OpenFermion: The Electronic Structure Package for Quantum Computers (1710.07629v5)

Abstract: Quantum simulation of chemistry and materials is predicted to be an important application for both near-term and fault-tolerant quantum devices. However, at present, developing and studying algorithms for these problems can be difficult due to the prohibitive amount of domain knowledge required in both the area of chemistry and quantum algorithms. To help bridge this gap and open the field to more researchers, we have developed the OpenFermion software package (www.openfermion.org). OpenFermion is an open-source software library written largely in Python under an Apache 2.0 license, aimed at enabling the simulation of fermionic models and quantum chemistry problems on quantum hardware. Beginning with an interface to common electronic structure packages, it simplifies the translation between a molecular specification and a quantum circuit for solving or studying the electronic structure problem on a quantum computer, minimizing the amount of domain expertise required to enter the field. The package is designed to be extensible and robust, maintaining high software standards in documentation and testing. This release paper outlines the key motivations behind design choices in OpenFermion and discusses some basic OpenFermion functionality which we believe will aid the community in the development of better quantum algorithms and tools for this exciting area of research.

• The paper presents OpenFermion as an open-source package that simplifies translating complex electronic structure problems into quantum computation formulations.
• It integrates with quantum chemistry tools and supports multiple fermion-to-qubit transformations, including Jordan-Wigner and Bravyi-Kitaev methods.
• The package enables researchers to simulate models like the Hubbard model and optimize quantum circuits for algorithms such as the variational quantum eigensolver.

OpenFermion: The Electronic Structure Package for Quantum Computers

The paper introduces OpenFermion, a comprehensive open-source software package designed to bridge the gap between quantum chemistry and quantum computing. It aims to facilitate the simulation of fermionic and bosonic models on quantum hardware, effectively translating complex electronic structure problems into quantum computation formulations. This package is critical for researchers venturing into quantum simulations, especially those lacking extensive domain knowledge in chemistry or quantum algorithms.

Overview

OpenFermion addresses a significant challenge in the quantum computing field: the complexity of developing and implementing algorithms for simulating quantum systems, particularly in chemistry and materials science. The paper details the motivations behind the package's development, emphasizing its role in democratizing research in quantum algorithm development by lowering the entry barrier for non-experts.

Functional Capabilities

OpenFermion is designed with a robust and extensible architecture, enabling several key functionalities:

• Integration with Quantum Chemistry Packages: OpenFermion seamlessly interfaces with existing electronic structure packages, such as Psi4 and PySCF, to retrieve necessary molecular integrals and other intermediate data required for quantum simulations.
• Transformation to Qubit Representations: The package supports various mappings of Fermionic states to qubit representations, crucial for implementing quantum algorithms. Notably, it includes Jordan-Wigner, Bravyi-Kitaev, and Bravyi-Kitaev Super Fast (BKSF) transformations, offering flexibility in methodology.
• Circuit Compilation: While focusing on representing mathematical formulations and transformations, OpenFermion delegates the physical compilation of quantum circuits to specialized plugins compatible with platforms like Cirq, Forest, and Strawberry Fields. These plugins bridge to hardware-specific or platform-specific operational layers.
• Data Structures: The package leverages efficient data structures, such as FermionOperator, QubitOperator, BosonOperator, InteractionOperator, and QuadraticHamiltonian, to manage different types of operators and interactions in quantum simulations.

Numerical and Theoretical Implications

Through packages like OpenFermion, simulations of fermionic systems reach new levels of accessibility and manageability, both numerically and theoretically. The package allows researchers to:

• Conduct simulations of well-known models such as the Hubbard model and the homogeneous electron gas, offering versatility in research applications.
• Perform optimization of quantum circuits for implementing algorithms like Trotter decomposition and variational quantum eigensolver (VQE), which are central to near-term quantum computing applications.
• Transform complex Hamiltonians efficiently, bridging theoretical formulations with practical implementations on quantum hardware.

Broad Impact and Future Directions

The introduction of OpenFermion represents a significant step in converging quantum computing and quantum chemistry. The package not only simplifies the complex task of algorithm development for non-experts but also lays the groundwork for more sophisticated simulations and research, potentially unlocking new insights into material properties and chemical reactions.

Practically, future developments in OpenFermion could involve enhancing integration with more quantum chemistry software, optimizing data handling for larger Hamiltonians through improved data structures, and expanding its plugin system to support emerging quantum computing platforms.

The open-source nature of OpenFermion ensures it can evolve with community contributions, fostering collaborative innovation. As quantum hardware continues to advance, OpenFermion stands to remain a pivotal resource in simulating quantum systems and advancing quantum technology applications across diverse scientific domains.