Fast Quantum Gates Achieve High Fidelity for Neutral Atoms Separated by 20 Um

Quantum computing with neutral atoms promises scalability, but creating rapid connections between distant qubits remains a significant challenge. Matteo Bergonzoni, from the University of Strasbourg and CNRS, Rosario Roberto Riso from the Norwegian University of Science and Technology, and Guido Pupillo, also of the University of Strasbourg and CNRS, now demonstrate a theoretical pathway to achieve precisely this. Their work details a method for creating fast, high-fidelity quantum gates between neutral atoms separated by tens of micrometers, a distance far exceeding the range of conventional approaches. By harnessing resonant interactions between Rydberg states, the team achieves gate performance comparable to existing technologies while dramatically increasing the potential connectivity of future quantum processors, paving the way for more complex and powerful quantum computations.

Resonant dipole-dipole spin-exchange interactions between Rydberg states underpin a new protocol for quantum computation. This method harnesses coherent excitation-exchange-deexcitation dynamics between qubits and Rydberg states within a single laser pulse, leveraging strong dipole-dipole interactions. Researchers employ optimal control methods to achieve theoretical gate fidelities and durations comparable to existing blockade-based gates, while extending the effective interaction range by an order of magnitude, enabling entanglement over distances well beyond the typical blockade radius and offering a potential route toward fast, high-connectivity quantum processors. Neutral atoms trapped in optical tweezers represent a leading platform for realising this approach.

Rydberg Qubits and High Fidelity Gates

Scientists are making significant progress in building quantum computers using neutral atoms, specifically Rydberg atoms, as qubits. This research focuses on creating high-fidelity entanglement and implementing quantum gates, particularly the iSWAP gate, and explores methods to overcome challenges inherent in these processes. A key focus is optimizing control pulses and mitigating errors caused by factors like atomic motion, decoherence, and unwanted interactions. Rydberg atoms and polar molecules are leveraged for their strong, long-range interactions, which facilitate the creation of entanglement.

Optimal control theory is used to design control pulses, such as laser pulses, that maximize the fidelity of quantum gates. Researchers address sources of error that degrade gate performance, including atomic or molecular motion, decoherence, van der Waals interactions, and ensuring atoms remain in the excited Rydberg state long enough to perform operations. Maximizing fidelity is a central goal, and the GRAPE software tool is used for optimizing control pulses, focusing on designing robust gates less sensitive to errors and variations in experimental conditions.

Fast, High-Fidelity Gates with Rydberg Atoms

Scientists have achieved a breakthrough in quantum gate fidelity and duration using resonant dipole-dipole spin-exchange interactions between neutral atoms, specifically Rydberg atoms. This work demonstrates a theoretical scheme for implementing fast and high-fidelity iSWAP gates between qubits separated by distances exceeding 20 micrometers. The team utilized optimal control methods to design pulses that minimize errors arising from spontaneous emission, a fundamental limitation in Rydberg atom quantum computing. Experiments reveal that time-optimal pulses, achieving zero infidelity, are obtained when the amplitudes of the two Rabi frequencies are equal and the laser phases are synchronized.

For a specific ratio of Rabi frequency to interaction strength, the shortest pulse duration was found, achieving a time-optimal gate with exceptionally low infidelity under ideal conditions. The research demonstrates that the effective interaction range scales with distance, enabling connectivity beyond the typical blockade radius and reaching distances of 20 micrometers for specific experimental parameters. Detailed analysis of error sources shows that the time-optimal pulses exhibit exceptional robustness against various perturbations. The team quantified the impact of van der Waals interactions, Rydberg decay, atomic motion, and photon recoil, demonstrating low infidelities even in the presence of these effects. The research shows that the impact of atomic motion and photon recoil are limited, and the robust pulses achieve an overall low infidelity, demonstrating a significant improvement in gate performance and stability, paving the way for building larger, more connected quantum processors with enhanced fidelity and scalability.

Fast Rydberg iSWAP Gates Over Micrometers

Scientists have demonstrated a new method for creating fast and high-fidelity iSWAP gates between neutral atoms, even when those atoms are separated by distances exceeding 20 micrometers. This achievement relies on carefully controlling resonant dipole-dipole spin-exchange interactions between Rydberg states of the atoms, utilizing optimally shaped laser pulses to drive the process. The resulting gate speeds are comparable to those of existing CZ gates operating at much shorter distances, representing a significant advance in the field of quantum computing. The team achieved these results by harnessing coherent excitation-exchange-deexcitation dynamics, enabling strong interactions over extended distances.

The demonstrated gate fidelities are compatible with the thresholds required for effective quantum error correction, paving the way for more robust and reliable quantum computations. Researchers identified potential sources of error, such as recoil-induced detuning and motional-electronic coupling, and proposed methods for mitigation, including fine-tuning laser frequencies and aligning beams along specific directions. Furthermore, the use of heavier atomic species can further suppress these errors. This work opens several promising avenues for future research. The approach could be extended to create multi-qubit gates, enhancing the efficiency of quantum error-correcting codes and reducing the complexity of quantum circuits. Importantly, these long-range gates facilitate efficient quantum information transport across neutral-atom registers without the need for intermediate operations or atomic movement, which is crucial for building fast, modular quantum computers. The high-fidelity gates may also enable the implementation of advanced low-density parity-check codes, offering improved encoding rates and reduced qubit overhead for quantum computations.