Quantum state
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Quantum states may be defined differently for different kinds of systems or problems. Two broad categories are
- wave functionsdescribing quantum systems using position or momentum variables and
- the more abstract vector quantum states.
Historical, educational, and application-focused problems typically feature wave functions; modern professional physics uses the abstract vector states. In both categories, quantum states divide into pure versus mixed states, or into coherent states and incoherent states. Categories with special properties include stationary states for time independence and quantum vacuum states in quantum field theory.
From the states of classical mechanics
As a tool for physics, quantum states grew out of states in
Similarly, quantum states consist of sets of dynamical variables that evolve under equations of motion. However, the values derived from quantum states are
Role in quantum mechanics
The process of describing a quantum system with quantum mechanics begins with identifying a set of variables defining the quantum state of the system.[1]: 204 The set will contain compatible and incompatible variables. Simultaneous measurement of a complete set of compatible variables prepares the system in a unique state. The state then evolves deterministically according to the equations of motion. Subsequent measurement of the state produces a sample from a probability distribution predicted by the quantum mechanical operator corresponding to the measurement.
The fundamentally statistical or probabilisitic nature of quantum measurements changes the role of quantum states in quantum mechanics compared to classical states in classical mechanics. In classical mechanics the initial state of one or more bodies is measured; the state evolves according to the equations of motion; measurements of the final state are compared to predictions. In quantum mechanics, ensembles of identically prepared quantum states evolve according to the equations of motion and many repeated measurements are compared to predicted probability distributions.[1]: 204
Measurements
Measurements, macroscopic operations on quantum states, filter the state.[1]: 196 Whatever the input quantum state might be, repeated identical measurements give consistent values. For this reason, measurements 'prepare' quantum states for experiments, placing the system in a partially defined state. Subsequent measurements may either further prepare the system – these are compatible measurements – or it may alter the state, redefining it – these are called incompatible or complementary measurements. For example, we may measure the momentum of a state along the axis any number of times and get the same result, but if we measure the position after once measuring the momentum, subsequent measurements of momentum are changed. The quantum state appears unavoidably altered by incompatible measurements. This is known as the uncertainty principle.
Eigenstates and pure states
The quantum state after a measurement is in an eigenstate corresponding to that measurement and the value measured.[1]: 202 Other aspects of the state may be unknown. Repeating the measurement will not alter the state. In some cases, compatible measurements can further refine the state, causing it to be an eigenstate corresponding to all these measurements.[2] A full set of compatible measurements produces a pure state. Any state that is not pure is called a mixed state[1]: 204 (See mixed states below).
The eigenstate solutions to the Schrödinger equation can be formed into pure states. Experiments rarely produce pure states. Therefore statistical mixtures of solutions must be compared to experiments.[1]: 204
Representations
The same physical quantum state can be expressed mathematically in different ways called representations.[1] The position wave function is one representation often seen first in introductions to quantum mechanics. The equivalent momentum wave function is another wave function based representation. Representations are analogous to coordinate systems[1]: 244 or similar mathematical devices like parametric equations. Selecting a representation will make some aspects of a problem easier at the cost of making other things difficult.
In formal quantum mechanics (see below) the theory develops in terms of abstract 'vector space', avoiding any particular representation. This allows many elegant concepts of quantum mechanics to be expressed and to be applied even in cases where no classical analog exists.[1]: 244
Wave function representations
Wave functions represent quantum states, particularly when they are functions of position or of momentum. Historically definitions of quantum states used wavefunctions before the more formal methods were developed.[3]: 268 The wave function is a complex-valued function of any complete set of commuting or compatible degrees of freedom. For example, one set could be the spatial coordinates of an electron. Preparing a system by measuring the complete set of compatible produces a pure quantum state. More common, incomplete preparation produces a mixed quantum state. Wave function solutions of Schrödinger's equations of motion for operators corresponding to measurements can readily be expressed as pure states; they must be combined with statistical weights matching experimental preparation to compute the expected probability distribution.[1]: 205
Pure states of wave functions
Numerical or analytic solutions in quantum mechanics can be expressed as pure states. These solution states, called
The
On the other hand, a system in a superposition of multiple different eigenstates does in general have quantum uncertainty for the given observable. Using bra–ket notation, this linear combination of eigenstates can be represented as:
Mixed states of wave functions
A mixed quantum state corresponds to a probabilistic mixture of pure states; however, different distributions of pure states can generate equivalent (i.e., physically indistinguishable) mixed states. A mixture of quantum states is again a quantum state.
