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THE CHECKLIST
Five ingredients for a quantum computer
1. Qubits
Well-defined two-level systems you can scale up.
2–3. Reset and coherence
Start in 0, and last much longer than a gate.
4–5. Gates and measurement
A universal set of gates, and readout of each qubit.
Physicist David DiVincenzo listed these in 2000; every platform is still judged against them.
A REAL QUBIT
Is it a good qubit?
Not enough
Having two positions, like a switch or a coin.
What's needed
A genuinely quantum two-level system that can hold superpositions.
Good candidates
Atoms, electron spins, superconducting circuits, photons.
Quantum behaviour, not just two states, is what makes a qubit.
START FROM ZERO
Reset: every calculation starts at 0
Passive reset
Wait for the qubit to relax in a cold chip.
Active reset
Measure it, and flip it if it reads 1.
Why cold matters
Heat would randomly kick qubits back to 1.
A clean start is the first step of every quantum program.
LIFETIME VS SPEED
Long life, fast gates
Coherence time
How long the qubit keeps its quantum state.
Gate time
How long one operation takes.
What counts
The ratio: how many gates fit before the qubit forgets.
Thousands of gates per lifetime is the minimum for error correction.
CONTROL AND READOUT
Talk to every qubit, one at a time
Control
Each qubit needs its own pulses, without disturbing its neighbours.
Readout
Fast, accurate measurement of each qubit.
At scale
Thousands of control lines are a major engineering challenge.
Every one of these ingredients turns into a fabrication and wiring problem later in the course.
UNIT 9 STUDY COMPLETE
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Ready for the Fab Challenge?
You can judge any qubit candidate against DiVincenzo's checklist.