing import Optional, Callable, Any, Union, List

from qiskit.circuit import QuantumCircuit
from qiskit.circuit.library import GroverOperator
from qiskit.quantum_info import Statevector


[docs]class AmplificationProblem: """The amplification problem is the input to amplitude amplification algorithms, like Grover. This class contains all problem-specific information required to run an amplitude amplification algorithm. It minimally contains the Grover operator. It can further hold some post processing on the optimal bitstring. """ def __init__( self, oracle: Union[QuantumCircuit, Statevector], state_preparation: Optional[QuantumCircuit] = None, grover_operator: Optional[QuantumCircuit] = None, post_processing: Optional[Callable[[str], Any]] = None, objective_qubits: Optional[Union[int, List[int]]] = None, is_good_state: Optional[ Union[Callable[[str], bool], List[int], List[str], Statevector] ] = None, ) -> None: r""" Args: oracle: The oracle reflecting about the bad states. state_preparation: A circuit preparing the input state, referred to as :math:`\mathcal{A}`. If None, a layer of Hadamard gates is used. grover_operator: The Grover operator :math:`\mathcal{Q}` used as unitary in the phase estimation circuit. If None, this operator is constructed from the ``oracle`` and ``state_preparation``. post_processing: A mapping applied to the most likely bitstring. objective_qubits: If set, specifies the indices of the qubits that should be measured. If None, all qubits will be measured. The ``is_good_state`` function will be applied on the measurement outcome of these qubits. is_good_state: A function to check whether a string represents a good state. By default if the ``oracle`` argument has an ``evaluate_bitstring`` method (currently only provided by the :class:`~qiskit.circuit.library.PhaseOracle` class) this will be used, otherwise this kwarg is required and **must** be specified. """ self._oracle = oracle self._state_preparation = state_preparation self._grover_operator = grover_operator self._post_processing = post_processing self._objective_qubits = objective_qubits if is_good_state is not None: self._is_good_state = is_good_state elif hasattr(oracle, "evaluate_bitstring"): self._is_good_state = oracle.evaluate_bitstring else: self._is_good_state = None @property def oracle(self) -> Union[QuantumCircuit, Statevector]: """Return the oracle. Returns: The oracle. """ return self._oracle @oracle.setter def oracle(self, oracle: Union[QuantumCircuit, Statevector]) -> None: """Set the oracle. Args: oracle: The oracle. """ self._oracle = oracle @property def state_preparation(self) -> QuantumCircuit: r"""Get the state preparation operator :math:`\mathcal{A}`. Returns: The :math:`\mathcal{A}` operator as `QuantumCircuit`. """ if self._state_preparation is None: state_preparation = QuantumCircuit(self.oracle.num_qubits) state_preparation.h(state_preparation.qubits) return state_preparation return self._state_preparation @state_preparation.setter def state_preparation(self, state_preparation: Optional[QuantumCircuit]) -> None: r"""Set the :math:`\mathcal{A}` operator. If None, a layer of Hadamard gates is used. Args: state_preparation: The new :math:`\mathcal{A}` operator or None. """ self._state_preparation = state_preparation @property def post_processing(self) -> Callable[[str], Any]: """Apply post processing to the input value. Returns: A handle to the post processing function. Acts as identity by default. """ if self._post_processing is None: return lambda x: x return self._post_processing @post_processing.setter def post_processing(self, post_processing: Callable[[str], Any]) -> None: """Set the post processing function. Args: post_processing: A handle to the post processing function. """ self._post_processing = post_processing @property def objective_qubits(self) -> List[int]: """The indices of the objective qubits. Returns: The indices of the objective qubits as list of integers. """ if self._objective_qubits is None: return list(range(self.oracle.num_qubits)) if isinstance(self._objective_qubits, int): return [self._objective_qubits] return self._objective_qubits @objective_qubits.setter def objective_qubits(self, objective_qubits: Optional[Union[int, List[int]]]) -> None: """Set the objective qubits. Args: objective_qubits: The indices of the qubits that should be measured. If None, all qubits will be measured. The ``is_good_state`` function will be applied on the measurement outcome of these qubits. """ self._objective_qubits = objective_qubits @property def is_good_state(self) -> Callable[[str], bool]: """Check whether a provided bitstring is a good state or not. Returns: A callable that takes in a bitstring and returns True if the measurement is a good state, False otherwise. """ if (self._is_good_state is None) or callable(self._is_good_state): return self._is_good_state # returns None if no is_good_state arg has been set elif isinstance(self._is_good_state, list): if all(isinstance(good_bitstr, str) for good_bitstr in self._is_good_state): return lambda bitstr: bitstr in self._is_good_state else: return lambda bitstr: all( bitstr[good_index] == "1" # type:ignore for good_index in self._is_good_state ) return lambda bitstr: bitstr in self._is_good_state.probabilities_dict() @is_good_state.setter def is_good_state( self, is_good_state: Union[Callable[[str], bool], List[int], List[str], Statevector] ) -> None: """Set the ``is_good_state`` function. Args: is_good_state: A function to determine whether a bitstring represents a good state. """ self._is_good_state = is_good_state @property def grover_operator(self) -> Optional[QuantumCircuit]: r"""Get the :math:`\mathcal{Q}` operator, or Grover operator. If the Grover operator is not set, we try to build it from the :math:`\mathcal{A}` operator and `objective_qubits`. This only works if `objective_qubits` is a list of integers. Returns: The Grover operator, or None if neither the Grover operator nor the :math:`\mathcal{A}` operator is set. """ if self._grover_operator is None: return GroverOperator(self.oracle, self.state_preparation) return self._grover_operator @grover_operator.setter def grover_operator(self, grover_operator: Optional[QuantumCircuit]) -> None: r"""Set the :math:`\mathcal{Q}` operator. If None, this operator is constructed from the ``oracle`` and ``state_preparation``. Args: grover_operator: The new :math:`\mathcal{Q}` operator or None. """ self._grover_operator = grover_operator