Welcome to the Qubit Kingdom.

Qubit code Encodes $$K$$-dimensional Hilbert space into a $$2^n$$-dimensional (i.e., $$n$$-qubit) Hilbert space. Usually denoted as $$((n,K))$$ or $$((n,K,d))$$, where $$d$$ is the code's distance. Protection: Corrects erasure errors on up to $$d-1$$ qubits. The number of correctable errors is often called the decoding radius, and it is upper bounded by half of the code distance. As a result, qubit codes cannot tolerate adversarial errors on more than $$(1-R)/4$$ registers.
This is a family of codes derived via an algorithm that takes as input any binary classical code and outputs a quantum code (note that this framework can be extended to $$q$$-ary codes). The algorithm is probabalistic but succeeds almost surely if the classical code is random. An explicit code construction does exist for linear distance codes encoding one logical qubit. For finite rate codes, there is no rigorous proof that the construction algorithm succeeds, and approximate constructions are described instead. Protection: Let $$C \subset \{0,1,\dots,q-1\}^n$$ be a classical code with distance $$d_x$$. Let $$d_z$$ satisfy $$q^n > 2 V_q(d_z-1) -1$$, where $$V_q(r)$$ is the volume of the $$q$$-ary Hamming ball of radius $$r$$. Then the algorithm produces a quantum code with distance $$d = \text{min}(d_x,d_z)$$. Asymptotically, the distance scales linearly with $$n$$. Parents: Qubit code, Hamiltonian-based code. Parent of: Calderbank-Shor-Steane (CSS) stabilizer code. Cousins: Qubit stabilizer code, Binary code. Cousin of: Codeword stabilized (CWS) code.
Also called a Pauli stabilizer code. An $$((n,2^k,d))$$ qubit stabilizer code is denoted as $$[[n,k]]$$ or $$[[n,k,d]]$$, where $$d$$ is the code's distance. Logical subspace is the joint eigenspace of commuting Pauli operators forming the code's stabilizer group $$\mathsf{S}$$. Traditionally, the logical subspace is the joint $$+1$$ eigenspace of a set of $$2^{n-k}$$ commuting Pauli operators which do not contain $$-I$$. The distance is the minimum weight of a Pauli string that implements a nontrivial logical operation in the code. Protection: Detects errors on up to $$d-1$$ qubits, and corrects erasure errors on up to $$d-1$$ qubits. More generally, define the normalizer $$\mathsf{N(S)}$$ of $$\mathsf{S}$$ to be the set of all operators that commute with all $$S\in\mathsf{S}$$. A stabilizer code can correct a Pauli error set $${\mathcal{E}}$$ if and only if $$E^\dagger F \notin \mathsf{N(S)}\setminus \mathsf{S}$$ for all $$E,F \in {\mathcal{E}}$$.
Code dynamically generated by unitary Clifford circuits defined on a lattice with some crystalline symmetry. A notable example is the circuit defined on a rotated square lattice with vertices corresponding to iSWAP gates and edges decorated by $$R_X[\pi/2]$$, a single-qubit rotation by $$\pi/2$$ around the $$X$$-axis. This circuit is invariant under space-time translations by a unit cell $$(T, a)$$ and all transformations of the square lattice point group $$D_4$$. Protection: The code protects against Pauli errors. The circuit composed of iSWAP and $$R_X[\pi/2]$$ gates on the square lattice is a “good scrambler” with non-fractal operator spreading and thus behaves like a random circuit in that regard, motivating the use of contiguous code distance as a proxy for code distance. Parent of: Floquet code.
Constructed using a variation of the stabilizer formalism designed to utilize pre-shared entanglement between sender and receiver. A code is typically denoted as $$[[n,k;c]]$$ or $$[[n,k,d;c]]$$, where $$d$$ is the distance of the underlying non-EA $$[[n,k,d]]$$ code, and $$c$$ is the number of required pre-shared maximally entangled Bell states. While other entangled states can be used, there is always a choice a generators such that the Bell state suffices while still using the fewest ebits. Parent of: Quantum polar code.
Also known as a hyperbolic pentagon code (HyPeC). Holographic code constructed out of a network of perfect tensors that tesselates hyperbolic space. Physical qubits are associated with uncontracted tensor legs at the boundary of the tesselation, while logical qubits are associated with uncontracted legs in the bulk. The code serves as a minimal model for several aspects of the AdS/CFT holographic duality and potentially a dF/CFT duality [8]. The construction below is described for qubits, but straightforward generalizations exist to modular qudits, oscillators, and rotors [9]. Protection: Protects against erasure errors and Pauli errors on the boundary qubits. Parents: Holographic code, Qubit stabilizer code.
