Infidelity Induced by Ground-Rydberg Decoherence of the Control Qubit in a Two-Qubit Rydberg-Blockade Gate

For neutral-atom qubits, two-qubit gates are typically realized via the Rydberg-blockade effect, which hints at the special status of Rydberg levels compared with regular qubit register states. However, the experimental and theoretical studies that have been done to reveal how the ground-Rydberg coherence of the control-qubit atom affects the fidelity of a two-qubit gate are not yet adequate, especially when all experimental two-qubit gates feature a single atom that stays in the Rydberg state for a certain time. Here, we perform a careful investigation of decoherence in ground-Rydberg transitions in a single atom with various pulse sequences. In particular, based on the commonly used controlled-Z (CZ) gate sequence proposed by D. Jaksch et al., Phys. Rev. Lett. 85, 2208 (2000), we observe that a control-qubit atom subject to a two-qubit entangling process with the H-CZ controlled-NOT gate scheme experiences a process that is essentially similar to ground-Rydberg Ramsey interference. Then, we build a straightforward theoretical model to link the decoherence time rgr of the control qubit (extracted from Ramsey fringes of the groundRydberg transition) to this CZ pulse sequence, and also analyze the typical origins of decoherence effects. Finally, we discuss the loss of fidelity of the CZ gate due to the limits imposed by the ground-Rydberg coherence properties, and the prospect of improving fidelity with refined gate protocols.

Automated summary

This study investigates the coherence of ground-Rydberg transitions in a single atom subjected to various pulse sequences, particularly focusing on the CZ gate sequence. It is found that the process experienced by the control qubit atom under a two-qubit entangling process with the H-CZ controlled-NOT gate scheme is essentially similar to ground-Rydberg Ramsey interference. A theoretical model linking the decoherence time of the control qubit to the CZ pulse sequence is established, shedding light on the origins of decoherence effects and potential ways to improve fidelity of the CZ gate.