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**Quantum decoherence** is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave function is used to explain various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent. A definite phase relationship is necessary to perform quantum computing on quantum information encoded in quantum states. Coherence is preserved under the laws of quantum physics.

- Mechanisms
- Phase-space picture
- Dirac notation
- Loss of interference and the transition from quantum to classical probabilities
- Density-matrix approach
- Operator-sum representation
- Semigroup approach
- Examples of non-unitary modelling of decoherence
- Rotational decoherence
- Depolarizing
- Dissipation
- Timescales
- Mathematical details
- Experimental observations
- Quantitative measurement
- Reducing environmental decoherence
- Criticism
- In interpretations of quantum mechanics
- See also
- References
- Further reading
- External links

If a quantum system were perfectly isolated, it would maintain coherence indefinitely, but it would be impossible to manipulate or investigate it. If it is not perfectly isolated, for example during a measurement, coherence is shared with the environment and appears to be lost with time; a process called quantum decoherence. As a result of this process, quantum behavior is apparently lost, just as energy appears to be lost by friction in classical mechanics.

Decoherence was first introduced in 1970 by the German physicist H. Dieter Zeh ^{ [1] } and has been a subject of active research since the 1980s.^{ [2] } Decoherence has been developed into a complete framework, but there is controversy as to whether it solves the measurement problem, as the founders of decoherence theory admit in their seminal papers.^{ [3] }

Decoherence can be viewed as the loss of information from a system into the environment (often modeled as a heat bath),^{ [4] } since every system is loosely coupled with the energetic state of its surroundings. Viewed in isolation, the system's dynamics are non-unitary (although the combined system plus environment evolves in a unitary fashion).^{ [5] } Thus the dynamics of the system alone are irreversible. As with any coupling, entanglements are generated between the system and environment. These have the effect of sharing quantum information with—or transferring it to—the surroundings.

Decoherence has been used to understand the possibility of the collapse of the wave function in quantum mechanics. Decoherence does not generate *actual* wave-function collapse. It only provides a framework for *apparent* wave-function collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wave function are decoupled from a coherent system and acquire phases from their immediate surroundings. A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue. With respect to the measurement problem, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".

Decoherence represents a challenge for the practical realization of quantum computers, since such machines are expected to rely heavily on the undisturbed evolution of quantum coherences. Simply put, they require that the coherence of states be preserved and that decoherence is managed, in order to actually perform quantum computation. The preservation of coherence, and mitigation of decoherence effects, are thus related to the concept of quantum error correction.

To examine how decoherence operates, an "intuitive" model is presented. The model requires some familiarity with quantum theory basics. Analogies are made between visualisable classical phase spaces and Hilbert spaces. A more rigorous derivation in Dirac notation shows how decoherence destroys interference effects and the "quantum nature" of systems. Next, the density matrix approach is presented for perspective.

An *N*-particle system can be represented in non-relativistic quantum mechanics by a wave function , where each *x _{i}* is a point in 3-dimensional space. This has analogies with the classical phase space. A classical phase space contains a real-valued function in 6

Different previously isolated, non-interacting systems occupy different phase spaces. Alternatively we can say that they occupy different lower-dimensional subspaces in the phase space of the joint system. The *effective* dimensionality of a system's phase space is the number of * degrees of freedom * present, which—in non-relativistic models—is 6 times the number of a system's *free* particles. For a macroscopic system this will be a very large dimensionality. When two systems (and the environment would be a system) start to interact, though, their associated state vectors are no longer constrained to the subspaces. Instead the combined state vector time-evolves a path through the "larger volume", whose dimensionality is the sum of the dimensions of the two subspaces. The extent to which two vectors interfere with each other is a measure of how "close" they are to each other (formally, their overlap or Hilbert space multiplies together) in the phase space. When a system couples to an external environment, the dimensionality of, and hence "volume" available to, the joint state vector increases enormously. Each environmental degree of freedom contributes an extra dimension.

