Curator's Take
This article introduces a promising new approach to quantum error suppression that could dramatically extend qubit coherence times by encoding information in nuclear spins with built-in noise protection. The researchers propose using antimony nuclei in silicon as "spin Kerr-cat" qubits that naturally suppress the most common source of quantum errors - dephasing noise - potentially achieving an extraordinary 100-second coherence time compared to microseconds for many current qubits. What makes this particularly exciting is that the error protection happens at the hardware level through clever use of the nuclear spin's natural symmetries, rather than requiring complex quantum error correction codes that consume many physical qubits. While the approach still needs experimental validation and faces challenges like achieving the required quadrupolar field enhancements, it represents an intriguing path toward more robust quantum hardware that could significantly reduce the overhead needed for fault-tolerant quantum computing.
— Mark Eatherly
Summary
The use of noise-robust qubit encodings provides a way of extending the lifetime of quantum information at the hardware level. In this work, we introduce the spin Kerr-cat encoding, which leverages a clock transition in the spectrum of quadrupolar nuclei (having spin length $I\geq 1$) to achieve a first-order suppression of noise leading to qubit dephasing. The basis states of the spin Kerr-cat qubit are given by the two lowest levels of a $\mathbb{Z}_2$-symmetric nuclear-spin Hamiltonian and are well approximated by spin cat states. We compute the dephasing time of the spin Kerr-cat qubit under a model of $1/f$ noise, as well as relaxation of the qubit due to breaking of the $\mathbb{Z}_2$ symmetry by charge-noise-induced fluctuations of the quadrupolar tensor. Using measured parameters for antimony (${}^{123}\mathrm{Sb}$) donors in silicon, we estimate that a coherence time of $T_2^*=100$ s could be achieved with this encoding. We propose a two-qubit gate mediated by hopping electrons and estimate that with an enhancement of measured quadrupolar splittings by a factor of $\approx 4$, a gate fidelity of $99\%$ could be achieved for spin Kerr-cat qubits encoded in ${}^{123}\mathrm{Sb}$ nuclear spins, neglecting errors that impact the electron while it is being shuttled and read out.