Código fonte de qiskit.algorithms.linear_solvers.hhl

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# (C) Copyright IBM 2020, 2022.
# This code is licensed under the Apache License, Version 2.0. You may
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"""The HHL algorithm."""

from typing import Optional, Union, List, Callable, Tuple
import numpy as np

from qiskit.circuit import QuantumCircuit, QuantumRegister, AncillaRegister
from qiskit.circuit.library import PhaseEstimation
from qiskit.circuit.library.arithmetic.piecewise_chebyshev import PiecewiseChebyshev
from qiskit.circuit.library.arithmetic.exact_reciprocal import ExactReciprocal
from qiskit.opflow import (
from qiskit.providers import Backend
from qiskit.quantum_info.operators.base_operator import BaseOperator
from qiskit.utils import QuantumInstance
from qiskit.utils.deprecation import deprecate_function

from .linear_solver import LinearSolver, LinearSolverResult
from .matrices.numpy_matrix import NumPyMatrix
from .observables.linear_system_observable import LinearSystemObservable

[documentos]class HHL(LinearSolver): r"""The deprecated systems of linear equations arise naturally in many real-life applications in a wide range of areas, such as in the solution of Partial Differential Equations, the calibration of financial models, fluid simulation or numerical field calculation. The problem can be defined as, given a matrix :math:`A\in\mathbb{C}^{N\times N}` and a vector :math:`\vec{b}\in\mathbb{C}^{N}`, find :math:`\vec{x}\in\mathbb{C}^{N}` satisfying :math:`A\vec{x}=\vec{b}`. A system of linear equations is called :math:`s`-sparse if :math:`A` has at most :math:`s` non-zero entries per row or column. Solving an :math:`s`-sparse system of size :math:`N` with a classical computer requires :math:`\mathcal{ O }(Ns\kappa\log(1/\epsilon))` running time using the conjugate gradient method. Here :math:`\kappa` denotes the condition number of the system and :math:`\epsilon` the accuracy of the approximation. The deprecated HHL is a quantum algorithm to estimate a function of the solution with running time complexity of :math:`\mathcal{ O }(\log(N)s^{2}\kappa^{2}/\epsilon)` when :math:`A` is a Hermitian matrix under the assumptions of efficient oracles for loading the data, Hamiltonian simulation and computing a function of the solution. This is an exponential speed up in the size of the system, however one crucial remark to keep in mind is that the classical algorithm returns the full solution, while the HHL can only approximate functions of the solution vector. The HHL class is deprecated as of Qiskit Terra 0.22.0 and will be removed no sooner than 3 months after the release date. It is replaced by the tutorial at `HHL <https://qiskit.org/textbook/ch-applications/hhl_tutorial.html>`_ Examples: .. jupyter-execute:: import warnings import numpy as np from qiskit import QuantumCircuit from qiskit.algorithms.linear_solvers.hhl import HHL from qiskit.algorithms.linear_solvers.matrices import TridiagonalToeplitz from qiskit.algorithms.linear_solvers.observables import MatrixFunctional with warnings.catch_warnings(): warnings.simplefilter('ignore') matrix = TridiagonalToeplitz(2, 1, 1 / 3, trotter_steps=2) right_hand_side = [1.0, -2.1, 3.2, -4.3] observable = MatrixFunctional(1, 1 / 2) rhs = right_hand_side / np.linalg.norm(right_hand_side) # Initial state circuit num_qubits = matrix.num_state_qubits qc = QuantumCircuit(num_qubits) qc.isometry(rhs, list(range(num_qubits)), None) with warnings.catch_warnings(): warnings.simplefilter('ignore') hhl = HHL() solution = hhl.solve(matrix, qc, observable) approx_result = solution.observable References: [1]: Harrow, A. W., Hassidim, A., Lloyd, S. (2009). Quantum algorithm for linear systems of equations. `Phys. Rev. Lett. 103, 15 (2009), 1–15. <https://doi.org/10.1103/PhysRevLett.103.150502>`_ [2]: Carrera Vazquez, A., Hiptmair, R., & Woerner, S. (2022). Enhancing the Quantum Linear Systems Algorithm Using Richardson Extrapolation. `ACM Transactions on Quantum Computing 3, 1, Article 2 <https://doi.org/10.1145/3490631>`_ """ @deprecate_function( """The HHL class is deprecated as of Qiskit Terra 0.22.0 and will be removed no sooner than 3 months after the release date. It is replaced by the tutorial at https://qiskit.org/textbook/ch-applications/hhl_tutorial.html" """ ) def __init__( self, epsilon: float = 1e-2, expectation: Optional[ExpectationBase] = None, quantum_instance: Optional[Union[Backend, QuantumInstance]] = None, ) -> None: r""" Args: epsilon: Error tolerance of the approximation to the solution, i.e. if :math:`x` is the exact solution and :math:`\tilde{x}` the one calculated by the algorithm, then :math:`||x - \tilde{x}|| \le epsilon`. expectation: The expectation converter applied to the expectation values before evaluation. If None then PauliExpectation is used. quantum_instance: Quantum Instance or Backend. If None, a Statevector calculation is done. """ super().