The original system's wave function can be expanded in many different ways as a sum of elements in a quantum superposition. Each expansion corresponds to a projection of the wave vector onto a basis. The basis can be chosen at will. Let us choose an expansion where the resulting basis elements interact with the environment in an element-specific way. Such elements will—with overwhelming probability—be rapidly separated from each other by their natural unitary time evolution along their own independent paths. After a very short interaction, there is almost no chance of any further interference. The process is effectively irreversible. The different elements effectively become "lost" from each other in the expanded phase space created by coupling with the environment; in phase space, this decoupling is monitored through the Wigner quasi-probability distribution. The original elements are said to have *decohered*. The environment has effectively selected out those expansions or decompositions of the original state vector that decohere (or lose phase coherence) with each other. This is called "environmentally-induced superselection", or einselection.^{ [6] } The decohered elements of the system no longer exhibit quantum interference between each other, as in a double-slit experiment. Any elements that decohere from each other via environmental interactions are said to be quantum-entangled with the environment. The converse is not true: not all entangled states are decohered from each other.

Any measuring device or apparatus acts as an environment, since at some stage along the measuring chain, it has to be large enough to be read by humans. It must possess a very large number of hidden degrees of freedom. In effect, the interactions may be considered to be quantum measurements. As a result of an interaction, the wave functions of the system and the measuring device become entangled with each other. Decoherence happens when different portions of the system's wave function become entangled in different ways with the measuring device. For two einselected elements of the entangled system's state to interfere, both the original system and the measuring in both elements device must significantly overlap, in the scalar product sense. If the measuring device has many degrees of freedom, it is *very* unlikely for this to happen.

As a consequence, the system behaves as a classical statistical ensemble of the different elements rather than as a single coherent quantum superposition of them. From the perspective of each ensemble member's measuring device, the system appears to have irreversibly collapsed onto a state with a precise value for the measured attributes, relative to that element. And this provided one explains how the Born rule coefficients effectively act as probabilities as per the measurement postulate, constitutes a solution to the quantum measurement problem.

Using Dirac notation, let the system initially be in the state

where the s form an einselected basis (*environmentally induced selected eigenbasis*^{ [6] }), and let the environment initially be in the state . The vector basis of the combination of the system and the environment consists of the tensor products of the basis vectors of the two subsystems. Thus, before any interaction between the two subsystems, the joint state can be written as

where is shorthand for the tensor product . There are two extremes in the way the system can interact with its environment: either (1) the system loses its distinct identity and merges with the environment (e.g. photons in a cold, dark cavity get converted into molecular excitations within the cavity walls), or (2) the system is not disturbed at all, even though the environment is disturbed (e.g. the idealized non-disturbing measurement). In general, an interaction is a mixture of these two extremes that we examine.

If the environment absorbs the system, each element of the total system's basis interacts with the environment such that

- evolves into

and so

- evolves into

The unitarity of time evolution demands that the total state basis remains orthonormal, i.e. the scalar or inner products of the basis vectors must vanish, since :

This orthonormality of the environment states is the defining characteristic required for einselection.^{ [6] }

In an idealised measurement, the system disturbs the environment, but is itself undisturbed by the environment. In this case, each element of the basis interacts with the environment such that

- evolves into the product

and so

- evolves into

In this case, unitarity demands that

where was used. *Additionally*, decoherence requires, by virtue of the large number of hidden degrees of freedom in the environment, that

As before, this is the defining characteristic for decoherence to become einselection.^{ [6] } The approximation becomes more exact as the number of environmental degrees of freedom affected increases.

Note that if the system basis were not an einselected basis, then the last condition is trivial, since the disturbed environment is not a function of , and we have the trivial disturbed environment basis . This would correspond to the system basis being degenerate with respect to the environmentally defined measurement observable. For a complex environmental interaction (which would be expected for a typical macroscale interaction) a non-einselected basis would be hard to define.

The utility of decoherence lies in its application to the analysis of probabilities, before and after environmental interaction, and in particular to the vanishing of quantum interference terms after decoherence has occurred. If we ask what is the probability of observing the system making a transition from to *before* has interacted with its environment, then application of the Born probability rule states that the transition probability is the squared modulus of the scalar product of the two states:

where , , and etc.

The above expansion of the transition probability has terms that involve ; these can be thought of as representing *interference* between the different basis elements or quantum alternatives. This is a purely quantum effect and represents the non-additivity of the probabilities of quantum alternatives.

