Wave function collapse
Glossary · History
In quantum mechanics, wave function collapse (also called collapse of the state vector or reduction of the wave packet) is the phenomenon in which a wave function—initially in a superposition of several different possible eigenstates—appears to reduce to a single one of those states after interaction with an observer. It is the essence of measurement in quantum mechanics, and connects the wave function with classical observables like position and momentum. In classical terms, it is the reduction of all possible physical states to a single possibility which is measured by the observer. Collapse is one of two processes by which quantum systems evolve in time; the other is continuous evolution via the Schrödinger equation. However in this role, collapse is merely a black box for thermodynamically irreversible interaction with a classical environment. Calculations of quantum decoherence predict apparent wave function collapse when a superposition forms between the quantum system's states and the environment's states. Significantly, the combined wave function of the system and environment continue to obey the Schrödinger equation.
When the Copenhagen interpretation was first expressed, Bohr postulated wave function collapse to cut the quantum world from the classical. This tactical move allowed quantum theory to develop without distractions from interpretational worries. Nevertheless it was debated, for if collapse were a fundamental physical phenomenon, rather than just an epiphenomenon of some other process, it would mean nature were fundamentally stochastic, i.e. nondeterministic, an undesirable property for a theory. This issue remained until quantum decoherence entered mainstream opinion after its reformulation in the 1980s. Decoherence explains the perception of wave function collapse in terms of interacting large- and small-scale quantum systems, and is commonly taught at the graduate level (e.g. the Cohen-Tannoudji textbook). The quantum filtering approach and the introduction of quantum causality non-demolition principle allows for a classical-environment derivation of wave function collapse from the stochastic Schrödinger equation.
Mathematical description 
Before collapse, the wave function may be any square-integrable function. This function is expressible as a linear combination of the eigenstates of any observable. Observables represent classical dynamical variables, and when one is measured by a classical observer, the wave function is projected onto a random eigenstate of that observable. The observer simultaneously measures the classical value of that observable to be the eigenvalue of the final state.
Mathematical background 
Where represents the Kronecker delta.
An observable (i.e. measurable parameter of the system) is associated with each eigenbasis, with each quantum alternative having a specific value or eigenvalue, ei, of the observable. A "measurable parameter of the system" could be the usual position r and the momentum p of (say) a particle, but also its energy E, z-components of spin (sz), orbital (Lz) and total angular (Jz) momenta etc. In the basis representation these are respectively .
The coefficients c1, c2, c3... are the probability amplitudes corresponding to each basis . These are complex numbers. The moduli square of ci, that is |ci|2 = ci*ci (* denotes complex conjugate), is the probability of measuring the system to be in the state .
For simplicity in the following, all wave functions are assumed to be normalized; the total probability of measuring all possible states is unity:
The process of collapse 
With these definitions it is easy to describe the process of collapse. For any observable, the wave function is initially some linear combination of the eigenbasis of that observable. When an external agency (an observer, experimenter) measures the observable associated with the eigenbasis , the wave function collapses from the full to just one of the basis eigenstates, , that is:
The probability of collapsing to a given eigenstate is the Born probability, . Post-measurement, other elements of the wave function vector, , have "collapsed" to zero, and .
More generally, collapse is defined for an operator with eigenbasis . If the system is in state , and is measured, the probability of collapsing the system to state (and measuring ) would be . Note that this is not the probability that the particle is in state : that's complete nonsense. It is in state until cast to an eigenstate of .
However, we never observe collapse to a single eigenstate of a continuous-spectrum operator (e.g. position, momentum, or a scattering Hamiltonian), because such eigenfunctions are non-normalizable. In these cases, the wave function will partially collapse to a linear combination of "close" eigenstates (necessarily involving a spread in eigenvalues) that embodies the imprecision of the measurement apparatus. The more precise the measurement, the tighter the range. Calculation of probability proceeds identically, except with an integral over the expansion coefficient . This phenomenon is unrelated to the uncertainty principle, although increasingly precise measurements of one operator (e.g. position) will naturally homogenize the expansion coefficient of wave function with respect to another, incompatible operator (e.g. momentum), lowering the probability of measuring any particular value of the latter.
