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Measurement in quantum mechanics

The framework of Quantum Mechanics admits a careful definition of measurement, and a thorough discussion of its practical and philosophical implications.


The mathematical formalism of measurement

The goal of a particular measurement of a particular system, in any experimental trial, is to obtain a characterization of this system in mutual agreement between all members of this system, and therefore by a particular method which is reproducible by all members of this system, at least in principle.

Measurable Quantities ("Observables") as Operators

Observable quantities are represented mathematically by a Hermitian operator, with its eigenvalues representing any definite result value which might be obtained as result of the measurement; and the state of the system during the trial as its corresponding eigenstate. This representation is possible and appropriate because

  1. Its eigenvalues are real, and the possible result values of a measurement are correspondingly real numbers.
  2. It can be unitarily diagonalized (See Spectral theorem). In other words, it has a basis of eigenvectors which spans the entire space of system states corresponding to any possible outcome of the measurement with a definite result value. Distinct system states in distinct trials, resulting in distinct definite result values, are thereby guaranteed to be represented by distinct eigenvectors, and the state of a system can be represented as a linear combination of eigenvectors of any suitable operator.
  3. Its trace is real, corresponding to the (appropriately weighted) real average of definite result values which may be obtained from an ensemble of trials.

Important examples are:

Many operators are pairwise noncommuting; that is, for a given set of observational data, from a particular trial, one may obtain a definite real result value for one quantity, but not for the other, or even for neither. Even if the state of the system in one particular trial corresponds to one particular eigenstate of one operator, it is then to be represented as a nontrivial linear combination of eigenstates of the other operator.

Eigenstates and projection

In Quantum mechanics, when you take a measurement of a system with state vector (wave function) |\psi\rang where the corresponding measurement operator {\hat O} has eigenstates |n\rang for n = 1,2,3,..., and if you found one definite result value ON and the state which the system had in this trial is consequently represented as |N\rang, the system may be said having been forced or "collapsed" into the state |N\rang.

The continious spectrum case is more problametic, since the basis has aleph eigenvectors, which might be represented by a Delta function, which is in fact not a function, and moreover, doesn't belong to Hilbert space of square square-integrable functions. The concept of dot particle is still causing problems in today's Physics, such as singularities and infinite values. In practical manners, each measurement has its resolution, and therefore the continous space may be divided into discrete segments. Other solutions is to approximate any lab experiments by a "box" potential (which bounds the volume in which the particle can be found, and thus ensure countable spectrum.

Wavefunction collpase

Given any quantum state which is a superposition of eigenstates
| \psi \rang = c_1 e^{-i E_1 t} | 1 \rang + c_2 e^{-i E_2 t} | 2 \rang + c_3 e^{-i E_3 t} | 3 \rang + \cdots \ \ ,
if we measure, for example, the energy of the system and receive En
(this result is chosen randomly according to probability given by
Prob \left( E_n \right) = \frac{ | A_n |^2 }{\sum_k{ | A_k |^2}} ),
than the system's quantum state instantly becomes
| \psi \rang = e^{-i E_2 t} | 2 \rang
so any further measurement of energy will always yield E2.

Figure 1. The process of wavefunction collapse illustrated mathematicly.

The process in which a quantum state is instantly becomes one (chosen randomly) of the eigenstates of the operator corresponding to the measured observable is called "collapse", or "wavefunction collapse". The collapse process has no trace or corresponding mathematical description in the mathematical formulation of quantum mechanics. Moreover, the Schrodinger equation, that determines the evolution of system in time, does not predicts such a process. Yet, the process of collapse was demonstrated in many experiments (such as the double slit experiment). The wavefunction collpase raises serious questions of determinism and locality, as demonstrated in the EPR paradox and later in GHZ entanglement.

There are two major approaches toward the "wavefunction collpase":

  1. Accept it as it is. This approach was supported by Niels Bohr and his Copenhagen interpretation which accept the collapse as one of the elemetary properties of nature (at least, for small enough systems). According to this, there is an inherent randomness embedded in nature, and physical obserables are exist only after they were measured (for example: as long as a particle's speed wasn't measured it doesn't have any defined speed).
  2. Reject it as a physical process and relate to it only as an illusion. This approach says that there is no collapse at all, and we only think there is. Those who support this approach usually offer another interpretation of quantum mechanics, which avoids the wavefunction collapse.