A mixed state for electron spins, in the density-matrix formulation, has the structure of a matrix that is Hermitian and positive semi-definite, and has trace 1.[4] A more complicated case is given (in bra–ket notation) by the singlet state, which exemplifies quantum entanglement:
A pure quantum state can be represented by a
The Schrödinger–HJW theorem classifies the multitude of ways to write a given mixed state as a convex combination of pure states.[7] Before a particularStatistical mixtures of states are a different type of linear combination. A statistical mixture of states is a statistical ensemble of independent systems. Statistical mixtures represent the degree of knowledge whilst the uncertainty within quantum mechanics is fundamental. Mathematically, a statistical mixture is not a combination using complex coefficients, but rather a combination using real-valued, positive probabilities of different states . A number represents the probability of a randomly selected system being in the state . Unlike the linear combination case each system is in a definite eigenstate.[8][9]
The expectation value of an observable A is a statistical mean of measured values of the observable. It is this mean, and the distribution of probabilities, that is predicted by physical theories.
There is no state that is simultaneously an eigenstate for all observables. For example, we cannot prepare a state such that both the position measurement Q(t) and the momentum measurement P(t) (at the same time t) are known exactly; at least one of them will have a range of possible values.[a] This is the content of the Heisenberg uncertainty relation.
Moreover, in contrast to classical mechanics, it is unavoidable that performing a measurement on the system generally changes its state.[10][11][b] More precisely: After measuring an observable A, the system will be in an eigenstate of A; thus the state has changed, unless the system was already in that eigenstate. This expresses a kind of logical consistency: If we measure A twice in the same run of the experiment, the measurements being directly consecutive in time,[c] then they will produce the same results. This has some strange consequences, however, as follows.
Consider two
Suppose that the system is in an eigenstate of B at the experiment's beginning. If we measure only B, all runs of the experiment will yield the same result. If we measure first A and then B in the same run of the experiment, the system will transfer to an eigenstate of A after the first measurement, and we will generally notice that the results of B are statistical. Thus: Quantum mechanical measurements influence one another, and the order in which they are performed is important.Another feature of quantum states becomes relevant if we consider a physical system that consists of multiple subsystems; for example, an experiment with two particles rather than one. Quantum physics allows for certain states, called entangled states, that show certain statistical correlations between measurements on the two particles which cannot be explained by classical theory. For details, see entanglement. These entangled states lead to experimentally testable properties (Bell's theorem) that allow us to distinguish between quantum theory and alternative classical (non-quantum) models.
Schrödinger picture vs. Heisenberg picture
One can take the observables to be dependent on time, while the state σ was fixed once at the beginning of the experiment. This approach is called the Heisenberg picture. (This approach was taken in the later part of the discussion above, with time-varying observables P(t), Q(t).) One can, equivalently, treat the observables as fixed, while the state of the system depends on time; that is known as the Schrödinger picture. (This approach was taken in the earlier part of the discussion above, with a time-varying state .) Conceptually (and mathematically), the two approaches are equivalent; choosing one of them is a matter of convention.
Both viewpoints are used in quantum theory. While non-relativistic
Formalism in quantum physics
Pure states as rays in a complex Hilbert space
Quantum physics is most commonly formulated in terms of linear algebra, as follows. Any given system is identified with some finite- or infinite-dimensional Hilbert space. The pure states correspond to vectors of norm 1. Thus the set of all pure states corresponds to the unit sphere in the Hilbert space, because the unit sphere is defined as the set of all vectors with norm 1.
Multiplying a pure state by a scalar is physically inconsequential (as long as the state is considered by itself). If a vector in a complex Hilbert space can be obtained from another vector by multiplying by some non-zero complex number, the two vectors in are said to correspond to the same
Bra–ket notation
Calculations in quantum mechanics make frequent use of
- The expression used to denote a state vector (which corresponds to a pure quantum state) takes the form (where the "" can be replaced by any other symbols, letters, numbers, or even words). This can be contrasted with the usual mathematical notation, where vectors are usually lower-case Latin letters, and it is clear from the context that they are indeed vectors.
