# Density matrix

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In quantum mechanics, a density matrix is a matrix that describes the quantum state of a physical system. It allows for the calculation of the probabilities of the outcomes of any measurement performed upon this system, using the Born rule. It is a generalization of the more usual state vectors or wavefunctions: while those can only represent pure states, density matrices can also represent mixed states. 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 statistical ensemble of possible preparations, and second when one wants to describe a physical system which is entangled with another, as its state can not be described by a pure state.

## Contents

Density matrices are thus crucial tools in areas of quantum mechanics that deal with mixed states, such as quantum statistical mechanics, open quantum systems, quantum decoherence, and quantum information.

## Definition and motivation

The density matrix is a representation of a linear operator called the density operator. The density matrix is obtained from the density operator by choice of basis in the underlying space. In practice, the terms density matrix and density operator are often used interchangeably.

In operator language, a density operator for a system is a positive semi-definite, Hermitian operator of trace one acting on the Hilbert space of the system. [1] [2] [3] This definition can be motivated by considering a situation where a pure state ${\displaystyle |\psi _{j}\rangle }$ is prepared with probability ${\displaystyle p_{j}}$, known as an ensemble. The probability of obtaining projective measurement result ${\displaystyle m}$ when using projectors ${\displaystyle \Pi _{m}}$ is given by [4] :99

${\displaystyle p(m)=\sum _{j}p_{j}\langle \psi _{j}|\Pi _{m}|\psi _{j}\rangle =\operatorname {tr} \left[\Pi _{m}\left(\sum _{j}p_{j}|\psi _{j}\rangle \langle \psi _{j}|\right)\right],}$

which makes the density operator, defined as

${\displaystyle \rho =\sum _{j}p_{j}|\psi _{j}\rangle \langle \psi _{j}|}$,

a convenient representation for the state of this ensemble. It is easy to check that this operator is positive semi-definite, Hermitian, and has trace one. Conversely, it follows from the spectral theorem that every operator with these properties can be written as ${\displaystyle \textstyle \sum _{j}p_{j}|\psi _{j}\rangle \langle \psi _{j}|}$ for some states ${\displaystyle |\psi _{j}\rangle }$ and coefficients ${\displaystyle p_{j}}$ that are non-negative and add up to one. [5] [4] :102 However, this representation will not be unique, as shown by the Schrödinger–HJW theorem.

Another motivation for the definition of density operators comes from considering local measurements on entangled states. Let ${\displaystyle |\Psi \rangle }$ be a pure entangled state in the composite Hilbert space ${\displaystyle {\mathcal {H}}_{1}\otimes {\mathcal {H}}_{2}}$. The probability of obtaining measurement result ${\displaystyle m}$ when measuring projectors ${\displaystyle \Pi _{m}}$ on the Hilbert space ${\displaystyle {\mathcal {H}}_{1}}$ alone is given by [4] :107

${\displaystyle p(m)=\langle \Psi |\Pi _{m}\otimes I|\Psi \rangle =\operatorname {tr} \left[\Pi _{m}\left(\operatorname {tr} _{2}|\Psi \rangle \langle \Psi |\right)\right],}$

where ${\displaystyle \operatorname {tr} _{2}}$ denotes the partial trace over the Hilbert space ${\displaystyle {\mathcal {H}}_{2}}$. This makes the operator

${\displaystyle \rho =\operatorname {tr} _{2}|\Psi \rangle \langle \Psi |}$

a convenient tool to calculate the probabilities of these local measurements. It is known as the reduced density matrix of ${\displaystyle |\Psi \rangle }$ on subsystem 1. It is easy to check that this operator has all the properties of a density operator. Conversely, the Schrödinger–HJW theorem implies that all density operators can be written as ${\displaystyle \operatorname {tr} _{2}|\Psi \rangle \langle \Psi |}$ for some state ${\displaystyle |\Psi \rangle }$.

