Qiskit Fermions¶
qiskit-fermions extends the Qiskit SDK with tools for working on fermionic
systems. Within its scope it provides the following:
Efficient data structures for the representation and manipulation of fermionic operators in different forms
A framework for implementing operator conversion methods (including fermion-to-qubit encodings)
A library of efficient implementations of common conversion methods
A framework and library of gates for expressing fermionic circuits
A transpilation pipeline integrated with the Qiskit transpiler process to synthesize fermionic circuits into qubit-based circuits
Additionally, qiskit-fermions integrates with other tools of the ecosystem,
such as:
ffsim for the high-level ansatz operators and efficient simulation of fermionic circuits (see Relationship to ffsim)
Get started¶
Several guides exist to help you get started with this package. For an overview of its breadth of features, visit the 1D Fermi-Hubbard guide.
Use case examples¶
Components of this package have been used in research related to the following papers:
Technical discussion¶
Design intentions¶
This package is deliberately designed to align with Qiskit: it builds on a core implemented in Rust and provides first-party language bindings to Python and C. Its API intends to draw parallels to Qiskit in order to seamlessly integrate into the workflows of users with experience in programming Qiskit.
A core principle to the design of qiskit-fermions was the decoupling of its
fermionic circuit representation from its qubitized form. To be more precise:
the fermionic circuits are meaningful by themselves and do not require a mapping
to qubit space to be interpretable.
Furthermore, the fermionic circuit representation cannot make any assumptions
about its fermion-to-qubit encoding applied later on. Consequently, while
Jordan-Wigner retains a dominant position and role, it is not assumed to be the
_default_ fermion-to-qubit encoding.
Relationship to ffsim¶
ffsim and qiskit-fermions
divide the work rather than overlap. ffsim owns the high-level ansatz operators
and their fast simulation: it constructs a UCJ or UCCSD operator from
coupled-cluster amplitudes or a parameter vector, and simulates fermionic states
in a compact fixed-particle-number space. qiskit-fermions owns the circuit
side: it expresses that operator as a FermionicCircuit on fermionic
modes and transpiles it into a qubit circuit through any fermion-to-qubit
encoding.
That last point is the reason to bring an ffsim operator here. ffsim’s own
Qiskit gates are specific to the Jordan-Wigner transformation, so lowering an
ansatz through a different encoding (a local encoding trading qubits for
shallower circuits, for instance) is what this package adds. Conversely, this
package does not reimplement the ansatz math, so UCJ and
UCC take an ffsim operator directly as their input.
On the simulation side, ffsim is the backend. This package’s gates and operators
implement the protocol methods ffsim’s simulation functions consume
(_apply_unitary_, _linear_operator_ and _trace_; see
qiskit_fermions.protocols), so ffsim’s tools work on them natively, with
no conversion step. Because ffsim depends
on PySCF, which does not support Windows, simulating on Windows means going
through the Windows Subsystem for Linux for now. Building operators,
mapping them and transpiling the resulting circuits need none of this and work
everywhere.
For the full picture, read the ffsim relationship guide.
Known issues¶
As long as the Qiskit C API has not yet reached feature parity with its Python
API, some components of this package remain exclusive to its Python API, too.
This includes the entire circuit library (qiskit_fermions.circuit) as
well as transpiler passes (qiskit_fermions.transpiler).
Future work¶
Migrate the circuit library and transpiler passes into the Rust core (and provide a C API for interacting with them)
Extend the library of efficient operator conversion implementations
Extend the library of efficient operator data structures
Contributing¶
The source code is available on GitHub.
The developer guide is located at CONTRIBUTING.md in the root of this project’s repository. By participating, you are expected to uphold Qiskit’s code of conduct.
Citing this package¶
If you use this package in your research, use the CITATION.bib file in this project’s repository to cite the appropriate reference(s).
License¶
Deprecation policy¶
This package follows semantic versioning. Breaking changes are made only occasionally, to improve the user experience. When possible, old interfaces are kept and marked as deprecated for as long as they can co-exist with the new ones. Each substantial improvement, breaking change, or deprecation is documented in the release notes.