Semiconductor Spin Qubits and Their Cooling
Spin qubits encode information in the spin of electrons confined in semiconductor quantum dots and, like superconducting qubits, operate at millikelvin temperatures.
Qubits in silicon
A semiconductor spin qubit stores quantum information in the spin state of one or a few electrons trapped in a quantum dot, a tiny electrostatically defined region in a semiconductor such as silicon. Spin up and spin down serve as the two computational states. Spin qubits are attractive because they are small and because they can be built with fabrication techniques related to those of the classical microelectronics industry.
Why they need to be cold
The energy splitting between spin states in an applied magnetic field is small, corresponding to well below one kelvin, so the qubits must be operated at tens of millikelvin in a dilution refrigerator to keep them in a definite state and to resolve their energy levels. Like superconducting qubits, they share the cold end of the fridge and inherit the same dependence on helium-3 cooling.
A cooling advantage
Spin qubits offer one notable thermal advantage: some can tolerate operation at slightly higher temperatures, around one kelvin, than superconducting qubits. Because cooling power is far more abundant at one kelvin than at ten millikelvin, operating there would ease the cooling budget and the cryo-CMOS integration, since control electronics could share the stage. This hot-qubit prospect is an active research direction motivated directly by the cooling constraints of scaling.
Their own challenges
Spin qubits face challenges distinct from superconducting ones: they are exquisitely sensitive to charge noise and to nuclear-spin noise in the host material, addressed by using isotopically purified silicon-28 that removes spinful nuclei. Their small size aids density but makes uniform fabrication and individual control demanding. As with every millikelvin platform, coherence, control, and cooling must be advanced together.
- Information stored in electron spin in a quantum dot
- Operate at tens of millikelvin like superconducting qubits
- Some tolerate around one kelvin, easing the cooling budget
- Sensitive to charge and nuclear-spin noise; use silicon-28