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The difference is not that one kind of material follows quantum mechanics and the other does not: quantum mechanics underlies every solid. A material is called a quantum material when quantum effects remain essential to explaining its important large-scale properties, beyond what familiar conventional models can capture. It is a practical distinction, not a strict division between quantum and non-quantum matter.
What distinguishes quantum materials from conventional materials?
For many familiar materials, simplified models describe everyday properties—such as electrical transport or mechanical behavior—well enough for practical purposes. Those models do not make the material fundamentally classical. They are approximations to a microscopic world governed by quantum mechanics, including the behavior of electrons, chemical bonding, and vibrations in the crystal lattice.
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The label quantum material is useful when quantum behavior is not just part of the underlying explanation but has a visible, consequential effect on the material’s properties. UC Berkeley describes the term as shorthand for solids whose properties cannot be captured even qualitatively without a quantum-mechanical description (UC Berkeley Department of Physics). The U.S. Department of Energy’s 2016 Basic Energy Sciences report puts the distinction succinctly: “All materials require quantum mechanics, but not all materials are quantum materials” (DOE workshop report).
What to look for in a material
There is no single quick test that sorts every sample into one category. Instead, ask whether a familiar model adequately explains the property in question, or whether a specifically quantum feature is needed to account for what is observed.
- Distinctive observable: Is there behavior such as superconductivity, unusual surface transport, or the absence of static magnetic order?
- Explanatory mechanism: Does the explanation depend on electronic wavefunction topology, strong interactions, reduced dimensionality, or a collective quantum state?
- Limits of a conventional picture: Does a simpler model explain the broad behavior but fail to capture the defining feature?
- State and conditions: Does the behavior depend on the material’s particular state or on conditions needed to reveal it?
These questions help identify what is distinctive; they do not establish a universal classification threshold. A material’s behavior can depend on its specific state and conditions.
Examples: different quantum effects, not one shared mechanism
Quantum materials are an umbrella category rather than a sharply bounded group on the periodic table. The field’s use of the term has expanded from strongly correlated solids to include two-dimensional and topological materials. Its members do not all exhibit the same physics.
Ordinary metals and superconductors
A classical electron-fluid picture can be a helpful introduction to electrical conduction in a metal, even though it does not give accurate values for every quantity. A superconductor, by contrast, enters a state in which electrical resistance is not measurable below a transition temperature. Berkeley also highlights the phenomenon as electrons flowing without measurable resistance below a nonzero temperature. Superconductivity is a characteristic quantum phenomenon, but its description should not be taken to mean every superconductor behaves identically under every condition.
Topological insulators and semimetals
In topological materials, the geometry of electronic wavefunctions enriches the usual description of electronic bands. Topological insulators and semimetals are examples of this family. The DOE report describes coherent transport protected at material surfaces as a distinctive feature of topological states. The relevant quantum behavior is tied to topology; it is not a property to assume for every material called quantum.
Two-dimensional materials such as graphene
In some materials, confining electrons to two-dimensional sheets helps produce distinctive properties. Graphene is a prominent example discussed in the quantum-materials literature. Reduced dimensionality is one recurring route to quantum behavior, not a requirement that every quantum material is a two-dimensional sheet.
Quantum spin liquids and spin ices
In many examples, electrons interact too strongly to be treated as independent particles. Those interactions can give rise to collective behavior and excitations called quasiparticles. A quantum spin liquid differs from a conventional magnet because its spins do not settle into a static ordered state. Spin ices are another representative family; neither example implies that all quantum materials share the same magnetic behavior.
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How researchers investigate quantum materials
Researchers combine measurements of a material’s structure and properties with computational methods. At SLAC, bright X-rays from the Stanford Synchrotron Radiation Lightsource and Linac Coherent Light Source are used alongside computation to probe and manipulate material structure and behavior (SLAC’s explainer on quantum materials). The DOE report likewise emphasizes characterization and theory capable of revealing subtle emergent and topological order.
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These are research approaches, not a universal step-by-step test for identifying an unknown sample. The useful evidence depends on the property and mechanism being examined.
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What the label does—and does not—tell you
Calling a material a quantum material does not mean it is automatically useful for quantum computing, nor does the label identify one common mechanism. Superconductivity, topology, reduced dimensionality, strong interactions, and collective excitations are different routes to distinctive behavior. Some material families also retain open questions about their fundamental behavior and their eventual usefulness for quantum information science.
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