Quantum Dots
A quantum dot is a nanoscale potential well that confines electrons to discrete levels, an artificial atom used to host spin qubits.
Artificial atoms
A quantum dot confines one or a few electrons in all three dimensions, so their energy levels become discrete like an atom's. Dots can be defined electrostatically by voltages on surface gates over a two-dimensional electron gas, grown self-assembled in III-V materials, or formed in nanowires. For qubits, gate-defined dots in silicon or SiGe dominate.
Charge stability
Gate voltages set how many electrons sit in a dot. A charge stability diagram maps electron number against voltages; qubit operation lives at a specific occupancy, usually a single electron or a two-electron singlet-triplet configuration. Tuning many dots to their target occupancy is a high-dimensional calibration problem increasingly handled by automated routines.
Qubit encodings
- Loss-DiVincenzo: one electron per dot, spin up/down is the qubit
- Singlet-triplet: two electrons across a double dot, immune to global magnetic fluctuations
- Exchange-only: three dots, all operations by exchange pulses, no microwaves needed
Coupling
Neighboring dots interact by the exchange interaction, tuned by the barrier between them. Longer-range coupling uses shuttling of electrons, capacitive coupling, or a shared microwave resonator, an approach that borrows the circuit-QED readout ideas of superconducting qubits.
Quantum dots are the workhorse of the silicon spin-qubit program. Their small footprint is the appeal; the challenge is that each dot needs precise, individually tuned control, so scaling depends on cutting the wiring and calibration burden per qubit.