What is decoherence? Introduction to decoherence

Understand: In a computer, a qubit is not an isolated system, it will interact with the external environment to attenuate quantum coherence, i.e., "decoherence".

Decoherence makes quantum computers different from traditional computers, where the computation time is limited. This is because it is difficult to maintain the coherence between qubi and Qingpai for a long time, and after a certain period of time, once it encounters the observation of an external entity, it will lose its coherence. The time between the time a qubit goes from a coherent state to when it loses its coherence is called the "decoherence time". If the decoherence time is not long enough, the calculation cannot be completed.

Therefore, extending the decoherence time is a major issue that must be solved in the future.

Quantum superposition collapses as a result of observations. Decoherence is the interference caused by the surrounding environmental noise that makes the qubit "worse", so the observation will also affect the coherence; To avoid decoherence, circuit components are isolated from their surroundings. However, there are still many reasons for the decoherence of the liquid that have not been identified.

Decoherence, commonly known as the "wave function collapse effect", is one of the fundamental mathematical properties of quantum mechanics. It refers to the phenomenon that the probability amplitude of the wave function, which was originally continuously distributed, degenerates instantaneously after "observation" into a δ function (Dirac function δ which is discretely distributed at a specific point, and the value of all other points is infinite, and the total area of the whole function graph is defined as 1). In an exaggeration, the decoherence effect refers to the fact that "when no one is looking at the moon, the moon only hangs in the sky with a certain probability; And when someone takes a look, the original uncertain existence of the moon suddenly becomes reality the moment people look at it. ”

Decoherence history was proposed by Murray Gell-Mann, James Hartle, and Robert Griffiths in 1984 to explain the theory of wave function collapse in quantum mechanics.

Decoherence history was proposed by Murray Gell-Mann, James Hartle, and Robert Griffiths in 1984 to explain the theory of Woober function collapse in quantum mechanics.

According to the popular science book "Does God Roll the Dice? - The introduction in the History of Quantum Physics (hereinafter referred to as "Part I"), together with some information on the Internet, I summarize the concept of this theory as follows.

First of all, it is listed by a number of concepts:

1.Quantum coherence: Quantum coherence, or "correlation between states".

One of them was a prediction made by Albert Einstein and his collaborators in 1935 based on a hypothetical experiment. In this hypothetical experiment, if an electron and a positron generated by energy fly in opposite directions in a high-energy accelerator, and when no one observes them, both of them are in a superposition of right and left spins, and if the electrons are observed to be spinning to the right, then the positrons must be in a state of left spin, that is, no matter how far apart they are, there must be a high correlation between the direction of rotation between them.