Bohr topos (changes) in nLab

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Contents

Idea

A Bohr topos is a topos associated with any quantum mechanical system, which is such that

• the quantum observables and quantum states of the quantum mechanical system, hence its quantum probability theory,

are equivalently

• the classical observables and classical states, hence its classical probability theory,

  internal to the Bohr topos, hence in the internal logic of the Bohr topos.

A detailed motivation/derivation of this from classical theorems on the foundations of standard quantum mechanics is at order-theoretic structure in quantum mechanics.

One might think of a Bohr topos as (part of) a formalization of the “coordination” of the physical theory of quantum mechanics, providing a formalized prescription of how to map the theory to propositions about (experimental) observables of the system. The internal logic of Bohr toposes has been argued (e.g. Heunen-Landsman-Spitters 09) to be a better formal context for such considerations than the old quantum logic going back to von Neumann.

The idea of Bohr toposes goes back to Butterfield-Hamilton-Isham, Isham-Döring 07) and Heunen-Landsman-Spitters 09. The concept is named after Niels Bohr, whose informal ideas about the nature of quantum mechanics (Bohr 1949, cf. Scheibe 1973, p. 24) it is supposed to formalize, see at interpretation of quantum mechanics – Bohr’s standpoint.

Sometimes in the literature the discussion of Bohr toposes is referred to as “the topos-theoretic formulation of physics”. But actually Bohr toposes currently formalize but one aspect of quantum mechanics, namely “the quantum mechanical phase space” in the form of the quantum observables and the quantum states. The plain Bohr topos does not even encode any dynamics, though in the spirit of AQFT a certain presheaf of Bohr toposes on spacetime does encode dynamics (Nuiten 11). For other and more comprehensive usages of topos theory in the formalization of physics see at geometry of physics and at higher category theory and physics.

Brief

To every quantum mechanical system is associated its Bohr topos: a ringed topos which plays the role of the quantum phase space. The idea of this construction – Bohrification – is that it naturally captures the geometric and logical aspects of quantum physics in terms of higher geometry/topos theory.

One way to understand Bohrification is as a generalization of the construction of the Gelfand spectrum of commutative C-star algebras to a context of noncommutative C *C^*-algebras. It assigns to a noncommutative C *C^*-algebra AA essentially a system of Gelfand dual spaces to each of its commutative subalgebras. Together, this system yields a generalized Gelfand spectrum in the form of a locale Σ̲ A\underline{\Sigma}_A internal to the sheaf topos 𝒯 A\mathcal{T}_A over the poset of commutative subalgebras of AA, or equivalently its externalization Σ A→𝒞(A)\Sigma_A \to \mathcal{C}(A) regarded as a locale over the locale of open subalgebras.

As a topos, the Bohr topos is just a presheaf topos, the topos of presheaves on this poset of commutative subalgebras of AA, but the point is that it is naturally a ringed topos with the original non-commutative algebra AA appearing as a commutative C *C^\ast-algebra internal to the Bohr topos. This allows to talk about quantum states on AA much like classical states, but internal to the Bohr topos.

In fact, under mild assumptions on AA, its poset of commutative subalgebras, and hence the Bohr topos over it, encodes precisely the Jordan algebra underlying AA. As discussed at Jordan algebra, this is precisely that part of AA which knows about the quantum observables themselves. In order to have the Bohr topos remember the full non-commutative algebra structure of AA, it needs to be equipped with the information about Hamiltonian flows induced on AA by automorphisms of the form exp(i[H,−])\exp(i [H,-]), where [−,−][-,-] is the commutator (that gets discarded as one passes to the Jordan algebra). According to Andreas Döring (private communication at MPI Bonn, April 2013), this can be formulated nicely in topos theory.

More detailed

The formalization of the notion quantum mechanical system (see there for details) with its states and observables is the following.

