CSS-T codes were recently introduced as quantum error-correcting codes that respect a transversal gate. A CSS-T code depends on a pair (
C1,C2) of binary linear codes
C1and
C2that satisfy certain conditions. We prove that
C1and
C2form a CSS-T pair if and only if
C2⊂Hull(C1)∩Hull(C12), where the hull of a code is the intersection of the code with its dual. We show that if (
C1,C2) is a CSS-T pair, and the code
C2is degenerated on
{i}, meaning that the
ith-entry is zero for all the elements in
C2, then the pair of punctured codes (
C1∣i,C2∣i) is also a CSS-T pair. Finally, we provide Magma code based on our results and quasi-cyclic codes as a step toward finding quantum LDPC or LDGM CSS-T codes computationally.
Designing quantum codes with both transversal non-Clifford gates and good error-correcting parameters is an important goal in fault-tolerant quantum computation. Here, we construct an explicit family of binary CSS codes with constant rate and linear distance that admit fully addressable transversal
T gates. Specifically, any prescribed tensor product of logical powers of
T is implemented by a tensor product of physical powers of
T, without any subsequent correction. Our construction combines algebraic-geometry codes with suitable binary embedding to obtain generalized divisibility of binary codes. This divisibility then enables fully addressable transversal
T gates. Furthermore, we formulate the minimum binary embedding length problem as a minimum-weight problem over an affine space and numerically improve the constants in the asymptotic rate and relative-distance bounds.
Abstract CSS-T codes were recently introduced as quantum error-correcting codes that respect a transversal gate. A CSS-T code depends on a CSS-T pair, which is a pair of binary codes
(C1,C2) ( C 1 , C 2 ) such that
C1 C 1 contains
C2 C 2 ,
C2 C 2 is even, and the shortening of the dual of
C1 C 1 with respect to the support of each codeword of
C2 C 2 is self-dual. In this paper, we give new conditions to guarantee that a pair of binary codes
(C1,C2) ( C 1 , C 2 ) is a CSS-T pair. We define the poset of CSS-T pairs and determine the minimal and maximal elements of the poset. We provide a propagation rule for nondegenerate CSS-T codes. We apply some main results to Reed–Muller, cyclic and extended cyclic codes. We characterize CSS-T pairs of cyclic codes in terms of the defining cyclotomic cosets. We find cyclic and extended cyclic codes to obtain quantum codes with better parameters than those in the literature.
For each level of the Clifford hierarchy from the third level onward, we construct explicit asymptotically good binary CSS codes for which a strongly transversal diagonal rotation, followed by a correction from the preceding level, implements the corresponding logical rotation. We achieve this using algebraic geometry codes over binary extension fields and performing an alphabet reduction that translates orthogonality conditions over the extension field to binary overlap conditions. In particular, we obtain the first asymptotically good family of binary triorthogonal codes. We also give a stronger construction with worse parameters for which the strongly transversal physical rotation implements the desired logical rotation, with no subsequent correction. Finally, adapting an error-correction-based distillation protocol, our binary
T-gate families yield direct constant-overhead
T-state block distillation from noisy
T-state inputs under sufficiently weak input noise and ideal stabilizer operations.
This paper introduces a framework for constructing Calderbank-Shor-Steane (CSS) codes that support fault-tolerant logical transversal
Z-rotations. Using this framework, we obtain asymptotically good CSS codes that fault-tolerantly realize the logical transversal Clifford group (i.e., transversal single-qubit Clifford gates are realized within a single code block, while transversal two-qubit Clifford gates are realized across two identical code blocks). Furthermore, investigating CSS-T codes, we: (a) demonstrate asymptotically good CSS-T codes wherein the transversal
T realizes the logical transversal
S†; (b) show that the condition
C2∗C1⊆C1⊥ is necessary but not sufficient for CSS-T codes; and (c) revise the characterizations of CSS-T codes wherein the transversal
T implements the logical identity and the logical transversal
T, respectively.
Calderbank-Shor-Steane (CSS) codes, constructed from nested classical codes
C2⊆C1, are typically optimized for good code parameters. However, practical quantum computing equally demands fault-tolerant logical gates. In this work, we characterize nested pairs
(C1,C2) whose resulting CSS codes realize a target logical diagonal gate via transversal physical
Z-rotations. In doing so, we recover a result of Camps-Moreno et al. that CSS codes can realize only logical single-qubit
Z-rotations and multi-qubit controlled
Z-rotations via transversal physical
Z-rotations. Building on our characterization, we develop the ''appending construction'', that takes as input an
[[n′,k′]] CSS code
Q′ and a target logical
Z-rotation (single-qubit or multi-controlled)
UL, and extends
Q′ by systematically appending
n′′ physical qubits to obtain an
[[n,k]] CSS code
Q with
n=n′+n′′ and
k=k′. The target logical gate
UL is realized in
Q by applying a well-chosen physical transversal
Z-rotation to the
n′′ appended physical qubits. Moreover, any logical gate realized via transversal physical
Z-rotations in the input code
Q′ remains transversally realizable in the extended code
Q. The CSS code
Q may incur a loss in minimum distance, but the loss can be controlled through the parameter choices made in the construction. By repeatedly applying the appending construction, we can extend any CSS code
Q′ to obtain a CSS code
Q that supports fault-tolerant implementations of multiple desired logical
Z-rotations. The cost to be paid for this is the increased physical qubit overhead as the number of target logical gates grows. To illustrate our methodology, we construct CSS code families that transversally realize addressable logical single-qubit and multi-controlled
Z-rotations.