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arXiv:1912.02977 · Physical Review A

Symmetric Rydberg controlled- Z gates with adiabatic pulses

Mark E. Saffman, Ilya I. Beterov, Archismita Dalal, Eduardo Javier Paez, Barry C. Sanders

Published Jun 3, 2020• 98 citations
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Abstract

We analyze neutral atom Rydberg ${C}_{Z}$ gates based on adiabatic pulses applied symmetrically to both atoms. Analysis with smooth pulse shapes and Cs atom parameters predicts the gates can create Bell states with fidelity $\mathcal{F}>0.999$ by using adiabatic rapid passage pulses. With globally optimized adiabatic pulse shapes, in a two-photon excitation process, we generate Bell states with fidelity $\mathcal{F}=0.997$. The analysis fully accounts for spontaneous emission from intermediate and Rydberg states, including the Rydberg lifetime in a room-temperature environment, but does not include errors arising from laser noise. The gate protocols do not require individual addressing and are shown to be robust against Doppler shifts due to atomic motion.

DOI
10.1103/physreva.101.062309

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We analyze neutral atom Rydberg CZ{C}_{Z}CZ​ gates based on adiabatic pulses applied symmetrically to both atoms. Analysis with smooth pulse shapes and Cs atom parameters predicts the gates can create Bell states with fidelity F>0.999\mathcal{F}>0.999F>0.999 by using adiabatic rapid passage pulses. With globally optimized adiabatic pulse shapes, in a two-photon excitation process, we generate Bell states with fidelity F=0.997\mathcal{F}=0.997F=0.997. The analysis fully accounts for spontaneous emission from intermediate and Rydberg states, including the Rydberg lifetime in a room-temperature environment, but does not include errors arising from laser noise. The gate protocols do not require individual addressing and are shown to be robust against Doppler shifts due to atomic motion.
We designed a scheme for a neutral atom Rydberg blockade CZC_ZCZ​ 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 CZC_ZCZ​ gate using three-photon excitation of rubidium atoms. In contrast to many modern CZC_ZCZ​ 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.
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Adiabatic controlled-ZZZ gate for neutral atoms with Förster-enhanced Rydberg dressing
We study an adiabatic controlled-ZZZ (CZ) gate for neutral atoms in which dipole-dipole coupling between nearly degenerate Rydberg pair states enhances the interaction between dressed ground states. This coupling produces an avoided crossing in the two-atom dressed-state spectrum. Approaching the crossing enhances the dressed interaction, but also reduces the dressed-state gap and increases the Rydberg admixture. We identify a Förster-enhanced operating range in which the required conditional phase is accumulated with adiabatic return and small transient Rydberg population. We then analyze the finite-time dynamics of the two-qubit computational states under a smooth pulse. Reducing the dressing strength weakens the dressed interaction, so maintaining the target phase at the same pulse duration requires operation closer to the avoided crossing, where the Rydberg admixture increases. This dependence is then used to select the shortest gate at a fixed peak Rabi frequency in the Förster-enhanced operating range that accumulates the CZ phase with high Bell-state fidelity while keeping the Rydberg population small. Changing the Förster defect shifts the avoided crossing, while the gate parameters change only weakly. For a specific rubidium Förster channel, the selected gate corresponds to experimentally relevant laser coupling, pulse duration, and interatomic separation. We also estimate errors from the finite Rydberg lifetime and from weak excitation of a neighboring spectator atom. For the parameters considered, the spectator-induced error remains below the finite-lifetime contribution.