Optimizing Two-Qubit Gates for Ultracold Fermions Enables High-Fidelity Control in Optical Lattices

The quest to build powerful quantum computers receives a boost from new research into controlling interactions between ultracold atoms, offering a pathway to more reliable quantum operations. Jan A. P. Reuter, Juhi Singh, Tommaso Calarco, and colleagues at Forschungszentrum Jülich GmbH and the University of Cologne, alongside Robert Zeier, demonstrate a method for optimising the fundamental ‘gates’ needed to manipulate quantum information using fermionic lithium atoms trapped in optical lattices. Their work advances beyond previous simulations of complex materials by accurately modelling how the interaction energy between atoms changes with their momentum, revealing a crucial factor influencing gate performance. This detailed understanding not only highlights potential limitations in achieving perfect gate fidelity, but also opens up possibilities for designing tailored quantum operations that exploit these momentum-dependent interactions for enhanced computational power.

Optimizing Fermion Qubit Interactions in Lattices

Ultracold neutral atoms trapped in optical lattices offer a promising platform for quantum simulation and computation. Achieving scalable quantum algorithms requires precise control over interactions between individual qubits. This work investigates optimizing two-qubit gate implementations for qubits encoded in hyperfine states of ultracold lithium-6 fermions within an optical lattice, focusing on minimizing errors caused by unintended excitation of atomic motion and unwanted interactions. The approach combines analytical calculations and numerical simulations to design optimized pulse sequences for driving the two-qubit gate.

Specifically, the team explores using shaped radio-frequency pulses to selectively address the qubit subspace while suppressing unwanted transitions. The optimization procedure incorporates realistic experimental constraints, such as finite pulse durations and limitations in controlling system parameters. A key aspect of the method is developing a robust control scheme that remains effective despite variations in atomic density and lattice spacing. The research demonstrates that carefully tailoring the pulse shape and amplitude allows achieving two-qubit gate fidelities exceeding 99. 9%. The results show a significant reduction in errors compared to conventional gate implementations, and the optimized pulse sequences prove robust against experimental imperfections. Furthermore, the team identifies the primary sources of error and proposes strategies for further improving gate performance, providing a pathway towards realizing high-fidelity quantum computations with ultracold fermionic atoms in optical lattices.

Fermionic Atoms Enable High-Fidelity Quantum Gates

This research explores the use of neutral fermionic atoms trapped in optical lattices as a platform for quantum simulation and computation. The central goal is to develop methods for performing quantum information processing using these atoms, which possess unique properties compared to qubits based on spins or superconducting circuits. A major focus is on designing and implementing high-fidelity two-qubit gates, essential for building scalable quantum computers, a particularly challenging task with fermionic systems. The research addresses the challenges of controlling and mitigating errors that arise in quantum simulations and computations, including those due to imperfections in the experimental setup and interactions between atoms.

The team employs optical lattices and tweezers to trap and control individual atoms, and explores superlattices for engineering Hubbard couplings. Advanced pulse shaping techniques and optimal control algorithms are used to precisely control the interactions between atoms and implement desired quantum gates. A wide range of numerical methods are used to simulate the dynamics of fermionic atoms, including time-splitting methods, balanced and rebalanced splitting, the Numerov algorithm, tensor network methods, and density functional theory. Wannier function analysis is used to understand the band structure of the optical lattice and design efficient quantum gates, and derivative methods are used to optimize control parameters.

Specific contributions include designing quantum gates based on controlled collisions between fermionic atoms, manipulating the Hubbard model parameters to create desired quantum states, and developing error mitigation strategies. The team focuses on designing control pulses that are robust to noise and imperfections, compensating for nonlinear distortions, and utilizing higher-order optimization algorithms to improve accuracy and efficiency. They also develop and implement improved numerical methods for simulating the dynamics of fermionic atoms and explore real-time feedback control loops to stabilize the system and correct for errors. This research contributes to the development of more powerful and accurate quantum simulators, which can be used to study complex many-body physics and materials science problems.

The development of high-fidelity quantum gates and error mitigation strategies is essential for building scalable quantum computers. The ability to control and manipulate fermionic atoms with high precision opens up new possibilities for exploring fundamental quantum phenomena. Quantum simulation with fermionic atoms can provide insights into the behavior of strongly correlated materials and other complex systems, and can lead to new discoveries in quantum chemistry.

Fermionic Qubit Gates with High Fidelity

This research demonstrates high-fidelity two-qubit gates using ultracold fermionic lithium atoms confined within a one-dimensional optical lattice. By optimising collision gates and accounting for realistic experimental constraints, the team achieved a short entangling gate suitable for implementation in current experimental setups. The method extends beyond previous Fermi-Hubbard simulations by incorporating momentum dependence in the interaction energy, revealing a distinct behaviour of interacting atoms depending on their initial positioning within the double well potential. The simulations account for factors such as laser recoil energies and the transfer function from electrical to optical signals, providing realistic predictions for experimental implementation.

Analysis of gate robustness considered asymmetric lattices, uncertainties in interaction energy, and state preparation errors. Importantly, the results indicate that optimising gates separately for initial states where atoms begin on the same or opposite sides of the double well could further enhance performance, offering tailored solutions for applications in quantum chemistry, simulation, and computing. Future work will focus on additional numerical and feedback-based optimisations, adapting control sequences to specific experimental conditions, and paving the way for efficient and robust gates in fermionic atom-based quantum computers and simulators.