Qiskit Aqua

Qiskit is an open-source framework for working with noisy quantum computers at the level of pulses, circuits, and algorithms.

Qiskit is made up elements that work together to enable quantum computing. This element is Aqua. Aqua provides a library of cross-domain algorithms upon which domain-specific applications can be built. Qiskit Chemistry has been created to utilize Aqua for quantum chemistry computations. Aqua is also showcased for other domains, such as Optimization, Artificial Intelligence, and Finance, with both code and notebook examples available in the qiskit-tutorials and qiskit-tutorials-community GitHub Repositories.

Aqua was designed to be extensible, and uses a pluggable framework where algorithms and support objects used by algorithms—such as optimizers, variational forms, and oracles—are derived from a defined base class for the type and discovered dynamically at run time.

Installation

Qiskit Aqua is part of the Qiskit software framework. We encourage installing Qiskit Aqua as part of Qiskit via the pip tool (a python package manager):

pip install qiskit

pip will handle all dependencies automatically for you, including the other Qiskit elements upon which Aqua is built, such as Qiskit Terra, and you will always install the latest (and well-tested) version.

To run chemistry experiments using Qiskit Chemistry, it is recommended that you to install a classical computation chemistry software program interfaced by Qiskit Chemistry. Several such programs are supported, and while logic to interface these programs is supplied by Qiskit Chemistry via the above pip installation, the dependent programs themselves need to be installed separately becausea they are not part of the Qiskit Chemistry installation bundle. Qiskit Chemistry comes with prebuilt support to interface the following classical computational chemistry software programs:

1. Gaussian 16™, a commercial chemistry program
2. PSI4, a chemistry program that exposes a Python interface allowing for accessing internal objects
3. PySCF, an open-source Python chemistry program
4. PyQuante, a pure cross-platform open-source Python chemistry program

Except for the Windows platform, PySCF is installed automatically as a dependency by the pip tool whenever Qiskit Chemistry is installed. The other classical computational chemistry software programs will have to be installed separately, even though Qiskit Chemistry includes the code for interfacing all of them. Please refer to the Qiskit Chemistry drivers installation instructions for details on how to integrate these drivers into Qiskit Chemistry.

A useful functionality integrated into Qiskit Chemistry is its ability to serialize a file in Hierarchical Data Format 5 (HDF5) format representing all the data extracted from one of the drivers listed above when executing an experiment. Qiskit Chemistry can then use that data to initiate the conversion of that data into a fermionic operator and then a qubit operator, which can then be used as an input to a quantum algorithm. Therefore, even without installing one of the drivers above, it is still possible to run chemistry experiments as long as you have a Hierarchical Data Format 5 (HDF5) file that has been previously created. Qiskit Chemistry's built-in HDF5 driver accepts such such HDF5 files as input.
A few sample HDF5 files for different are provided in the chemistry folder of the Qiskit Tutorials repository.

To install from source, follow the instructions in the contribution guidelines.

Creating Your First Quantum Program in Qiskit Aqua

Now that Qiskit Aqua is installed, it's time to begin working with it. We are ready to try out an experiment using Qiskit Aqua:

$ python

from qiskit import Aer
from qiskit.aqua.components.oracles import LogicalExpressionOracle
from qiskit.aqua.algorithms import Grover

sat_cnf = """
c Example DIMACS 3-sat
p cnf 3 5
-1 -2 -3 0
1 -2 3 0
1 2 -3 0
1 -2 -3 0
-1 2 3 0
"""

backend = Aer.get_backend('qasm_simulator')
oracle = LogicalExpressionOracle(sat_cnf)
algorithm = Grover(oracle)
result = algorithm.run(backend)
print(result["result"])

The code above demonstrates how Grover’s search algorithm can be used with the LogicalExpressionOracle to find one satisfying assignment for the Satisfiability (SAT) problem instance encoded in the DIMACS CNF format. The input string sat_cnf corresponds to the following Conjunctive Normal Form (CNF):

(¬x₁ ∨ ¬x₂ ∨ ¬x₃) ∧ (x₁ ∨ ¬x₂ ∨ x₃) ∧ (x₁ ∨ x₂ ∨ ¬x₃) ∧ (x₁ ∨ ¬x₂ ∨ ¬x₃) ∧ (¬x₁ ∨ x₂ ∨ x₃)

The Python code above prints out one possible solution for this CNF. For example, output 1, -2, 3 indicates that logical expression (x₁ ∨ ¬x₂ ∨ x₃) satisfies the given CNF.