(should be reviewed and cleaned up)

Suppose we knew that a particle had been confined throughout in a box potential (see, for example, the particle in a box problem) and we had found its energy value to be EN; with the corresponding system state |\psi_N\rang = |N\rang = \sqrt{ \frac{2}{L} }~{\rm sin}\left(\frac{N \pi x}{L}\right) as solution of the Schrödinger equation, under assumption of a box potential. Suppose further, that in one particular trial over the course of obtaining the energy measurement, we had met the particle at a particular distance value S from one potential wall of the box; corresponding to system state |\psi_S\rang = |S\rang = \delta( S - x ).

The state functions |\psi_N\rang and |\psi_S\rang are distinct functions (of distance x), but they are in general not orthogonal to each other:

\lang \psi_S | \psi_N\rang = \lang S | N\rang = \int_0^L dx~\delta( S - x)~\sqrt{ \frac{2}{L} }~{\rm sin}\left(\frac{N \pi x}{L}\right) = \sqrt{ \frac{2}{L} }~{\rm sin}\left(\frac{N \pi S}{L}\right).

The two trials from which observations were collected in order to obtain these measured values S and EN were therefore distinct trials; a meeting between the particle and "us" (or someone who will be able to assert the distance value S) is instantaneous, while a definite value of energy EN is established only in the limit of a long-lasting trial.

Completeness of eigenvectors of Hermitian operators guarantees that either system state, being the eigenvector to one measurement operator, can be expressed as a linear combination of eigenvectors of the other measurement operator:

|S\rang = \sum_n | n \rang \left\langle n | S \right\rangle = \frac{2}{L}~\sum_n {\rm sin}\left(\frac{n \pi x}{L}\right)~{\rm sin}\left(\frac{n \pi S}{L}\right) = \delta( S - x ), and

|N\rang = \int ds~|s\rang \left\langle s | N \right\rangle = \int ds~\delta( s - x )~\sqrt{ \frac{2}{L} }~{\rm sin}\left(\frac{N \pi s}{L}\right) = \sqrt{ \frac{2}{L} }~{\rm sin}\left(\frac{N \pi x}{L}\right).

The evolution of states is described by the Schrödinger equation, and in the given example with energy eigenvalues En it follows that

|\psi( t )\rang = \sum_n |n\rang \lang n|\psi_S\rang ~e^{-i t E_n/\hbar},

where t represents the duration since the meeting had been observed, based on which the distance value S was measured. Consequently

\lang n|\psi( t )\rang = \lang n|\psi_S\rang ~e^{-i t E_n/\hbar} = \sqrt{ \frac{2}{L} }~{\rm sin}\left(\frac{n \pi S}{L}\right)~e^{-i t E_n/\hbar} ~{\not =}~ 0

at least for several distinct energy eigenstates |n\rang, for all values t, and for all 0 < S < L.

The particle state |\psi_S \rang therefore can not have evolved (in the above technical sense) into state |\psi_N \rang (which is orthogonal to all energy eigenstates, except itself), for any duration t. While this conclusion may be characterized accordingly instead as "the wave function of the particle having been projected, or having collapsed into" the energy eigenstate |\psi_N\rang, it is perhaps worth emphasizing that any definite value of energy EN can be established only in the limit of a long-lasting trial, i. e. not for any one particular value of t.

Philosophical problems of Quantum measurements

What physical interaction is indeed a measurement?

A major conceptual prolem of quantum mechanics and a specially the Copenhagen interpretation is that they don't give a distinctive criteria for reach physical interaction can qualify as measurement and cause a wavefunction to collapse. This is best illustrated at Schrödinger's cat paradox.

There are still major philosophical and metaphysical question regarding this issues:

  • Does a measurement depends on the existant of a self-aware observer?
  • What interactions are strong enough to consist a measurement?
  • The concept of weak measurements .
  • Macroscopic systems (such as chairs or cats) do not exhibit the odd quantum properties, which can only be observed in microscopic particles such as electrons or photons. The question, when the system is "big enough" to behave classicly and not quantic?

Does measurement actually determine the state?

The question of whether a measurement actually determines the state, is deeply related to the Wavefunction collapse.

According to the Copenhagen interpretation, the answer is an unqualified "yes".

See also:

The Quantum entanglement problem

See EPR paradox.

See also

Last updated: 10-24-2004 05:10:45