Quantum & After, No. 6: What a Quantum Computer Can’t Do

What quantum computers can’t do and how error correction with logical qubits works.

A quantum computer is constrained not by competition between technologies but by physics: qubits are highly sensitive to their environment and easily lose the states required for computation.

This Quantum & After installment covers physical and logical qubits, decoherence, and error correction.

The short answer

Quantum computers will not replace conventional ones, fit in your pocket, or run familiar programs. It is not only about today’s level of maturity: qubits are highly sensitive to external noise, and errors accumulate quickly as computations grow in complexity. Without error correction, long algorithms lose accuracy. To reduce errors, quantum information is distributed across several physical qubits to form a more resilient logical qubit.

Why

Returning to the example from the previous installment. Imagine a coin that hasn’t landed yet: we don’t know whether it will end up heads or tails. In quantum systems, uncertainty is different: a qubit can be in a superposition of two states. Any external influence can disrupt this state and cause an error. In hardware, such errors come from thermal noise, vibrations, electromagnetic interference, and imperfections in materials and control. That is why superconducting processors from Google and IBM operate at temperatures on the order of 10–20 mK, well below the ~2.72 K cosmic microwave background. Such cooling reduces thermal noise but requires complex cryogenic systems.