To calculate the probability of observing the system making a quantum leap from to *after* has interacted with its environment, then application of the Born probability rule states that we must sum over all the relevant possible states of the environment *before* squaring the modulus:

The internal summation vanishes when we apply the decoherence/einselection condition , and the formula simplifies to

If we compare this with the formula we derived before the environment introduced decoherence, we can see that the effect of decoherence has been to move the summation sign from inside of the modulus sign to outside. As a result, all the cross- or quantum interference-terms

have vanished from the transition-probability calculation. The decoherence has irreversibly converted quantum behaviour (additive probability amplitudes) to classical behaviour (additive probabilities).^{ [6] }^{ [7] }^{ [8] }

In terms of density matrices, the loss of interference effects corresponds to the diagonalization of the "environmentally traced-over" density matrix.^{ [6] }

The effect of decoherence on density matrices is essentially the decay or rapid vanishing of the off-diagonal elements of the partial trace of the joint system's density matrix, i.e. the trace, with respect to *any* environmental basis, of the density matrix of the combined system *and* its environment. The decoherence irreversibly converts the "averaged" or "environmentally traced-over"^{ [6] } density matrix from a pure state to a reduced mixture; it is this that gives the *appearance* of wave-function collapse. Again, this is called "environmentally induced superselection", or einselection.^{ [6] } The advantage of taking the partial trace is that this procedure is indifferent to the environmental basis chosen.

Initially, the density matrix of the combined system can be denoted as

where is the state of the environment. Then if the transition happens before any interaction takes place between the system and the environment, the environment subsystem has no part and can be traced out, leaving the reduced density matrix for the system:

Now the transition probability will be given as

where , , and etc.

Now the case when transition takes place after the interaction of the system with the environment. The combined density matrix will be

To get the reduced density matrix of the system, we trace out the environment and employ the decoherence/einselection condition and see that the off-diagonal terms vanish (a result obtained by Erich Joos and H. D. Zeh in 1985):^{ [9] }

Similarly, the final reduced density matrix after the transition will be

The transition probability will then be given as

which has no contribution from the interference terms

The density-matrix approach has been combined with the Bohmian approach to yield a *reduced-trajectory approach*, taking into account the system reduced density matrix and the influence of the environment.^{ [10] }

Consider a system *S* and environment (bath) *B*, which are closed and can be treated quantum-mechanically. Let and be the system's and bath's Hilbert spaces respectively. Then the Hamiltonian for the combined system is

where are the system and bath Hamiltonians respectively, is the interaction Hamiltonian between the system and bath, and are the identity operators on the system and bath Hilbert spaces respectively. The time-evolution of the density operator of this closed system is unitary and, as such, is given by

where the unitary operator is . If the system and bath are not entangled initially, then we can write . Therefore, the evolution of the system becomes

The system–bath interaction Hamiltonian can be written in a general form as

where is the operator acting on the combined system–bath Hilbert space, and are the operators that act on the system and bath respectively. This coupling of the system and bath is the cause of decoherence in the system alone. To see this, a partial trace is performed over the bath to give a description of the system alone:

is called the *reduced density matrix* and gives information about the system only. If the bath is written in terms of its set of orthogonal basis kets, that is, if it has been initially diagonalized, then . Computing the partial trace with respect to this (computational) basis gives

where are defined as the *Kraus operators* and are represented as (the index combines indices and ):

This is known as the * operator-sum representation * (OSR). A condition on the Kraus operators can be obtained by using the fact that ; this then gives

This restriction determines whether decoherence will occur or not in the OSR. In particular, when there is more than one term present in the sum for , then the dynamics of the system will be non-unitary, and hence decoherence will take place.

A more general consideration for the existence of decoherence in a quantum system is given by the *master equation*, which determines how the density matrix of the *system alone* evolves in time (see also the Belavkin equation ^{ [11] }^{ [12] }^{ [13] } for the evolution under continuous measurement). This uses the Schrödinger picture, where evolution of the *state* (represented by its density matrix) is considered. The master equation is

where is the system Hamiltonian along with a (possible) unitary contribution from the bath, and is the *Lindblad decohering term*.^{ [5] } The Lindblad decohering term is represented as

The are basis operators for the *M*-dimensional space of bounded operators that act on the system Hilbert space and are the *error generators*.^{ [14] } The matrix elements represent the elements of a positive semi-definite Hermitian matrix; they characterize the decohering processes and, as such, are called the *noise parameters*.^{ [14] } The semigroup approach is particularly nice, because it distinguishes between the unitary and decohering (non-unitary) processes, which is not the case with the OSR. In particular, the non-unitary dynamics are represented by , whereas the unitary dynamics of the state are represented by the usual Heisenberg commutator. Note that when , the dynamical evolution of the system is unitary. The conditions for the evolution of the system density matrix to be described by the master equation are:^{ [5] }

- the evolution of the system density matrix is determined by a one-parameter semigroup,
- the evolution is "completely positive" (i.e. probabilities are preserved),
- the system and bath density matrices are
*initially*decoupled.

