# Source code for pyquil.api._wavefunction_simulator

```
##############################################################################
# Copyright 2018 Rigetti Computing
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
##############################################################################
from typing import List, Union, Optional
import numpy as np
from six import integer_types
from pyquil.api._base_connection import ForestConnection
from pyquil.api._error_reporting import _record_call
from pyquil.paulis import PauliSum, PauliTerm
from pyquil.quil import Program
from pyquil.wavefunction import Wavefunction
[docs]class WavefunctionSimulator:
@_record_call
def __init__(self, connection: ForestConnection = None,
random_seed: Optional[int] = None) -> None:
"""
A simulator that propagates a wavefunction representation of a quantum state.
:param connection: A connection to the Forest web API.
:param random_seed: A seed for the simulator's random number generators. Either None (for
an automatically generated seed) or a non-negative integer.
"""
if connection is None:
connection = ForestConnection()
self.connection = connection
if random_seed is None:
self.random_seed = None
elif isinstance(random_seed, integer_types) and random_seed >= 0:
self.random_seed = random_seed
else:
raise TypeError("random_seed should be None or a non-negative int")
[docs] @_record_call
def wavefunction(self, quil_program: Program) -> Wavefunction:
"""
Simulate a Quil program and return the wavefunction.
.. note:: If your program contains measurements or noisy gates, this method may not do what
you want. If the execution of ``quil_program`` is **non-deterministic** then the
final wavefunction only represents a stochastically generated sample and the
wavefunctions returned by *different* ``wavefunction`` calls *will generally be
different*.
:param quil_program: A Quil program.
:return: A Wavefunction object representing the state of the QVM.
"""
return self.connection._wavefunction(quil_program=quil_program,
random_seed=self.random_seed)
[docs] @_record_call
def expectation(self, prep_prog: Program,
pauli_terms: Union[PauliSum, List[PauliTerm]]) -> Union[float, np.ndarray]:
"""
Calculate the expectation value of Pauli operators given a state prepared by prep_program.
If ``pauli_terms`` is a ``PauliSum`` then the returned value is a single ``float``,
otherwise the returned value is an array of values, one for each ``PauliTerm`` in the
list.
.. note:: If your program contains measurements or noisy gates, this method may not do what
you want. If the execution of ``quil_program`` is **non-deterministic** then the
final wavefunction from which the expectation value is calculated only represents
a stochastically generated sample and the wavefunctions returned by *different*
``wavefunction`` calls *will generally be different*.
:param prep_prog: A program that prepares the state on which we measure the expectation.
:param pauli_terms: A Pauli representation of a quantum operator.
:return: Either a float or array floats depending on ``pauli_terms``.
"""
is_pauli_sum = False
if isinstance(pauli_terms, PauliSum):
progs, coeffs = pauli_terms.get_programs()
is_pauli_sum = True
else:
coeffs = np.array([pt.coefficient for pt in pauli_terms])
progs = [pt.program for pt in pauli_terms]
bare_results = self.connection._expectation(prep_prog, progs, random_seed=self.random_seed)
results = coeffs * bare_results
if is_pauli_sum:
return np.sum(results)
return results
[docs] @_record_call
def run_and_measure(self, quil_program: Program, qubits: List[int] = None,
trials: int = 1) -> np.ndarray:
"""
Run a Quil program once to determine the final wavefunction, and measure multiple times.
Alternatively, consider using ``wavefunction`` and calling ``sample_bitstrings`` on the
resulting object.
For a large wavefunction and a low-medium number of trials, use this function.
On the other hand, if you're sampling a small system many times you might want to
use ``Wavefunction.sample_bitstrings``.
.. note:: If your program contains measurements or noisy gates, this method may not do what
you want. If the execution of ``quil_program`` is **non-deterministic** then the
final wavefunction from which the returned bitstrings are sampled itself only
represents a stochastically generated sample and the outcomes sampled from
*different* ``run_and_measure`` calls *generally sample different bitstring
distributions*.
:param quil_program: The program to run and measure
:param qubits: An optional list of qubits to measure. The order of this list is
respected in the returned bitstrings. If not provided, all qubits used in
the program will be measured and returned in their sorted order.
:param int trials: Number of times to sample from the prepared wavefunction.
:return: An array of measurement results (0 or 1) of shape (trials, len(qubits))
"""
if qubits is None:
qubits = sorted(quil_program.get_qubits(indices=True))
return self.connection._run_and_measure(quil_program=quil_program, qubits=qubits,
trials=trials,
random_seed=self.random_seed)
```