Qubits and quantum devices
Can blocking sound waves make qubit-damaging defects live longer?
Open access · cc by · source: Europe PMC
Atomic-scale defects inside superconducting qubit junctions lived about a hundred times longer when their frequency fell inside an engineered band where sound waves (phonons) cannot travel.
Study at a glance
- Design
- Other — Cryogenic experiment: transmon qubits with junctions on suspended silicon platforms surrounded by a phononic bandgap shield, used as sensors to swap into and time the decay of individual defects.
- N
- N=56 · 56 two-level-system defects with unique frequencies, measured across seven transmon devices on two chips (including thermal cycling to redistribute frequencies).
- Population
- Two-level-system defects in the aluminium oxide barriers of Josephson junctions on silicon-on-insulator chips
- Outcome
- Defect energy relaxation time T1 versus frequency relative to the acoustic bandgap; defect coherence T2*; temperature dependence of T1
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
Across 56 defects, lifetimes split into two families, and the long-lived family sat mostly inside a frequency band that matched the simulated acoustic bandgap. The median lifetime rose from 4.4 μs outside the band to 505 μs inside, and one defect reached about 1100 μs. However, the defect's dephasing time stayed near 0.91 μs, limited by low-frequency noise, and its quality factor dropped sharply above about 75 mK.
Methodology
The team built frequency-tunable transmon qubits whose Josephson junctions sit on suspended silicon platforms tethered by a patterned 'cross-shield' structure with an acoustic bandgap. In a dilution refrigerator, they used each qubit to find strongly coupled two-level-system (TLS) defects in the junctions, swapped excitations into them, and measured how long they stayed excited. They repeated this across devices and thermal cycles, then studied one long-lived defect's coherence and temperature dependence.
Limitations
Longer energy lifetime did not bring longer coherence, so the defects are not yet useful memories without techniques like dynamical decoupling, which were not tried. Several outliers remain, attributed to fabrication variation or dipole orientation but not directly tested. The low-temperature plateau in defect lifetime is only tentatively explained by quasiparticles, and the qubits themselves had short lifetimes of about 3 μs, so the paper does not show improved qubit performance.
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.
Longer energy lifetime is not longer coherence.
Atomic-scale defects (two-level systems) can be protected by phononic bandgaps: median defect lifetime rose from 4.4 µs outside the gap to 505 µs inside, but dephasing stayed near 0.91 µs.
Evidence for the claim as stated.
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