• The system as such is encoded by a C-star algebra AA;

• a self-adjoint operator a∈Aa \in A is thought of as being an observable of the system: a kind of observation that one can make about it;

• a ℂ\mathbb{C}-linear map ρ:A→ℂ\rho : A \to \mathbb{C} (which is “positive” and “normalized”) is thought of as being a state that the system can be in – a physical configuration (or rather: a probability distribution of such);

• the pairing (a,ρ)↦ρ(a)∈ℂ(a, \rho) \mapsto \rho(a) \in \mathbb{C} is thought of as being the value of the observable aa made on the system in state ρ\rho: for instance the total energy of the system as measured in some chosen unit;

• a one-parameter group of automorphisms ℝ→Aut(A)\mathbb{R} \to Aut(A) (inner automorphisms for “localized” systems, see below) is thought of as being an evolution of the system, for instance in time or more general in spacetime.

(Often in the literature quantum mechanical systems are instead dually conceived of in terms of Hilbert spaces of pure states. The relation between these two descriptions is established by the GNS-construction which allows to pass from one to the other.)

Notice that these axioms are naturally thought of as exhibiting the C *C^\ast-algebra AA as a formal dual of the would-be quantum phase space of the physical system – not quite an ordinary topological space (which by Gelfand duality it would be if AA were commutative) but still a kind of generalized space. Traditionally it is common to regard this as a space in noncommutative geometry. Notice that if we do so – hence if we regard the object SpecA∈C *Alg opSpec A \in C^\ast Alg^{op} that is the incarnation of A∈C *AlgA \in C^\ast Alg in the opposite category – then the definition of observable above reduces to the evident notion of real-valued functions on quantum phase space: such a function is a morphism SpecA→ℝSpec A \to \mathbb{R} in C *Alg opC^\ast Alg^{op}, which dually is a C *C^\ast-algebra homomorphism C(ℝ) 0→AC(\mathbb{R})_0 \to A (where on the left we have the unitization of functions with compact support). That such correspond to self-adjoint operators of AA is the statement of functional calculus for C *C^\ast-algebras.

More subtle is the interpretation of the axiom for states. Historically this had been been subject to some discussion: by the spectral theorem two different observables a 1,a 2∈Aa_1, a_2 \in A have a compatible set of observable values if and only if these elements commute with each other in AA. Generally, a set {a i} i\{a_i\}_i of observables has a jointly consistent set of observable values if and only if the sub-C *C^\ast-algebra ⟨{a i} i⟩⊂A\langle \{a_i\}_i\rangle \subset A generated by them is commutative. Therefore for the phenomenological interpretation of the axioms it seems to make no sense to demand that a state ρ:A→ℂ\rho : A \to \mathbb{C} be linear on non-commuting observables: if a 1a_1 and a 2a_2 do not commute, it is not a-priori clear that it makes sense to require that ρ(a 1+a 2)=ρ(a 1)+ρ(a 2)\rho(a_1 + a_2) = \rho(a_1) + \rho(a_2). This might experimentally fail, and hold only for commuting a 1,a 2a_1, a_2.

Therefore the notion of quasi-state was introduced: a quasi-state on AA is defined to be a (positive and normalized) function ρ:A→ℂ\rho : A \to \mathbb{C} which is required to be ℂ\mathbb{C}-linear only on all commutative subalgebras of AA. Operationally, quasi-states should be the genuine states!

One would therefore tend to think that the terminology has been chosen in an unfortunate way. While maybe true, it turns out – non-trivially – that in a major class of cases of interest the distinction does not matter: namely Gleason's theorem states that for HH a separable complex Hilbert space with dimH>2dim H \gt 2 and A=B(H)A = B(H) the C *C^\ast-algebra of bounded operators on H\H, all quasi-states on AA are automatically states: a function that is linear on all commutative subalgebras is automatically also linear on all of AA.

While this means that the distinction between states and quasi-states disappears in a major case of interest, it does not disappear in all cases of interest. In particular, other foundational theorems about quantum mechanics concern the collection of commutative subalgebras, too.