Decoherence can be modelled as a non-unitary process by which a system couples with its environment (although the combined system plus environment evolves in a unitary fashion).^{ [5] } Thus the dynamics of the system alone, treated in isolation, are non-unitary and, as such, are represented by irreversible transformations acting on the system's Hilbert space . Since the system's dynamics are represented by irreversible representations, then any information present in the quantum system can be lost to the environment or heat bath. Alternatively, the decay of quantum information caused by the coupling of the system to the environment is referred to as decoherence.^{ [4] } Thus decoherence is the process by which information of a quantum system is altered by the system's interaction with its environment (which form a closed system), hence creating an entanglement between the system and heat bath (environment). As such, since the system is entangled with its environment in some unknown way, a description of the system by itself cannot be made without also referring to the environment (i.e. without also describing the state of the environment).

Consider a system of *N* qubits that is coupled to a bath symmetrically. Suppose this system of *N* qubits undergoes a rotation around the eigenstates of . Then under such a rotation, a random phase will be created between the eigenstates , of . Thus these basis qubits and will transform in the following way:

This transformation is performed by the rotation operator

Since any qubit in this space can be expressed in terms of the basis qubits, then all such qubits will be transformed under this rotation. Consider a qubit in a pure state . This state will decohere, since it is not "encoded" with the dephasing factor . This can be seen by examining the density matrix averaged over all values of :

where is a probability density. If is given as a Gaussian distribution

then the density matrix is

Since the off-diagonal elements—the coherence terms—decay for increasing , then the density matrices for the various qubits of the system will be indistinguishable. This means that no measurement can distinguish between the qubits, thus creating decoherence between the various qubit states. In particular, this dephasing process causes the qubits to collapse onto the axis. This is why this type of decoherence process is called **collective dephasing**, because the *mutual* phases between *all* qubits of the *N*-qubit system are destroyed.

**Depolarizing** is a non-unitary transformation on a quantum system which maps pure states to mixed states. This is a non-unitary process, because any transformation that reverses this process will map states out of their respective Hilbert space thus not preserving positivity (i.e. the original probabilities are mapped to negative probabilities, which is not allowed). The 2-dimensional case of such a transformation would consist of mapping pure states on the surface of the Bloch sphere to mixed states within the Bloch sphere. This would contract the Bloch sphere by some finite amount and the reverse process would expand the Bloch sphere, which cannot happen.

**Dissipation** is a decohering process by which the populations of quantum states are changed due to entanglement with a bath. An example of this would be a quantum system that can exchange its energy with a bath through the interaction Hamiltonian. If the system is not in its ground state and the bath is at a temperature lower than that of the system's, then the system will give off energy to the bath, and thus higher-energy eigenstates of the system Hamiltonian will decohere to the ground state after cooling and, as such, will all be non-degenerate. Since the states are no longer degenerate, they are not distinguishable, and thus this process is irreversible (non-unitary).

Decoherence represents an extremely fast process for macroscopic objects, since these are interacting with many microscopic objects, with an enormous number of degrees of freedom, in their natural environment. The process is needed if we are to understand why we tend not to observe quantum behaviour in everyday macroscopic objects and why we do see classical fields emerge from the properties of the interaction between matter and radiation for large amounts of matter. The time taken for off-diagonal components of the density matrix to effectively vanish is called the **decoherence time**. It is typically extremely short for everyday, macroscale processes.^{ [6] }^{ [7] }^{ [8] } A modern basis-independent definition of the decoherence time relies on the short-time behavior of the fidelity between the initial and the time-dependent state ^{ [15] } or, equivalently, the decay of the purity .^{ [16] }

We assume for the moment that the system in question consists of a subsystem *A* being studied and the "environment" , and the total Hilbert space is the tensor product of a Hilbert space describing *A* and a Hilbert space describing , that is,

