Entangling an arbitrary pair of qubits in a long ion crystal
It is well established that the collective motion of ion crystals can be used as a quantum bus for multiqubit entanglement. However, as the number of ions increases, it becomes difficult to directly entangle ions far apart and resolve all motional modes of the ion crystal. We introduce a scalable and flexible scheme for efficient entanglement between any pair of ions within a large ion chain, using an evenly distributed 50-ion crystal as an example. By performing amplitude and frequency modulation, we find high-fidelity pulse sequences that primarily drive a transverse motional mode with a wavelength of four ion spacings. We present two 500μs pulses that can in theory suppress gate errors due to residual motion to below 10-4, and observe a trade-off between gate power and robustness against unwanted frequency offsets.
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