Notably, one may wonder about the evident strengthening of the notion of quasi-states to that of a map ρ:A→ℂ\rho : A \to \mathbb{C} which is not just linear but also an algebra homomorphism on each commuting subalgebra. Notice that by Gelfand duality every commutative C *C^\ast-algebra CC is the algebra of continuous functions on some topological space sp(C)sp(C). Under this duality a state on CC is a probability distribution on sp(C)sp(C), while an algebra homomorphism C→ℂC \to \mathbb{C} is a point of X CX_C. Therefore a quasi-state which is commutative-subalgebra-wise an algebra homomorphism may be thought of as encoding a collection of precise numerical values (as opposed to just expectation values) of all possible observables. Such a hypothetical quasi-state is sometimes called a collection of hidden variables of the quantum mechanical system: its existence would mean that despite the apparent probabilistic nature of quantum mechanics, there are “hidden” non-probabilistic states. But there are not. This is the statement of the Kochen-Specker theorem: under precisely the assumptions that make Gleason's theorem work, there is no quasi-state which is commutative-subalgebra-wise an algebra homomorphism.

In summary, this means:

The foundational characteristics of quantum physics are encoded in notions of functions on the algebra of observables AA which are linear and positive only commutative-subalgebra-wise .

Since the notion of commutative-subalgebra-wise homomorphism is at the heart of quantum physics, it seems worthwhile to consider natural formalizations of this notion. There is indeed a very natural and general abstract one: whenever any notion of function is defined only locally it is natural to consider the sheaf of such functions over all possible local patches.

The historically motivating example, and possibly still the most widely familiar one, is that of holomorphic functions on a complex manifold: there are in general very few holomorphic functions defined over all of a complex manifold, but plenty of them defined over any small enough subset. And it is of fundamental interest to consider the collections of holomorphic functions over each such subset, and how these restrict to each other under restriction of subsets. This collection of local data is a sheaf of functions on the complex manifold.

There is an evident analog setup of this situation that applies in the present case of interest, that of functions defined on commutative subalgebras:

For AA any C-star algebra, write 𝒞(A)\mathcal{C}(A) for the set of all its commutative C *C^\ast-subalgebras. This is naturally a poset under inclusion of subalgebras. A (co)presheaf of this set is a functor 𝒞(A)→Set\mathcal{C}(A) \to Set. Any such functor we may think of as a collection of commutative-subalgebra-wise data on AA, consistent with restriction of subalgebras. The collection of all such functors – which we write [𝒞(A),Set][\mathcal{C}(A), Set] – is a category called a presheaf topos.

Inside this topos, all the above discussion of foundations of quantum mechanics finds a natural simple equivalent reformulation:

First of all, the non-commutative C *C^\ast-algebra AA naturally induces a commutative C-star algebra object A̲\underline{A} internal to [𝒞(A),Set][\mathcal{C}(A), Set]: namely the copresheaf defined by the tautological assignment

A̲:(C∈𝒞(A))↦C. \underline{A} : (C \in \mathcal{C}(A)) \mapsto C \,.

In words this means nothing but that the collection of all commutative subalgebras of AA may naturally be regarded as a single commutative C *C^\ast-algebra internal to the topos [𝒞(A),Set][\mathcal{C}(A), Set].

Below we shall discuss (here) that in a precise sense this commutative internal A̲\underline{A} captures precisely all the kinematical information encoded in the quantum mechanical system of AA – everything related to states and observables but not information about (time) evolution. So everything we have discussed so far.

The pair of these two ingredients

Bohr(A):=([𝒞(A),Set],A̲) Bohr(A) := ([\mathcal{C}(A), Set], \underline{A})

constitutes what is called a ringed topos – a special case of the notion of a locally ringed topos. This notion is a fundamental notion for generalized spaces in higher geometry. The most advanced general theory of higher geometry (Lurie09) is based on modelling spaces as ringed toposes.

We shall call this ringed topos the Bohr topos of AA.

This terminology is meant to indicate that one may think of this construction as formalizing faithfully and usefully a heuristic that has been emphasized by Nils Niels Bohr – one of the founding fathers of quantum mechanics – and is known as the doctrine of classical concepts (Scheibe 1973) in quantum mechanics. This states that nonclassical/noncommutative as the logic/geometry of quantum mechanics may be, it is to be probed and detected by classical/commutative logic/geometry.

