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arXiv:2410.07641 · Newton

Certifying the quantumness of a nuclear spin qudit through its uniform precession

Arjen Vaartjes, Martin Nurizzo, Lin Htoo Zaw, Benjamin Wilhelm, Xi Yu, Danielle Holmes, Daniel Schwienbacher, Anders Kringhøj, Mark R. van Blankenstein, Alexander Jakob, Fay E. Hudson, Kohei M. Itoh, Riley J. Murray, Robin Blume-Kohout, Namit Anand, Andrew S. Dzurak, David N. Jamieson, Valerio Scarani, Andrea Morello

Published Feb 14, 2025• 8 citations
arXivHTMLPDF

Abstract

Spin precession is a textbook example of the dynamics of a quantum system that exactly mimics its classical counterpart. Here, we challenge this view by certifying the quantumness of exotic states of a nuclear spin through its uniform precession. The key to this result is measuring the positivity, instead of the expectation value, of the x -projection of the precessing spin, and using a spin >1/2 qudit that is not restricted to semi-classical spin coherent states. The experiment is performed on a single spin-7/2 123 Sb nucleus, implanted in a silicon nanoelectronic device, amenable to high-fidelity preparation, control, and projective single-shot readout. Using Schrödinger cat states and other bespoke states of the nucleus, we violate the classical bound by 18 standard deviations, proving that no classical probability distribution can explain the statistic of this spin precession and highlighting our ability to prepare quantum resource states with high fidelity in a single-atomic-scale qudit.

DOI
10.1016/j.newton.2025.100017

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Spin precession is a textbook example of the dynamics of a quantum system that exactly mimics its classical counterpart. Here, we challenge this view by certifying the quantumness of exotic states of a nuclear spin through its uniform precession. The key to this result is measuring the positivity, instead of the expectation value, of the x -projection of the precessing spin, and using a spin >1/2 qudit that is not restricted to semi-classical spin coherent states. The experiment is performed on a single spin-7/2 123 Sb nucleus, implanted in a silicon nanoelectronic device, amenable to high-fidelity preparation, control, and projective single-shot readout. Using Schrödinger cat states and other bespoke states of the nucleus, we violate the classical bound by 18 standard deviations, proving that no classical probability distribution can explain the statistic of this spin precession and highlighting our ability to prepare quantum resource states with high fidelity in a single-atomic-scale qudit.
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We propose a scheme to generate spin cat states, i.e., superpositions of maximally separated quasiclassical states on a single high-dimensional nuclear spin in a solid-state device. We exploit a strong quadrupolar nonlinearity to drive the nucleus significantly faster than usual gate sequences, achieving collapses and revivals two orders of magnitude faster than the dephasing timescale. Furthermore, these states are engineered without entanglement with an ancilla, hence, are robust against error propagation. With our multitone control, we can realize arbitrary high-spin rotations within an experimentally feasible regime, as well as transform a spin coherent state to a spin cat state using only phase modulation, opening the possibility of storing and manipulating high-fidelity cat states. Published by the American Physical Society 2024
Projective measurement of single electron and nuclear spins has evolved from a gedanken experiment to a problem relevant for applications in atomic-scale technologies like quantum computing. Although several approaches allow for detection of a spin of single atoms and molecules, multiple repetitions of the experiment that are usually required for achieving a detectable signal obscure the intrinsic quantum nature of the spin's behavior. We demonstrated single-shot, projective measurement of a single nuclear spin in diamond using a quantum nondemolition measurement scheme, which allows real-time observation of an individual nuclear spin's state in a room-temperature solid. Such an ideal measurement is crucial for realization of, for example, quantum error correction protocols in a quantum register.
Most classical dynamical systems are chaotic. The trajectories of two identical systems prepared in infinitesimally different initial conditions diverge exponentially with time. Quantum systems, instead, exhibit quasiperiodicity due to their discrete spectrum. Nonetheless, the dynamics of quantum systems whose classical counterparts are chaotic are expected to show some features that resemble chaotic motion. Among the many controversial aspects of the quantum-classical boundary, the emergence of chaos remains among the least experimentally verified. Time-resolved observations of quantum chaotic dynamics are particularly rare, and as yet unachieved in a single particle, where the subtle interplay between chaos and quantum measurement could be explored at its deepest levels. We present here a realistic proposal to construct a chaotic driven top from the nuclear spin of a single donor atom in silicon, in the presence of a nuclear quadrupole interaction. This system is exquisitely measurable and controllable, and possesses extremely long intrinsic quantum coherence times, allowing for the observation of subtle dynamical behavior over extended periods. We show that signatures of chaos are expected to arise for experimentally realizable parameters of the system, allowing the study of the relation between quantum decoherence and classical chaos, and the observation of dynamical tunneling.
We report repeated single-shot measurements of the two-electron spin state in a GaAs double quantum dot. The readout allows measurement with a fidelity above 90% with a approximately 7 micros cycle time. Hyperfine-induced precession between singlet and triplet states of the two-electron system are directly observed, as nuclear Overhauser fields are quasistatic on the time scale of the measurement cycle. Repeated measurements on millisecond to second time scales reveal the evolution of the nuclear environment.