Concept · physics
Qubits
6 studies1 discoveryEvidence last moved Sep 27, 2026
A qubit is a two-level quantum system used to store and process information; its usefulness depends on coherence times, gate fidelities and how precisely many qubits can be made alike. The evidence here covers silicon spin qubits, superconducting transmons, defects that limit them, and small processors used in experiments.
Qubit progress is often summarised in one number, but these papers show trade-offs between speed and decay, between frequency targeting and yield, and between longer lifetimes and coherence. They help students see which noise sources limit each platform.
Studies
6
Findings
5
8 supporting · 0 challenging · 1 qualifying citations
Open tensions
1
Latest change
Concept page published
Qubits
Currently
What we know
- Silicon's low nuclear-spin noise gives long coherence.
- Knowing the dominant noise source guides what to fix.
- Post-fabrication tuning helps frequency crowding.
- Longer energy lifetime is not longer coherence.
- Circuit depth is limited by accumulated gate error.
Largest unresolved question
Platform differences: spin-qubit studies measure dephasing and single-qubit fidelity on single devices, whereas transmon work reports processor-scale two-qubit fidelities; the numbers are not directly comparable across platforms.
Common misconceptions
Driving a qubit faster always gives better gates.
In the silicon double dot, stronger driving shortened decay; the best quality factor was near a 10 MHz Rabi frequency, not maximum drive.
Removing nuclear spins removes magnetic noise from silicon qubits.
In the isotopically enhanced triple dot, remaining magnetic noise seemed to come from electronic paramagnetism rather than 29Si.
Extending a defect's lifetime makes it a good quantum memory.
Phononic shielding raised defect lifetimes over 100-fold but dephasing stayed near 1 µs.
Related
Claim ledger
What the evidence shows
Drawn from 6 studies in this library. Mix labels say which citation roles are present; they are not a strength score. Supports means evidence for a finding; Challenges means evidence against a stated position; Qualifies marks scope.
Silicon's low nuclear-spin noise gives long coherence.
Silicon spin qubits reach microsecond dephasing times: a natural-silicon double dot had T2* = 1.83 µs and 99.6% single-qubit Clifford fidelity, and an isotopically enhanced triple dot reached 2.31 µs versus about 10 ns in GaAs.
- Can ordinary silicon host fast, accurate spin qubits?— Single-qubit gates only; one device.
Study Role Design N Population Outcome Can ordinary silicon host fast, accurate spin qubits? Supports OtherLow-temperature device experiment on a Si/SiGe double quantum dot with a cobalt micromagnet, driven by electric dipole spin resonance One device; most data are from the left dot, with the right dot showing similar results in the supplement Single electron spins in a natural-silicon Si/SiGe double quantum dot at about 120 mK Rabi frequency, Rabi decay time, dephasing time T2*, quality factor Q and single-qubit Clifford gate fidelity Does purified silicon make an all-electric spin qubit quieter? Supports OtherSingle-device experiment on a Si/SiGe triple quantum dot at 20 mK: single-shot spin readout, double-dot singlet-triplet dephasing, triple-dot Rabi/Ramsey fringes and a composite Y-pulse echo One triple-dot device; the 10,000 figure refers to single-shot measurements in a readout histogram, not samples Electron spins in a gate-defined triple quantum dot in a 28Si-enriched Si/SiGe quantum well (800 ppm residual 29Si) Magnetic dephasing time T2*, echo T2, charge-noise amplitude, readout visibility Knowing the dominant noise source guides what to fix.
Remaining noise is often not nuclear: the triple-dot device's magnetic noise appeared to come from electronic paramagnetism, with 1/f charge noise near 15 µV, and in the natural-silicon device faster driving shortened decay, attributed to microwave heating.
- Does purified silicon make an all-electric spin qubit quieter?
- Can ordinary silicon host fast, accurate spin qubits?
Study Role Design N Population Outcome Does purified silicon make an all-electric spin qubit quieter? Supports OtherSingle-device experiment on a Si/SiGe triple quantum dot at 20 mK: single-shot spin readout, double-dot singlet-triplet dephasing, triple-dot Rabi/Ramsey fringes and a composite Y-pulse echo One triple-dot device; the 10,000 figure refers to single-shot measurements in a readout histogram, not samples Electron spins in a gate-defined triple quantum dot in a 28Si-enriched Si/SiGe quantum well (800 ppm residual 29Si) Magnetic dephasing time T2*, echo T2, charge-noise amplitude, readout visibility Can ordinary silicon host fast, accurate spin qubits? Supports OtherLow-temperature device experiment on a Si/SiGe double quantum dot with a cobalt micromagnet, driven by electric dipole spin resonance One device; most data are from the left dot, with the right dot showing similar results in the supplement Single electron spins in a natural-silicon Si/SiGe double quantum dot at about 120 mK Rabi frequency, Rabi decay time, dephasing time T2*, quality factor Q and single-qubit Clifford gate fidelity Post-fabrication tuning helps frequency crowding.
