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.
Source
Isotopically enhanced triple-quantum-dot qubit
Study at a glance
- Design
- Other — Single-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
- N
- One triple-dot device; the 10,000 figure refers to single-shot measurements in a readout histogram, not samples
- Population
- Electron spins in a gate-defined triple quantum dot in a 28Si-enriched Si/SiGe quantum well (800 ppm residual 29Si)
- Outcome
- Magnetic dephasing time T2*, echo T2, charge-noise amplitude, readout visibility
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The team built a triple quantum dot holding one electron in each dot in a silicon quantum well enriched in spin-free 28Si. They read the spin state in a single shot using Pauli spin blockade and a charge sensor. Operating two dots as a singlet-triplet qubit, they measured magnetic dephasing; with all three dots they drove Rabi and Ramsey oscillations using exchange pulses only, and built a composite 'Y-pulse' echo to measure charge noise.
What they found
The magnetic dephasing time was 2.31 microseconds at zero field, far above about 10 ns in GaAs and 360 to 900 ns in natural silicon dots. Surprisingly, the remaining magnetic noise seemed to come from electronic paramagnetism rather than 29Si nuclei, with echo times rising from 7 to 700 microseconds as field increased. The Y-echo data fitted a 1/f charge-noise model with an amplitude of about 15 microvolts, and readout visibility was around 98%.
The limits
What it doesn't show
It is a single device, and the authors say the experiments do little to identify the physical sources of either the magnetic or the charge noise; the paramagnetic source is only guessed to come from fabrication. Gate fidelities, two-qubit operations and randomized benchmarking were not measured, only argued to be feasible. The singlet-triplet splitting on one side of the device was too small to use, hinting at valley-splitting variability.
Key terms
- Exchange-only qubit
- A qubit encoded in three electron spins that is controlled purely by voltage-tuned exchange interactions, without microwaves or magnetic gradients.
- Pauli spin blockade
- Readout where a triplet spin state cannot move into a doubly occupied dot, converting spin into a detectable charge difference.
- T2* (dephasing time)
- How long an ensemble of measurements keeps phase coherence before slowly varying noise washes it out.
- Spin echo
- A refocusing pulse midway through evolution that cancels the effect of slowly varying noise.
- 1/f noise
- Noise whose power grows at low frequency, common for charge fluctuations in semiconductor devices.
Flashcards
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Quiz yourself
What makes 28Si attractive for spin qubits?
Common questions
Why purify silicon isotopically?
29Si nuclei have spin and create random magnetic fields; removing them reduces the magnetic noise that dephases electron spins.
Why is all-electrical control valuable?
Voltage pulses are easy to confine to individual devices, unlike microwaves or micromagnet gradients, which helps scaling to many qubits.
Why was a special Y-pulse needed?
The two exchange axes in a triple dot are not orthogonal, so a composite of four pulses was built to rotate about an axis orthogonal to both.
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