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Quantum coherence is the preservation of phase relationships between parts of a quantum state. Those relationships let quantum alternatives interfere with one another. When interactions with the environment disturb them, the interference can fade—a process called decoherence.
What does quantum coherence mean?
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 produce interference. The National Academies explains that coherence is necessary for quantum interference, superposition, and entanglement in its report on quantum computing.
A compact way to think about it is: coherence lets quantum alternatives retain phase relationships that allow them to interfere. If those relationships are weakened, the interference is reduced. This is more precise than saying a particle is simply “in two places”; coherence describes a relationship among components of a state, not a location by itself.
How is coherence different from superposition?
Superposition means a quantum state is expressed as a combination of possible states. Coherence concerns whether the phase relationships among those components are preserved. The terms are connected, but they are not interchangeable: a superposition’s relative phases are what allow characteristic interference effects, while environmental interaction can degrade those relationships.
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For intuition, NIST researcher Tara Fortier writes, “Superposition is a little like flipping a coin.” The analogy can suggest that a quantum system is not always adequately described by one classical outcome, but a quantum object is not literally a coin rotating between heads and tails. NIST discusses the analogy and its limits in its quantum explainer.
How does coherence relate to waves?
In wave language, coherence describes how well-defined phase relationships between waves remain. Spatial coherence concerns phase relationships across different positions; temporal coherence concerns how they persist over time. These ideas connect to quantum coherence because phase determines how contributions combine, but a quantum state is not merely a classical wave in disguise. The National Academies discusses coherence in both quantum-information and chemistry contexts in its quantum-computing report and quantum information science material.
What causes quantum decoherence?
Decoherence is the loss or degradation of coherent phase relationships, usually because a quantum system interacts with uncontrolled degrees of freedom in its environment. Stray fields, temperature fluctuations, and other disturbances can entangle a system with its surroundings and make its interference harder to observe. It is not simply a person looking at the system: measurement is a physical interaction, and no human consciousness is needed for environmental coupling to affect a state. NIST describes fragile quantum states losing their quantum behavior through environmental interaction in its decoherence overview; ISO likewise defines decoherence as a process affecting quantum systems in its quantum terminology standard.
Why does coherence matter in quantum computing?
Quantum computers use qubits that can be prepared in superpositions and entangled with other qubits. Coherence helps preserve the phase relationships needed for quantum operations and interference. But it is only one ingredient: a useful computer also needs reliable state preparation, control, readout, and error management. Coherence does not mean a device automatically evaluates every answer and reveals the correct one. NIST notes that practical quantum states are fragile and can be disrupted by stray fields or temperature fluctuations in its quantum computing overview.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsHow do scientists study coherence and decoherence?
Interference in three-level atoms
NIST describes experiments with driven three-level atoms in which multiple quantum pathways contribute to whether a probe experiences gain or loss. The work illustrates how coherence and interference can be studied and points to research possibilities such as short-wavelength coherent radiation, sensitive magnetometers, and sub-Doppler laser cooling. These are research contexts, not evidence that consumer devices offer a directly controllable coherence feature. See NIST’s account of coherence and interference in driven three-level atoms.
A trapped atom and engineered environments
A 2002 Nature study recorded by NIST measured decoherence in superposed motional states of a single trapped atom, with decoherence induced using engineered reservoirs. It is a concrete demonstration that researchers can investigate how coupling to an environment affects a quantum state; its results concern that specific trapped-atom system, not every quantum platform. The study is listed by NIST here.
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Is there a standard coherence time?
No single duration applies to all quantum systems. A coherence time depends on the platform, the particular state, the way the system is controlled, and how coherence is measured. A trapped-atom result or a value reported for one device should not be treated as a universal clock for quantum technology. The National Academies’ discussion of coherence in chemistry and quantum information provides broader context in its quantum information science material.
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