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Quantum coherence is the preservation of phase relationships between parts of a quantum state. Those relationships allow quantum alternatives to interfere, producing effects that a simple mixture of possibilities cannot. When interactions with the environment disrupt the relationships, coherence is lost or reduced—a process called decoherence.
What quantum coherence means
A quantum state can be described as a combination of possible basis states, each with a complex amplitude. The relative phases among those amplitudes matter: they determine how the alternatives combine and whether they reinforce or cancel one another. Coherence is the preservation of those relationships.
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This idea also appears in wave physics. Spatial coherence describes how phase relationships are maintained across different positions; temporal coherence describes how they are maintained over time. These are useful connected descriptions, but a quantum state should not be treated as merely a classical wave in disguise.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThe National Academies summarizes the role of coherence this way: “Coherence is necessary for quantum phenomena such as quantum interference, superposition, and entanglement.” (National Academies, Quantum Computing: Progress and Prospects; National Academies chapter on chemistry and quantum information science)
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How coherence differs from superposition
A superposition is a combination of quantum states. Coherence concerns whether the phase relationships among the components of that combination are preserved. The terms are closely related, but they are not interchangeable: a state’s superposition alone does not explain the interference effects that depend on its relative phases.
When coherent alternatives meet, their probability amplitudes can interfere. If their phases are changed or their relationships are erased, the interference pattern can change or disappear. This is one reason quantum behavior can differ from a classical mixture, in which the system is described as having one possibility or another without phase relationships between them.
What causes decoherence?
Decoherence is the degradation or loss of coherence, usually because a quantum system interacts with uncontrolled degrees of freedom in its surroundings. Stray fields, temperature fluctuations, and other environmental disturbances can become entangled with or otherwise carry information about the system, suppressing observable interference. It is not simply a person looking at the system, and “measurement” does not require a conscious observer.
NIST explains that fragile quantum states can lose their quantum behavior through interaction with the environment (NIST’s quantum computing explainer; NIST on decoherence). The exact behavior depends on the system and its coupling to its surroundings.
Why coherence matters in quantum computing
Qubits can be prepared in superpositions and entangled with other qubits. Preserving coherence helps maintain the phase relationships needed for quantum interference and for the intended operation of a quantum circuit. But coherence is only one ingredient: a useful quantum computer also needs reliable state preparation, control, readout, and error management. Coherence does not mean a computer automatically calculates every possible answer and reveals them all, nor does it by itself guarantee a speedup.
Quantum devices are vulnerable to disturbances such as stray fields and temperature fluctuations. Engineers therefore work to control the environment and manage errors so that useful operations can be completed before unwanted interactions spoil the state (NIST’s quantum computing explainer; National Academies report).
Where coherence appears in experiments
Interference in driven atoms
NIST describes experiments with driven three-level atoms in which multiple quantum paths contribute to observed probe gain or loss. The work illustrates how coherent pathways and interference can shape an atomic response. Possible research applications discussed include short-wavelength coherent radiation, sensitive magnetometers, and sub-Doppler laser cooling; these are research contexts, not evidence that a consumer device offers a directly controllable coherence feature (NIST on coherence and interference in driven three-level atoms).
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Decoherence in a trapped atom
A 2002 Nature study recorded by NIST measured decoherence in superposed motional states of a single trapped atom, using engineered reservoirs. It is a concrete example of decoherence being studied experimentally and of how system–environment coupling matters. Its specific system does not establish a general coherence duration for other atoms or quantum technologies (NIST record of the trapped-atom study).
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Is there a standard coherence time?
No single duration applies to every quantum system. A coherence time must be tied to a particular platform, state, and measurement method. The trapped-atom experiment is one specific study, not a universal benchmark. When comparing technologies, coherence duration is only one relevant factor alongside control speed, error rates, scalability, and measurement.
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Common analogies—and their limits
A spinning coin can suggest that a quantum system may not be adequately described by one definite classical outcome before measurement. NIST researcher Tara Fortier puts the analogy this way: “Superposition is a little like flipping a coin.” The analogy is for superposition, not a definition of coherence; a quantum object is not literally a coin spinning between heads and tails (NIST explainer).
Schrödinger’s cat is another metaphor for the strangeness of quantum description, not a claim that real cats can be both alive and dead. Such images can introduce the subject, but phase relationships, interference, and environmental interaction provide the more precise account (NIST explainer).
Further reading
For a more technical treatment of quantum information science and coherence, see the National Academies report Quantum Computing: Progress and Prospects. ISO also provides a concise definition of decoherence (ISO quantum computing terminology).
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