Quantum decoherence 
Wave function collapse is not fundamental from the perspective of quantum decoherence. There are several equivalent approaches to deriving collapse, like the density matrix approach, but each has the same effect: decoherence irreversibly converts the "averaged" or "environmentally traced over" density matrix from a pure state to a reduced mixture, giving the appearance of wave function collapse.
History and context 
The concept of wavefunction collapse was introduced by Werner Heisenberg in his 1927 paper on the uncertainty principle, "Über den anschaulichen Inhalt der quantentheoretischen Kinematic und Mechanik", and incorporated into the mathematical formulation of quantum mechanics by John von Neumann, in his 1932 treatise Mathematische Grundlagen der Quantenmechanik. Consistent with Heisenberg, von Neumann postulated that there were two processes of wave function change:
- The probabilistic, non-unitary, non-local, discontinuous change brought about by observation and measurement, as outlined above.
- The deterministic, unitary, continuous time evolution of an isolated system that obeys the Schrödinger equation (or a relativistic equivalent, i.e. the Dirac equation).
In general, quantum systems exist in superpositions of those basis states that most closely correspond to classical descriptions, and, in the absence of measurement, evolve according to the Schrödinger equation. However, when a measurement is made, the wave function collapses—from an observer's perspective—to just one of the basis states, and the property being measured uniquely acquires the eigenvalue of that particular state, . After the collapse, the system again evolves according to the Schrödinger equation.
He was able to prove the possibility of a quantum mechanical measurement scheme consistent with wave function collapse. However, he did not prove the necessity of such a collapse. Although von Neumann's projection postulate is often presented as a normative description of quantum measurement, it was conceived by taking into account experimental evidence available during the 1930s (in particular the Compton-Simon experiment has been paradigmatic), and many important present-day measurement procedures do not satisfy it (so-called measurements of the second kind).
The existence of the wave function collapse is required in
- the Copenhagen interpretation
- the objective collapse interpretations
- the transactional interpretation
- the von Neumann interpretation in which consciousness causes collapse.
On the other hand, the collapse is considered a redundant or optional approximation in
- the Consistent histories approach, self-dubbed "Copenhagen done right"
- the Bohm interpretation
- the Many-worlds interpretation
- the Ensemble Interpretation
The cluster of phenomena described by the expression wave function collapse is a fundamental problem in the interpretation of quantum mechanics, and is known as the measurement problem. The problem is deflected by the Copenhagen Interpretation, which postulates that this is a special characteristic of the "measurement" process. Everett's many-worlds interpretation deals with it by discarding the collapse-process, thus reformulating the relation between measurement apparatus and system in such a way that the linear laws of quantum mechanics are universally valid; that is, the only process according to which a quantum system evolves is governed by the Schrödinger equation or some relativistic equivalent.
Originating from Everett's theory, but no longer tied to it, is the physical process of decoherence, which causes an apparent collapse. Decoherence is also important for the consistent histories interpretation. A general description of the evolution of quantum mechanical systems is possible by using density operators and quantum operations. In this formalism (which is closely related to the C*-algebraic formalism) the collapse of the wave function corresponds to a non-unitary quantum operation.
The significance ascribed to the wave function varies from interpretation to interpretation, and varies even within an interpretation (such as the Copenhagen Interpretation). If the wave function merely encodes an observer's knowledge of the universe then the wave function collapse corresponds to the receipt of new information. This is somewhat analogous to the situation in classical physics, except that the classical "wave function" does not necessarily obey a wave equation. If the wave function is physically real, in some sense and to some extent, then the collapse of the wave function is also seen as a real process, to the same extent.
See also 
- Arrow of time
- Interpretation of quantum mechanics
- Quantum decoherence
- Quantum interference
- Schrödinger's cat
- Zeno effect
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