Hardware & Errors
Spin Qubit
A qubit stored in the spin of a single electron or nucleus, typically in a silicon chip made with semiconductor processes.
Definition
A spin qubit uses the tiny magnetic orientation of a single electron, up or down, as the 0 and 1 of a qubit. The electron is trapped in a nanoscale structure on a chip, often silicon, that looks like a transistor. That similarity is the main attraction: spin qubits could be manufactured in the same factories that make computer chips, and they are small enough that millions could in principle fit on one die.
In depth
Spin qubits began with the 1998 proposal by Daniel Loss and David DiVincenzo for quantum computing with electron spins in quantum dots. Early experiments used gallium arsenide, where nuclear spins in the host material cause rapid decoherence. Silicon changed that, because its most common isotope, silicon-28, has no nuclear spin, and purified silicon-28 wafers give a magnetically quiet environment. Coherence times of milliseconds to seconds are possible for spins in purified silicon.
Performance has improved quickly. In 2022, three groups in Delft, at RIKEN, and at Princeton reported two-qubit gate fidelities above 99 percent in silicon devices. In 2025, Diraq and imec reported fidelities above 99 percent on devices made on 300 millimeter wafers in an industrial CMOS line, and Intel's Tunnel Falls chip put 12 spin qubits in a package available to researchers. Operating temperatures are about 100 millikelvin, and some devices have worked above 1 kelvin, which would ease cooling.
Challenges include uniformity, since tiny variations in the fabricated dots shift qubit frequencies, and wiring, because each qubit needs several control lines. Two-qubit gates depend on exchange coupling that decays quickly with distance, so qubits must be close, and crosstalk must be managed. Researchers are developing cryogenic control chips and shuttling methods that move electrons across a chip to connect distant qubits.
Technical Definition
In a gate-defined quantum dot, electrodes on top of a silicon layer create a potential well that traps one electron. The qubit states are the spin-up and spin-down states in a magnetic field. Single-qubit gates use electron spin resonance driven by microwave fields or electric-dipole spin resonance with a magnetic field gradient, and two-qubit gates use the exchange interaction between neighboring dots.
Visual Explanation: An Analogy
Imagine a bar magnet so small that it is a single electron, held in a cage on a chip. Pointing it up or down is a bit, and tilting it in between is a superposition. The cage is built with the same techniques used to make transistors.
Key facts
| Item | Detail |
|---|---|
| Proposal | Loss and DiVincenzo, 1998 |
| Host material | Often silicon, ideally isotopically purified silicon-28 |
| Two-qubit fidelity | Above 99 percent demonstrated in 2022 and on 300 mm wafers in 2025 |
| Operating temperature | About 100 millikelvin, with demonstrations above 1 kelvin |
| Size | Tens of nanometers per qubit, much smaller than superconducting qubits |
Where the idea comes from
After the 1998 proposal, single-spin control in quantum dots was demonstrated in the 2000s and 2010s, with silicon devices at UNSW in 2012 and 2014. Industrial interest rose as Intel, imec, and CEA-Leti began fabricating devices, and the first fidelities above the error-correction threshold were reported in 2022.
Real-World Use Cases
- Building quantum processors with semiconductor manufacturing.
- Research toward millions of qubits on one chip.
- Integrating qubits with cryogenic control electronics.
- Quantum sensing and memory using donor spins in silicon.
How it differs from related ideas
- Compared with transmons, spin qubits are far smaller and compatible with CMOS, but earlier in development and harder to wire.
- Compared with trapped ions, they are faster and smaller, but have lower gate fidelities so far.
- Donor spins in silicon, such as phosphorus atoms, have longer coherence than gate-defined dots but are harder to place.
Common Misconceptions
- Spin qubits are not a mature technology. They are advancing quickly but have only demonstrated up to a dozen or so qubits.
- Being made in silicon does not mean they are easy to mass-produce. Uniformity is a major challenge.
- They are not the same as quantum dots in displays. The dots are used here to hold a single electron.
Frequently asked questions
What is a spin qubit?
A qubit that stores information in the spin of a single electron or nucleus, usually trapped in a silicon quantum dot.
Why is silicon used?
Purified silicon-28 has no nuclear spins, which gives a quiet magnetic environment, and silicon manufacturing is mature.
Who is building spin qubit processors?
Intel, Diraq, imec, Quantum Motion, Silicon Quantum Computing, and several universities.