computer science//quantum computing//decoherence

Decoherence is the loss of a quantum state's superposition through its unwanted interaction with the environment, which turns the delicate combination of amplitudes into an ordinary random mixture, and it is the main reason quantum computers are hard to build. A qubit is only useful while it stays coherent, and every computation has to finish, or be protected, before the environment wins.

Decoherence is the loss of a quantum state's superposition through its unwanted interaction with the environment, which turns the delicate combination of amplitudes into an ordinary random mixture, and it is the main reason quantum computers are hard to build. A qubit is only useful while it stays coherent, and every computation has to finish, or be protected, before the environment wins.

The environment is anything the qubit couples to that the computer does not control: stray electromagnetic fields, thermal photons in the wiring, vibrations, defects in the chip's materials, neighbouring qubits, the control pulses themselves when they are slightly off. Each interaction leaks a little information about the qubit's state into the surroundings, and information that has leaked cannot interfere any more (superposition).

Two time constants measure it. T1T_1T1​, the relaxation time, is how long an excited qubit takes to decay to its ground state (a 1 drifting to 0); T2T_2T2​, the dephasing time, is how long its phase survives, and it is never longer than 2T12T_12T1​. For today's superconducting qubits both are of the order of tens to a hundred microseconds; trapped ions keep coherence for seconds and run their gates more slowly.

What counts is the ratio of coherence time to gate time. A superconducting gate takes tens of nanoseconds, so a qubit with a 100 µs coherence allows a few thousand operations before its state is mostly noise, which bounds the depth of any circuit run without error correction.

The defences are physical and logical. Physically: refrigerate to about ten millikelvin so thermal photons are rare, shield against magnetic fields and radiation, filter the control lines, choose cleaner materials. Logically: spread each logical qubit across many physical ones and keep measuring for errors (quantum error correction).

A qubit must be isolated from everything except the controls that drive it.

Every line that lets the computer reach in is also a path for noise to get in, which is why scaling to many qubits is a wiring, cooling and materials problem as much as a physics one.

For an engineer used to classical electronics, decoherence is the analogue of noise with one harsh difference: classical bits can be copied and compared to correct errors, while an unknown quantum state cannot be copied at all (the no-cloning theorem), so the correction has to work without ever looking at the data directly.