## Pure and mixed states

A pure quantum state is a state that can not be written as a probabilistic mixture, or convex combination, of other quantum states. [3] There are several equivalent characterizations of pure states in the language of density operators. [6] :73 A density operator represents a pure state if and only if:

• it can be written as an outer product of a state vector ${\displaystyle |\psi \rangle }$ with itself, that is,
${\displaystyle \rho =|\psi \rangle \langle \psi |.}$
${\displaystyle \rho =\rho ^{2}}$.
${\displaystyle \operatorname {tr} (\rho ^{2})=1}$.

It is important to emphasize the difference between a probabilistic mixture of quantum states and their superposition. If a physical system is prepared to be either in state ${\displaystyle |\psi _{1}\rangle }$ or ${\displaystyle |\psi _{2}\rangle }$, with equal probability, it can be described by the mixed state

${\displaystyle \rho ={\frac {1}{2}}{\begin{pmatrix}1&0\\0&1\end{pmatrix}},}$

where ${\displaystyle |\psi _{1}\rangle }$ and ${\displaystyle |\psi _{2}\rangle }$ are assumed orthogonal and of dimension 2, for simplicity. On the other hand, a quantum superposition of these two states with equal probability amplitudes results in the pure state ${\displaystyle |\psi \rangle =(|\psi _{1}\rangle +|\psi _{2}\rangle )/{\sqrt {2}},}$ with density matrix

${\displaystyle |\psi \rangle \langle \psi |={\frac {1}{2}}{\begin{pmatrix}1&1\\1&1\end{pmatrix}}.}$

Unlike the probabilistic mixture, this superposition can display quantum interference. [4] :81

Geometrically, the set of density operators is a convex set, and the pure states are the extremal points of that set. The simplest case is that of a two-dimensional Hilbert space, known as a qubit. An arbitrary state for a qubit can be written as a linear combination of the Pauli matrices, which provide a basis for ${\displaystyle 2\times 2}$ self-adjoint matrices: [7] :126

${\displaystyle \rho ={\frac {1}{2}}\left(I+r_{x}\sigma _{x}+r_{y}\sigma _{y}+r_{z}\sigma _{z}\right),}$

where the real numbers ${\displaystyle (r_{x},r_{y},r_{z})}$ are the coordinates of a point within the unit ball and

${\displaystyle \sigma _{x}={\begin{pmatrix}0&1\\1&0\end{pmatrix}},\quad \sigma _{y}={\begin{pmatrix}0&-i\\i&0\end{pmatrix}},\quad \sigma _{z}={\begin{pmatrix}1&0\\0&-1\end{pmatrix}}.}$

Points with ${\displaystyle r_{x}^{2}+r_{y}^{2}+r_{z}^{2}=1}$ represent pure states, while mixed states are represented by points in the interior. This is known as the Bloch sphere picture of qubit state space.

### Example: light polarization

An example of pure and mixed states is light polarization. An individual photon can be described as having right or left circular polarization, described by the orthogonal quantum states ${\displaystyle |\mathrm {R} \rangle }$ or ${\displaystyle |\mathrm {L} \rangle }$ a superposition of the two: it can be in any state ${\displaystyle \alpha |\mathrm {R} \rangle +\beta |\mathrm {L} \rangle }$ (with ${\displaystyle |\alpha |^{2}+|\beta |^{2}=1}$), corresponding to linear, circular, or elliptical polarization. Consider now a vertically polarized photon, described by the state ${\displaystyle |\mathrm {V} \rangle =(|\mathrm {R} \rangle +|\mathrm {L} \rangle )/{\sqrt {2}}}$. If we pass it through a circular polarizer which allows either only ${\displaystyle |\mathrm {R} \rangle }$ polarized light, or only ${\displaystyle |\mathrm {L} \rangle }$ polarized light, half of the photons are absorbed in both cases. This may make it seem like half of the photons are in state ${\displaystyle |\mathrm {R} \rangle }$ and the other half in state ${\displaystyle |\mathrm {L} \rangle }$, but this is not correct: if we pass ${\displaystyle (|\mathrm {R} \rangle +|\mathrm {L} \rangle )/{\sqrt {2}}}$ though a linear polarizer there's no absorption whatsoever, but if we pass either state ${\displaystyle |\mathrm {R} \rangle }$ or ${\displaystyle |\mathrm {L} \rangle }$ half of the photons are absorbed.

