# This code is part of Qiskit.
#
# (C) Copyright IBM 2018, 2019.
#
# 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.
"""
Qiskit Aer qasm simulator backend.
"""
import copy
import logging
from qiskit.providers.models import QasmBackendConfiguration
from ..version import __version__
from .aerbackend import AerBackend
from .backend_utils import (cpp_execute, available_methods,
MAX_QUBITS_STATEVECTOR)
# pylint: disable=import-error, no-name-in-module
from .controller_wrappers import qasm_controller_execute
logger = logging.getLogger(__name__)
class QasmSimulator(AerBackend):
"""
Noisy quantum circuit simulator backend.
**Configurable Options**
The `QasmSimulator` supports multiple simulation methods and
configurable options for each simulation method. These may be set using the
appropriate kwargs during initialization. They can also be set of updated
using the :meth:`set_options` method.
Run-time options may also be specified as kwargs using the :meth:`run` method.
These will not be stored in the backend and will only apply to that execution.
They will also override any previously set options.
For example, to configure a density matrix simulator with a custom noise
model to use for every execution
.. code-block:: python
noise_model = NoiseModel.from_backend(backend)
backend = QasmSimulator(method='density_matrix',
noise_model=noise_model)
**Simulating an IBMQ Backend**
The simulator can be automatically configured to mimic an IBMQ backend using
the :meth:`from_backend` method. This will configure the simulator to use the
basic device :class:`NoiseModel` for that backend, and the same basis gates
and coupling map.
.. code-block:: python
backend = QasmSimulator.from_backend(backend)
**Simulation Method Option**
The simulation method is set using the ``method`` kwarg.
Supported simulation methods are:
* ``"statevector"``: A dense statevector simulation that can sample
measurement outcomes from *ideal* circuits with all measurements at
end of the circuit. For noisy simulations each shot samples a
randomly sampled noisy circuit from the noise model.
``"statevector_cpu"`` is an alias of ``"statevector"``.
* ``"statevector_gpu"``: A dense statevector simulation that provides
the same functionalities with ``"statevector"``. GPU performs the computation
to calculate probability amplitudes as CPU does. If no GPU is available,
a runtime error is raised.
* ``"density_matrix"``: A dense density matrix simulation that may
sample measurement outcomes from *noisy* circuits with all
measurements at end of the circuit. It can only simulate half the
number of qubits as the statevector method.
* ``"density_matrix_gpu"``: A dense density matrix simulation that provides
the same functionalities with ``"density_matrix"``. GPU performs the computation
to calculate probability amplitudes as CPU does. If no GPU is available,
a runtime error is raised.
* ``"stabilizer"``: An efficient Clifford stabilizer state simulator
that can simulate noisy Clifford circuits if all errors in the noise model are also
Clifford errors.
* ``"extended_stabilizer"``: An approximate simulated based on a
ranked-stabilizer decomposition that decomposes circuits into stabilizer
state terms. The number of terms grows with the number of
non-Clifford gates.
* ``"matrix_product_state"``: A tensor-network statevector simulator that
uses a Matrix Product State (MPS) representation for the state.
* ``"automatic"``: The default behavior where the method is chosen
automatically for each circuit based on the circuit instructions,
number of qubits, and noise model.
**Additional Backend Options**
The following simulator specific backend options are supported
* ``method`` (str): Set the simulation method (Default: ``"automatic"``).
* ``precision`` (str): Set the floating point precision for
certain simulation methods to either ``"single"`` or ``"double"``
precision (default: ``"double"``).
* ``zero_threshold`` (double): Sets the threshold for truncating
small values to zero in the result data (Default: 1e-10).
* ``validation_threshold`` (double): Sets the threshold for checking
if initial states are valid (Default: 1e-8).
* ``max_parallel_threads`` (int): Sets the maximum number of CPU
cores used by OpenMP for parallelization. If set to 0 the
maximum will be set to the number of CPU cores (Default: 0).