Creating Your First Qiskit Chemistry Programming Experiment

Now that Qiskit Aqua is installed, it's time to begin working with it. We are ready to try out an experiment using Qiskit Chemistry:

from qiskit.chemistry import FermionicOperator
from qiskit.chemistry.drivers import PySCFDriver, UnitsType

# Use PySCF, a classical computational chemistry software
# package, to compute the one-body and two-body integrals in
# molecular-orbital basis, necessary to form the Fermionic operator
driver = PySCFDriver(atom='H .0 .0 .0; H .0 .0 0.735',
                    unit=UnitsType.ANGSTROM,
                    basis='sto3g')
molecule = driver.run()
num_particles = molecule.num_alpha + molecule.num_beta
num_spin_orbitals = molecule.num_orbitals * 2

# Build the qubit operator, which is the input to the VQE algorithm in Aqua
ferOp = FermionicOperator(h1=molecule.one_body_integrals, h2=molecule.two_body_integrals)
map_type = 'PARITY'
qubitOp = ferOp.mapping(map_type)
qubitOp = qubitOp.two_qubit_reduced_operator(num_particles)
num_qubits = qubitOp.num_qubits

# set the backend for the quantum computation
from qiskit import Aer
backend = Aer.get_backend('statevector_simulator')

# setup a classical optimizer for VQE
from qiskit.aqua.components.optimizers import L_BFGS_B
optimizer = L_BFGS_B()

# setup the initial state for the variational form
from qiskit.chemistry.aqua_extensions.components.initial_states import HartreeFock
init_state = HartreeFock(num_qubits, num_spin_orbitals, num_particles)

# setup the variational form for VQE
from qiskit.aqua.components.variational_forms import RYRZ
var_form = RYRZ(num_qubits, initial_state=init_state)

# setup and run VQE
from qiskit.aqua.algorithms import VQE
algorithm = VQE(qubitOp, var_form, optimizer)
result = algorithm.run(backend)
print(result['energy'])

The program above uses a quantum computer to calculate the ground state energy of molecular Hydrogen, H₂, where the two atoms are configured to be at a distance of 0.735 angstroms. The molecular configuration input is generated using PySCF. First, Qiskit Chemisrtry transparently executes PySCF, and extracts from it the one- and two-body molecular-orbital integrals; an inexpensive operation that scales well classically and does not require the use of a quantum computer. These integrals are then used to create a quantum fermionic-operator representation of the molecule. In this specific example, we use a parity mapping to generate a qubit operator from the fermionic one, with a unique precision-preserving optimization that allows for two qubits to be tapered off; a reduction in complexity that is particularly advantageous for NISQ computers. The qubit operator is then passed as an input to the Variational Quantum Eigensolver (VQE) algorithm, instantiated with a Limited-memory Broyden-Fletcher-Goldfarb-Shanno Bound (L-BFGS-B) classical optimizer and the RyRz variational form. The Hartree-Fock state is utilized to initialize the variational form. This example emphasizes the use of Qiskit Aqua and Qiskit Chemistry's programmatic interface by illustrating the constructor calls that initialize the VQE QuantumAlgorithm, along with its supporting components—consisting of the L-BFGS-B Optimizer, RyRz VariationalForm, and Hartree-Fock InitialState. The Aer statevector simulator backend is passed as a parameter to the run method of the VQE algorithm object, which means that the backend will be executed with default parameters. To customize the backend, you can wrap it into a QuantumInstance object, and then pass that object to the run method of the QuantumAlgorithm, as explained above. The QuantumInstance API allows you to customize run-time properties of the backend, such as the number of shots, the maximum number of credits to use, a dictionary with the configuration settings for the simulator, a dictionary with the initial layout of qubits in the mapping, and the Terra PassManager that will handle the compilation of the circuits. For the full set of options, please refer to the documentation of the Aqua QuantumInstance API.

Using Real Device

You can also use Qiskit to execute your code on a real quantum chip. In order to do so, you need to configure Qiskit to use the credentials in your IBM Q account. Please consult the relevant instructions in the Qiskit Terra GitHub repository for more details.

Qiskit Chemistry Wizard and Command-line Interfaces

Qiskit Chemistry is a modular and extensible software framework that allows researchers to contribute new components to it and extend its functionality. For this reason, Qiskit Chemistry exposes all the Application Programming Interfaces (APIs) necessary to access its functionality programmatically.

Those users who are interested in executing Qiskit as a tool should install Qiskit Aqua Interfaces via the pip tool. This software package contains command-line and graphical user interfaces to easily configure an experiment and executing without having to write any line of code. Both interfaces come with a schema-based configuration-correctness mechanism. Furthermore, the Graphical User Interface (GUI) includes capabilities for automatic code generation.

Contribution Guidelines

If you'd like to contribute to Qiskit, please take a look at our contribution guidelines. This project adheres to Qiskit's code of conduct. By participating, you are expected to uphold to this code. Please also follow the style guidelines.

We use GitHub issues for tracking requests and bugs. Please join the Qiskit Slack community and use the Aqua Slack channel for discussion and simple questions. For questions that are more suited for a forum, we use the Qiskit tag in Stack Overflow.

Next Steps

Now you're set up and ready to check out some of the other examples from the qiskit/aqua and community/aqua and qiskit/chemistry and community/chemistry folders of the qiskit-tutorials GitHub Repository.

Authors and Citation

Aqua was inspired, authored and brought about by the collective work of a team of researchers. Aqua continues to grow with the help and work of many people, who contribute to the project at different levels. If you use Qiskit, please cite as per the included BibTeX file.

License

This project uses the Apache License 2.0.

Some of the code embedded in Qiskit Chemistry to interface some of the computational chemistry software drivers requires additional licensing:

• The Gaussian 16 driver contains work licensed under the Gaussian Open-Source Public License.