Symmetric $C_Z$ gate for ultracold neutral atoms based on counterdiabatic driving at Rydberg excitation

We designed a scheme for a neutral atom Rydberg blockade $C_Z$ gate based on the double sequence of adiabatic pulses applied symmetrically to both atoms and using counterdiabatic driving for Rydberg excitation. This provides a substantial reduction in the quantum gate operation time compared to previously proposed double adiabatic schemes, and makes our scheme competitive with modern time-optimal protocols for high-fidelity entangling gates with neutral atoms. Our approach creates a bridge between fully adiabatic and time-optimal gate schemes. The use of adiabatic passage reduces the sensitivity of gate fidelity to variations in laser intensity, while counterdiabatic driving provides short gate times. The intensity and phase profiles of the laser pulse acting on the atoms are described analytically depending only on the gate duration. We demonstrated the applicability of this scheme for single-photon and two-photon schemes of Rydberg excitation in rubidium and cesium atoms, and, for the first time, discussed the implementation of a $C_Z$ gate using three-photon excitation of rubidium atoms. In contrast to many modern $C_Z$ gate protocols, our scheme does not generate intrinsic single-qubit phase shifts, although they still appear in two-photon configuration. We also designed a numerically optimized amplitude-robust gate with an analytically defined phase profile of the laser pulse and compared its performance with the counteradiabatic gate scheme.

💡 Research Summary

In this work the authors present a new protocol for implementing a high‑fidelity, symmetric CZ gate with neutral‑atom qubits that leverages Rydberg blockade while dramatically shortening the gate duration. The starting point is the well‑known double‑adiabatic pulse sequence, in which two identical smooth pulses are applied symmetrically to both atoms. This sequence is robust against variations of the laser intensity because the population transfer follows an adiabatic rapid passage, but it suffers from long gate times (tens of microseconds) that are comparable to the finite lifetime of the Rydberg state and to the limited blockade strength.

To overcome this limitation the authors introduce a “shortcut to adiabaticity” (STA) based on counter‑diabatic (CD) driving. In a two‑level system described by a time‑dependent Rabi frequency Ω₀(t) and detuning δ(t), the CD term is Ω_CD(t)=−2 θ̇(t), where θ(t)=½ arctan