High-fidelity multiqubit gates with Rydberg atoms via level-crossing-free Rapid adiabatic passage

How can researchers improve the fidelity and robustness of multiqubit entangled state preparation in Rydberg atom systems? Conventional Rapid Adiabatic Passage (RAP) techniques often suffer from nonadiabatic transitions when the system passes through a level crossing (resonance), which limits the theoretical fidelity and requires strict parameter control.

The authors propose a novel RAP scheme that eliminates level crossings entirely. By utilizing an antisymmetric Rabi frequency (a zero-area pulse) and an even-symmetric detuning, the system avoids the resonance point where the energy gap is minimal. This design keeps the system in a large energy gap throughout the evolution. The protocol uses two sequential pulses separated by a pi-pulse to prepare various entangled states, including two-qubit Bell states, three-qubit W states, four-qubit GHZ states, and six-qubit honeycomb W states.

Numerical simulations demonstrate that this level-crossing-free approach achieves exceptionally high fidelities: 0.9997 for Bell and three-qubit W states, 0.997 for four-qubit GHZ states, and 0.9995 for six-qubit honeycomb W states. The scheme is highly robust against pulse parameter fluctuations, maintaining fidelities above 0.99 even with 5% variations in parameters. The zero-area pulse shape also provides first-order suppression of laser intensity noise, making it more resilient to experimental imperfections than traditional RAP methods.

This method offers a more efficient and robust pathway for scaling up quantum information processing in neutral-atom platforms. By removing the need to navigate the sensitive resonance region, the scheme simplifies pulse optimization and reduces the complexity of preparing complex entangled states, which are essential resources for quantum computing, communication, and metrology.