Namely in terms of the Bohr topos we have the following equivalent reformulations of the foundational facts about quantum physics discussed above, now internally in Bohr(A)Bohr(A).

and

Non-existence of hidden quantum variables. The Gelfand spectrum sp(A̲)sp(\underline{A}) of A̲\underline{A} internal to the Bohr topos has no global point. (Kochen-Specker).

These two statements might be taken as suggesting that a quantum mechanical system AA is naturally regarded in terms of its Bohr topos Bohr(A)Bohr(A) – somewhat more naturally than as a C *C^\ast-algebra AA. (The second, in a slightly different setup, was emphasized in IshamHamiltonButterfield, which inspired all of the following discussion, the first in Spitters). In fact, thinking of ringed toposes as generalized spaces in higher geometry, it suggests that the Bohr topos Bohr(A)Bohr(A) itself is the quantum phase space of the quantum mechanical system in question.

To which extent this perspective is genuinely useful is maybe still to be established. For pointers to the literature see the references below. Discussion along the above lines may suggest that this perspective is indeed useful, but what is probably still missing is a statement about quantum physics that can be formulated and proven in terms of Bohr toposes, while being hardly conceivable or at least more unnatural without. It is probably currently not clear if such statements have been found.

One potential such statement has been suggested in (Nuiten 11) after discussion with Spitters:

In the formalization of quantum field theory by the Haag-Kastler axioms – called AQFT – every quantum field theory is entirely encoded in terms of its local net of observables over spacetime XX. This is a copresheaf of C-star algebras

A:Op(X)→C *Alg A : Op(X) \to C^\ast Alg

which assigns to every open subset U⊂XU \subset X of spacetime the quantum subsystem A UA_U of quantum fields supported in that region. By the above, we may consider for each of these quantum systems their associated quantum phase spaces given by the correspondong Bohr toposes Bohr(A U)Bohr(A_U). This yields a presheaf

Bohr(A):Op(X) op→C com *TopSpace↪C com *Topos Bohr(A) : Op(X)^{op} \to C^\ast_{com} TopSpace \hookrightarrow C^\ast_{com}Topos

of ringed toposes whose internal ring object has the structure of a commutative C *C^\ast-algebra. With the copresheaf thus turned into a presheaf it is natural to ask under which conditions this is a sheaf: under which conditions this presheaf satisfies descent.

In (Nuiten 11) the following is observed: if AA satisfies what is called the split property (a strong form of the time slice axiom) then the Bohr-presheaf of quantum phase spaces satisfies spatial descent by local geometric morphisms precisely if the original copresheaf of observables A:Op(X)→C *AlgA : Op(X) \to C^\ast Alg is indeed local – spatially and causally. So this means that a natural property of quantum physics – spatial and causal locality – corresponds from the perspective of Bohr toposes to a natural property of presheaves of quantum phase spaces: descent.

One can probably view this as further suggestive evidence that indeed quantum physics is naturally regarded from the point of view of the Bohr topos. But for seeing where this perspective is headed, it seems that more insights along these lines would be useful.

Outline

The discussion below proceeds in the following steps (following (Nuiten 11))

1. Bohr topos of a quantum mechanics system

   This discusses the Bohr topos incarnation of a quantum mechanical system – the topos-theoretic quantum phase space – and its functoriality.

2. Kinematics in a Bohr topos

   This discusses how the classical kinematics internal to a Bohr topos is the external quantum kinematics of the underlying quantum mechanical system.

3. (Pre-)Sheaf of Bohr toposes of a quantum field theory

   This discusses how the presheaves of Bohr toposes obtained by applying Bohrification to a local net of observables of a quantum field theory.

Bohr topos of a quantum mechanical system

We discuss the definitions and some basic properties of Bohr toposes: certain ringed toposes – in fact ringed spaces – associated with any (possibly non-commutative) algebra. We formulate the construction for C-star algebras, since this is the standard model for quantum mechanical systems, but actually much of it does not depend on either the topology or the star-algebra structure until we come to the discussion of the kinematics in a Bohr topos below.

