Atomic Gate Design Boosts Stability Tenfold for Quantum Computing

Researchers at the Russia and Novosibirsk State University in collaboration with Russia and Institute of Laser Physics SB RAS and Russia Novosibirsk State Technical University, have developed a numerically optimised method for creating a key controlled-Z gate utilising ultracold neutral atoms, sharply improving resilience to variations in laser intensity. K. V. Kozenko and colleagues achieved this by designing analytically defined laser pulse shapes, enhancing robustness by almost an order of magnitude compared with existing techniques. The research details a gate protocol applicable to individual atom addressing during Rydberg excitation, mitigating the impact of atomic motion and laser beam instability on gate accuracy. The investigation confirms the benefits of this approach for both single-photon and two-photon Rydberg excitation schemes, particularly within trapped atom systems at finite temperatures.

Numerical optimisation delivers record fidelity for neutral atom quantum gates

Gate fidelity reached 99.35%, a substantial improvement over previously reported values and crossing a threshold necessary for scalable quantum computation. This level of precision resulted from numerical optimisation of a controlled-Z (CZ) gate scheme for neutral atoms, enhancing strong resistance against variations in the Rabi frequency, the rate at which a laser alters an atom’s quantum state, by almost an order of magnitude. The Rabi frequency is a crucial parameter in quantum control, directly influencing the speed and accuracy of gate operations. Small fluctuations in laser power or beam pointing stability can cause significant deviations in the Rabi frequency, leading to errors. This new method demonstrably reduces the sensitivity of the CZ gate to these fluctuations. The optimised protocol is directly applicable to individual atom addressing during Rydberg excitation, a process where atoms are excited to higher energy levels using lasers, mitigating the impact of atomic motion and laser beam instability, previously limiting factors in gate accuracy. Individual addressing is achieved through tightly focused laser beams, and maintaining precise control over these beams is essential for reliable qubit manipulation.

Single-photon Rydberg excitation of cesium atoms with ultraviolet light previously achieved entanglement fidelities of 96.7%, and this new protocol builds on that work. The Rydberg state, possessing a large dipole moment, facilitates strong interactions between atoms, forming the basis for the controlled-Z gate. However, these interactions are sensitive to the precise distance between atoms and the laser parameters. Experiments utilising single-photon Rydberg excitation of strontium atoms achieved even higher fidelities of 99.6% and 99.7%, demonstrating the potential for further refinement. Strontium, with its different atomic properties, allows for exploration of the protocol’s versatility and optimisation for various atomic species. The optimised gate protocol functions effectively with both single-photon and two-photon Rydberg excitation schemes, the latter commonly used when working with rubidium and cesium atoms. Two-photon excitation involves sequentially exciting the atom with two different laser wavelengths, offering greater control over the excitation process and potentially reducing unwanted transitions.

Neutral atoms are increasingly favoured as qubits, the basic units of quantum information, due to their long coherence times and individual addressability. Coherence time refers to how long a qubit can maintain its quantum state before decoherence occurs, and neutral atoms exhibit remarkably long coherence times, making them ideal candidates for quantum computation. Individual addressability allows for precise control over each qubit, enabling complex quantum algorithms. A practical quantum computer demands not just stable qubits, but gates, operations that manipulate those qubits, that are robust against real-world imperfections. These imperfections include variations in laser intensity, atomic motion due to the finite temperature of the trapped atoms, and fluctuations in atomic energy levels. This optimisation represents a valuable step forward, demonstrably strengthening the durability of a key quantum operation against common imperfections in neutral atom systems, while acknowledging that achieving truly error-free quantum gates remains a significant hurdle. The controlled-Z gate is a fundamental building block for many quantum algorithms, and its improved performance directly translates to enhanced computational capabilities.

The enhanced robustness of the optimised controlled-Z gate now reduces the critical need to minimise the impact of fluctuating atomic energy levels, lessening the accumulation of errors during complex calculations. The analytical design of the laser pulse shapes is key to this robustness. By carefully tailoring the amplitude and phase of the laser pulse, the gate operation becomes less sensitive to variations in the Rabi frequency. A carefully designed laser pulse sequence allows for more reliable entanglement of individual atoms, a key step towards scalable quantum computation. Entanglement, a uniquely quantum phenomenon, allows qubits to be correlated in a way that is impossible in classical systems, and is essential for performing quantum computations. Current investigations explore the limits of this approach by examining different atomic species and excitation parameters, and the method demonstrates flexible application across different experimental setups. This includes varying the trapping potential, laser wavelengths, and atomic densities. Further work will focus on scaling up the number of qubits and implementing more complex quantum algorithms, utilising the improved gate fidelity to reduce the overall error rate of quantum computations. Scaling up the number of qubits presents significant challenges, including maintaining individual addressability and minimising crosstalk between qubits. The ultimate goal is to build a fault-tolerant quantum computer capable of solving problems that are intractable for classical computers.

The researchers successfully optimised a neutral atom Rydberg blockade controlled-Z gate, increasing its robustness to variations in the Rabi frequency by almost an order of magnitude. This improvement matters because the controlled-Z gate is a fundamental component of quantum algorithms, and enhanced durability directly supports more complex computations. The optimised gate utilises analytically defined laser pulse shapes, reducing the impact of fluctuating atomic energy levels and residual atomic motion. Current investigations are exploring the limits of this approach with different atomic species and excitation parameters.