Buffer-atom-mediated quantum logic gates with off-resonant modulated driving

Connectivity of two-qubit logic gates plays a crucial and indispensable role in quantum computation research. For the cold atom qubit platform, while the two-qubit Rydberg blockade gate has recently made rapid experimental progress, a pressing challenge is to improve connectivity in pursuit of genuine scalability without sacrificing speed or fidelity. A significant advancement in this direction can be achieved by introducing an extra buffer atom to extend the two-qubit gate beyond purely nearest-neighbor two-body interactions. The buffer atom couples with the two qubit atoms through nearest-neighbor interactions, even though the qubit atoms do not directly exert any physical influence on each other. The established method of off-resonant modulated driving (ORMD) is not only convenient but also lays the groundwork for this latest development. Although the atomic linkage structure here exhibits more complex interactions compared to previous two-body systems, the population can satisfactorily return to the ground state after the ground-Rydberg transition with a properly designed modulation waveform. This can be achieved through one-photon and two-photon ground-Rydberg transitions in common practices. Furthermore, with buffer atom relay or similar structures, it is possible to realize a two-qubit entangling gate between two distant qubit atoms. In addition to demonstrating that such solutions are feasible, the representative modulation patterns are analyzed, showcasing the versatility of buffer-atom-mediated two-qubit gates. From a broader perspective, these efforts enhance the resemblance between the cold atom qubit platform and the superconducting qubit system, with the buffer atom functioning like wires and junctions.