What Is Quantum Entanglement?

When pushed to explain why quantum computers can outspeed classical computers, stories about quantum computing often invoke a mysterious property called “entanglement.” Qubits, the reader is assured, can somehow be quantum mechanically entangled such that they depend on one another. If more detail is needed, the reader is told that entanglement links qubits no matter how far apart they are—so long as the qubits are “coherent.”