Stardust Log

A record of stardust, quietly accumulating.
← Back to the glossary
Glossary / Quantum computing

Entanglement

A state in which two or more qubits or particles cannot be described separately, however far apart they are: they form a single whole.

For two entangled particles, the result of measuring one is strongly correlated with the result of measuring the other, at any distance. In 1935 Einstein, Podolsky and Rosen argued that this showed quantum mechanics to be incomplete[1]. In 1964 John Bell showed, as an inequality that experiments can test, that the correlations cannot be fully explained by supposing the particles carried their answers from the start[2]. Experiments since then have agreed with quantum mechanics.

In a quantum computer, entanglement is the resource that computation draws on. The most strongly entangled states of two qubits are called Bell states, and how accurately a machine can prepare them is a basic test of its quality. Error correction, too, works by entangling many qubits so that the information is spread across them.

Related terms

  • QubitThe basic unit of information in a quantum computer. Rather than being either 0 or 1, it can be in a superposition of both.
  • Quantum error correctionSpreading the information of fragile qubits across many of them, so that errors can be detected and fixed.
  • Logical qubitA single, robust qubit made by bundling many physical qubits together and running error correction on them.

Sources

  1. A. Einstein, B. Podolsky, N. Rosen, “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?,” Phys. Rev. 47, 777–780 (1935).
  2. J. S. Bell, “On the Einstein Podolsky Rosen paradox,” Physics Physique Fizika 1, 195–200 (1964).