- Dirac defined two kinds of vector, bra and ket, dual to each other.[e]
- Each ket is uniquely associated with a so-called bra, denoted , which corresponds to the same physical quantum state. Technically, the bra is the adjoint of the ket. It is an element of the dual space, and related to the ket by the Riesz representation theorem. In a finite-dimensional space with a chosen orthonormal basis, writing as a column vector, is a row vector; to obtain it just take the transpose and entry-wise complex conjugate of .
- Scalar products[f][g] (also called brackets) are written so as to look like a bra and ket next to each other: . (The phrase "bra-ket" is supposed to resemble "bracket".)
Spin
The
As a consequence, the quantum state of a particle with spin is described by a
Many-body states and particle statistics
The quantum state of a system of N particles, each potentially with spin, is described by a complex-valued function with four variables per particle, corresponding to 3
Here, the spin variables mν assume values from the set
The treatment of
(particles with half-integer spin). The above N-particle function must either be symmetrized (in the bosonic case) or anti-symmetrized (in the fermionic case) with respect to the particle numbers. If not all N particles are identical, but some of them are, then the function must be (anti)symmetrized separately over the variables corresponding to each group of identical variables, according to its statistics (bosonic or fermionic).Electrons are fermions with S = 1/2, photons (quanta of light) are bosons with S = 1 (although in the vacuum they are massless and can't be described with Schrödinger mechanics).
When symmetrization or anti-symmetrization is unnecessary, N-particle spaces of states can be obtained simply by tensor products of one-particle spaces, to which we will return later.
Basis states of one-particle systems
As with any Hilbert space, if a basis is chosen for the Hilbert space of a system, then any ket can be expanded as a linear combination of those basis elements. Symbolically, given basis kets , any ket can be written
One property worth noting is that the normalized states are characterized by
Expansions of this sort play an important role in measurement in quantum mechanics. In particular, if the are
A particularly important example is the
Pure states vs. bound states
Though closely related, pure states are not the same as bound states belonging to the pure point spectrum of an observable with no quantum uncertainty. A particle is said to be in a bound state if it remains localized in a bounded region of space for all times. A pure state is called a bound state if and only if for every there is a
Superposition of pure states
As mentioned above, quantum states may be superposed. If and are two kets corresponding to quantum states, the ket
One example of superposition is the
Another example of the importance of relative phase in quantum superposition is
Mixed states
A pure quantum state is a state which can be described by a single ket vector, as described above. A mixed quantum state is a
Mixed states arise in quantum mechanics in two different situations: first, when the preparation of the system is not fully known, and thus one must deal with a
Mixed states inevitably arise from pure states when, for a composite quantum system with an entangled state on it, the part is inaccessible to the observer. The state of the part is expressed then as the partial trace over .
A mixed state cannot be described with a single ket vector. Instead, it is described by its associated density matrix (or density operator), usually denoted ρ. Note that density matrices can describe both mixed and pure states, treating them on the same footing. Moreover, a mixed quantum state on a given quantum system described by a Hilbert space can be always represented as the partial trace of a pure quantum state (called a
The density matrix describing a mixed state is defined to be an operator of the form
A simple criterion for checking whether a density matrix is describing a pure or mixed state is that the trace of ρ2 is equal to 1 if the state is pure, and less than 1 if the state is mixed.[k][16] Another, equivalent, criterion is that the von Neumann entropy is 0 for a pure state, and strictly positive for a mixed state.
The rules for measurement in quantum mechanics are particularly simple to state in terms of density matrices. For example, the ensemble average (expectation value) of a measurement corresponding to an observable A is given by
According to Eugene Wigner,[17] the concept of mixture was put forward by Lev Landau.[18][14]: 38–41
Mathematical generalizations
States can be formulated in terms of observables, rather than as vectors in a vector space. These are positive normalized linear functionals on a C*-algebra, or sometimes other classes of algebras of observables. See State on a C*-algebra and Gelfand–Naimark–Segal construction for more details.
See also
Notes
- ^ To avoid misunderstandings: Here we mean that Q(t) and P(t) are measured in the same state, but not in the same run of the experiment.
- ^ Dirac (1958),[12] p. 4: "If a system is small, we cannot observe it without producing a serious disturbance."