Unpolarized light (such as the light from an incandescent light bulb) cannot be described as any state of the form ${\displaystyle \alpha |\mathrm {R} \rangle +\beta |\mathrm {L} \rangle }$ (linear, circular, or elliptical polarization). Unlike polarized light, it passes through a polarizer with 50% intensity loss whatever the orientation of the polarizer; and it cannot be made polarized by passing it through any wave plate. However, unpolarized light can be described as an statistical ensemble, e. g. as each photon having either ${\displaystyle |\mathrm {R} \rangle }$ polarization or ${\displaystyle |\mathrm {L} \rangle }$ polarization with probability 1/2. The same behavior would occur if each photon had either vertical polarization ${\displaystyle |\mathrm {V} \rangle }$ or horizontal polarization ${\displaystyle |\mathrm {H} \rangle }$ with probability 1/2. These two ensembles are completely indistinguishable experimentally, and therefore they are considered the same mixed state. For this example of unpolarized light, the density operator equals [6] :75

${\displaystyle \rho ={\frac {1}{2}}|\mathrm {R} \rangle \langle \mathrm {R} |+{\frac {1}{2}}|\mathrm {L} \rangle \langle \mathrm {L} |={\frac {1}{2}}|\mathrm {H} \rangle \langle \mathrm {H} |+{\frac {1}{2}}|\mathrm {V} \rangle \langle \mathrm {V} |={\frac {1}{2}}{\begin{pmatrix}1&0\\0&1\end{pmatrix}}.}$

There are also other ways to generate unpolarized light: one possibility is to introduce uncertainty in the preparation of the photon, for example, passing it through a birefringent crystal with a rough surface, so that slightly different parts of the light beam acquire different polarizations. Another possibility is using entangled states: a radioactive decay can emit two photons traveling in opposite directions, in the quantum state ${\displaystyle (|\mathrm {R} ,\mathrm {L} \rangle +|\mathrm {L} ,\mathrm {R} \rangle )/{\sqrt {2}}}$. The joint state of the two photons together is pure, but the density matrix for each photon individually, found by taking the partial trace of the joint density matrix, is completely mixed. [4] :106

## Equivalent ensembles and purifications

A given density operator does not uniquely determine which ensemble of pure states gives rise to it; in general there are infinitely many different ensembles generating the same density matrix. [8] Those cannot be distinguished by any measurement. [9] The equivalent ensembles can be completely characterized: let ${\displaystyle \{p_{j},|\psi _{j}\rangle \}}$ be an ensemble. Then for any complex matrix ${\displaystyle U}$ such that ${\displaystyle U^{\dagger }U=I}$ (a partial isometry), the ensemble ${\displaystyle \{q_{i},|\varphi _{i}\rangle \}}$ defined by

${\displaystyle {\sqrt {q_{i}}}\left|\varphi _{i}\right\rangle =\sum _{j}U_{ij}{\sqrt {p_{j}}}\left|\psi _{j}\right\rangle }$

will give rise to the same density operator, and all equivalent ensembles are of this form.

A closely related fact is that a given density operator has infinitely many different purifications, which are pure states that generate the density operator when a partial trace is taken. Let

${\displaystyle \rho =\sum _{j}p_{j}|\psi _{j}\rangle \langle \psi _{j}|}$

be the density operator generated by the ensemble ${\displaystyle \{p_{j},|\psi _{j}\rangle \}}$, with states ${\displaystyle |\psi _{j}\rangle }$ not necessarily orthogonal. Then for all partial isometries ${\displaystyle U}$ we have that

${\displaystyle |\Psi \rangle =\sum _{j}{\sqrt {p_{j}}}|\psi _{j}\rangle U|a_{j}\rangle }$

is a purification of ${\displaystyle \rho }$, where ${\displaystyle |a_{j}\rangle }$ is an orthogonal basis, and furthermore all purifications of ${\displaystyle \rho }$ are of this form.

