Description
A concatenated code whose outer code is a qubit code. In other words, a qubit code that can be thought of as a concatenation of an inner qubit code and an outer qubit code. An inner \(C_{\text{in}} = ((n_1,K,d_1))\) and outer \(C_{\text{out}} = ((n_2,2,d_2))\) qubit code yield an \(((n_1 n_2, K, d \geq d_1d_2))\) concatenated qubit code.
Concatenating an \(((n,2,d))\) qubit code can be done recursively, with the \(r\)th level of concatenation yielding an \(((n^r,2,d^r))\) code.
For qubit CSS codes, concatenation is generalized by the mapping cone framework of quantum code embedding [1].
Protection
Any distance-three recursively concatenated code protects against an open set of errors [2]. Concatenating stabilizer codes can help protect against catastrophic errors such as cosmic rays [3].Decoding
Adaptive syndrome extraction for a concatenation of a small error-detecting code and a high-rate, high-distance QLDPC code [4].The effective channel for a concatenation of codes is the composition of the codes’ effective channels [5].Message passing algorithm for concatenated codes can be equivalent to ML decoding [6].In the statistical mechanical mapping of maximum-likelihood decoding, recursively concatenated codes yield classical models on hierarchical lattices, and optimal decoding admits a hierarchical message-passing formulation that, in the fully postselected limit, reduces to an exact real-space renormalization group flow [7].Fault Tolerance
Fault-tolerant message passing between devices [8].Blocklet concatenation uses concatenation and transversal gates in a way that is tailored to FBQC platforms [9].Threshold
The first methods to achieve a concatenated threshold against local stochastic noise use concatenated qubit stabilizer codes [10–17]; see the book [18].Cousins
- Hamiltonian-based code— Concatenated stabilizer code Hamiltonians have been investigated [19].
- Fusion-based quantum computing (FBQC) code— Blocklet concatenation uses concatenation and transversal gates in a way that is tailored to FBQC platforms [9].
- Permutationally self-dual (PSD) CSS code— For a suitable concatenation map, every level of the recursive concatenation of an \([[n_0,1,d_0]]\) PSD seed code is finite-size Kramers-Wannier self-dual under the statistical mechanical mapping of bit-flip decoding [7; Lemma 1]. If the resulting zero-rate family has a unique threshold, this self-duality constrains its optimal bit-flip threshold [7].
- Gauss’ law code— The Gauss’ law code can be concatenated to form a stabilizer code for fault-tolerant quantum simulation of the underlying gauge theory [20,21].
- Amplitude-damping (AD) code— Using the dual-rail code as an outer code with an inner \([[n,k,d]]\) qubit code yields an \([[2n,k]]\) code correcting \(d-1\) qubit AD errors [22].
- EA qubit stabilizer code— There exist concatenated EA qubit stabilizer codes that saturate the EA quantum Griesmer and Plotkin bounds [23].
- \(((n,2,2))\) Bravyi-Lee-Li-Yoshida PI code— The Bravyi-Lee-Li-Yoshida PI code can be concatenated to yield codes that have higher distance and that admit codewords with vanishing entanglement [24; Appx. D] (cf. [25]).
- \([[2^D,D,2]]\) hypercube quantum code— The hypercube quantum code can be concatenated with a two-qubit quantum repetition code to yield a \([[2^{D+1},D,4]]\) error-detecting code family [26]. It can also be concatenated with \(D\) distance-two \(D\)-dimensional toric/surface-code blocks to yield a \([[2^D(2^D+1),D,4]]\) error-correcting code family that admits a transversal implementation of the logical \(C^{D-1}Z\) gate [26].
- \([[15,1,3]]\) quantum RM code— The concatenated \([[15,1,3]]\) code has a measurement threshold less than one [27].
- \([[4,2,2]]\) Four-qubit code— Concatenations of \([[4,2,2]]\) and \(C_6\) codes yield fault-tolerant quantum computation schemes [28] admitting a post-selected threshold [29,30] (see also Ref. [31]). Concatenating quantum Hamming codes on top of the \([[4,2,2]]\) and \(C_6\) codes yields fault-tolerant quantum computation with constant space and quasi-polylogarithmic time overheads [32]. In the optimized protocol of Ref. [32], a level-five \(C_4/C_6\) code underlies concatenated quantum Hamming codes \(\mathcal{Q}_5,\mathcal{Q}_6,\mathcal{Q}_7,\mathcal{Q}_7\), yielding a \(2.5\%\) threshold and space overheads \(162\) and \(373\) physical qubits per logical qubit at physical error rate \(0.1\%\) for logical CNOT error rates \(10^{-10}\) and \(10^{-24}\), respectively. Concatenating the \([[4,2,2]]\) code with the surface code is equivalent to removing stabilizer generators from the 4.8.8 color code [33]. The \([[4,2,2]]\) code can be concatenated with two copies of the surface code to yield the 4.6.12 color code [33].
- \([[5,1,3]]\) Five-qubit perfect code— The recursively concatenated five-qubit code has a measurement threshold of one [27]. Code performance against general Pauli channels has been worked out [5,34].
- \([[6,2,2]]\) \(C_6\) code— Concatenations of \([[4,2,2]]\) and \(C_6\) codes yield fault-tolerant quantum computation schemes [28] admitting a post-selected threshold [29,30] (see also Ref. [31]) and the Meier-Eastin-Knill (MEK) magic-state distillation protocols [35]. Concatenating quantum Hamming codes on top of the \([[4,2,2]]\) and \(C_6\) codes yields fault-tolerant quantum computation with constant space and quasi-polylogarithmic time overheads [32]. In the optimized protocol of Ref. [32], a level-five \(C_4/C_6\) code underlies concatenated quantum Hamming codes \(\mathcal{Q}_5,\mathcal{Q}_6,\mathcal{Q}_7,\mathcal{Q}_7\), yielding a \(2.5\%\) threshold and space overheads \(162\) and \(373\) physical qubits per logical qubit at physical error rate \(0.1\%\) for logical CNOT error rates \(10^{-10}\) and \(10^{-24}\), respectively.
