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Entanglement and quantum information

Can special quantum states avoid scrambling and keep entanglement?

Dong H, Desaules JY, Gao Y, et al. · Science advances · 2023

Open access · cc by · source: Europe PMC

In a chaotic ladder of superconducting qubits, certain specially built states keep oscillating and stay structured instead of thermalizing, and the entanglement of one family of them can be dialled with disorder.

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Study at a glance

Design
Other — Theory of exact scar eigenstates in a two-row qubit ladder, tested by quench dynamics and state tomography on a tunable-coupler superconducting processor, with numerical simulations including device imperfections
N
No sample size; circuits of two rows with up to eight qubits each
Population
Transmon superconducting qubits in a ladder with tunable-sign couplings
Outcome
Population imbalance, subsystem fidelity revivals and entanglement entropy over time for special versus generic initial states

Structured fields used in claim comparison tables when every cited study has a complete layer.

Key findings

A product initial state overlapping the first scar family showed population oscillations lasting around a microsecond, whereas a generic state decayed to zero within about 50 ns. Fidelity of a two-site subsystem revived repeatedly with a period of about 80 ns and entanglement grew only slowly. An entangled initial state probing the second family behaved distinctly, and increasing one coupling raised the revival fidelity by about 0.3, as theory predicted.

Methodology

The authors built a model of two rows of qubits where the bottom row's couplings have the opposite sign to the top row's, and showed it hosts two families of exact 'rainbow' scar states made of Bell pairs across the rows. The second family depends explicitly on the disorder in the couplings. They realised the model on a superconducting processor with up to eight qubits per row, started it in chosen initial states, and tracked local populations, subsystem fidelity and entanglement entropy via tomography.

Limitations

Device imperfections (unwanted diagonal couplings and incomplete suppression of higher qubit levels) caused revivals to decay slowly, so the perfect behaviour of the ideal model was not observed. Disorder was kept weak so the system stayed chaotic; strong-disorder regimes were not tested. Circuits were small, and the stability of these scars in much larger or higher-dimensional systems remains open.

How this study connects

Role on claims

Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.

  • SupportsQuantum entanglementconcept

    Special states and paths can control how entanglement evolves.

    Entanglement dynamics can be engineered: scar states on a superconducting qubit ladder showed slow entanglement growth and revivals lasting about a microsecond versus 50 ns decay for generic states, and loops around an exceptional point in a photonic walk switched Bell states with fidelities of at least 84%.

    Evidence for the claim as stated.

  • SupportsQubitsconcept

    Circuit depth is limited by accumulated gate error.

    Current processors already support physics experiments but are limited by gate errors: scar-state revivals on a superconducting ladder decayed due to device imperfections, and three-copy measurement circuits failed on all tested devices.

    Evidence for the claim as stated.

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