## Measurement

Let ${\displaystyle A}$ be an observable of the system, and suppose the ensemble is in a mixed state such that each of the pure states ${\displaystyle \textstyle |\psi _{j}\rangle }$ occurs with probability ${\displaystyle p_{j}}$. Then the corresponding density operator equals

${\displaystyle \rho =\sum _{j}p_{j}|\psi _{j}\rangle \langle \psi _{j}|.}$

The expectation value of the measurement can be calculated by extending from the case of pure states:

${\displaystyle \langle A\rangle =\sum _{j}p_{j}\langle \psi _{j}|A|\psi _{j}\rangle =\sum _{j}p_{j}\operatorname {tr} \left(|\psi _{j}\rangle \langle \psi _{j}|A\right)=\operatorname {tr} \left(\sum _{j}p_{j}|\psi _{j}\rangle \langle \psi _{j}|A\right)=\operatorname {tr} (\rho A),}$

where ${\displaystyle \operatorname {tr} }$ denotes trace. Thus, the familiar expression ${\displaystyle \langle A\rangle =\langle \psi |A|\psi \rangle }$ for pure states is replaced by

${\displaystyle \langle A\rangle =\operatorname {tr} (\rho A)}$

for mixed states. [6] :73

Moreover, if ${\displaystyle A}$ has spectral resolution

${\displaystyle A=\sum _{i}a_{i}P_{i},}$

where ${\displaystyle P_{i}}$ is the projection operator into the eigenspace corresponding to eigenvalue ${\displaystyle a_{i}}$, the post-measurement density operator is given by [10] [11]

${\displaystyle \rho _{i}'={\frac {P_{i}\rho P_{i}}{\operatorname {tr} \left[\rho P_{i}\right]}}}$

when outcome i is obtained. In the case where the measurement result is not known the ensemble is instead described by

${\displaystyle \;\rho '=\sum _{i}P_{i}\rho P_{i}.}$

If one assumes that the probabilities of measurement outcomes are linear functions of the projectors ${\displaystyle P_{i}}$, then they must be given by the trace of the projector with a density operator. Gleason's theorem shows that in Hilbert spaces of dimension 3 or larger the assumption of linearity can be replaced with an assumption of non-contextuality. [12] This restriction on the dimension can be removed by assuming non-contextuality for POVMs as well, [13] [14] but this has been criticized as physically unmotivated. [15]

## Entropy

The von Neumann entropy ${\displaystyle S}$ of a mixture can be expressed in terms of the eigenvalues of ${\displaystyle \rho }$ or in terms of the trace and logarithm of the density operator ${\displaystyle \rho }$. Since ${\displaystyle \rho }$ is a positive semi-definite operator, it has a spectral decomposition such that ${\displaystyle \rho =\textstyle \sum _{i}\lambda _{i}|\varphi _{i}\rangle \langle \varphi _{i}|}$, where ${\displaystyle |\varphi _{i}\rangle }$ are orthonormal vectors, ${\displaystyle \lambda _{i}\geq 0}$, and ${\displaystyle \textstyle \sum \lambda _{i}=1}$. Then the entropy of a quantum system with density matrix ${\displaystyle \rho }$ is

${\displaystyle S=-\sum _{i}\lambda _{i}\ln \lambda _{i}=-\operatorname {tr} (\rho \ln \rho ).}$

This definition implies that the von Neumann entropy of any pure state is zero. [16] :217 If ${\displaystyle \rho _{i}}$ are states that have support on orthogonal subspaces, then the von Neumann entropy of a convex combination of these states,

${\displaystyle \rho =\sum _{i}p_{i}\rho _{i},}$

is given by the von Neumann entropies of the states ${\displaystyle \rho _{i}}$ and the Shannon entropy of the probability distribution ${\displaystyle p_{i}}$:

