CQS-12 Tutorial: Lieven Vandersypen - Quantum computing with spins – past, present and future

Tutorial talk from the 12th Rochester Conference on Coherence and Quantum Science on June 24 2025 at University of Rochester. Hosted by the Center for Coherence and Quantum Science (https://www.sas.rochester.edu/quantum...) Speaker: Lieven Vandersypen, QuTech, Delft University of Technology Abstract:The long and winding road from the early quantum computing experiments to the major industry efforts today has been truly exciting. Over all these years, the state of a spin-1/2 particle not only served as the textbook example of a quantum bit, but also as a leading qubit implementation. In this tutorial, we will briefly survey past work and then focus on ongoing efforts and future directions towards a large-scale quantum computer. Early explorations of quantum algorithms using liquid-state nuclear magnetic resonance relied on spin-1/2 nuclei in specially selected and synthesized molecules. These efforts culminated in the first implementation of Shor’s celebrated factoring algorithm in 2001. Since then, efforts have focused on electron and nuclear spins hosted in semiconductor quantum dots or color centers, for their potential for ultimate scalability. In this talk, I will present our vision of a large-scale spin-based quantum processor, and ongoing work to realize this vision. Today, single-qubit gate fidelities with semiconductor spins are routinely above 99.9%, sometimes surpassing 99.99%. In small systems, two-qubit gates fidelities exceed 99.5%. Universal high-fidelity control, initialization and readout was shown with six qubits, including their operation in quantum algorithms. Meanwhile, quantum dot arrays of 16 dots have been demonstrated. Whereas quantum dot spins usually sit just 100 nm apart during a two-qubit gate, we realized also two-qubit operations on spins 250 micron apart, mediated by microwave photons in a superconducting on-chip resonator. We also shuttled electrons across a device, covering an effective distance of 10 micron in less than 200 ns and with 99.5% fidelity. Moving electrons around, we can imagine bringing any two electrons close together for two-qubit operations, enabling arbitrary connectivity. Already, we realized a 99% fidelity two-qubit gate between mobile spins, and teleport a quantum state across the device. When combined, the progress along these various fronts can lead the way to scalable networks of highfidelity spin qubit registers for fault-tolerant quantum computation. The same quantum dot platform serves to simulate Fermi-Hubbard physics and spin models in two dimensions. [1] L.M.K. Vandersypen, et al., Nature 414, 883 (2001) [2] L.M.K. Vandersypen, et al., npj Quantum Information 3, 34 (2017). [3] X. Xue et al., Nature 601, 343 (2022). [4] S. Philips, M. Madzik et al., Nature 609, 919–924, (2022). [5] F. Unseld, B. Undseth et al., arXiv:2412.05171. [6] X. Zhang, E. Morozova, et al., Nature Nano 20, 209 (2025). [7] T.-K. Hsiao, P. Cova Fariña, et al., Phys. Rev. X 14, 011048 (2024). [8] M. De Smet, Y. Matsumoto, et al., Nature Nano, in print, arXiv:2406.07267. [9] J. Dijkema, X. Xue, et al., Nature Phys., 21, 23 (2025) [10] L.M.K. Vandersypen, and M.A. Eriksson, Physics Today 72 (8), 38 (2019)