Can special quantum states avoid scrambling and keep entanglement?
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.
Source
Disorder-tunable entanglement at infinite temperature
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
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What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Thermalization
- The process by which an isolated many-body system loses memory of its initial state and local observables approach thermal values.
- Quantum many-body scar
- A special eigenstate in an otherwise chaotic system that does not thermalize, causing persistent revivals from certain initial states.
- Entanglement entropy
- A measure of how entangled a subsystem is with the rest, computed from the subsystem's reduced density matrix.
- Bell pair
- Two qubits in a maximally entangled state.
- Quantum state tomography
- Reconstructing a quantum state's density matrix from many measurements in different bases.
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Quiz yourself
What is a quantum many-body scar?
Common questions
What does 'infinite temperature' mean here?
The scar states sit throughout the energy spectrum, including the middle where generic states look like infinite-temperature thermal states, yet they still do not thermalize.
Why does the sign of coupling matter?
Opposite-sign couplings on the two rows make their spectra mirror images, which is what allows the Bell-pair rainbow states to be exact eigenstates.
How is this different from many-body localization?
Localization uses strong disorder to stop thermalization and limits entanglement, while here the system stays chaotic and only special states avoid thermalization, with large, tunable entanglement.
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