${\displaystyle S(\rho )=H(p_{i})+\sum _{i}p_{i}S(\rho _{i}).}$

When the states ${\displaystyle \rho _{i}}$ do not have orthogonal supports, the sum on the right-hand side is strictly greater than the von Neumann entropy of the convex combination ${\displaystyle \rho }$. [4] :518

Given a density operator ${\displaystyle \rho }$ and a projective measurement as in the previous section, the state ${\displaystyle \rho '}$ defined by the convex combination

${\displaystyle \rho '=\sum _{i}P_{i}\rho P_{i},}$

which can be interpreted as the state produced by performing the measurement but not recording which outcome occurred, [7] :159 has a von Neumann entropy larger than that of ${\displaystyle \rho }$, except if ${\displaystyle \rho =\rho '}$. It is however possible for the ${\displaystyle \rho '}$ produced by a generalized measurement, or POVM, to have a lower von Neumann entropy than ${\displaystyle \rho }$. [4] :514

## The von Neumann equation for time evolution

Just as the Schrödinger equation describes how pure states evolve in time, the von Neumann equation (also known as the Liouville–von Neumann equation) describes how a density operator evolves in time. The von Neumann equation dictates that [17] [18] [19]

${\displaystyle i\hbar {\frac {\partial \rho }{\partial t}}=[H,\rho ]~,}$

where the brackets denote a commutator.

Note that this equation only holds when the density operator is taken to be in the Schrödinger picture, even though this equation seems at first look to emulate the Heisenberg equation of motion in the Heisenberg picture, with a crucial sign difference:

${\displaystyle i\hbar {\frac {dA^{(\mathrm {H} )}}{dt}}=-\left[H,A^{(\mathrm {H} )}\right]~,}$

where ${\displaystyle A^{(\mathrm {H} )}(t)}$ is some Heisenberg picture operator; but in this picture the density matrix is not time-dependent, and the relative sign ensures that the time derivative of the expected value ${\displaystyle \langle A\rangle }$ comes out the same as in the Schrödinger picture. [3]

If the Hamiltonian is time-independent, the von Neumann equation can be easily solved to yield

${\displaystyle \rho (t)=e^{-iHt/\hbar }\rho (0)e^{iHt/\hbar }.}$

For a more general Hamiltonian, if ${\displaystyle G(t)}$ is the wavefunction propagator over some interval, then the time evolution of the density matrix over that same interval is given by

${\displaystyle \rho (t)=G(t)\rho (0)G(t)^{\dagger }.}$

## Wigner functions and classical analogies

The density matrix operator may also be realized in phase space. Under the Wigner map, the density matrix transforms into the equivalent Wigner function,

${\displaystyle W(x,p)\,\ {\stackrel {\mathrm {def} }{=}}\ \,{\frac {1}{\pi \hbar }}\int _{-\infty }^{\infty }\psi ^{*}(x+y)\psi (x-y)e^{2ipy/\hbar }\,dy.}$

The equation for the time evolution of the Wigner function, known as Moyal equation, is then the Wigner-transform of the above von Neumann equation,

${\displaystyle {\frac {\partial W(q,p,t)}{\partial t}}=-\{\{W(q,p,t),H(q,p)\}\},}$

where ${\displaystyle H(q,p)}$ is the Hamiltonian, and ${\displaystyle \{\{\cdot ,\cdot \}\}}$ is the Moyal bracket, the transform of the quantum commutator.

The evolution equation for the Wigner function is then analogous to that of its classical limit, the Liouville equation of classical physics. In the limit of vanishing Planck's constant ${\displaystyle \hbar }$, ${\displaystyle W(q,p,t)}$ reduces to the classical Liouville probability density function in phase space.

