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--- projects:quantum:category-qc-foundation [2025/04/16 15:30] – kymki
+++ projects:quantum:category-qc-foundation [2025/04/30 09:26] (current) – [Emmanuel Jeandel, Simon Perdrix & Renaud Vilmart (2020)] kymki
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+      <div class="tooltip">This post is very close to being completely finished. Not that rambly at all.</div>
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 ===== Categorical Quantum Computing Series: Foundational Papers =====
+==== Introduction ====
 Below is a series of papers that have been setting the direction of category theory applied to quantum computing. The overview provides an easy reading that can give you (the reader) an idea of why you would want to read any of the chosen publications. The papers were selected in terms of general "impact" (not really clearly defined measure, but how formative the texts were for the field). If you have papers that you would like to see in this list, please do contact me.
+Early works by Abramsky, Coecke, and others established the core semantic framework (dagger compact categories) and demonstrated its relevance by reconstructing quantum protocols and no-cloning within that abstract setting. The introduction of classical structures and CPM by 2007–2008 extended the framework to encompass measurements, mixed states, and classical data, enabling a more realistic modeling of quantum algorithms and protocols. Around 2010–2011, the focus shifted to powerful graphical calculi (like ZX) and to connecting the categorical approach with other fields (via expository works and textbook-style treatments). The 2010s saw proofs of completeness (Selinger 2012) and unification of classical/quantum channels (CP* in 2014), showing the framework is mathematically robust and comprehensive. Most recently, achievements like the completeness of the ZX-calculus (2020) illustrate how far the field has come – from abstract theory to practical diagrammatic reasoning that fully captures quantum computing.
 ==== Samson Abramsky & Bob Coecke (2004) ====
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 This theoretical result addresses the completeness of the categorical axiomatics with respect to standard quantum mechanics. Selinger proved that any equation between morphisms that holds in all finite-dimensional Hilbert spaces (the usual semantics of quantum computing) can already be derived from the abstract axioms of a dagger compact closed category. In other words, the category **FdHilb** (finite-dimensional Hilbert spaces with linear maps) is a complete model of the dagger compact closed category axioms – there are no “extra” equations in Hilbert space beyond those provable from the categorical framework. This paper ensures that the diagrammatic reasoning developed in categorical quantum mechanics is complete: if two quantum processes are equal (as linear maps), their equality can be proven using only the categorical axioms and graph transformations. Completeness was a non-trivial property to establish and gives strong validation that the chosen axioms fully capture finite-dimensional quantum theory. This work thus solidifies the foundational footing of the entire approach, confirming that one does not lose anything by working abstractly – the categorical semantics is equivalent to the usual Hilbert space semantics for all practical purposes.
-. **Bob Coecke, Chris Heunen & Aleks Kissinger (2014)** – *“Categories of Quantum and Classical Channels.”* **Quantum Inf. Process. 13**(11): 2567–2609, 2014. [ArXiv:1408.0049]. This paper introduced the **CP***-construction, a significant generalization of Selinger’s CPM construction to incorporate both quantum and classical information in one category ([](https://arxiv.org/pdf/1408.0049#:~:text=We%20introduce%20the%20CP,%E2%80%93construction%20yields)) ([](https://arxiv.org/pdf/1408.0049#:~:text=finite,elegant%20abstract%20notions%20of%20preparation)). The authors build a unified categorical framework where objects can represent **hybrid classical-quantum systems** (interpreted as abstract C*-algebras), and morphisms represent **completely positive maps** that may carry classical data as well ([](https://arxiv.org/pdf/1408.0049#:~:text=Selinger%E2%80%99s%20CPM%E2%80%93construction,algebras%20and%20completely%20positive%20maps)) ([](https://arxiv.org/pdf/1408.0049#:~:text=finite,elegant%20abstract%20notions%20of%20preparation)). The CP* construction fully embeds the pure quantum CPM category as a special case while also embedding the category of classical stochastic processes, thereby allowing classical and quantum channels to coexist and interact in a single symmetric monoidal category ([](https://arxiv.org/pdf/1408.0049#:~:text=finite,elegant%20abstract%20notions%20of%20preparation)). This yields elegant abstract notions of **state preparation, measurement, and conditional dynamics** within one compositional model ([](https://arxiv.org/pdf/1408.0049#:~:text=the%20objects%20in%20the%20image,and%20more%20general%20quantum%20channels)) ([](https://arxiv.org/pdf/1408.0049#:~:text=classical%E2%80%9D%20state%20spaces,and%20more%20general%20quantum%20channels)). In essence, CP* provides a categorical semantics for quantum systems with classical outcomes or controls – a “von Neumann style” category of quantum **observables and channels** as opposed to purely unitary evolutions ([](https://arxiv.org/pdf/1408.0049#:~:text=dagger%20compact,a%20%E2%80%9Cvon%20Neumann%20style%20category%E2%80%9D)) ([](https://arxiv.org/pdf/1408.0049#:~:text=by%20considering%20the%20C%2A,corresponding%20to%20the%20entire%20state)). This work has been influential in areas like quantum foundations and quantum protocols, as it places classical-quantum interaction on equal footing and has inspired further developments in categorical probability and **operational theories**.
