A valid Calderbank-Shor-Steane (CSS) error correction code requires two classical linear codes for the preparation of the initial state (codewords). This code allow to correct for certain errors caused by an unwanted interaction which produces a degraded quantum state. However, this initial seven qubits encoding can be obtained from a maximally entangled Bell state (0000000>+|1111111)/√ through an operation Hint whose explicit expression is derived in the present work. The price the CSS syndrome has to pay due to its classical grounds is that the operator Hint is not unitary. In other words, Hint is not a valid quantum gate i. e. this does not represent a logical operation. Consequently, the final state is not completely robust for the standard cryptography of Quantum Computation. Besides to be a non unitary operator, Hint, is not reversible introducing with this dissipative effects that destroy the coherence in the quantum computer. Additionally, this operator is not invariant under rotations of the protector qubits inducing then preferred directions of the propagation of the logical information. These are indeed the reasons that prompt us for extending the semi classical CSS quantum error correction codes formalism to a pure quantum Hamming codes.
A valid Calderbank-Shor-Steane (CSS) error correction code requires two classical linear codes for the preparation of the initial state (codewords). This code allow to correct for certain errors caused by an unwanted interaction which produces a degraded quantum state. However, this initial seven qubits encoding can be obtained from a maximally entangled Bell state (0000000>+|1111111)/√ through an operation Hint whose explicit expression is derived in the present work. The price the CSS syndrome has to pay due to its classical grounds is that the operator Hint is not unitary. In other words, Hint is not a valid quantum gate i. e. this does not represent a logical operation. Consequently, the final state is not completely robust for the standard cryptography of Quantum Computation. Besides to be a non unitary operator, Hint, is not reversible introducing with this dissipative effects that destroy the coherence in the quantum computer. Additionally, this operator is not invariant under rotations of the protector qubits inducing then preferred directions of the propagation of the logical information. These are indeed the reasons that prompt us for extending the semi classical CSS quantum error correction codes formalism to a pure quantum Hamming codes.
Stabilizer states are extensively studied in quantum information theory for their structures based on the Pauli group. Calderbank-Shor-Steane (CSS) stabilizer states are of particular importance in their application to fault-tolerant quantum computation (FTQC). However, how to fault-tolerantly prepare arbitrary CSS stabilizer states for general CSS stabilizer codes is still unknown, and their preparation can be highly costly in computational resources. In this paper, we show how to prepare a large class of CSS stabilizer states useful for FTQC. We propose distillation protocols using syndrome encoding by classical codes or quantum CSS codes. Along the same lines, we show that classical coding techniques can reduce the ancilla consumption in Steane syndrome extraction by using additional transversal controlled-not gates and classical computing power. In the scenario of a fixed ancilla consumption rate, we can increase the frequency of quantum error correction and effectively lower the error rate.
Since quantum states are limited in their ease of manipulation, quantum computing has great advantages when used for computation. These interactions, even if infinitesimal, are the very reason why errors arise, be it bit-flip, phase-flip, or composite, thus diminishing the accuracy in computation. Quantum error correction (QEC) intervenes systematically to guard against the loss of coherence from quantum information. Amongst the various QEC protocols developed so far, Steane code [ [7], [1], [3]] is a very famous CSS stabilizer code that was derived from a classical Hamming code. It will encode a single logical qubit into seven physical qubits, which corrects all single-qubit errors by separating X and Z syndrome measurements. The Steane code enables the implementation of transversal Clifford gates, making it an ideal playground to study fault-tolerant quantum computation. Although surface codes remain the codes of choice for scaling implementations, the Steane code will forever remain one of the cornerstone codes due to its clear structure of error correction and ease in teaching.
