The stabiliser fragment of quantum theory is a foundational building block for quantum error correction and the fault-tolerant compilation of quantum programs. In this article, we develop a sound, universal and complete denotational semantics for stabiliser operations which include measurement, classically-controlled Pauli operators, and affine classical operations, in which quantum error-correcting codes are first-class objects. The operations are interpreted as certain affine relations over finite fields. This offers a conceptually motivated and computationally-tractable alternative to the standard operator-algebraic semantics of quantum programs (whose time complexity grows exponentially as the state space increases in size). We demonstrate the power of the resulting semantics by describing a small, proof-of-concept assembly language for stabiliser programs with fully-abstract denotational semantics.
The stabiliser fragment of quantum theory is a foundational building block for quantum error correction and the fault-tolerant compilation of quantum programs. In this article, we develop a sound, universal and complete denotational semantics for stabiliser operations which include measurement, classically-controlled Pauli operators, and affine classical operations, in which quantum error-correcting codes are first-class objects. The operations are interpreted as certain affine relations over finite fields. This offers a conceptually motivated and computationally-tractable alternative to the standard operator-algebraic semantics of quantum programs (whose time complexity grows exponentially as the state space increases in size). We demonstrate the power of the resulting semantics by describing a small, proof-of-concept assembly language for stabiliser programs with fully-abstract denotational semantics.
The stabiliser fragment of quantum theory is a foundational building block for quantum error correction, and hence for the fault-tolerant compilation of quantum programs. In this article, we develop a sound, universal, and complete denotational semantics for stabiliser operations, including measurement, classically controlled Pauli operators, and affine classical computation, thereby supporting an explicit treatment of quantum error-correcting codes. We interpret stabiliser operations as affine relations over finite fields, yielding a semantics that reflects the algebraic structure underlying stabiliser quantum error correction. Because stabiliser quantum mechanics has a well-behaved algebraic structure, our relational semantics is conceptually transparent and computationally tractable when compared to standard denotational models for general quantum programs. We demonstrate the resulting semantics by describing a small, low-level assembly language for stabiliser programs with fully abstract denotational semantics.
Qubits are a great way to build a quantum computer, but a limited way to program one. We replace the usual "states and gates" formalism with a "props and ops" (propositions and operators) model in which (a) the C*-algebra of observables supplies the syntax; (b) states, viewed as linear functionals, give the semantics; and (c) a novel diagrammatic calculus unifies the two. The first part develops the basic objects of the framework, encoding consistent patterns of operator correlation, recovering Hilbert space via the GNS construction, and re-deriving the Bloch sphere as the set of all consistent correlations of operators in the Pauli algebra. We then turn to intervention, showing how measurement modifies state, proving an operator-algebraic version of the Knill-Laflamme conditions, and expressing stabilizer codes with the same diagrammatic machinery. This provides a concise, representation-agnostic account of quantum error correction. The result is a self-contained foundation in which C*-algebras, and their dual Hilbert spaces, offer a rich and universal substrate for quantum programming; forthcoming papers will build a high-level language and quantum software applications on top of this substrate.
Quantum computers are expected to have advantages over classical computers in solving a range of high impact problems, but they are highly susceptible to errors due to environmental noise. The Pauli Stabiliser formalism generalises classical error-correction methods and makes use of quantum error correction codes to protect quantum information. In this thesis, we introduce the XP stabiliser formalism, which is a generalisation of the Pauli stabiliser formalism with a number of useful applications. Quantum algorithms are typically written in terms of quantum circuits which involve a series of unitary gates followed by measurements which form the output of the computation. To implement quantum algorithms reliably, we need to perform unitary gates fault-tolerantly so that errors do not propagate in an uncontrolled way. Transversal logical operators are one way of applying unitary gates fault-tolerantly on Pauli stabiliser codes. Identifying transversal logical operators for a given Pauli stabiliser code is challenging, and existing methods have exponential complexity in one or more of the parameters of the code. Making use of the XP formalism, we present efficient algorithms which identify all transversal logical operators that are diagonal in the computational basis for any Pauli stabiliser code. We also show how to construct codes with a transversal implementation of any desired diagonal logical operator. The Pauli stabiliser formalism can also be used to efficiently represent certain quantum states, but many states of interest lie outside the formalism. In the XP formalism, a wider range of states can be represented than in the Pauli stabiliser formalism, including hypergraph states which have interesting non-local properties. The braiding of non-Abelian anyons is a proposed pathway to universal fault-tolerant quantum computation. Certain XP stabiliser codes are known to harbour non-Abelian anyons, and can be studied within the new formalism.
We propose a notion of quantum control in a quantum programming language which permits the superposition of finitely many quantum operations without performing a measurement. This notion takes the form of a conditional construct similar to the "if" statement in classical programming languages. We show that adding such a quantum "if" statement to the QPLprogramming language simplifies the presentation of several quantum algorithms. This motivates the possibility of extending the denotational semantics of QPL to include this form of quantum alternation. We give a denotational semantics for this extension of QPL based on Kraus decompositions rather than on superoperators. Finally, we clarify the relation between quantum alternation and recursion, and discuss the possibility of lifting the semantics defined by Kraus operators to the superoperator semantics defined by Selinger.
Coherent control, aka quantum control, is a central concept in quantum computing that is attracting increasing attention from both the quantum foundations and quantum software communities. Defining coherent control in the presence of recursion and measurement has long been known to be a major challenge. In particular, no-go results have been established for standard semantical domains like completely positive maps. We address this problem by introducing the first quantum programming language with recursion that allows for the coherent control of arbitrary quantum operations. We equip this language with both an operational and a denotational semantics that we prove to be adequate. To design these semantics, we show that combining coherent control, recursion, and measurement crucially requires describing the evolution of subprograms in the absence of input. To address this, the operational semantics takes into account a default evolution branch, while the denotational semantics uses the concept of coherent quantum operation, based on vacuum extensions. We strengthen the validity of our approach by developing an observational equivalence: two programs are equivalent if their probability of termination is the same in any context. The denotational semantics is shown to be fully abstract with respect to this observational equivalence.
Fault-tolerant quantum compilers implement gates through Pauli-product measurements and classical feedback. Verifying such a compiler requires reasoning about the quantum state and the recorded outcomes together: a correction reads an outcome, a gate's internal outcomes must be hidden from its caller, and an application's outcomes must survive optimization. We introduce QPPM, a language for Pauli-product measurement programs, with a weakest-precondition semantics over store-indexed matrix assertions, following quantum Hoare logic with classical variables. Every program terminates, so the Hoare logic is a syntactic presentation of weakest preconditions whose soundness and relative completeness are immediate. Its value is that component specifications compose. All results are mechanized in Lean.
We verify lowering of Clifford+T circuits to QPPM with private outcomes and supplied magic states, and an optimizer that eliminates measurements, fuses and reorders rotations, and schedules logical rounds while preserving every outcome that subsequent code can read. A compositional approximation relation bounds the effect of synthesis error on specifications and on output trace distance, and a certificate-checked pipeline selects, among synthesized circuits that meet the user's error limit, the one with the lowest certified depth bound. On QASMBench, verified fusion and scheduling reduce logical measurement depth by a median of 17.9\%, and complete source-to-QPPM allowances are certified for all 35 valid selected small-suite inputs.