This is a reasonably good approximation in the case where *A* and are relatively independent (e.g. there is nothing like parts of *A* mixing with parts of or conversely). The point is, the interaction with the environment is for all practical purposes unavoidable (e.g. even a single excited atom in a vacuum would emit a photon, which would then go off). Let's say this interaction is described by a unitary transformation *U* acting upon . Assume that the initial state of the environment is , and the initial state of *A* is the superposition state

where and are orthogonal, and there is no entanglement initially. Also, choose an orthonormal basis for . (This could be a "continuously indexed basis" or a mixture of continuous and discrete indexes, in which case we would have to use a rigged Hilbert space and be more careful about what we mean by orthonormal, but that's an inessential detail for expository purposes.) Then, we can expand

and

uniquely as

and

respectively. One thing to realize is that the environment contains a huge number of degrees of freedom, a good number of them interacting with each other all the time. This makes the following assumption reasonable in a handwaving way, which can be shown to be true in some simple toy models. Assume that there exists a basis for such that and are all approximately orthogonal to a good degree if *i* ≠ *j* and the same thing for and and also for and for any *i* and *j* (the decoherence property).

This often turns out to be true (as a reasonable conjecture) in the position basis because how *A* interacts with the environment would often depend critically upon the position of the objects in *A*. Then, if we take the partial trace over the environment, we would find the density state^{[ clarification needed ]} is approximately described by

that is, we have a diagonal mixed state, there is no constructive or destructive interference, and the "probabilities" add up classically. The time it takes for *U*(*t*) (the unitary operator as a function of time) to display the decoherence property is called the **decoherence time**.

The decoherence rate depends on a number of factors, including temperature or uncertainty in position, and many experiments have tried to measure it depending on the external environment.^{ [17] }

The process of a quantum superposition gradually obliterated by decoherence was quantitatively measured for the first time by Serge Haroche and his co-workers at the École Normale Supérieure in Paris in 1996.^{ [18] } Their approach involved sending individual rubidium atoms, each in a superposition of two states, through a microwave-filled cavity. The two quantum states both cause shifts in the phase of the microwave field, but by different amounts, so that the field itself is also put into a superposition of two states. Due to photon scattering on cavity-mirror imperfection, the cavity field loses phase coherence to the environment.

Haroche and his colleagues measured the resulting decoherence via correlations between the states of pairs of atoms sent through the cavity with various time delays between the atoms.

In July 2011, researchers from University of British Columbia and University of California, Santa Barbara were able to reduce environmental decoherence rate "to levels far below the threshold necessary for quantum information processing" by applying high magnetic fields in their experiment.^{ [19] }^{ [20] }^{ [21] }

In August 2020 scientists reported that that ionizing radiation from environmental radioactive materials and cosmic rays may substantially limit the coherence times of qubits if they aren't shielded adequately which may be critical for realizing fault-tolerant superconducting quantum computers in the future.^{ [22] }^{ [23] }^{ [24] }

Criticism of the adequacy of decoherence theory to solve the measurement problem has been expressed by Anthony Leggett: "I hear people murmur the dreaded word "decoherence". But I claim that this is a major red herring".^{ [25] } Concerning the experimental relevance of decoherence theory, Leggett has stated: "Let us now try to assess the decoherence argument. Actually, the most economical tactic at this point would be to go directly to the results of the next section, namely that it is experimentally refuted! However, it is interesting to spend a moment enquiring why it was reasonable to anticipate this in advance of the actual experiments. In fact, the argument contains several major loopholes".^{ [26] }

Before an understanding of decoherence was developed, the Copenhagen interpretation of quantum mechanics treated wave-function collapse as a fundamental, *a priori* process. Decoherence as a possible *explanatory mechanism* for the *appearance* of wave function collapse was first developed by David Bohm in 1952, who applied it to Louis DeBroglie's pilot-wave theory, producing Bohmian mechanics,^{ [27] }^{ [28] } the first successful hidden-variables interpretation of quantum mechanics. Decoherence was then used by Hugh Everett in 1957 to form the core of his many-worlds interpretation.^{ [29] } However, decoherence was largely ignored for many years (with the exception of Zeh's work),^{ [1] } and not until the 1980s^{ [30] }^{ [31] } did decoherent-based explanations of the appearance of wave-function collapse become popular, with the greater acceptance of the use of reduced density matrices.^{ [9] }^{ [7] } The range of decoherent interpretations have subsequently been extended around the idea, such as consistent histories. Some versions of the Copenhagen interpretation have been modified to include decoherence.