* ``max_parallel_experiments`` (int): Sets the maximum number of
qobj experiments that may be executed in parallel up to the
max_parallel_threads value. If set to 1 parallel circuit
execution will be disabled. If set to 0 the maximum will be
automatically set to max_parallel_threads (Default: 1).
* ``max_parallel_shots`` (int): Sets the maximum number of
shots that may be executed in parallel during each experiment
execution, up to the max_parallel_threads value. If set to 1
parallel shot execution will be disabled. If set to 0 the
maximum will be automatically set to max_parallel_threads.
Note that this cannot be enabled at the same time as parallel
experiment execution (Default: 0).
* ``max_memory_mb`` (int): Sets the maximum size of memory
to store a state vector. If a state vector needs more, an error
is thrown. In general, a state vector of n-qubits uses 2^n complex
values (16 Bytes). If set to 0, the maximum will be automatically
set to half the system memory size (Default: 0).
* ``optimize_ideal_threshold`` (int): Sets the qubit threshold for
applying circuit optimization passes on ideal circuits.
Passes include gate fusion and truncation of unused qubits
(Default: 5).
* ``optimize_noise_threshold`` (int): Sets the qubit threshold for
applying circuit optimization passes on ideal circuits.
Passes include gate fusion and truncation of unused qubits
(Default: 12).
These backend options only apply when using the ``"statevector"``
simulation method:
* ``statevector_parallel_threshold`` (int): Sets the threshold that
the number of qubits must be greater than to enable OpenMP
parallelization for matrix multiplication during execution of
an experiment. If parallel circuit or shot execution is enabled
this will only use unallocated CPU cores up to
max_parallel_threads. Note that setting this too low can reduce
performance (Default: 14).
* ``statevector_sample_measure_opt`` (int): Sets the threshold that
the number of qubits must be greater than to enable a large
qubit optimized implementation of measurement sampling. Note
that setting this two low can reduce performance (Default: 10)
These backend options only apply when using the ``"stabilizer"``
simulation method:
* ``stabilizer_max_snapshot_probabilities`` (int): set the maximum
qubit number for the
`~qiskit.providers.aer.extensions.SnapshotProbabilities`
instruction (Default: 32).
These backend options only apply when using the ``"extended_stabilizer"``
simulation method:
* ``extended_stabilizer_measure_sampling`` (bool): Enable measure
sampling optimization on supported circuits. This prevents the
simulator from re-running the measure monte-carlo step for each
shot. Enabling measure sampling may reduce accuracy of the
measurement counts if the output distribution is strongly
peaked (Default: False).
* ``extended_stabilizer_mixing_time`` (int): Set how long the
monte-carlo method runs before performing measurements. If the
output distribution is strongly peaked, this can be decreased
alongside setting extended_stabilizer_disable_measurement_opt
to True (Default: 5000).
* ``"extended_stabilizer_approximation_error"`` (double): Set the error
in the approximation for the extended_stabilizer method. A
smaller error needs more memory and computational time
(Default: 0.05).
* ``extended_stabilizer_norm_estimation_samples`` (int): Number of
samples used to compute the correct normalization for a
statevector snapshot (Default: 100).
* ``extended_stabilizer_parallel_threshold`` (int): Set the minimum
size of the extended stabilizer decomposition before we enable
OpenMP parallelization. If parallel circuit or shot execution
is enabled this will only use unallocated CPU cores up to
max_parallel_threads (Default: 100).
These backend options only apply when using the ``"matrix_product_state"``
simulation method:
* ``matrix_product_state_max_bond_dimension`` (int): Sets a limit
on the number of Schmidt coefficients retained at the end of
the svd algorithm. Coefficients beyond this limit will be discarded.
(Default: None, i.e., no limit on the bond dimension).
* ``matrix_product_state_truncation_threshold`` (double):
Discard the smallest coefficients for which the sum of
their squares is smaller than this threshold.
(Default: 1e-16).
* ``mps_sample_measure_algorithm`` (str):
Choose which algorithm to use for ``"sample_measure"``. ``"mps_probabilities"``
means all state probabilities are computed and measurements are based on them.
