1. Quantum Mechanics

The ideas a quantum computer runs on: superposition, entanglement and measurement, and what they let us do with information.

This part works through four ideas, each followed by a short quiz:

• Entanglement. Two qubits that share one state. Why a maximally entangled pair can share none of its entanglement with a third party (its monogamy), and how measuring the pair gives correlated outcomes: in the Bell state \((|00\rangle + |11\rangle)/\sqrt{2}\), both qubits measured in the \(Z\) basis, or both in the \(X\) basis, always agree.

• Teleportation. Moving an unknown qubit state from one place to another with one entangled pair and two classical bits.

• Measurement. The probability of each outcome, and what measuring one part of an entangled state does to the rest.

• Repeaters. How a chain of short entangled links carries quantum information across distances an optical fibre cannot.

The mathematics behind these ideas, from ket notation to the Bloch sphere, is introduced in The Hardware of Quantum Computers, where it is first put to work, and the quantum machine learning series returns to it with code.

Sources

This part draws on TU Delft’s courses on quantum internet and quantum computing on edX. I am grateful to TU Delft, the University of Toronto and edX for making that teaching open to everyone.