In this paper, we demonstrate a direct correspondence between phase-free ZX diagrams, a graphical notation for representing and manipulating a certain class of linear maps on qubits, and Calderbank-Shor-Steane (CSS) codes, a large family of quantum error correcting codes constructed from classical codes, including for example the Steane code, surface codes, and colour codes. The stabilisers of a CSS code have an especially nice structure arising from a pair of orthogonal F2-linear subspaces, or in the case of maximal CSS codes, a single subspace and its orthocomplement. On the other hand, phase-free ZX diagrams can always be efficiently reduced to a normal form given by the basis elements of an F2-linear subspace. Here, we will show that these two ways of describing a quantum state by an F2-linear subspace S are in fact the same. Namely, the maximal CSS code generated by S fixes the quantum state whose ZX normal form is also given by S. This insight gives us an immediate translation from stabilisers of a maximal CSS code into a ZX diagram describing its associated state. We show that we can extend this translation to stabilisers and logical operators of any (possibly non-maximal) CSS code by "bending wires". To demonstrate the utility of this translation, we give a simple picture of the surface code and a fully graphical derivation of the action of physical lattice surgery operations on the space of logical qubits, completing the ZX presentation of lattice surgery initiated by de Beudrap and Horsman.
Stabilizer codes lie at the heart of modern quantum-error-correcting codes (QECCs). Of particular importance is a class called Calderbank-Shor-Steane (CSS) codes, which includes many important examples such as toric codes, color codes, and fractons. Recent studies have revealed that the decoding transition for these QECCs could be intrinsically captured by calculating information-theoretic quantities from the mixed state. Here we perform a simple analytic calculation of the coherent information for general CSS codes under local incoherent Pauli errors via diagonalization of the density matrices and mapping to classical statistical mechanical (SM) models. Our result establishes a rigorous connection between the decoding transition of the quantum code and the phase transition in the random classical SM model. It is also directly confirmed for CSS codes that exact error correction is possible if and only if the maximum-likelihood decoder always succeeds in the thermodynamic limit. Thus, the fundamental threshold is saturated by the optimal decoder.
Flag qubits have recently been proposed in syndrome extraction circuits to detect high-weight errors arising from fewer faults. The use of flag qubits allows the construction of fault-tolerant protocols with the fewest number of ancillas known to date. In this work, we prove some critical properties of Calderbank-Shor-Steane (CSS) codes constructed from classical cyclic codes that enable the construction of a flag fault-tolerant error correction scheme. We then develop fault-tolerant protocols as well as a family of circuits for flag fault-tolerant error correction and operator measurement, requiring only four ancilla qubits and applicable to cyclic CSS codes of distance 3. The measurement protocol can be further used for logical Clifford gate implementation via quantum gate teleportation. We also provide examples of cyclic CSS codes with large encoding rates.
Calderbank-Shor-Steane (CSS) codes are a versatile quantum error-correcting family built out of commuting X- and Z-type checks. We introduce CSS-like codes on G-valued qudits for any finite group G that reduce to qubit CSS codes for G=Z2 yet generalize the Kitaev quantum double model for general groups. The X-checks of our group-CSS codes correspond to left and/or right multiplication by group elements, while Z-checks project onto solutions to group word equations. We describe quantum-double models on oriented two-dimensional CW complexes (which need not cellulate a manifold) and prove that, when G is non-Abelian and simple, every G-covariant group-CSS code with suitably upper-bounded Z-check weight and lower-bounded Z-distance reduces to a CW quantum double. We describe the codespace and logical operators of CW quantum doubles via the same intuition used to obtain logical structure of surface codes. We obtain distance bounds for codes on non-Abelian simple groups from the graph underlying the CW complex, and construct intrinsically non-Abelian code families with asymptotically optimal rate and distances. Adding "ghost vertices" to the CW complex generalizes quantum double models with defects and rough boundary conditions whose logical structure can be understood without reference to non-Abelian anyons or defects. Several non-invertible symmetry-protected topological states, both with ordinary and higher-form symmetries, are the unique codewords of simply-connected CW quantum doubles with a single ghost vertex.