- \([[6,4,2]]\) error-detecting code— Concatenations of this code with itself yield the level-\(r\) \([[6^r,4^r,2^r]]\) many-hypercube code [36]. The \([[6,4,2]]\) code can be concatenated with the surface code to yield the 6.6.6 color code [33; Appx. A].
- \([[8,2,3]]\) Hermitian code— Applying concatenated symplectic doubling to the \([[8,2,3]]\) Hermitian code yields a \([[32,4,6]]\) self-dual CSS code [37; Corr. 2].
- \([[8,3,2]]\) Smallest interesting color code— Concatenating \([[8,3,2]]\) blocks with triples of qubits drawn from three cyclically rotated 3D surface/toric codes yields a 3D toric/color family with parameters \([[8n,3,2d]]\) and transversal logical \(CCZ\) implemented by physical \(T\) gates on the outer \([[8,3,2]]\) blocks [26].
- \([[9,1,3]]\) Surface-17 code— Recursively concatenating the surface-17 code yields a zero-rate family with distance \(d = n^{\log_9 3} = \sqrt{n}\) [7]. Its bit-flip phase boundary nearly coincides with that of the toric code [7].
- Mapping cone code— The mapping cone framework can be regarded as a generalization of code concatenation for CSS codes [1].
- Phantom code— Concatenating a phantom inner code with a one-logical-qubit outer quantum code preserves phantomness.
- Quantum Logic Code (QLC)— Using the concatenation convention of the Zoo, the \(\ell>0\) members are obtained by concatenating the QLC cores (outer codes) with the Steane code (inner code), and the construction also permits any self-dual doubly-even \([[n_i,1,d_i]]\) inner code with suitable transversal Clifford gates [38].
- Layer code— Each pair of surface-code squares in a layer code is fused (or quasi-concatenated) with perpendicular surface-code squares via lattice surgery.
- Weakly divisible CSS code— Particular coset-divisible codes can be used as outer codes in layered constructions that implement a fault-tolerant \(T\) gate on the five-qubit or Steane code [39; Sec. V].
- Raussendorf-Bravyi-Harrington (RBH) cluster-state code— Concatenation of the RBH code with small codes such as the \([[2,1]]\) repetition code, \([[4,1,1,2]]\) subsystem code, or Steane code can improve thresholds [40].
- Hypergraph product (HGP) code— There is a fault-tolerant universal computation scheme for hypergraph-product codes concatenated with the \([[4,2,2]]\) code in which the full syndrome measurement on the lower hypergraph product code is performed only if an error is detected at the upper four-qubit code [4].
- \([[2^r-1, 2^r-2r-1, 3]]\) quantum Hamming code— Concatenating a growing sequence of quantum Hamming codes yields fault-tolerant quantum computation with constant space overhead and quasi-polylogarithmic time overhead [41]. Concatenating quantum Hamming codes on top of the \([[4,2,2]]\) and \(C_6\) codes yields fault-tolerant quantum computation with constant space and quasi-polylogarithmic time overheads [32]. In the optimized protocol of Ref. [32], a level-five \(C_4/C_6\) code underlies concatenated quantum Hamming codes \(\mathcal{Q}_5,\mathcal{Q}_6,\mathcal{Q}_7,\mathcal{Q}_7\), yielding a \(2.5\%\) threshold and space overheads \(162\) and \(373\) physical qubits per logical qubit at physical error rate \(0.1\%\) for logical CNOT error rates \(10^{-10}\) and \(10^{-24}\), respectively. A modified tower of interleaved quantum Hamming codes with reserved qubits and recursive hookless Pauli-product measurements yields fault-tolerant quantum computation on a 1D nearest-neighbor qubit line with asymptotic rate above \(5\%\), constant space overhead, quasi-polylogarithmic time overhead, and a threshold [42]. Quantum Hamming codes can also be concatenated with surface codes [43].
- 3D color code— On closed 3-manifolds, the 3D color code is equivalent to multiple decoupled copies of the 3D surface code via a local constant-depth Clifford circuit [44–46]. This process can be viewed as an ungauging [47–56] of certain symmetries. This mapping can also be done via code concatenation [57].
- Dense twist-defect surface code— Each column of the twist-defect dense packing is concatenated with an \([[n,n-2,2]]\) error-detecting code as the inner code, in the concatenation convention of the Zoo, which roughly doubles the distance [58]. The inner stabilizer generators are \(X^{\otimes n}\) and \(Z^{\otimes n}\), and two of the \(n\) outer code blocks serve as yoke qubits, so that the \(j\)th encoded qubit can be given the logical basis \(X_j X_{n-1}\) and \(Z_j Z_{n-2}\). This is the twist-defect analogue of the yoked surface code, which uses a column of one-qubit rotated surface-code patches in place of the dense packing. Unlike in that case, the logical operators of the dense packing generally lie in the interior, and must be moved to a boundary by twist-defect lattice surgery before the parity checks can be measured.
- Subsystem homological product code— Concatenated CSS stabilizer codes are gauge-fixed SP codes [59; Thm. 4].
Primary Hierarchy
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- Victor V. Albert (2026-09-23) — most recent
- Victor V. Albert (2026-09-22)
- Victor V. Albert (2026-07-29)
- Victor V. Albert (2026-06-08)
- Victor V. Albert (2024-07-16)
Cite as:
“Concatenated qubit code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2026. https://errorcorrectionzoo.org/c/qubit_concatenated, arXiv:2606.11484