arXiv subjects

Erika Magnusson

Publications and source records attributed to Erika Magnusson.

3 recordsLinked to original sources

arXiv2026

Constant-Depth Quantum Imaginary Time Evolution Using Dynamic Fan-out Circuits

Dynamic quantum circuits combine mid-circuit measurement with classical feed-forward, enabling circuit constructions with reduced entangling-gate depth. Here, we investigate their use in Quantum Imaginary Time Evolution (QITE), where circuit depth and parameter growth limit practical implementations of ground-state preparation. For dense classical optimization Hamiltonians, we introduce a reduced-parameter QITE ansatz that restricts entanglement generation via a small set of control qubits, enabling each QITE layer to be implemented with constant two-qubit gate depth using fan-out-based dynamic circuits. In noiseless simulations of exact cover and set partitioning instances, the reduced ansatz yields a higher success probability than standard QITE approaches. We implement unitary, dynamic fan-out, and semi-classical adaptive variants on IBM superconducting hardware. The semi-classical variant performs favorably to the unitary implementation, while the fully dynamic construction exposes the trade-offs between entangling-depth reduction and measurement and feed-forward overhead associated to dynamic circuit implementations. Using a fidelity threshold of 0.5 relative to the noiseless QITE ansatz, we show that dynamic fan-out based QITE would outperform unitary implementations on current devices when the measurement and two-qubit gate errors are reduced by 65% and the feedback latency is halved.

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arXiv2025

Multireference error mitigation for quantum computation of chemistry

Quantum error mitigation (QEM) strategies are essential for improving the precision and reliability of quantum chemistry algorithms on noisy intermediate-scale quantum devices. Reference-state error mitigation (REM) is a cost-effective chemistry-inspired QEM method that performs exceptionally well for weakly correlated problems. However, the effectiveness of REM is often limited when applied to strongly correlated systems. Here, we introduce multireference-state error mitigation (MREM), an extension of REM that systematically captures quantum hardware noise in strongly correlated ground states by utilizing multireference states. A pivotal aspect of MREM is using Givens rotations to efficiently construct quantum circuits to generate multireference states. To strike a balance between circuit expressivity and noise sensitivity, we employ compact wavefunctions composed a few dominant Slater determinants. These truncated multireference states, engineered to exhibit substantial overlap with the target ground state, can effectively enhance error mitigation in variational quantum eigensolver experiments. We demonstrate the effectiveness of MREM through comprehensive simulations of molecular systems $\mathrm{H_2O, ~N_2, ~and ~F_2}$, underscoring its ability to realize significant improvements in computational accuracy compared to the original REM method. MREM broadens the scope of error mitigation to encompass a wider variety of molecular systems, including those exhibiting pronounced electron correlation.

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arXiv2024

Towards Efficient Quantum Computing for Quantum Chemistry: Reducing Circuit Complexity with Transcorrelated and Adaptive Ansatz Techniques

The near-term utility of quantum computers is hindered by hardware constraints in the form of noise. One path to achieving noise resilience in hybrid quantum algorithms is to decrease the required circuit depth -- the number of applied gates -- to solve a given problem. This work demonstrates how to reduce circuit depth by combining the transcorrelated (TC) approach with adaptive quantum ansätze and their implementations in the context of variational quantum imaginary time evolution (AVQITE). The combined TC-AVQITE method is used to calculate ground state energies across the potential energy surfaces of H$_4$, LiH, and H$_2$O. In particular, H$_4$ is a notoriously difficult case where unitary coupled cluster theory, including singles and doubles excitations, fails to provide accurate results. Adding TC yields energies close to the complete basis set (CBS) limit while reducing the number of necessary operators -- and thus circuit depth -- in the adaptive ansätze. The reduced circuit depth furthermore makes our algorithm more noise-resilient and accelerates convergence. Our study demonstrates that combining the TC method with adaptive ansätze yields compact, noise-resilient, and easy-to-optimize quantum circuits that yield accurate quantum chemistry results close to the CBS limit.

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