C *C^*-algebras

This appears as (vdBergHeunen, def. 11,12).

The Bohr site

Notice the following fact about Alexandroff spaces:

This appears as (Nuiten 11, lemma 2.6).

The Bohr topos

The general notion of morphisms between toposes are geometric morphisms. But those that remember the morphisms of Bohr sites are essential geometric morphisms.

The next proposition asserts that all essential geometric morphisms between Bohr toposes arise this way:

For such essential geometric morphisms to be parts of morphisms in C *ToposC^\ast Topos, def. 9 we need that their inverse images respect internal C *C^\ast-algebras:

Using this we now discuss morphisms of Bohr toposes in C *ToposC^\ast Topos.

This is (Nuiten 11, lemma 2.7). (Essentially this argument also appears as (vdBergHeunen, prop. 33), where however the extra right adjoint is not made use of and instead the variances of the morphisms involved in the definition of C *ToposC^\ast Topos are redefined in order to make the statement come out.)

Using prop. 4 the above prop. 6 has the following partial converse.

Kinematics in a Bohr topos

A key aspect of the Bohr topos Bohr(A)Bohr(A) of a quantum mechanical system (as defined above) is that that classical kinematics and classical probability theory of the commutative internal C *C^*-algebra A̲∈Bohr(A)\underline{A} \in Bohr(A) is the quantum kinematics and quantum probability theory of AA.

In fact, the very definition of Bohr(A)Bohr(A) provides a formal context in which Gleason's theorem has a natural formulation:

A function that preserves certain structure locally – here: over each commutative subalgebra – is precisely an internal fully structure preserving homomorphism in the presheaf topos over these local objects – here: over commutative subalgebras. Hence we have the following direct topos-theoretic equivalent reformulation of Gleason’s theorem.

The phase space

The idea is that for A∈C *AlgA \in C^* Alg, the Bohr topos Bohr(A)=(Sh(Alex(𝒞(A))),A̲)∈C *TopSpace⊂C *ToposBohr(A) = (Sh(Alex(\mathcal{C}(A))), \underline{A}) \in C^* TopSpace \subset C^* Topos is the corresponding quantum phase space. More precisely, we may think of the internal commutative C *C^*-algebra A̲∈Bohr(A)\underline{A} \in Bohr(A) as the formal dual to the quantum phase space.

The following discussion makes this precise by exhibiting this formal dual as an internal locale. Since Bohr(A)Bohr(A) is a spatial topos, this internal locale is in fact an ordinary topological space bundle Σ→Alex𝒞(A)\Sigma \to Alex \mathcal{C}(A) over the Alexandroff space Alex𝒞(A)Alex \mathcal{C}(A).

This appears as (HLSW, theorem 1).

This is highlighted in (vdBergHeunen).

This is (Spitters06, theorem 9, corollary 10).

This is effectively the functoriality of the internal constructive Gelfand duality applied to the above observation. The statement appears as (vdBergHeunen, theorem 35).

The observables

For A∈C *AlgA \in C^\ast Alg a C*-algebra, then in quantum physics a self-adjoint operator a∈Aa \in A is a quantum observable. The following statement asserts that quantum observables on AA are in a precise sense the ℝ\mathbb{R}-valued “functions” on the Bohr topos of AA.

Write C(ℝ)C(\mathbb{R}) for the C *C^\ast-algebra of continuous complex functions on the real line. We think of Bohr(C(ℝ))Bohr(C(\mathbb{R})) as the incarnation of ℝ\mathbb{R} in the context of Bohr toposes.

The states

(…)

(Pre-)Sheaf of Bohr toposes of a quantum field theory

Notice that in the context of AQFT a quantum field theory is encoded by a local net of C-star algebras on spacetime.

This appears as (Nuiten 11, def. 17).

