Doppler-resilient ground-Rydberg transition and its application in high-fidelity entangling gates with neutral atoms

The motion-induced dephasing is a severe problem that limits the accuracy of a quantum control process by using external laser fields in neutral Rydberg atoms. This dephasing is a major issue that limits the realizable fidelity of a quantum entangling gate with neutral atoms when there is a {\it gap} time for the Rydberg atom to drift freely. We find that such a dephasing can be largely suppressed by using a transition in a V'-type dual-rail configuration. The left~(right) arm of this V’ represents a transition to a Rydberg state $\mathrm{\mid }{r}_{1(2)}⟩$ with a Rabi frequency $\mathrm{\Omega }{e}^{ikz}(\mathrm{\Omega }{e}^{-ikz})$, where $z$ is frozen without atomic drift, but changes linearly in each experimental cycle. Such a configuration is equivalent to a transition between the ground state and a hybrid and time-dependent Rydberg state with a Rabi frequency $\sqrt{2}\mathrm{\Omega }$, such that there is no phase error whenever the state returns to the ground state. We study two applications of this method. First, it is possible to faithfully transfer the atomic state between a hyperfine ground state $\mathrm{\mid }1⟩$ and Rydberg states $\mathrm{\mid }{r}_{1(2)}⟩$ with no {\it gap} time between the excitation and deexcitation. Second, by adding infrared laser fields to induce transition between $\mathrm{\mid }{r}_{1(2)}⟩$ and a nearby Rydberg state $\mathrm{\mid }{r}_3⟩$ via a largely detuned low-lying intermediate state in the {\it gap} time, the atom can keep its internal state in the Rydberg level as well as adjust the population branching in $\mathrm{\mid }{r}_{1(2)}⟩$ during the {\it gap} time. This allows an almost perfect Rydberg deexcitation after the {\it gap} time, making it possible to recover a high fidelity in the Rydberg blockade gate. The theory paves the way for high-fidelity quantum control over neutral Rydberg atoms without cooling qubits to the motional ground states in optical traps.