Schrödinger cat states of a nuclear spin qudit in silicon

High-dimensional quantum systems are a valuable resource for quantum
information processing. They can be used to encode error-correctable
logical qubits, for instance in continuous-variable states of oscillators
such as microwave cavities or the motional modes of trapped ions. Powerful
encodings include ‘Schrödinger cat’ states, and superpositions of widely
displaced coherent states, which also embody the challenge of quantum
effects at the large scale. Alternatively, recent proposals suggest
encoding logical qubits in high-spin atomic nuclei, which can host
hardware-efficient versions of continuous-variable codes on a
finite-dimensional system. Here we demonstrate the creation and
manipulation of Schrödinger cat states using the spin-7/2 nucleus of a
single antimony (123Sb) atom, embedded and operated within a silicon
nanoelectronic device. We use a coherent multi-frequency control scheme to
produce spin rotations that preserve the SU(2) symmetry of the qudit, and
constitute logical Pauli operations for logical qubits encoded in the
Schrödinger cat states. The Wigner function of the cat states exhibits
parity oscillations with a contrast up to 0.982(5), and state fidelities
up to 0.913(2). These results demonstrate high-fidelity preparation of
nonclassical resource states and logical control in a single atomic-scale
object, opening up applications in quantum information processing and
quantum error correction within a scalable, manufacturable semiconductor
platform.