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The **Born rule** (also called **Born's rule**) is a postulate of quantum mechanics which gives the probability that a measurement of a quantum system will yield a given result.^{[1]} In its simplest form, it states that the probability density of finding a system in a given state, when measured, is proportional to the square of the amplitude of the system's wavefunction at that state. It was formulated and published by German physicist Max Born in July, 1926.

The Born rule states that if an observable corresponding to a self-adjoint operator with discrete spectrum is measured in a system with normalized wave function (see Bra–ket notation), then:

- the measured result will be one of the eigenvalues of , and
- the probability of measuring a given eigenvalue will equal , where is the projection onto the eigenspace of corresponding to .

- (In the case where the eigenspace of corresponding to is one-dimensional and spanned by the normalized eigenvector , is equal to , so the probability is equal to . Since the complex number is known as the
*probability amplitude*that the state vector assigns to the eigenvector , it is common to describe the Born rule as saying that probability is equal to the amplitude-squared (really the amplitude times its own complex conjugate). Equivalently, the probability can be written as .)

In the case where the spectrum of is not wholly discrete, the spectral theorem proves the existence of a certain projection-valued measure , the spectral measure of . In this case:

- the probability that the result of the measurement lies in a measurable set is given by .

A wave function for a single structureless particle in space position implies that the probability density function for a measurement of the particles's position at time is:

In some applications, this treatment of the Born rule is generalized using positive-operator-valued measures. A POVM is a measure whose values are positive semi-definite operators on a Hilbert space. POVMs are a generalisation of von Neumann measurements and, correspondingly, quantum measurements described by POVMs are a generalisation of quantum measurement described by self-adjoint observables. In rough analogy, a POVM is to a PVM what a mixed state is to a pure state. Mixed states are needed to specify the state of a subsystem of a larger system (see purification of quantum state); analogously, POVMs are necessary to describe the effect on a subsystem of a projective measurement performed on a larger system. POVMs are the most general kind of measurement in quantum mechanics and can also be used in quantum field theory.^{[2]} They are extensively used in the field of quantum information.

In the simplest case, of a POVM with a finite number of elements acting on a finite-dimensional Hilbert space, a POVM is a set of positive semi-definite matrices on a Hilbert space that sum to the identity matrix,^{[3]}^{: 90 }:

The POVM element is associated with the measurement outcome , such that the probability of obtaining it when making a measurement on the quantum state is given by:

where is the trace operator. This is the POVM version of the Born rule. When the quantum state being measured is a pure state this formula reduces to:

The Born rule, together with the unitarity of the time evolution operator (or, equivalently, the Hamiltonian being Hermitian), implies the unitarity of the theory, which is considered required for consistency. For example, unitarity ensures that the probabilities of all possible outcomes sum to 1 (though it is not the only option to get this particular requirement^{[clarification needed]}).

The Born rule was formulated by Born in a 1926 paper.^{[4]} In this paper, Born solves the Schrödinger equation for a scattering problem and, inspired by Albert Einstein and Einstein’s probabilistic rule for the photoelectric effect,^{[5]} concludes, in a footnote, that the Born rule gives the only possible interpretation of the solution. (The main body of the article says that the amplitude "gives the probability" [*bestimmt die Wahrscheinlichkeit*], while the footnote added in proof says that the probability is proportional to the square of its magnitude.) In 1954, together with Walther Bothe, Born was awarded the Nobel Prize in Physics for this and other work.^{[5]} John von Neumann discussed the application of spectral theory to Born's rule in his 1932 book.^{[6]}

Gleason's theorem shows that the Born rule can be derived from the usual mathematical representation of measurements in quantum physics together with the assumption of non-contextuality. Andrew M. Gleason first proved the theorem in 1957,^{[7]} prompted by a question posed by George W. Mackey.^{[8]}^{[9]} This theorem was historically significant for the role it played in showing that wide classes of hidden-variable theories are inconsistent with quantum physics.^{[10]}

Several other researchers have also tried to derive the Born rule from more basic principles. A number of derivations have been proposed in the context of the many-worlds interpretation. These include the decision-theory approach pioneered by David Deutsch^{[11]} and later developed by Hilary Greaves^{[12]} and David Wallace;^{[13]} and an "envariance" approach by Wojciech H. Zurek.^{[14]} These proofs have, however, been criticized as circular.^{[15]} In 2018, an approach based on self-locating uncertainty was suggested by Charles Sebens and Sean M. Carroll;^{[16]} this has also been criticized.^{[17]} Simon Saunders, in 2021, produced a branch counting derivation of the Born rule. The crucial feature of this approach is to define the branches so that they all have the same magnitude or 2-norm. The ratios of the numbers of branches thus defined give the probabilities of the various outcomes of a measurement, in accordance with the Born rule.^{[18]}

In 2019, Lluís Masanes, Thomas Galley, and Markus Müller proposed a derivation based on postulates including the possibility of state estimation.^{[19]}^{[20]}

It has also been claimed that pilot-wave theory can be used to statistically derive the Born rule, though this remains controversial.^{[21]}

Within the QBist interpretation of quantum theory, the Born rule is seen as an extension of the normative principle of coherence, which ensures self-consistency of probability assessments across a whole set of such assessments. It can be shown that an agent who thinks they are gambling on the outcomes of measurements on a sufficiently quantum-like system but refuses to use the Born rule when placing their bets is vulnerable to a Dutch book.^{[22]}