The journey of a high-spin donor in silicon, from quantum chaos to fault-tolerant qubits
ORAL · Invited
Abstract
In this talk I will tell the instructive story of how a purely curiosity-driven idea ended up delivering a fault-tolerant encoded qubit in silicon.
My group is best known for its work on donor spin qubits in silicon, but I have always had a deep fascination for quantum aspects of chaos. Ten years ago, I realized that a quadrupolar nucleus subjected to a strong drive could embody the “driven top” Hamiltonian, which represents a prototypical model for quantum chaos. The use of ion implantation to introduce donors in silicon makes it easy to change atomic species, and we converged on the choice of 123Sb, which has a large spin I = 7/2 and a strong quadrupole moment. We wrote a proposal for how to perform the experiment [1] and we proceeded to fabricate and measure the devices.
We were first sidetracked by accidental the (re)discovery of nuclear electric resonance [2] – a method to control nuclear spins with electric field first predicted in 1961 but never convincingly observed in experiments. Then, we became aware of exciting new ideas to use high-spin systems to embed error-correcting codes [3,4]. We developed the devices and the control methods to execute such ideas, using Schroedinger cat states of the 123Sb nucleus [5,6]. We have now demonstrated the fault-tolerant operation of a cat-qubit encoded in the 123Sb nucleus, where the fidelity of logical gates on the encoded qubit outperforms that of gates on the unencoded circuit.
Finally, we performed the quantum chaos experiments, exploiting the ability to produce near-instantaneous “kicks” through a multi-level virtual phase gate. These experiments have interesting implications for digital quantum simulations.
This story is a reminder of the importance of curiosity in enabling breakthroughs in science.
[1] V. Mourik et al., Phys. Rev. E 98, 042206 (2018)
[2] S. Asaad, V. Mourik et al, Nature 579, 205 (2020)
[3] J. Gross, Phys. Rev. Lett. 127, 010504 (2021)
[4] J. Gross et al., Phys. Rev. App. 22, 014006 (2024)
[5] P. Gupta et al., Phys. Rev. Res. 6, 013101 (2024)
[6] X. Yu et al, Nature Physics 21, 362 (2025)
My group is best known for its work on donor spin qubits in silicon, but I have always had a deep fascination for quantum aspects of chaos. Ten years ago, I realized that a quadrupolar nucleus subjected to a strong drive could embody the “driven top” Hamiltonian, which represents a prototypical model for quantum chaos. The use of ion implantation to introduce donors in silicon makes it easy to change atomic species, and we converged on the choice of 123Sb, which has a large spin I = 7/2 and a strong quadrupole moment. We wrote a proposal for how to perform the experiment [1] and we proceeded to fabricate and measure the devices.
We were first sidetracked by accidental the (re)discovery of nuclear electric resonance [2] – a method to control nuclear spins with electric field first predicted in 1961 but never convincingly observed in experiments. Then, we became aware of exciting new ideas to use high-spin systems to embed error-correcting codes [3,4]. We developed the devices and the control methods to execute such ideas, using Schroedinger cat states of the 123Sb nucleus [5,6]. We have now demonstrated the fault-tolerant operation of a cat-qubit encoded in the 123Sb nucleus, where the fidelity of logical gates on the encoded qubit outperforms that of gates on the unencoded circuit.
Finally, we performed the quantum chaos experiments, exploiting the ability to produce near-instantaneous “kicks” through a multi-level virtual phase gate. These experiments have interesting implications for digital quantum simulations.
This story is a reminder of the importance of curiosity in enabling breakthroughs in science.
[1] V. Mourik et al., Phys. Rev. E 98, 042206 (2018)
[2] S. Asaad, V. Mourik et al, Nature 579, 205 (2020)
[3] J. Gross, Phys. Rev. Lett. 127, 010504 (2021)
[4] J. Gross et al., Phys. Rev. App. 22, 014006 (2024)
[5] P. Gupta et al., Phys. Rev. Res. 6, 013101 (2024)
[6] X. Yu et al, Nature Physics 21, 362 (2025)
*Australian Research Council (Grants No. DP150101863, DP180100969, DP210103769, FL240100181), US Army Research Office (Contract No. W911NF-23-1-0113), Australian Department of Industry, Innovation and Science (Grant No. AUSMURI000002).
–
Presenters
-
Andrea Morello
- University of New South Wales