11.2 What does it take to be a qubit?

In his 2000 paper “The Physical Implementation of Quantum Computation,” David P. DiVincenzo laid out five “requirements for the implementation of quantum computation.” 71 DiVincenzo, David P.

In his words, they are:

1. A scalable physical system with well characterized qubits
2. The ability to initialize the state of the qubits to a simple fiducial state, such as |000…⟩
3. Long relevant decoherence times, much longer than the gate operation time
4. A “universal” set of quantum gates
5. A qubit-specific measurement capability

Let’s discuss what each of these means, following his lead.

Scalable physical system

In the physical system we manufacture for quantum computing, we need to create a qubit with two clearly delineated states, |0⟩ and |1⟩. Other states may be possible...