__init__() self._epsilon = epsilon # Tolerance for the different parts of the algorithm as per [1] self._epsilon_r = epsilon / 3 # conditioned rotation self._epsilon_s = epsilon / 3 # state preparation self._epsilon_a = epsilon / 6 # hamiltonian simulation self._scaling = None # scaling of the solution self._sampler = None self.quantum_instance = quantum_instance self._expectation = expectation # For now the default reciprocal implementation is exact self._exact_reciprocal = True # Set the default scaling to 1 self.scaling = 1 @property def quantum_instance(self) -> Optional[QuantumInstance]: """Get the quantum instance. Returns: The quantum instance used to run this algorithm. """ return None if self._sampler is None else self._sampler.quantum_instance @quantum_instance.setter def quantum_instance(self, quantum_instance: Optional[Union[QuantumInstance, Backend]]) -> None: """Set quantum instance. Args: quantum_instance: The quantum instance used to run this algorithm. If None, a Statevector calculation is done. """ if quantum_instance is not None: self._sampler = CircuitSampler(quantum_instance) else: self._sampler = None @property def scaling(self) -> float: """The scaling of the solution vector.""" return self._scaling @scaling.setter def scaling(self, scaling: float) -> None: """Set the new scaling of the solution vector.""" self._scaling = scaling @property def expectation(self) -> ExpectationBase: """The expectation value algorithm used to construct the expectation measurement from the observable.""" return self._expectation @expectation.setter def expectation(self, expectation: ExpectationBase) -> None: """Set the expectation value algorithm.""" self._expectation = expectation def _get_delta(self, n_l: int, lambda_min: float, lambda_max: float) -> float: """Calculates the scaling factor to represent exactly lambda_min on nl binary digits. Args: n_l: The number of qubits to represent the eigenvalues. lambda_min: the smallest eigenvalue. lambda_max: the largest eigenvalue. Returns: The value of the scaling factor. """ formatstr = "#0" + str(n_l + 2) + "b" lambda_min_tilde = np.abs(lambda_min * (2**n_l - 1) / lambda_max) # floating point precision can cause problems if np.abs(lambda_min_tilde - 1) < 1e-7: lambda_min_tilde = 1 binstr = format(int(lambda_min_tilde), formatstr)[2::] lamb_min_rep = 0 for i, char in enumerate(binstr): lamb_min_rep += int(char) / (2 ** (i + 1)) return lamb_min_rep def _calculate_norm(self, qc: QuantumCircuit) -> float: """Calculates the value of the euclidean norm of the solution. Args: qc: The quantum circuit preparing the solution x to the system. Returns: The value of the euclidean norm of the solution. """ # Calculate the number of qubits nb = qc.qregs[0].size nl = qc.qregs[1].size na = qc.num_ancillas # Create the Operators Zero and One zero_op = (I + Z) / 2 one_op = (I - Z) / 2 # Norm observable observable = one_op ^ TensoredOp((nl + na) * [zero_op]) ^ (I ^ nb) norm_2 = (~StateFn(observable) @ StateFn(qc)).eval() return np.real(np.sqrt(norm_2) / self.scaling) def _calculate_observable( self, solution: QuantumCircuit, observable: Optional[Union[LinearSystemObservable, BaseOperator]] = None, observable_circuit: Optional[QuantumCircuit] = None, post_processing: Optional[ Callable[[Union[float, List[float]]], Union[float, List[float]]] ] = None, ) -> Tuple[Union[float, List[float]], Union[float, List[float]]]: """Calculates the value of the observable(s) given. Args: solution: The quantum circuit preparing the solution x to the system. observable: Information to be extracted from the solution. observable_circuit: Circuit to be applied to the solution to extract information. post_processing: Function to compute the value of the observable. Returns: The value of the observable(s) and the circuit results before post-processing as a tuple. """ # Get the number of qubits nb = solution.qregs[0].size nl = solution.qregs[1].size na = solution.num_ancillas # if the observable is given construct post_processing and observable_circuit if observable is not None: observable_circuit = observable.observable_circuit(nb) post_processing = observable.post_processing if isinstance(observable, LinearSystemObservable): observable = observable.observable(nb) # in the other case use the identity as observable else: observable = I ^ nb # Create the Operators Zero and One zero_op = (I + Z) / 2 one_op = (I - Z) / 2 is_list = True if not isinstance(observable_circuit, list): is_list = False observable_circuit = [observable_circuit] observable = [observable] expectations = [] for circ, obs in zip(observable_circuit, observable): circuit = QuantumCircuit(solution.num_qubits) circuit.append(solution, circuit.qubits) circuit.append(circ, range(nb)) ob = one_op ^ TensoredOp((nl + na) * [zero_op]) ^ obs expectations.append(~StateFn(ob) @ StateFn(circuit)) if is_list: # execute all in a list op to send circuits in batches expectations = ListOp(expectations) else: expectations = expectations[0] # check if an expectation converter is given if self._expectation is not None: expectations = self._expectation.convert(expectations) # if otherwise a backend was specified, try to set the best expectation value elif self._sampler is not None: if is_list: op = expectations.oplist[0] else: op = expectations self._expectation = ExpectationFactory.build(op, self._sampler.quantum_instance) if self._sampler is not None: expectations = self._sampler.convert(expectations) # evaluate expectation_results = expectations.eval() # apply post_processing result = post_processing(expectation_results, nb, self.scaling) return result, expectation_results
[documentos] def construct_circuit( self, matrix: Union[List, np.ndarray, QuantumCircuit], vector: Union[List, np.ndarray, QuantumCircuit], neg_vals: Optional[bool] = True, ) -> QuantumCircuit: """Construct the HHL circuit. Args: matrix: The matrix specifying the system, i.e. A in Ax=b. vector: The vector specifying the right hand side of the equation in Ax=b. neg_vals: States whether the matrix has negative eigenvalues. If False the computation becomes cheaper. Returns: The HHL circuit. Raises: ValueError: If the input is not in the correct format. ValueError: If the type of the input matrix is not supported. """ # State preparation circuit - default is qiskit if isinstance(vector, QuantumCircuit): nb = vector.num_qubits vector_circuit = vector elif isinstance(vector, (list, np.ndarray)): if isinstance(vector, list): vector = np.array(vector) nb = int(np.log2(len(vector))) vector_circuit = QuantumCircuit(nb) vector_circuit.isometry(vector / np.linalg.norm(vector), list(range(nb)), None) # If state preparation is probabilistic the number of qubit flags should increase nf = 1 # Hamiltonian simulation circuit - default is Trotterization if isinstance(matrix, QuantumCircuit): matrix_circuit = matrix elif isinstance(matrix, (list, np.ndarray)): if isinstance(matrix, list): matrix = np.array(matrix) if matrix.shape[0] != matrix.shape[1]: raise ValueError("Input matrix must be square!") if np.log2(matrix.shape[0]) % 1 != 0: raise ValueError("Input matrix dimension must be 2^n!") if not np.allclose(matrix, matrix.conj().T): raise ValueError("Input matrix must be hermitian!") if matrix.shape[0] != 2**vector_circuit.num_qubits: raise ValueError( "Input vector dimension does not match input " "matrix dimension! Vector dimension: " + str(vector_circuit.num_qubits) + ". Matrix dimension: " + str(matrix.shape[0]) ) matrix_circuit = NumPyMatrix(matrix, evolution_time=2 * np.pi) else: raise ValueError(f"Invalid type for matrix: {type(matrix)}.") # Set the tolerance for the matrix approximation if hasattr(matrix_circuit, "tolerance"): matrix_circuit.tolerance = self._epsilon_a # check if the matrix can calculate the condition number and store the upper bound if ( hasattr(matrix_circuit, "condition_bounds") and matrix_circuit.condition_bounds() is not None ): kappa = matrix_circuit.condition_bounds()[1] else: kappa = 1 # Update the number of qubits required to represent the eigenvalues # The +neg_vals is to register negative eigenvalues because # e^{-2 \pi i \lambda} = e^{2 \pi i (1 - \lambda)} nl = max(nb + 1, int(np.ceil(np.log2(kappa + 1)))) + neg_vals # check if the matrix can calculate bounds for the eigenvalues if hasattr(matrix_circuit, "eigs_bounds") and matrix_circuit.eigs_bounds() is