Silicon Spin-Qubit Readout
Reading a spin qubit converts the spin state into a charge signal, sensed by a nearby charge detector or by dispersive gate reflectometry.
Spin-to-Charge Conversion
A single electron spin produces far too small a magnetic signal to measure directly. Readout instead converts spin information into a charge signal, which is comparatively easy to detect. Two conversion methods dominate. Energy-selective, or Elzerman, readout aligns the two spin levels across the Fermi level of a reservoir so that only the higher-energy spin can tunnel out, producing a charge change that is detected. Pauli spin blockade uses two electrons: the Pauli exclusion principle allows a charge transition between dots only for certain joint spin states, so a blocked or unblocked charge movement reveals the spin parity.
Charge Sensors
- A single-electron transistor or quantum point contact placed beside the dots changes its conductance when a single charge moves.
- The sensor is biased and its current or conductance is monitored continuously.
- Sensitivity down to a fraction of an electron charge is achievable.
Dispersive Gate Reflectometry
A more scalable approach dispenses with a separate charge sensor. A radio-frequency tone is reflected off a resonant circuit connected to one of the dot gates. The quantum capacitance of the dot, which depends on whether a charge tunneling event can occur, shifts the resonance and therefore the reflected signal's phase and amplitude. Because it reuses the gate electrodes already present, gate-based reflectometry reduces the number of extra components needed per qubit, an important consideration when scaling to large arrays.
Speed and Fidelity
Readout must be fast compared with the spin relaxation time and must resolve the small signal above amplifier noise. Cryogenic amplifiers, and increasingly Josephson parametric amplifiers, improve the signal-to-noise ratio and shorten the integration time. High-fidelity single-shot readout of individual spins has been demonstrated, and combining it with multiplexed reflectometry is a leading path toward reading many spin qubits without a wire per qubit.
Efficient readout is as central to spin-qubit scaling as control, and it shares much of the same amplifier and filtering technology used elsewhere in this library.