Bucket brigade and conveyor-mode coherent electron spin shuttling in Si/SiGe quantum dots

Long-range quantum links between spin qubits in gate-defined quantum dots are a key aspect in architectures that scale to the thousands of qubits that are required for practical quantum computation. Such coherent qubit connections, which we envision to span on the order of ten to tens of microns, provide space for on-chip classical control electronics [1, 2] and thereby alleviate the wiring bottleneck. 

 In this work we create a quantum link by shuttling a single electron spin across a linear array of six tunnel-coupled quantum dots in an isotopically enriched 28Si/SiGe heterostructure. An electron can be shuttled through the array in bucket brigade mode by sequentially pulsing both the electrochemical potential of each quantum dot and the interdot tunnel barriers [3,4,5]. Alternatively, sinusoidal voltage signals can be applied to all the channel gates to create a traveling-wave potential (conveyor-mode) [6,7]. We benchmark both bucket brigade and conveyor-mode shuttling while transporting the electron back and forth across the device. In bucket brigade, the (echoed) spin can be shuttled with an average single hop fidelity of 99.45% (99.62%), consistent with earlier work on shuttling between two dots [4,5]. The fidelity is significantly boosted when operating in conveyor mode, where one round trip from quantum dot two to five and back using a conveyor operated at 300 MHz yields a spin echo fidelity of up to 99.76%. This corresponds to a fidelity of 99.96% for transport over the same distance as in an interdot hop. 

 [1] J.M. Taylor et al., Nature Physics 1, 177–183 (2005) 

[2] L.M.K. Vandersypen et al., npj Quantum Information 3, 34 (2017) 

[3] T. Fujita et al., npj Quantum Information 3, 22 (2017) 

[4] J. Yoneda et al., Nature Communications 12, 4114 (2021) 

[5] A. Noiri et al., Nature Communications 13, 5740 (2022) 

[6] I. Seidler et al., npj Quantum Information 8, 100 (2022) 

[7] T. Struck et al., arXiv:2307.04897 (2023)