Glossary of elementary quantum mechanics

This is a glossary for the terminology often encountered in undergraduate quantum mechanics courses.



Kinematical postulates

a complete set of wavefunctions
A basis of the Hilbert space of wavefunctions with respect to a system.
The Hermitian conjugate of a ket is called a bra. . See "bra–ket notation".
Bra–ket notation
The bra–ket notation is a way to represent the states and operators of a system by angle brackets and vertical bars, for example, and .
Density matrix
Physically, the density matrix is a way to represent pure states and mixed states. The density matrix of pure state whose ket is is .
Mathematically, a density matrix has to satisfy the following conditions:
Density operator
Synonymous to "density matrix".
Dirac notation
Synonymous to "bra–ket notation".
Hilbert space
Given a system, the possible pure state can be represented as a vector in a Hilbert space. Each ray (vectors differ by phase and magnitude only) in the corresponding Hilbert space represent a state.[nb 1]
A wavefunction expressed in the form is called a ket. See "bra–ket notation".
Mixed state
A mixed state is a statistical ensemble of pure state.
Pure state:
Mixed state:
Normalizable wavefunction
A wavefunction is said to be normalizable if . A normalizable wavefunction can be made to be normalized by .
Normalized wavefunction
A wavefunction is said to be normalized if .
Pure state
A state which can be represented as a wavefunction / ket in Hilbert space / solution of Schrödinger equation is called pure state. See "mixed state".
Quantum numbers
a way of representing a state by several numbers, which corresponds to a complete set of commuting observables.
A common example of quantum numbers is the possible state of an electron in a central potential: , which corresponds to the eigenstate of observables (in terms of ), (magnitude of angular momentum), (angular momentum in -direction), and .
Spin wavefunction

Part of a wavefunction of particle(s). See "total wavefunction of a particle".


Synonymous to "spin wavefunction".

Spatial wavefunction

Part of a wavefunction of particle(s). See "total wavefunction of a particle".

A state is a complete description of the observable properties of a physical system.
Sometimes the word is used as a synonym of "wavefunction" or "pure state".
State vector
synonymous to "wavefunction".
Statistical ensemble
A large number of copies of a system.
A sufficiently isolated part in the universe for investigation.
Tensor product of Hilbert space
When we are considering the total system as a composite system of two subsystems A and B, the wavefunctions of the composite system are in a Hilbert space , if the Hilbert space of the wavefunctions for A and B are and respectively.
Total wavefunction of a particle
For single-particle system, the total wavefunction of a particle can be expressed as a product of spatial wavefunction and the spinor. The total wavefunctions are in the tensor product space of the Hilbert space of the spatial part (which is spanned by the position eigenstates) and the Hilbert space for the spin.
The word "wavefunction" could mean one of following:
  1. A vector in Hilbert space which can represent a state; synonymous to "ket" or "state vector".
  2. The state vector in a specific basis. It can be seen as a covariant vector in this case.
  3. The state vector in position representation, e.g. , where is the position eigenstate.


Main article: Schrödinger equation
See "degenerate energy level".
Degenerate energy level
If the energy of different state (wavefunctions which are not scalar multiple of each other) is the same, the energy level is called degenerate.
There is no degeneracy in 1D system.
Energy spectrum
The energy spectrum refers to the possible energy of a system.
For bound system (bound states), the energy spectrum is discrete; for unbound system (scattering states), the energy spectrum is continuous.
related mathematical topics: Sturm–Liouville equation
The operator represents the total energy of the system.
Schrödinger equation
-- (1)
(1) is sometimes called "Time-Dependent Schrödinger equation" (TDSE).
Time-Independent Schrödinger Equation (TISE)
A modification of the Time-Dependent Schrödinger equation as an eigenvalue problem. The solutions are energy eigenstate of the system.
-- (2)