Assume that a net of observables A:Op(X)→C *Alg incA : Op(X) \to C^\ast Alg_{inc} satisfies the split property. Then it is precisely if the corresponding presheaf of Bohr toposes Bohr(A):Op(X) op→AC *Alg inc→BohrC *TopBohr(A) : Op(X)^{op} \stackrel{A}{\to} C^\ast Alg_{inc} \stackrel{Bohr}{\to} C^\ast Top satisfies spatial descent by local geometric morphisms (meaning that for every spatial hyperslice Σ⊂X\Sigma \subset X the induced presheaf Bohr(A)| Σ:Op(Σ) op→C *TopBohr(A)|_\Sigma : Op(\Sigma)^{op} \to C^\ast Top satisfies descent by local geometric morphisms.)

(…)

This appears as (Nuiten 11, theorem 4.2).

Contravariant functors on open subsets

Above is discussed the notion of Bohr topos given by covariant functors on the poset of commutative subalgebras of a C*-algebra. The fact that the functors here are covariant is related to the fact that the algebra itself naturally exists inside the presheaf topos.

Alternatively one can explore the situation for contravariant functors on the poset of commutative subalgebras (Isham-Döring 07). The resulting presheaf topos then does not directly contain the given C *C^\ast-algebra, but by Gelfand duality, does directly contain an internal locale which is its Gelfand spectrum. This is called the “spectral presheaf”.

• quantum logic

quantum probability theory – observables and states

• states

  • classical state

  • quantum state

    • space of states (in geometric quantization)

    • state on a star-algebra, quasi-state

    • qbit, Bell state

      dimer, tensor network state

    • quantum state preparation

    • probability amplitude, quantum fluctuation

    • pure state

      • wave function

      • bra-ket

      • Bell state

    • quantum superposition, quantum interference

    • quantum entanglement

    • quantum measurement

      wave function collapse

      Born rule

      deferred measurement principle

      quantum reader monad

      measurement problem

    • superselection sector

    • mixed state, density matrix

      entanglement entropy

      holographic entanglement entropy

    • coherent quantum state

  • ground state, excited state

  • quasi-free state

  • Fock space, second quantization

  • vacuum, vacuum state

    • Hadamard state

    • vacuum diagram

    • vacuum expectation value, vacuum amplitude, vacuum fluctuation

    • vacuum energy

    • vacuum polarization

    • interacting vacuum

    • thermal vacuum, KMS state

    • vacuum stability

      • false vacuum, tachyon, Coleman-De Luccia instanton

    • theta vacuum

    • perturbative string theory vacuum

      • non-geometric string theory vacuum

      • landscape of string theory vacua

  • entangled state

  • tensor network state

    • matrix product state

    • tree tensor network state

• observables

  • quantum observable, beable

  • algebra of observables, star-algebra

  • Bohr topos

  • quantum operator (in geometric quantization)

  • quantum operation, quantum effect, effect algebra

  • in quantum field theory

    • local observable

    • polynomial observable

      • linear observable

        • field observable

    • regular observable

    • microcausal observable

    • normal-ordered product, time-ordered products, retarded product

    • Wick algebra

    • scattering amplitude

    • interacting field algebra of observables, Bogoliubov's formula

• GNS construction

• theorems

  • order-theoretic structure in quantum mechanics

    • Gleason's theorem

    • Alfsen-Shultz theorem

    • Harding-Döring-Hamhalter theorem

    • Kochen-Specker theorem

    • Nuiten's lemma

  • Wigner's theorem

  • no-cloning theorem

  • Bell's theorem

References

The assertion by Bohr that all experiments in quantum mechanics must be possible to describe in “classical terms” is in

• Nils Niels Bohr, Discussion with Einstein on Epistemological Problems in Atomic Physics, in: P. A. Schilpp (ed.) Albert Einstein, Philosopher-Scientist (Evanston: Library of Living Philosophers) (1949) 201-241 []

  “however far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms.”

Niels Bohr’s views on quantum mechanics that give the construction of Bohrification its name are reviewed further in

• Erhard Scheibe, Bohr’s interpretation of quantum mechanics, Chapter I in: The logical analysis of quantum mechanics, Oxford: Pergamon Press (1973)

For more see at interpretation of quantum mechanics the section Bohr’s standpoint.