It is more efficient for a large number of shots, small number of qubits and low
entanglement. ``"mps_apply_measure"`` creates a copy of the mps structure and
makes a measurement on it. It is more effients for a small number of shots, high
number of qubits, and low entanglement. If the user does not specify the algorithm,
a heuristic algorithm is used to select between the two algorithms.
(Default: "mps_heuristic").
These backend options apply in circuit optimization passes:
* ``fusion_enable`` (bool): Enable fusion optimization in circuit
optimization passes [Default: True]
* ``fusion_verbose`` (bool): Output gates generated in fusion optimization
into metadata [Default: False]
* ``fusion_max_qubit`` (int): Maximum number of qubits for a operation generated
in a fusion optimization [Default: 5]
* ``fusion_threshold`` (int): Threshold that number of qubits must be greater
than or equal to enable fusion optimization [Default: 14]
"""
_DEFAULT_CONFIGURATION = {
'backend_name': 'qasm_simulator',
'backend_version': __version__,
'n_qubits': MAX_QUBITS_STATEVECTOR,
'url': 'https://github.com/Qiskit/qiskit-aer',
'simulator': True,
'local': True,
'conditional': True,
'open_pulse': False,
'memory': True,
'max_shots': int(1e6),
'description': 'A C++ QasmQobj simulator with noise',
'coupling_map': None,
'basis_gates': [
'u1', 'u2', 'u3', 'u', 'p', 'r', 'rx', 'ry', 'rz', 'id', 'x',
'y', 'z', 'h', 's', 'sdg', 'sx', 't', 'tdg', 'swap', 'cx',
'cy', 'cz', 'csx', 'cp', 'cu1', 'cu2', 'cu3', 'rxx', 'ryy',
'rzz', 'rzx', 'ccx', 'cswap', 'mcx', 'mcy', 'mcz', 'mcsx',
'mcp', 'mcu1', 'mcu2', 'mcu3', 'mcrx', 'mcry', 'mcrz',
'mcr', 'mcswap', 'unitary', 'diagonal', 'multiplexer',
'initialize', 'kraus', 'roerror', 'delay'
],
'gates': []
}
_AVAILABLE_METHODS = None
def __init__(self,
configuration=None,
properties=None,
provider=None,
**backend_options):
self._controller = qasm_controller_execute()
# Update available methods for class
if QasmSimulator._AVAILABLE_METHODS is None:
QasmSimulator._AVAILABLE_METHODS = available_methods(
self._controller, [
'automatic', 'statevector', 'statevector_gpu',
'statevector_thrust', 'density_matrix',
'density_matrix_gpu', 'density_matrix_thrust',
'stabilizer', 'matrix_product_state', 'extended_stabilizer'
])
if configuration is None:
configuration = self._method_configuration()
super().__init__(configuration,
properties=properties,
available_methods=QasmSimulator._AVAILABLE_METHODS,
provider=provider,
backend_options=backend_options)
@classmethod
def from_backend(cls, backend, **options):
"""Initialize simulator from backend."""
# pylint: disable=import-outside-toplevel
# Avoid cyclic import
from ..noise.noise_model import NoiseModel
# Get configuration and properties from backend
configuration = copy.copy(backend.configuration())
properties = copy.copy(backend.properties())
# Customize configuration name
name = configuration.backend_name
configuration.backend_name = 'qasm_simulator({})'.format(name)
# Use automatic noise model if none is provided
if 'noise_model' not in options:
noise_model = NoiseModel.from_backend(backend)
if not noise_model.is_ideal():
options['noise_model'] = noise_model
# Initialize simulator
sim = cls(configuration=configuration,
properties=properties,
**options)
return sim
def _execute(self, qobj):
"""Execute a qobj on the backend.
Args:
qobj (QasmQobj): simulator input.
Returns:
dict: return a dictionary of results.
"""
return cpp_execute(self._controller, qobj)
def _set_option(self, key, value):
"""Set the simulation method and update configuration.
Args:
key (str): key to update
value (any): value to update.