Decoherence does not claim to provide a mechanism for the actual wave-function collapse; rather it puts forth a reasonable framework for the appearance of wave-function collapse. The quantum nature of the system is simply "leaked" into the environment so that a total superposition of the wave function still exists, but exists – at least for all practical purposes^{ [32] } — beyond the realm of measurement.^{ [33] } Of course, by definition, the claim that a merged but unmeasurable wave function still exists cannot be proven experimentally. Decoherence is needed to understand why a quantum system begins to obey classical probability rules after interacting with its environment (due to the suppression of the interference terms when applying Born's probability rules to the system).

**Quantum teleportation** is a technique for transferring quantum information from a sender at one location to a receiver some distance away. While teleportation is commonly portrayed in science fiction as a means to transfer physical objects from one location to the next, quantum teleportation only transfers quantum information. Moreover, the sender may not know the location of the recipient, and does not know which particular quantum state will be transferred.

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.

In quantum mechanics, **wave function collapse** occurs when a wave function—initially in a superposition of several eigenstates—reduces to a single eigenstate due to interaction with the external world. This interaction is called an "observation". It is the essence of a measurement in quantum mechanics which connects the wave function with classical observables like position and momentum. Collapse is one of two processes by which quantum systems evolve in time; the other is the continuous evolution via the Schrödinger equation. Collapse is a black box for a thermodynamically irreversible interaction with a classical environment. Calculations of quantum decoherence show that when a quantum system interacts with the environment, the superpositions *apparently* reduce to mixtures of classical alternatives. Significantly, the combined wave function of the system and environment continue to obey the Schrödinger equation. More importantly, this is not enough to explain wave function collapse, as decoherence does not reduce it to a single eigenstate.

In physics, an **operator** is a function over a space of physical states onto another space of physical states. The simplest example of the utility of operators is the study of symmetry. Because of this, they are very useful tools in classical mechanics. Operators are even more important in quantum mechanics, where they form an intrinsic part of the formulation of the theory.

In quantum physics, a **measurement** is the testing or manipulation of a physical system in order to yield a numerical result. The predictions that quantum physics makes are in general probabilistic. The mathematical tools for making predictions about what measurement outcomes may occur were developed during the 20th century and make use of linear algebra and functional analysis.

In quantum mechanics and computing, the **Bloch sphere** is a geometrical representation of the pure state space of a two-level quantum mechanical system (qubit), named after the physicist Felix Bloch.

In quantum information theory, a **quantum channel** is a communication channel which can transmit quantum information, as well as classical information. An example of quantum information is the state of a qubit. An example of classical information is a text document transmitted over the Internet.

In quantum mechanics, **einselections**, short for "environment-induced superselection", is a name coined by Wojciech H. Zurek for a process which is claimed to explain the appearance of wavefunction collapse and the emergence of classical descriptions of reality from quantum descriptions. In this approach, classicality is described as an emergent property induced in open quantum systems by their environments. Due to the interaction with the environment, the vast majority of states in the Hilbert space of a quantum open system become highly unstable due to entangling interaction with the environment, which in effect monitors selected observables of the system. After a decoherence time, which for macroscopic objects is typically many orders of magnitude shorter than any other dynamical timescale, a generic quantum state decays into an uncertain state which can be expressed as a mixture of simple pointer states. In this way the environment induces effective superselection rules. Thus, einselection precludes stable existence of pure superpositions of pointer states. These 'pointer states' are stable despite environmental interaction. The einselected states lack coherence, and therefore do not exhibit the quantum behaviours of entanglement and superposition.

**LOCC**, or **local operations and classical communication**, is a method in quantum information theory where a local (product) operation is performed on part of the system, and where the result of that operation is "communicated" classically to another part where usually another local operation is performed conditioned on the information received.

In functional analysis and quantum measurement theory, a **positive operator-valued measure** (**POVM**) is a measure whose values are positive semi-definite operators on a Hilbert space. POVMs are a generalisation of projection-valued measures (PVM) and, correspondingly, quantum measurements described by POVMs are a generalisation of quantum measurement described by PVMs.

