How Does Decoherence Remove Quantum Behaviour?
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- Ages 12+
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Decoherence is the loss of quantum interference when a system interacts with its environment, for example when many stray particles scatter off an electron. For a qubit, an outside disturbance can ruin a superposition or entanglement, which is why quantum states are hard to keep.
Contents
- With interference: clear bars
- After decoherence: faint bars
What you will learn
- State what decoherence is in plain words.
- Explain why interacting with the environment removes interference.
- Say why qubits need protection from noise.
Before you start
It helps to read What Is Quantum Superposition? first. Other words are explained as they appear.
What is decoherence?
Decoherence is the suppression of quantum interference. According to the Stanford Encyclopedia of Philosophy, interference phenomena are a well-known and crucial aspect of quantum mechanics, famously exemplified by the two-slit experiment. There are many situations, however, in which interference effects are suppressed, artificially or spontaneously.
The theory of decoherence is precisely the study of such situations. The Stanford source says the modern foundation of decoherence as a subject in its own right was laid by H.-D. Zeh in the early nineteen-seventies.
In short: interference is a crucial aspect of quantum mechanics, and decoherence is when it gets suppressed.
- Electron
- Barrier with two slits
- Screen with a banded pattern
How does the environment remove interference?
The environment removes interference because it interacts with the system and becomes entangled with the components of the wave. According to the Stanford source, “environmental” decoherence is decoherence that arises through suitable interaction of a system with its environment.
Follow an electron through two slits to a screen. If some other systems interact with the electron between the slits and the screen, leading to enough entanglement with the components of the wave going through the two slits, then the detection probabilities on the screen are as if we had placed detectors at the slits. For example, there could be sufficiently many stray particles that scatter off the electron.
At that point, according to the Stanford source, interference can be produced only in a suitable experiment that includes the larger system made of the electron and the stray particles. This suppression of interference is what is called decoherence.
A related remark. The Stanford source reminds us that if Alice and Bob share a pair of entangled systems, the probabilities for the results of any measurements Bob might make do not depend on whether or not Alice also makes any measurements. The source adds that the pattern of detections on the screen cannot distinguish mere entanglement with some other systems from the actual use of those systems for detection at the slits.
- Electron
- Barrier with two slits
- Screen with a blurred pattern
- Stray particles (red dots)
Why does decoherence matter for quantum computers?
Because qubits are fragile, and noise from the environment can ruin their state. According to the NIST explainer, qubits are exquisite but fragile. A stray electric or magnetic field, temperature fluctuations or even a cosmic ray can ruin a superposition or entanglement.
When interference in the system of interest is suppressed through entanglement with its environment, we should not say the qubit necessarily acquired one definite value. Stanford notes that interference may still be considered in the larger system including the environment. Decoherence and a measurement yielding a particular outcome are therefore distinct concepts.
- Qubit inside a protective box
- Electromagnetic disturbances (broken red lines)
- Thermal disturbance (yellow circle with rays)
Real-world example
Think of ripples on a still pond. Two ripples meet and make a regular pattern: that is interference. If many small sticks stir the water at random, the pattern blurs and is no longer clear.
This is only an intuition-building comparison, since decoherence in the Stanford source concerns quantum waves, not water waves.
Common misunderstandings
Decoherence does not necessarily need a detector. According to the Stanford source, the result on the screen is as if detectors had been placed at the slits, even though only other systems interacted with the electron.
Decoherence is not only a problem for quantum computers. The Stanford source places it in quantum mechanics in general, through the two-slit experiment.
A qubit that is easily disturbed does not mean all is lost. The NIST source says builders must find ways to protect qubits from outside disturbances.
Frequently asked questions
What is decoherence?
The suppression of quantum interference when a system interacts with its environment.
What causes decoherence?
Interaction with the environment, for example many stray particles scattering off an electron, leading to entanglement with the components of the wave.
Why do qubits lose their state so easily?
A stray electric or magnetic field, temperature fluctuations or a cosmic ray can ruin a superposition or entanglement.
Who laid the foundations of decoherence?
According to the Stanford source, H.-D. Zeh laid the modern foundation in the early nineteen-seventies.
Summary
- Decoherence is the suppression of quantum interference when a system interacts with its environment.
- The environment removes interference by becoming entangled with the components of the wave.
- Qubits are fragile, so they must be protected from outside noise.
Terms in this article
English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.
- decoherence
- The suppression of quantum interference when a system interacts with its environment.
- interferencegiao thoa
- The pattern produced when the components of a wave add up, as in the two-slit experiment.
- environmentmôi trường
- The other systems surrounding the system in question and interacting with it.
- qubit
- The unit of quantum information, easily ruined by disturbances.
- superpositionchồng chập
- A state combining several states a quantum system can have.
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Sources
These sources are written for older readers. If they feel hard, read them with an adult.
- The Role of Decoherence in Quantum Mechanics(opens in a new tab)
Stanford Encyclopedia of PhilosophyTier A: government, standards or research bodiesRead on November 6, 2026
- Quantum Computing Explained(opens in a new tab)
NISTTier A: government, standards or research bodiesRead on November 6, 2026