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Qumode-Based Variational Quantum Eigensolver for Molecular Excited States.

Rishab Dutta¹, Cameron Cianci^2,3, Alexander V Soudackov¹

¹Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States.

Journal of Chemical Theory and Computation

|January 15, 2026

Summary

This summary is machine-generated.

We introduce a new quantum algorithm, the qumode subspace variational quantum eigensolver (QSS-VQE), for calculating molecular excited states. This method uses bosonic qumodes to potentially outperform traditional qubit-based quantum simulations.

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Area of Science:

Quantum Computing
Computational Chemistry
Quantum Simulation

Background:

Accurate computation of molecular excited states is crucial for understanding chemical reactions and designing new materials.
Current quantum algorithms for electronic structure calculations face challenges in terms of resource requirements and expressivity.
Circuit quantum electrodynamics (cQED) architectures offer native control over both qubits and bosonic qumodes.

Purpose of the Study:

To introduce and evaluate the qumode subspace variational quantum eigensolver (QSS-VQE), a novel hybrid quantum-classical algorithm.
To leverage the Fock basis of bosonic qumodes for enhanced quantum simulation of molecular excited states.
To compare the performance of qumode-based approaches against conventional qubit-based methods.

Main Methods:

Developed the QSS-VQE algorithm, a hybrid quantum-classical approach.
Mapped the electronic structure Hamiltonian to a qubit representation and embedded it into the Fock space of bosonic qumodes.
Utilized native universal gate sets of qubit-qumode architectures for variational ansatze construction.
Performed simulations of molecular excited states, including dihydrogen and a cytosine conical intersection.

Main Results:

Demonstrated the feasibility of QSS-VQE for computing molecular excited states.
Showcased efficient state preparation and reduced quantum resource requirements using bosonic qumodes.
Identified specific regimes where qumode-based implementations offer advantages over purely qubit-based methods.
Assessed the expressivity of qumode gates through simulations of a bosonic model Hamiltonian.

Conclusions:

QSS-VQE presents a promising avenue for enhanced quantum simulation of complex molecular systems.
Leveraging bosonic degrees of freedom in quantum computation can lead to improved performance for specific quantum chemistry problems.
The native capabilities of qubit-qumode architectures can be effectively utilized for advanced quantum simulations.