Maybe the first article to propose to use intuitionistic logic/topos theory for the description of quantum physics is

• Murray Adelman, John Corbett, A Sheaf Model for Intuitionistic Quantum Mechanics, Appl. Cat. Struct. 3 (1995) 79-104 [[doi:10.1007/BF00872949](https://doi.org/10.1007/BF00872949)]

The term Bohrification and the investigations associated with it were initiated in

• Chris Heunen, Klaas Landsman, Bas Spitters, Bohrification of operator algebras and quantum logic, Synthese 186 3 (2012) 719-752 [[arXiv:0905.2275](http://arxiv.org/abs/0905.2275), doi;10.1007/s11229-011-9918-4]

see also related comments in disucssion of the Born rule here:

• Klaas Landsman, The Born rule and its interpretation, in: Compendium of Quantum Physics, Springer (2009) 64-70 [[doi:10.1007/978-3-540-70626-7_20](https://doi.org/10.1007/978-3-540-70626-7_20), pdf, pdf]

See also:

• Chris Heunen, Klaas Landsman, Bas Spitters, Bohrification, in: Deep Beauty – Understanding the Quantum World through Mathematical Innovation, Cambridge University Press (2009) 271-314 [[arXiv:0909.3468](http://arxiv.org/abs/0909.3468), doi:10.1017/CBO9780511976971.008]

The computation of the internal Gelfand spectrum Σ̲\underline{\Sigma} was initiated in

• Chris Heunen, Klaas Landsman, Bas Spitters A topos for algebraic quantum theory, Comm. Math. Phys. 291 (2009) 63-110 [[arXiv:0709.4364](http://arxiv.org/abs/0709.4364), doi:10.1007/s00220-009-0865-6]

with some results in section 5 and 6 of

• Martijn Caspers, Gelfand spectra of C *C^*-algebras in topos theory, MSc thesis, Nijmegen (2008) [[pdf](http://www.math.ru.nl/~landsman/scriptieMartijn.pdf), pdf]

and completed in

• Sander Wolters, Contravariant vs covariant quantum logic: A comparison of two topos-theoretic approaches to quantum theory, Comm. Math. Phys. 317 (2013) 3–53 [[arXiv:1010.2031](http://arxiv.org/abs/1010.2031), doi:10.1007/s00220-012-1652-3]

An outline of the full proof is given in

• Chris Heunen, Klaas Landsman, Bas Spitters, Sander Wolters, The Gelfand spectrum of a noncommutative C *C^\ast-algebra: a topos-theoretic approach, Journal of the Australian Mathematical Society 90 (2011) 39-52 [[arxiv:1010.2050](http://arxiv.org/abs/1010.2050), doi:10.1017/S1446788711001157]

Applications and examples for AA a matrix algebra are discussed in

• Martijn Caspers, Chris Heunen, Klaas Landsman, Bas Spitters, Intuitionistic Quantum Logic of an nn-level System, Foundations of Physics 39 7 (2009) 731-759 [[arXiv:0902.3201](https://arxiv.org/abs/0902.3201), doi:10.1007/s10701-009-9308-7]

The functoriality of Bohrification is observed in

• Benno van den Berg, Chris Heunen, Noncommutativity as a colimit, Applied Categorical Structures 20 4 (2012) 393-414 [[arXiv:1003.3618](http://arxiv.org/abs/1003.3618), doi:10.1007/s10485-011-9246-3]

The application of the double negation topology to make Bohrification coincide with ordinary Gelfand duality on commutative C *C^*-algebras is discussed in

• Bas Spitters, The space of measurement outcomes as a spectrum for non-commutative algebras, EPTCS 26 (2010) 127-133 [[arXiv:1006.1432](http://arxiv.org/abs/1006.1432), doi:10.4204/EPTCS.26.12]

The generalization of Bohrification from quantum mechanics to quantum field theory (AQFT):

• Joost Nuiten, Bohrification of local nets of observables, in: Proceedings of QPL 2011, EPTCS 95, 2012 (2012) 211-218 [[arXiv:1109.1397](http://arxiv.org/abs/1109.1397), doi:10.4204/EPTCS.95.15]