## Example applications

Density matrices are a basic tool of quantum mechanics, and appear at least occasionally in almost any type of quantum-mechanical calculation. Some specific examples where density matrices are especially helpful and common are as follows:

• Statistical mechanics uses density matrices, most prominently to express the idea that a system is prepared at a nonzero temperature. Constructing a density matrix using a canonical ensemble gives a result of the form ${\displaystyle \rho =\exp(-\beta H)/Z(\beta )}$, where ${\displaystyle \beta }$ is the inverse temperature ${\displaystyle (k_{\rm {B}}T)^{-1}}$ and ${\displaystyle H}$ is the system's Hamiltonian. The normalization condition that the trace of ${\displaystyle \rho }$ be equal to 1 defines the partition function to be ${\displaystyle Z(\beta )=\mathrm {tr} \exp(-\beta H)}$. If the number of particles involved in the system is itself not certain, then a grand canonical ensemble can be applied, where the states summed over to make the density matrix are drawn from a Fock space. [20] :174
• Quantum decoherence theory typically involves non-isolated quantum systems developing entanglement with other systems, including measurement apparatuses. Density matrices make it much easier to describe the process and calculate its consequences. Quantum decoherence explains why a system interacting with an environment transitions from being a pure state, exhibiting superpositions, to a mixed state, an incoherent combination of classical alternatives. This transition is fundamentally reversible, as the combined state of system and environment is still pure, but for all practical purposes irreversible, as the environment is a very large and complex quantum system, and it is not feasible to reverse their interaction. Decoherence is thus very important for explaining the classical limit of quantum mechanics, but cannot explain wave function collapse, as all classical alternatives are still present in the mixed state, and wave function collapse selects only one of them. [21]
• Similarly, in quantum computation, quantum information theory, open quantum systems, and other fields where state preparation is noisy and decoherence can occur, density matrices are frequently used. Noise is often modelled via a depolarizing channel or an amplitude damping channel. Quantum tomography is a process by which, given a set of data representing the results of quantum measurements, a density matrix consistent with those measurement results is computed. [22] [23]
• When analyzing a system with many electrons, such as an atom or molecule, an imperfect but useful first approximation is to treat the electrons as uncorrelated or each having an independent single-particle wavefunction. This is the usual starting point when building the Slater determinant in the Hartree–Fock method. If there are ${\displaystyle N}$ electrons filling the ${\displaystyle N}$ single-particle wavefunctions ${\displaystyle |\psi _{i}\rangle }$, then the collection of ${\displaystyle N}$ electrons together can be characterized by a density matrix ${\textstyle \sum _{i=1}^{N}|\psi _{i}\rangle \langle \psi _{i}|}$.

## C*-algebraic formulation of states

It is now generally accepted that the description of quantum mechanics in which all self-adjoint operators represent observables is untenable. [24] [25] For this reason, observables are identified with elements of an abstract C*-algebra A (that is one without a distinguished representation as an algebra of operators) and states are positive linear functionals on A. However, by using the GNS construction, we can recover Hilbert spaces which realize A as a subalgebra of operators.

Geometrically, a pure state on a C*-algebra A is a state which is an extreme point of the set of all states on A. By properties of the GNS construction these states correspond to irreducible representations of A.

The states of the C*-algebra of compact operators K(H) correspond exactly to the density operators, and therefore the pure states of K(H) are exactly the pure states in the sense of quantum mechanics.

The C*-algebraic formulation can be seen to include both classical and quantum systems. When the system is classical, the algebra of observables become an abelian C*-algebra. In that case the states become probability measures, as noted in the introduction.

## History

The formalism of density operators and matrices was introduced in 1927 by John von Neumann [26] and independently, but less systematically, by Lev Landau [27] and later in 1946 by Felix Bloch. [28] Von Neumann introduced the density matrix in order to develop both quantum statistical mechanics and a theory of quantum measurements. The name density matrix itself relates to its classical correspondence to a phase-space probability measure (probability distribution of position and momentum) in classical statistical mechanics, which was introduced by Wigner in 1932. [1]

In contrast, the motivation that inspired Landau was the impossibility of describing a subsystem of a composite quantum system by a state vector. [27]