Dynamics related to single particle in a potential / other spatial properties

In this situation, the SE is given by the form
It can be derived from (1) by considering and
Bound state
A state is called bound state if its position probability density at infinite tends to zero for all the time. Roughly speaking, we can expect to find the particle(s) in a finite size region with certain probability. More precisely, when , for all .
There is a criterion in terms of energy:
Let be the expectation energy of the state. It is a bound state iff .
Position representation and momentum representation
Position representation of a wavefunction: ,
momentum representation of a wavefunction: ;
where is the position eigenstate and the momentum eigenstate respectively.
The two representations are linked by Fourier transform.
Probability amplitude
Synonymous to "probability density".
Probability current
Having the metaphor of probability density as mass density, then probability current is the current:
The probability current and probability density together satisfy the continuity equation:
Probability density
Given the wavefunction of a particle, is the probability density at position and time . means the probability of finding the particle near .
Scattering state
The wavefunction of scattering state can be understood as a propagating wave. See also "bound state".
There is a criterion in terms of energy:
Let be the expectation energy of the state. It is a scattering state iff .
Square-integrable is a necessary condition for a function being the position/momentum representation of a wavefunction of a bound state of the system.
Given the position representation of a state vector of a wavefunction, square-integrable means:
1D case: .
3D case: .
Stationary state
A stationary state of a bound system is an eigenstate of Hamiltonian operator. Classically, it corresponds to standing wave. It is equivalent to the following things:[nb 2]
  • an eigenstate of the Hamiltonian operator
  • an eigenfunction of Time-Independent Schrödinger Equation
  • a state of definite energy
  • a state which "every expectation value is constant in time"
  • a state whose probability density () does not change with respect to time, i.e.

Measurement postulates

Born's rule
The probability of the state collapse to an eigenstate of an observable is given by .
"Collapse" means the sudden process which the state of the system will "suddenly" change to an eigenstate of the observable during measurement.
An eigenstate of an operator is a vector satisfied the eigenvalue equation: , where is a scalar.
Usually, in bra–ket notation, the eigenstate will be represented by its corresponding eigenvalue if the corresponding observable is understood.
Expectation value
The expectation value of the observable M with respect to a state is the average outcome of measuring with respect to an ensemble of state .
can be calculated by:
If the state is given by a density matrix , .
Hermitian operator
An operator satisfying .
Equivalently, for all allowable wavefunction .
Mathematically, it is represented by a Hermitian operator.
Quantum Zeno effect
The phenomenon that a frequent measurement leads to "freezing" of the state.

Indistinguishable particles

Intrinsically identical particles
If the intrinsic properties (properties that can be measured but are independent of the quantum state, e.g. charge, total spin, mass) of two particles are the same, they are said to be (intrinsically) identical.
Indistinguishable particles
If a system shows measurable differences when one of its particles is replaced by another particle, these two particles are called distinguishable.
Bosons are particles with integer spin (s = 0, 1, 2, ... ). They can either be elementary (like photons) or composite (such as mesons, nuclei or even atoms). There are five known elementary bosons: the four force carrying gauge bosons γ (photon), g (gluon), Z (Z boson) and W (W boson), as well as the Higgs boson.
Fermions are particles with half-integer spin (s = 1/2, 3/2, 5/2, ... ). Like bosons, they can be elementary or composite particles. There are two types of elementary fermions: quarks and leptons, which are the main constituents of ordinary matter.
Anti-symmetrization of wavefunctions
Symmetrization of wavefunctions
Pauli exclusion principle

Quantum statistical mechanics

Bose–Einstein distribution
Bose–Einstein condensation
Bose–Einstein condensation state (BEC state)
Fermi energy
Fermi–Dirac distribution
Slater determinant


Bell's inequality
Entangled state
separable state
no cloning theorem

Rotation: spin/angular momentum

angular momentum
Clebsch–Gordan coefficients
singlet state and triplet state

Approximation methods

adiabatic approximation
Born–Oppenheimer approximation
WKB approximation
time-dependent perturbation theory
time-independent perturbation theory

Historical Terms / semi-classical treatment

Ehrenfest theorem
A theorem connecting the classical mechanics and result dervied from Schrödinger equation.
first quantization
wave–particle duality

Uncategorized terms

uncertainty principle
Canonical commutation relations
Path integral

See also


  1. Exception: superselection rules
  2. Some textbooks (e.g. Cohen Tannoudji, Liboff) define "stationary state" as "an eigenstate of a Hamiltonian" without specific to bound states.


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