Transmon frequencies can be tuned after fabrication: laser annealing hit 89.5% of targets with about 4.7 MHz tuning precision, did not hurt coherence (T1 about 79 µs), and gave a 65-qubit processor with a median two-qubit fidelity of 98.7%.
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.
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.
- Can special quantum states avoid scrambling and keep entanglement?
- Can measuring two quantum copies together beat measuring them one by one?
Study Role Design N Population Outcome Can special quantum states avoid scrambling and keep entanglement? Supports OtherTheory 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 No sample size; circuits of two rows with up to eight qubits each Transmon superconducting qubits in a ladder with tunable-sign couplings Population imbalance, subsystem fidelity revivals and entanglement entropy over time for special versus generic initial states Can measuring two quantum copies together beat measuring them one by one? Supports OtherTheory-designed measurement circuits run on several quantum processors (IBM, Rigetti, trapped-ion, photonic) to estimate two small qubit rotations under controlled decoherence No participant sample; each unknown angle was estimated 400 times, each estimate averaging 512 circuit repetitions (341 for three-copy circuits) Qubits on the Fraunhofer IBM Q System One, 11 cloud IBM Q processors, Rigetti Aspen-9, the AQTION trapped-ion processor and the JenQuant photonic processor Mean squared error of simultaneous estimates of two rotation angles, compared with Nagaoka and Holevo bounds
Debates
Tensions and limits
Some items are genuine disagreements on the same question. Others mark different assays, populations, or outcomes.
Platform differences: spin-qubit studies measure dephasing and single-qubit fidelity on single devices, whereas transmon work reports processor-scale two-qubit fidelities; the numbers are not directly comparable across platforms.
Evidence for
Evidence against
PaperFren reads this as a limit on how far one study travels — different assays, populations, or outcomes — not a forced fight between papers.
Timeline
How understanding moved
Study years are when the paper was published. Evidence edits are dated changes to this page's claims. Explanations are when PaperFren added a Discovery — not a claim that the science happened that day.
2026
- Silicon spin qubits reached microsecond coherence both with and without isotope purification
Concept page published
Qubits
Change log
What changed
Dated edits to this page's evidence: studies added or removed from a claim, claims added or withdrawn, and new explanations tagged here. Rewordings are not listed.
- Concept page published
Papers
6 studies in this library bear on Qubits, ordered by citations.
- Can ordinary silicon host fast, accurate spin qubits?
A carefully designed tiny magnet let electron spins in ordinary (non-purified) silicon be flipped fast enough and cleanly enough to pass the error threshold for fault-tolerant quantum computing.
- Does purified silicon make an all-electric spin qubit quieter?
Using isotopically purified silicon, a three-dot spin qubit controlled only by voltages kept its phase hundreds of times longer than in gallium arsenide, leaving charge noise as the main limit.
- Can a laser fix mistuned qubits on a quantum chip?
Briefly heating individual qubit junctions with a laser sets their frequencies precisely enough to avoid clashes between neighbours, without harming how long the qubits stay coherent.
- Can measuring two quantum copies together beat measuring them one by one?
Measuring two copies of a noisy qubit jointly with an entangling circuit estimated two rotation angles more precisely than any one-at-a-time measurement can, on real quantum computers.
- Can blocking sound waves make qubit-damaging defects live longer?
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.
- 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.
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Questions
What is still open
Platform differences: spin-qubit studies measure dephasing and single-qubit fidelity on single devices, whereas transmon work reports processor-scale two-qubit fidelities; the numbers are not directly comparable across platforms.
Study this conceptflashcards and short-answer questions
Why is a single-qubit fidelity of 99.6% not enough to claim a working quantum computer?
The silicon double-dot study reached 99.6% average Clifford fidelity with addressable qubits, but showed no two-qubit gate, which computation requires. Results come from one device. Processor-scale work on transmons reports two-qubit gate fidelities (median 98.7% on 65 qubits), which is the harder benchmark.
Distinguish T1 and dephasing using the phononic-bandgap defect experiment.
T1 is energy relaxation; dephasing is loss of phase coherence. Shielding defects inside an acoustic bandgap raised their median lifetime from 4.4 µs to 505 µs, because phonon emission was blocked. But dephasing stayed near 0.91 µs, set by low-frequency noise. So blocking one decay channel did not fix coherence.