not None: lambda_min, lambda_max = matrix_circuit.eigs_bounds() # Constant so that the minimum eigenvalue is represented exactly, since it contributes # the most to the solution of the system. -1 to take into account the sign qubit delta = self._get_delta(nl - neg_vals, lambda_min, lambda_max) # Update evolution time matrix_circuit.evolution_time = 2 * np.pi * delta / lambda_min / (2**neg_vals) # Update the scaling of the solution self.scaling = lambda_min else: delta = 1 / (2**nl) print("The solution will be calculated up to a scaling factor.") if self._exact_reciprocal: reciprocal_circuit = ExactReciprocal(nl, delta, neg_vals=neg_vals) # Update number of ancilla qubits na = matrix_circuit.num_ancillas else: # Calculate breakpoints for the reciprocal approximation num_values = 2**nl constant = delta a = int(round(num_values ** (2 / 3))) # Calculate the degree of the polynomial and the number of intervals r = 2 * constant / a + np.sqrt(np.abs(1 - (2 * constant / a) ** 2)) degree = min( nb, int( np.log( 1 + ( 16.23 * np.sqrt(np.log(r) ** 2 + (np.pi / 2) ** 2) * kappa * (2 * kappa - self._epsilon_r) ) / self._epsilon_r ) ), ) num_intervals = int(np.ceil(np.log((num_values - 1) / a) / np.log(5))) # Calculate breakpoints and polynomials breakpoints = [] for i in range(0, num_intervals): # Add the breakpoint to the list breakpoints.append(a * (5**i)) # Define the right breakpoint of the interval if i == num_intervals - 1: breakpoints.append(num_values - 1) reciprocal_circuit = PiecewiseChebyshev( lambda x: np.arcsin(constant / x), degree, breakpoints, nl ) na = max(matrix_circuit.num_ancillas, reciprocal_circuit.num_ancillas) # Initialise the quantum registers qb = QuantumRegister(nb) # right hand side and solution ql = QuantumRegister(nl) # eigenvalue evaluation qubits if na > 0: qa = AncillaRegister(na) # ancilla qubits qf = QuantumRegister(nf) # flag qubits if na > 0: qc = QuantumCircuit(qb, ql, qa, qf) else: qc = QuantumCircuit(qb, ql, qf) # State preparation qc.append(vector_circuit, qb[:]) # QPE phase_estimation = PhaseEstimation(nl, matrix_circuit) if na > 0: qc.append(phase_estimation, ql[:] + qb[:] + qa[: matrix_circuit.num_ancillas]) else: qc.append(phase_estimation, ql[:] + qb[:]) # Conditioned rotation if self._exact_reciprocal: qc.append(reciprocal_circuit, ql[::-1] + [qf[0]]) else: qc.append( reciprocal_circuit.to_instruction(), ql[:] + [qf[0]] + qa[: reciprocal_circuit.num_ancillas], ) # QPE inverse if na > 0: qc.append(phase_estimation.inverse(), ql[:] + qb[:] + qa[: matrix_circuit.num_ancillas]) else: qc.append(phase_estimation.inverse(), ql[:] + qb[:]) return qc
[documentos] def solve( self, matrix: Union[List, np.ndarray, QuantumCircuit], vector: Union[List, np.ndarray, QuantumCircuit], observable: Optional[ Union[ LinearSystemObservable, BaseOperator, List[LinearSystemObservable], List[BaseOperator], ] ] = None, observable_circuit: Optional[Union[QuantumCircuit, List[QuantumCircuit]]] = None, post_processing: Optional[ Callable[[Union[float, List[float]]], Union[float, List[float]]] ] = None, ) -> LinearSolverResult: """Tries to solve the given linear system of equations. Args: matrix: The matrix specifying the system, i.e. A in Ax=b. vector: The vector specifying the right hand side of the equation in Ax=b. observable: Optional information to be extracted from the solution. Default is the probability of success of the algorithm. observable_circuit: Optional circuit to be applied to the solution to extract information. Default is `None`. post_processing: Optional function to compute the value of the observable. Default is the raw value of measuring the observable. Raises: ValueError: If an invalid combination of observable, observable_circuit and post_processing is passed. Returns: The result object containing information about the solution vector of the linear system. """ # verify input if observable is not None: if observable_circuit is not None or post_processing is not None: raise ValueError( "If observable is passed, observable_circuit and post_processing cannot be set." ) solution = LinearSolverResult() solution.state = self.construct_circuit(matrix, vector) solution.euclidean_norm = self._calculate_norm(solution.state) if observable is not None or observable_circuit is not None: solution.observable, solution.circuit_results = self._calculate_observable( solution.state, observable, observable_circuit, post_processing ) return solution