## Notes and references

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9. Ochs, Wilhelm (1981-11-01). "Some comments on the concept of state in quantum mechanics". Erkenntnis. 16 (3): 339–356. doi:10.1007/BF00211375. ISSN   1572-8420.
10. Lüders, Gerhart (1950). "Über die Zustandsänderung durch den Messprozeß". Annalen der Physik . 443: 322. doi:10.1002/andp.19504430510. Translated by K. A. Kirkpatrick as Lüders, Gerhart (2006-04-03). "Concerning the state-change due to the measurement process". Annalen der Physik . 15 (9): 663–670. arXiv:. Bibcode:2006AnP...518..663L. doi:10.1002/andp.200610207. S2CID   119103479.
11. Busch, Paul; Lahti, Pekka (2009), Greenberger, Daniel; Hentschel, Klaus; Weinert, Friedel (eds.), "Lüders Rule", Compendium of Quantum Physics, Springer Berlin Heidelberg, pp. 356–358, doi:10.1007/978-3-540-70626-7_110, ISBN   978-3-540-70622-9
12. Gleason, Andrew M. (1957). "Measures on the closed subspaces of a Hilbert space". Indiana University Mathematics Journal . 6 (4): 885–893. doi:. MR   0096113.
13. Busch, Paul (2003). "Quantum States and Generalized Observables: A Simple Proof of Gleason's Theorem". Physical Review Letters . 91 (12): 120403. arXiv:. Bibcode:2003PhRvL..91l0403B. doi:10.1103/PhysRevLett.91.120403. PMID   14525351. S2CID   2168715.
14. Caves, Carlton M.; Fuchs, Christopher A.; Manne, Kiran K.; Renes, Joseph M. (2004). "Gleason-Type Derivations of the Quantum Probability Rule for Generalized Measurements". Foundations of Physics . 34 (2): 193–209. arXiv:. Bibcode:2004FoPh...34..193C. doi:10.1023/B:FOOP.0000019581.00318.a5. S2CID   18132256.
15. Andrzej Grudka; Paweł Kurzyński (2008). "Is There Contextuality for a Single Qubit?". Physical Review Letters. 100 (16): 160401. arXiv:. doi:10.1103/PhysRevLett.100.160401. PMID   18518167. S2CID   13251108.
16. Rieffel, Eleanor G.; Polak, Wolfgang H. (2011-03-04). Quantum Computing: A Gentle Introduction. MIT Press. ISBN   978-0-262-01506-6.
17. Breuer, Heinz; Petruccione, Francesco (2002), The theory of open quantum systems, p. 110, ISBN   978-0-19-852063-4
18. Schwabl, Franz (2002), Statistical mechanics, p. 16, ISBN   978-3-540-43163-3
19. Müller-Kirsten, Harald J.W. (2008), Classical Mechanics and Relativity, World Scientific, pp. 175–179, ISBN   978-981-283-251-1
20. Schlosshauer, M. (2019). "Quantum Decoherence". Physics Reports. 831: 1–57. arXiv:. Bibcode:2019PhR...831....1S. doi:10.1016/j.physrep.2019.10.001. S2CID   208006050.
21. Granade, Christopher; Combes, Joshua; Cory, D. G. (2016-01-01). "Practical Bayesian tomography". New Journal of Physics. 18 (3): 033024. arXiv:. Bibcode:2016NJPh...18c3024G. doi:10.1088/1367-2630/18/3/033024. ISSN   1367-2630. S2CID   88521187.
22. Ardila, Luis; Heyl, Markus; Eckardt, André (28 December 2018). "Measuring the Single-Particle Density Matrix for Fermions and Hard-Core Bosons in an Optical Lattice". Physical Review Letters. 121 (260401): 6. arXiv:. Bibcode:2018PhRvL.121z0401P. doi:10.1103/PhysRevLett.121.260401. PMID   30636128. S2CID   51684413.
23. See appendix, Mackey, George Whitelaw (1963), Mathematical Foundations of Quantum Mechanics, Dover Books on Mathematics, New York: Dover Publications, ISBN   978-0-486-43517-6
24. Emch, Gerard G. (1972), Algebraic methods in statistical mechanics and quantum field theory, Wiley-Interscience, ISBN   978-0-471-23900-0
25. von Neumann, John (1927), "Wahrscheinlichkeitstheoretischer Aufbau der Quantenmechanik", Göttinger Nachrichten, 1: 245–272
26. "The Damping Problem in Wave Mechanics (1927)". Collected Papers of L.D. Landau. 1965. pp. 8–18. doi:10.1016/B978-0-08-010586-4.50007-9. ISBN   978-0-08-010586-4.
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The Sudarshan-Glauber P representation is a suggested way of writing down the phase space distribution of a quantum system in the phase space formulation of quantum mechanics. The P representation is the quasiprobability distribution in which observables are expressed in normal order. In quantum optics, this representation, formally equivalent to several other representations, is sometimes championed over alternative representations to describe light in optical phase space, because typical optical observables, such as the particle number operator, are naturally expressed in normal order. It is named after George Sudarshan and Roy J. Glauber, who worked on the topic in 1963. Despite many useful applications in laser theory and coherence theory, the Glauber–Sudarshan P representation has the drawback that it is not always positive, and is not a true probability function.