The **time-evolving block decimation** (**TEBD**) algorithm is a numerical scheme used to simulate one-dimensional quantum many-body systems, characterized by at most nearest-neighbour interactions. It is dubbed Time-evolving Block Decimation because it dynamically identifies the relevant low-dimensional Hilbert subspaces of an exponentially larger original Hilbert space. The algorithm, based on the Matrix Product States formalism, is highly efficient when the amount of entanglement in the system is limited, a requirement fulfilled by a large class of quantum many-body systems in one dimension.

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.

In 1927, a year after the publication of the Schrödinger equation, Hartree formulated what are now known as the **Hartree equations** for atoms, using the concept of *self-consistency* that Lindsay had introduced in his study of many electron systems in the context of Bohr theory. Hartree assumed that the nucleus together with the electrons formed a spherically symmetric field. The charge distribution of each electron was the solution of the Schrödinger equation for an electron in a potential , derived from the field. Self-consistency required that the final field, computed from the solutions was self-consistent with the initial field and he called his method the **self-consistent field** method.

A **decoherence-free subspace** (**DFS**) is a subspace of a quantum system's Hilbert space that is invariant to non-unitary dynamics. Alternatively stated, they are a small section of the system Hilbert space where the system is decoupled from the environment and thus its evolution is completely unitary. DFSs can also be characterized as a special class of quantum error correcting codes. In this representation they are *passive* error-preventing codes since these subspaces are encoded with information that (possibly) won't require any *active* stabilization methods. These subspaces prevent destructive environmental interactions by isolating quantum information. As such, they are an important subject in quantum computing, where (coherent) control of quantum systems is the desired goal. Decoherence creates problems in this regard by causing loss of coherence between the quantum states of a system and therefore the decay of their interference terms, thus leading to loss of information from the (open) quantum system to the surrounding environment. Since quantum computers cannot be isolated from their environment and information can be lost, the study of DFSs is important for the implementation of quantum computers into the real world.

A **quantum t-design** is a probability distribution over either pure quantum states or unitary operators which can duplicate properties of the probability distribution over the Haar measure for polynomials of degree t or less. Specifically, the average of any polynomial function of degree t over the design is exactly the same as the average over Haar measure. Here the Haar measure is a uniform probability distribution over all quantum states or over all unitary operators. Quantum t-designs are so called because they are analogous to t-designs in classical statistics, which arose historically in connection with the problem of design of experiments. Two particularly important types of t-designs in quantum mechanics are projective and unitary t-designs.

**Entanglement distillation** is the transformation of *N* copies of an arbitrary entangled state into some number of approximately pure Bell pairs, using only *local operations and classical communication* (LOCC).

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.

The **min-entropy**, in information theory, is the smallest of the Rényi family of entropies, corresponding to the most conservative way of measuring the unpredictability of a set of outcomes, as the negative logarithm of the probability of the *most likely* outcome. The various Rényi entropies are all equal for a uniform distribution, but measure the unpredictability of a nonuniform distribution in different ways. The min-entropy is never greater than the ordinary or Shannon entropy and that in turn is never greater than the Hartley or max-entropy, defined as the logarithm of the *number* of outcomes with nonzero probability.

The **Swap test** is a procedure in quantum computation that is used to check how much two quantum states differ.

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**.

- 1 2 H. Dieter Zeh, "On the Interpretation of Measurement in Quantum Theory",
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- Lidar, D. A.; Chuang, I. L.; Whaley, K. B. (1998). "Decoherence-Free Subspaces for Quantum Computation".
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*Phys. Rev. Lett*.**119**(13): 130401. arXiv: 1706.06943 . Bibcode:2017PhRvL.119m0401B. doi:10.1103/PhysRevLett.119.130401. PMID 29341721. S2CID 206299205. - ↑ Xu, Z.; García-Pintos, L. P.; Chenu, A.; del Campo, A. (2019). "Extreme Decoherence and Quantum Chaos".
*Phys. Rev. Lett*.**122**(1): 014103. arXiv: 1810.02319 . Bibcode:2019PhRvL.122a4103X. doi:10.1103/PhysRevLett.122.014103. PMID 31012673. S2CID 53628496. - ↑ Dan Stahlke. "Quantum Decoherence and the Measurement Problem" (PDF). Retrieved 23 July 2011.
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Our theory also predicted that we could suppress the decoherence, and push the decoherence rate in the experiment to levels far below the threshold necessary for quantum information processing, by applying high magnetic fields. (...)Magnetic molecules now suddenly appear to have serious potential as candidates for quantum computing hardware", said Susumu Takahashi, assistant professor of chemistry and physics at the University of Southern California. "This opens up a whole new area of experimental investigation with sizeable potential in applications, as well as for fundamental work".