+==== Bob Coecke, Chris Heunen & Aleks Kissinger (2014) ====
+[[https://arxiv.org/pdf/1408.0049|Categories of Quantum and Classical Channels]]
+This paper introduced the **CP***-construction, a significant generalization of Selinger’s CPM construction to incorporate both quantum and classical information in one category. The authors build a unified categorical framework where objects can represent hybrid classical-quantum systems (interpreted as abstract C*-algebras), and morphisms represent completely positive maps that may carry classical data as well. The CP* construction fully embeds the pure quantum CPM category as a special case while also embedding the category of classical stochastic processes, thereby allowing classical and quantum channels to coexist and interact in a single symmetric monoidal category. This yields elegant abstract notions of state preparation, measurement, and conditional dynamics within one compositional model. In essence, CP* provides a categorical semantics for quantum systems with classical outcomes or controls – a “von Neumann style” category of quantum observables and channels as opposed to purely unitary evolutions. This work has been influential in areas like quantum foundations and quantum protocols, as it places classical-quantum interaction on equal footing and has inspired further developments in categorical probability and operational theories.
+==== John Baez & Mike Stay (2010) ====
+[[https://arxiv.org/abs/0903.0340|Physics, Topology, Logic and Computation: A Rosetta Stone]] In **New Structures for Physics** (eds. B. Coecke), Lecture Notes in Physics 813, pp. 95–172. [ArXiv:0903.0340]. This widely cited exposition connected the categorical quantum computing program with broader areas, helping to spread these ideas. Baez and Stay survey how **closed symmetric monoidal categories** and their diagrammatic calculus provide a common language for quantum physics, logic, and computation. In particular, they explain Abramsky and Coecke’s categorical quantum mechanics in a tutorial style, drawing an analogy between Feynman diagrams, cobordisms in topology, and proof nets in logic. The paper does not introduce new theorems specific to quantum computing, but it synthesizes many concepts (traced monoidal categories, dagger compact categories, etc.) in an accessible way. By acting as a “Rosetta Stone,” it helped researchers in various fields understand the categorical approach to quantum theory and recognize its significance. This work thus contributed to the evolution of the field by broadening its audience and establishing a clear conceptual bridge between category theory and quantum computation, influencing education and further interdisciplinary research.
-. **John Baez & Mike Stay (2010)** – *“Physics, Topology, Logic and Computation: A Rosetta Stone.”* In **New Structures for Physics** (eds. B. Coecke), Lecture Notes in Physics 813, pp. 95–172. [ArXiv:0903.0340]. This widely cited exposition connected the categorical quantum computing program with broader areas, helping to spread these ideas. Baez and Stay survey how **closed symmetric monoidal categories** and their diagrammatic calculus provide a common language for quantum physics, logic, and computation ([[0903.0340] Physics, Topology, Logic and Computation: A Rosetta Stone](https://arxiv.org/abs/0903.0340#:~:text=,closed%20symmetric)). In particular, they explain Abramsky and Coecke’s categorical quantum mechanics in a tutorial style, drawing an analogy between **Feynman diagrams**, **cobordisms in topology**, and **proof nets in logic** ([[0903.0340] Physics, Topology, Logic and Computation: A Rosetta Stone](https://arxiv.org/abs/0903.0340#:~:text=,closed%20symmetric)). The paper does not introduce new theorems specific to quantum computing, but it synthesizes many concepts (traced monoidal categories, dagger compact categories, etc.) in an accessible way. By acting as a “Rosetta Stone,” it helped researchers in various fields understand the categorical approach to quantum theory and recognize its significance. This work thus contributed to the *evolution of the field* by broadening its audience and establishing a clear conceptual bridge between category theory and quantum computation, influencing education and further interdisciplinary research.