In physics, a superoperator is a linear operator acting on a vector space of linear operators.

Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. An individual photon can be described as having right or left circular polarization, or a superposition of the two. Equivalently, a photon can be described as having horizontal or vertical linear polarization, or a superposition of the two.

In many-body theory, the term Green's function is sometimes used interchangeably with correlation function, but refers specifically to correlators of field operators or creation and annihilation operators.

In quantum mechanics, the expectation value is the probabilistic expected value of the result (measurement) of an experiment. It can be thought of as an average of all the possible outcomes of a measurement as weighted by their likelihood, and as such it is not the most probable value of a measurement; indeed the expectation value may have zero probability of occurring. It is a fundamental concept in all areas of quantum physics.

A symmetric, informationally complete, positive operator-valued measure (SIC-POVM) is a special case of a generalized measurement on a Hilbert space, used in the field of quantum mechanics. A measurement of the prescribed form satisfies certain defining qualities that makes it an interesting candidate for a "standard quantum measurement", utilized in the study of foundational quantum mechanics, most notably in QBism. Furthermore, it has been shown that applications exist in quantum state tomography and quantum cryptography, and a possible connection has been discovered with Hilbert's twelfth problem.

In quantum mechanics, and especially quantum information theory, the purity of a normalized quantum state is a scalar defined as

In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution in time exhausts all that can be predicted about the system's behavior. A mixture of quantum states is again a quantum state. Quantum states that cannot be written as a mixture of other states are called pure quantum states, while all other states are called mixed quantum states. A pure quantum state can be represented by a ray in a Hilbert space over the complex numbers, while mixed states are represented by density matrices, which are positive semidefinite operators that act on Hilbert spaces.

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

The Maxwell–Bloch equations, also called the optical Bloch equations describe the dynamics of a two-state quantum system interacting with the electromagnetic mode of an optical resonator. They are analogous to the Bloch equations which describe the motion of the nuclear magnetic moment in an electromagnetic field. The equations can be derived either semiclassically or with the field fully quantized when certain approximations are made.

The entropy of entanglement is a measure of the degree of quantum entanglement between two subsystems constituting a two-part composite quantum system. Given a pure bipartite quantum state of the composite system, it is possible to obtain a reduced density matrix describing knowledge of the state of a subsystem. The entropy of entanglement is the Von Neumann entropy of the reduced density matrix for any of the subsystems. If it is non-zero, i.e. the subsystem is in a mixed state, it indicates the two subsystems are entangled.

In quantum information theory and quantum optics, the Schrödinger–HJW theorem is a result about the realization of a mixed state of a quantum system as an ensemble of pure quantum states and the relation between the corresponding purifications of the density operators. The theorem is named after physicists and mathematicians Erwin Schrödinger, Lane P. Hughston, Richard Jozsa and William Wootters. The result was also found independently by Nicolas Hadjisavvas building upon work by Ed Jaynes, while a significant part of it was likewise independently discovered by N. David Mermin. Thanks to its complicated history, it is also known by various other names such as the GHJW theorem, the HJW theorem, and the purification theorem.