Review:

• Klaas Landsman, Topos theory and quantum logic, Section 12 of: Foundations of quantum theory – From classical concepts to Operator algebras, Springer Open (2017) [[doi:10.1007/978-3-319-51777-3](https://link.springer.com/book/10.1007/978-3-319-51777-3), pdf]

The original suggestion to interpret the Kochen-Specker theorem in the topos over the poset of commutative subalgebras (there taken to be presheaves instead of copresheaves) is due to

• Jeremy Butterfield, John Hamilton, Chris Isham, A topos perspective on the Kochen-Specker theorem, I. quantum states as generalized valuations, Internat. J. Theoret. Phys. 37 11 (1998) 2669-2733 [[doi:10.1023/A:1026680806775](http://dx.doi.org/10.1023/A:1026680806775), arXiv:quant-ph/9803055, MR2000c:81027]

  II. conceptual aspects and classical analogues Int. J. of Theor. Phys. 38 3 (1999) 827-859 [[MR2000f:81012](http://www.ams.org/mathscinet-getitem?mr=1697983), doi:10.1023/A:1026652817988]

  III. Von Neumann algebras as the base category, Int. J. of Theor. Phys. 39 6 (2000) 1413-1436 [[arXiv:quant-ph/9911020](http://arxiv.org/abs/quant-ph/9911020), MR2001k:81016,doi:10.1023/A:1003667607842]

  IV. Interval valuations, Internat. J. Theoret. Phys. 41 4 (2002) 613-639 [[MR2003g:81009](http://www.ams.org/mathscinet-getitem?mr=1902067), doi]

with some review and outlook in

• Chris Isham, Jeremy Butterfield, Some Possible Roles for Topos Theory in Quantum Theory and Quantum Gravity, Found. Phys. 30 (2000) 1707-1735 [[arXiv:gr-qc/9910005](https://arxiv.org/abs/gr-qc/9910005), doi:10.1023/A:1026406502316]

and then

• Andreas Döring, Chris Isham, A Topos Foundation for Theories of Physics

  I. Formal Languages for Physics, J. Math. Phys. 49 (2008) 053515 [[arXiv:quant-ph/0703060](http://arxiv.org/abs/quant-ph/0703060), doi:10.1063/1.2883740]

  II. Daseinisation and the Liberation of Quantum Theory, J. Math. Phys. 49 (2008) 053516 [[arXiv:quant-ph/0703062](http://arxiv.org/abs/quant-ph/0703062), doi:10.1063/1.2883742]

  III. The Representation of Physical Quantities With Arrows, J. Math. Phys. 49 (2008) 053517 [[arXiv:quant-ph/0703064](https://arxiv.org/abs/quant-ph/0703064), doi:10.1063/1.2883777]

  IV. Categories of Systems, J. Math. Phys. 49 (2008) 053518 [[arXiv:quant-ph/0703066](http://arxiv.org/abs/quant-ph/0703066), doi:10.1063/1.2883826]

Further development in:

• John Harding, Chris Heunen, Topos quantum theory with short posets, Order 38 111–125 (2021) [[arXiv:1903.01897](https://arxiv.org/abs/1903.01897), doi:10.1007/s11083-020-09531-6]

Discussion of aspects of the process of quantization in terms of Bohr toposes is in

• Kunji Nakayama, Sheaves in Quantum Topos Induced by Quantization [[arXiv:1109.1192](http://arxiv.org/abs/1109.1192)]

A variant of the Bohr topos construction meant to take more of the topology of the underlying C *C^\ast-algebra into account has been suggested for finite-dimensional C *C^\ast-algebra in

• Guillaume Raynaud, Fibred contextual quantum physics, PhD thesis, University of Birmingham, 2014. [[web](http://www.cs.bham.ac.uk/~sjv/grpage.php)]

and generalized to arbitrary C *C^\ast-algebras in

• Simon Henry, A Geometric Bohr topos [[arXiv:1502.01896](https://arxiv.org/abs/1502.01896)]