- ^ i.e. separated by a zero delay. One can think of it as stopping the time, then making the two measurements one after the other, then resuming the time. Thus, the measurements occurred at the same time, but it is still possible to tell which was first.
- ^ For concreteness' sake, suppose that A = Q(t1) and B = P(t2) in the above example, with t2 > t1 > 0.
- ^ Dirac (1958),[12] p. 20: "The bra vectors, as they have been here introduced, are quite a different kind of vector from the kets, and so far there is no connexion between them except for the existence of a scalar product of a bra and a ket."
- ^ Dirac (1958),[12] p. 19: "A scalar product ⟨B|A⟩ now appears as a complete bracket expression."
- ^ Gottfried (2013),[13] p. 31: "to define the scalar products as being between bras and kets."
- ^ Note that a state is a superposition of different basis states , so and are elements of the same Hilbert space. A particle in state is located precisely at position , while a particle in state can be found at different positions with corresponding probabilities.
- ^ Landau (1965),[14] p. 17: "∫ Ψf′ Ψf* dq = δ(f′ − f)" (the left side corresponds to ⟨f|f′⟩), "∫ δ(f′ − f) df′ = 1".
- ^ In the continuous case, the basis kets are not unit kets (unlike the state ): They are normalized according to [i] i.e., (a Dirac delta function), which means that
- ^ Note that this criterion works when the density matrix is normalized so that the trace of ρ is 1, as it is for the standard definition given in this section. Occasionally a density matrix will be normalized differently, in which case the criterion is
References
- ^ ISBN 0486409244.
- ^ Cohen-Tannoudji, Claude; Diu, Bernard; Laloë, Franck (1977). Quantum Mechanics. Wiley. pp. 231–235.
- ISBN 0-486-26126-3.
- ISBN 978-0-262-01506-6.
- OCLC 318268606.
- ISBN 0-7923-2549-4.
- S2CID 15995449.
- ^ "Statistical Mixture of States". Archived from the original on September 23, 2019. Retrieved November 9, 2021.
- ^ "The Density Matrix". Archived from the original on January 15, 2012. Retrieved January 24, 2012.
- ^ Heisenberg, W. (1927). Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik, Z. Phys. 43: 172–198. Translation as 'The actual content of quantum theoretical kinematics and mechanics'. Also translated as 'The physical content of quantum kinematics and mechanics' at pp. 62–84 by editors John Wheeler and Wojciech Zurek, in Quantum Theory and Measurement (1983), Princeton University Press, Princeton NJ.
- ^ Bohr, N. (1927/1928). The quantum postulate and the recent development of atomic theory, Nature Supplement April 14 1928, 121: 580–590.
- ^ Dirac, P.A.M.(1958). The Principles of Quantum Mechanics, 4th edition, Oxford University Press, Oxford UK.
- ^ ISBN 9780387955766.
- ^ a b Lev Landau; Evgeny Lifshitz (1965). Quantum Mechanics — Non-Relativistic Theory (PDF). Course of Theoretical Physics. Vol. 3 (2nd ed.). London: Pergamon Press.
- ISBN 978-3-319-14044-5.
- ^ Blum, Density matrix theory and applications, page 39.
- ^ Eugene Wigner (1962). "Remarks on the mind-body question" (PDF). In I.J. Good (ed.). The Scientist Speculates. London: Heinemann. pp. 284–302.[permanent dead link] Footnote 13 on p.180
- S2CID 125732617. English translation reprinted in: D. Ter Haar, ed. (1965). Collected papers of L.D. Landau. Oxford: Pergamon Press. p.8–18
Further reading
The concept of quantum states, in particular the content of the section Formalism in quantum physics above, is covered in most standard textbooks on quantum mechanics.
For a discussion of conceptual aspects and a comparison with classical states, see:
- Isham, Chris J (1995). Lectures on Quantum Theory: Mathematical and Structural Foundations. ISBN 978-1-86094-001-9.
For a more detailed coverage of mathematical aspects, see:
- ISBN 978-3-540-17093-8. 2nd edition. In particular, see Sec. 2.3.
For a discussion of purifications of mixed quantum states, see Chapter 2 of John Preskill's lecture notes for Physics 219 at Caltech.
For a discussion of geometric aspects see:
- Bengtsson I; Życzkowski K (2006). Geometry of Quantum States. Cambridge: Cambridge University Press., second, revised edition (2017)