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- ↑ "Breakthrough removes major hurdle for quantum computing".
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*New Scientist*. Retrieved 7 September 2020. - ↑ "Cosmic rays may soon stymie quantum computing".
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*Physical Review D*, 24, pp. 1516–1525 (1981). - ↑ Wojciech H. Zurek, Environment-Induced Superselection Rules,
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*The Road to Reality*, pp. 802–803: "...the environmental-decoherence viewpoint [...] maintains that state vector reduction [the R process] can be understood as coming about because the environmental system under consideration becomes inextricably entangled with its environment. [...] We think of the environment as extremely complicated and essentially 'random' [...], accordingly we sum over the unknown states in the environment to obtain a density matrix [...] Under normal circumstances, one must regard the density matrix as some kind of approximation to the whole quantum truth. For there is no general principle providing an absolute bar to extracting information from the environment. [...] Accordingly, such descriptions are referred to as FAPP [for all practical purposes]". - ↑ Huw Price (1996),
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- Schlosshauer, Maximilian (2007).
*Decoherence and the Quantum-to-Classical Transition*(1st ed.). Berlin/Heidelberg: Springer. - Joos, E.; et al. (2003).
*Decoherence and the Appearance of a Classical World in Quantum Theory*(2nd ed.). Berlin: Springer. - Omnes, R. (1999).
*Understanding Quantum Mechanics*. Princeton: Princeton University Press. - Zurek, Wojciech H. (2003). "Decoherence and the transition from quantum to classical – REVISITED", arXiv : quant-ph/0306072 (An updated version of PHYSICS TODAY, 44:36–44 (1991) article)
- Schlosshauer, Maximilian (23 February 2005). "Decoherence, the Measurement Problem, and Interpretations of Quantum Mechanics".
*Reviews of Modern Physics*.**76**(2004): 1267–1305. arXiv: quant-ph/0312059 . Bibcode:2004RvMP...76.1267S. doi:10.1103/RevModPhys.76.1267. S2CID 7295619. - J. J. Halliwell, J. Perez-Mercader, Wojciech H. Zurek, eds,
*The Physical Origins of Time Asymmetry*, Part 3: Decoherence, ISBN 0-521-56837-4 - Berthold-Georg Englert, Marlan O. Scully & Herbert Walther,
*Quantum Optical Tests of Complementarity*, Nature, Vol 351, pp 111–116 (9 May 1991) and (same authors)*The Duality in Matter and Light*Scientific American, pg 56–61, (December 1994). Demonstrates that complementarity is enforced, and quantum interference effects destroyed, by irreversible object-apparatus correlations, and not, as was previously popularly believed, by Heisenberg's uncertainty principle itself. - Mario Castagnino, Sebastian Fortin, Roberto Laura and Olimpia Lombardi,
*A general theoretical framework for decoherence in open and closed systems*, Classical and Quantum Gravity, 25, pp. 154002–154013, (2008). A general theoretical framework for decoherence is proposed, which encompasses formalisms originally devised to deal just with open or closed systems.

- Decoherence.info by Erich Joos
- http://plato.stanford.edu/entries/qm-decoherence/
- Schlosshauer, Maximilian (2005). "Decoherence, the measurement problem, and interpretations of quantum mechanics".
*Reviews of Modern Physics*.**76**(4): 1267–1305. arXiv: quant-ph/0312059 . doi:10.1103/RevModPhys.76.1267. S2CID 7295619. - Dass, Tulsi (2005). "Measurements and Decoherence". arXiv: quant-ph/0505070 .
- A detailed introduction from a graduate student's website at Drexel University
- Quantum Bug : Qubits might spontaneously decay in seconds
*Scientific American*(October 2005) - Quantum Decoherence and the Measurement Problem

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