Raises:
AerError: if key is 'method' and val isn't in available methods.
"""
# If key is noise_model we also change the simulator config
# to use the noise_model basis gates by default.
if key == 'noise_model' and value is not None:
self._set_configuration_option('basis_gates', value.basis_gates)
# If key is method we update our configurations
if key == 'method':
method_config = self._method_configuration(value)
self._set_configuration_option('description', method_config.description)
self._set_configuration_option('backend_name', method_config.backend_name)
self._set_configuration_option('n_qubits', method_config.n_qubits)
# Take intersection of method basis gates and noise model basis gates
# if there is a noise model which has already set the basis gates
basis_gates = method_config.basis_gates
if 'noise_model' in self.options:
noise_basis_gates = self.options['noise_model'].basis_gates
basis_gates = list(
set(basis_gates).intersection(noise_basis_gates))
self._set_configuration_option('basis_gates', basis_gates)
# Set all other options from AerBackend
super()._set_option(key, value)
def _validate(self, qobj):
"""Semantic validations of the qobj which cannot be done via schemas.
Warn if no measurements in circuit with classical registers.
"""
for experiment in qobj.experiments:
# If circuit contains classical registers but not
# measurements raise a warning
if experiment.config.memory_slots > 0:
# Check if measure opts missing
no_measure = True
for op in experiment.instructions:
if not no_measure:
break # we don't need to check any more ops
if no_measure and op.name == "measure":
no_measure = False
# Print warning if clbits but no measure
if no_measure:
logger.warning(
'No measurements in circuit "%s": '
'count data will return all zeros.',
experiment.header.name)
@staticmethod
def _method_configuration(method=None):
"""Return QasmBackendConfiguration."""
# Default configuration
config = QasmBackendConfiguration.from_dict(
QasmSimulator._DEFAULT_CONFIGURATION)
# Statevector methods
if method in ['statevector', 'statevector_gpu', 'statevector_thrust']:
config.description = 'A C++ QasmQobj statevector simulator with noise'
# Density Matrix methods
elif method in [
'density_matrix', 'density_matrix_gpu', 'density_matrix_thrust'
]:
config.n_qubits = config.n_qubits // 2
config.description = 'A C++ QasmQobj density matrix simulator with noise'
config.basis_gates = [
'u1', 'u2', 'u3', 'u', 'p', 'r', 'rx', 'ry', 'rz', 'id', 'x',
'y', 'z', 'h', 's', 'sdg', 'sx', 't', 'tdg', 'swap', 'cx',
'cy', 'cz', 'cp', 'cu1', 'rxx', 'ryy',
'rzz', 'rzx', 'ccx', 'unitary', 'diagonal', 'kraus', 'superop'
'roerror', 'delay'
]
# Matrix product state method
elif method == 'matrix_product_state':
config.description = 'A C++ QasmQobj matrix product state simulator with noise'
config.basis_gates = [
'u1', 'u2', 'u3', 'u', 'p', 'cp', 'cx', 'cz', 'id', 'x', 'y', 'z', 'h', 's',
'sdg', 'sx', 't', 'tdg', 'swap', 'ccx', 'unitary', 'roerror', 'delay'
]
# Stabilizer method
elif method == 'stabilizer':
config.n_qubits = 5000 # TODO: estimate from memory
config.description = 'A C++ QasmQobj Clifford stabilizer simulator with noise'
config.basis_gates = [
'id', 'x', 'y', 'z', 'h', 's', 'sdg', 'sx', 'cx', 'cy', 'cz',
'swap', 'roerror', 'delay'
]
# Extended stabilizer method
elif method == 'extended_stabilizer':
config.n_qubits = 63 # TODO: estimate from memory
config.description = 'A C++ QasmQobj ranked stabilizer simulator with noise'
config.basis_gates = [
'cx', 'cz', 'id', 'x', 'y', 'z', 'h', 's', 'sdg', 'sx', 'swap',
'u0', 'u1', 'p', 'ccx', 'ccz', 'roerror', 'delay'
]
return config