+==== Emmanuel Jeandel, Simon Perdrix & Renaud Vilmart (2020) ====
-. **Emmanuel Jeandel, Simon Perdrix & Renaud Vilmart (2020)** – *“Completeness of the ZX-Calculus.”* **Logical Methods in Comp. Sci. 16**(2:11):1–72. [ArXiv:1903.06035]. This recent paper resolves a long-standing open question by proving the **ZX-calculus is complete for all pure qubit quantum mechanics** ([Completeness of the ZX-Calculus](https://lmcs.episciences.org/6532/pdf#:~:text=completeness%2C%20which%20roughly%20ensures%20the,provided%20axiomatisation%20is%20complete%20for)). The ZX-calculus (introduced by Coecke & Duncan in 2011) was known to be *universal* but not fully complete (certain true quantum equations had no diagrammatic proof using the original rules). Jeandel *et al.* provided the first complete axiomatisation: they show that any equality of linear maps (matrices) on qubits can be derived using ZX-diagram rewrite rules ([Completeness of the ZX-Calculus](https://lmcs.episciences.org/6532/pdf#:~:text=equational%20presentation,Thanks%20to%20a)). The proof proceeds by first enriching the ZX-calculus with additional rewrite rules to make it complete for the Clifford+T fragment (a universal gate set) ([Completeness of the ZX-Calculus](https://lmcs.episciences.org/6532/pdf#:~:text=improve%20on%20the%20known,of%20the%20language%2C%20namely%20Clifford%2BT)), and then leveraging a translation to a related graphical language (the ZW-calculus) to reach completeness for arbitrary quantum operations ([Completeness of the ZX-Calculus](https://lmcs.episciences.org/6532/pdf#:~:text=Thanks%20to%20a%20system%20of,Clifford%2BT%2C%20to%20a%20class%20of)). The result implies that the diagrammatic calculus now fully matches the expressive power of matrix mathematics – **if two quantum circuits are mathematically equivalent, the ZX-calculus can prove it**. This accomplishment marks a maturation of the categorical approach: the diagrammatic methods are not only convenient and intuitive but also **no less powerful than algebraic methods**. It has spurred further research and practical tools in quantum circuit optimization, equipping the field with a robust graphical proof technique for quantum computing theory.
+ [[https://lmcs.episciences.org/6532/pdf|Completeness of the ZX-Calculus]]
-**Evolution of the Field:** These papers collectively chart the development of categorical quantum computing from its inception (2004) to its modern advancements (2020). Early works by Abramsky, Coecke, and others established the core semantic framework (dagger compact categories) and demonstrated its relevance by reconstructing quantum protocols and no-cloning within that abstract setting. The introduction of classical structures and CPM by 2007–2008 extended the framework to encompass measurements, mixed states, and classical data, enabling a more realistic modeling of quantum algorithms and protocols. Around 2010–2011, the focus shifted to powerful **graphical calculi** (like ZX) and to connecting the categorical approach with other fields (via expository works and textbook-style treatments). The 2010s saw proofs of completeness (Selinger 2012) and unification of classical/quantum channels (CP* in 2014), showing the framework is mathematically robust and comprehensive. Most recently, achievements like the completeness of the ZX-calculus (2020) illustrate how far the field has come – from abstract theory to practical diagrammatic reasoning that fully captures quantum computing. Each of these key papers has played an influential role in building the **theoretical foundation of categorical quantum computing**, and together they provide a coherent picture of the field’s evolution.
+This recent paper resolves a long-standing open question by proving the ZX-calculus is complete for all pure qubit quantum mechanics. The ZX-calculus (introduced by Coecke & Duncan in 2011) was known to be universal but not fully complete (certain true quantum equations had no diagrammatic proof using the original rules). Jeandel et al. provided the first complete axiomatisation: they show that any equality of linear maps (matrices) on qubits can be derived using ZX-diagram rewrite rules. The proof proceeds by first enriching the ZX-calculus with additional rewrite rules to make it complete for the Clifford+T fragment (a universal gate set), and then leveraging a translation to a related graphical language (the ZW-calculus) to reach completeness for arbitrary quantum operations. The result implies that the diagrammatic calculus now fully matches the expressive power of matrix mathematics – if two quantum circuits are mathematically equivalent, the ZX-calculus can prove it. This accomplishment marks a maturation of the categorical approach: the diagrammatic methods are not only convenient and intuitive but also no less powerful than algebraic methods. It has generated further research and practical tools in quantum circuit optimization, now with graphical proof technique for quantum computing theory.