This family of codes strictly generalizes stabilizer codes. They are usually denoted by $$\mathcal{Q} = (\mathcal{G},\mathcal{C})$$ where $$\mathcal{G}$$ is a graph and $$\mathcal{C}$$ is a $$(n,K,d)$$ binary classical code. From the graph we form the unique graph state (stabilizer state) $$|G \rangle$$. From the classical code we form Pauli $$Z$$-type operators $$W_i = Z^{c_{i,1}} \otimes \cdots \otimes Z^{c_{i,n}}$$, where $$c_{i,j}$$ is the $$j$$-th bit of the $$i$$-th classical codeword. The CWS codewords are then $$| i \rangle = W_i | G \rangle$$. Protection: Code distance $$\mathcal{Q} = ( \mathcal{G},\mathcal{C})$$ is upper bounded by the distance of the classical code $$\mathcal{C}$$. The diagonal distance is upper bounded by $$\delta + 1$$, where $$\delta$$ is the minimum degree of $$\mathcal{G}$$. Computing the distance is generally NP-complete, and is NP-hard for non-degenerate codes [11]. Parents: Qubit code. Parent of: Qubit stabilizer code. Cousin of: XP stabilizer code.
Fermionic code Finite-dimensional quantum error-correcting code encoding a logical Hilbert space into a physical Fock space of fermionic modes. Codes are typically described using Majorana operators, which are linear combinations of fermionic creation and annihilation operators [12]. Parents: Qubit code. Parent of: Majorana stabilizer code. Cousins: Bosonic code.
The XP Stabilizer formalism is a generalization of the XS and Pauli stabilizer formalisms, with stabilizer generators taken from the group $$\{\omega I, X, P\}^{\otimes n}$$. Here, $$\omega$$ is a $$2N$$ root of unity, and $$P = \text{diag} ( 1, \omega^2)$$. The codespace is a $$+1$$ eigenspace of a set of XP stabilizer generators, which need not commute to define a valid codespace. Parents: Qubit code. Parent of: Qubit stabilizer code, XS stabilizer code.
An $$[[n,k,d]]$$ stabilizer code admitting a set of stabilizer generators that are either $$Z$$-type or $$X$$-type Pauli strings. The stabilizer generator matrix is of the form \begin{align} H=\begin{pmatrix}0 & H_{Z}\\ H_{X} & 0 \end{pmatrix} \label{eq:parity} \end{align} such that the rows of the two blocks must be orthogonal \begin{align} H_X H_Z^T=0~. \label{eq:comm} \end{align} The above condition guarantees that the $$X$$-stabilizer generators, defined in the symplectic representation as rows of $$H_X$$, commute with the $$Z$$-stabilizer generators associated with $$H_Z$$. Protection: Detects errors on $$d-1$$ qubits, corrects errors on $$\left\lfloor (d-1)/2 \right\rfloor$$ qubits.
A generally non-CSS derivative of the surface code defined by applying a constant-depth Clifford circuit to the original (CSS) surface code. Unlike the surface code, CDSCs include codes whose thresholds and subthreshold performance are enhanced under noise biased towards dephasing. Examples of CDSCs include the XY code, XZZX code, and random CDSCs. Protection: As a stabilizer code, $$[[n=O(d^2), k=O(1), d]]$$.
Fusion Based Quantum Computing, or FBQC, describes a fault tolerant way to produce fusion networks, or large entangled states starting from small constant-sized entangled resource states along with destructive measurements called fusions. These large states can be produced asychronously in the fusion framework and can be used as resources, as in measurement-based quantum computation (MBQC), or as logical states of topological codes. The difference from ordinary MBQC is that error-correction is baked into the state-generation protocol. Protection: Protects against erasure, Pauli errors, photon loss, fusion failure from non-determinism, and faulty resource states. Redundancy in fusion outcomes is captured by the check operator group. Fusion measurement outcomes form a syndrome that allows to correct for Pauli errors. There is no physical error correction, and decoding output is simply used to update the Pauli frame. Parents: Qubit stabilizer code.
Class of stabilizer codes on a length-$$L$$ cubic lattice with one or two qubits per site. We also require that the stabilizer group $$\mathsf{S}$$ is translation invariant and generated by two types of operators with support on a cube. In the non-CSS case, these two are related by spatial inversion. For CSS codes, we require that the product of all corner operators is the identity. We lastly require that there are no non-trival ''string operators'', meaning that single-site operators are a phase, and any period one logical operator $$l \in \mathsf{S}^{\perp}$$ is just a phase. Haah showed in his original construction that there is exactly one non-CSS code of this form, and 17 CSS codes [19]. The non-CSS code is labeled code 0, and the rest are numbered from 1 - 17. Codes 1-4, 7, 8, and 10 do not have string logical operators [19][20]. Protection: Cubic codes protect against simultaneous independent Pauli errors on different sites (not qubits, since there can be 2 qubits per site). Codes 0-4 are known to have distance $$d \ge L$$, meaning they can achieve macroscopic code distance as $$L\to\infty$$. Parents: Qubit stabilizer code, Fracton code. Cousins: Color code, Kitaev surface code.
Majorana fermion stabilizer codes are stabilizer codes whose stabilizers are products of an even number of Majorana fermion operators, analogous to Pauli strings for a traditional stabilizer code and referred to as Majorana stabilizers. The codespace is the mutual $$+1$$ eigenspace of all Majorana stabilizers. In such systems, Majorana fermions may either be considered individually or paired into creation and annihilation operators for fermionic modes. Codes can be denoted as $$[[n,k,d]]_{f}$$ [22], where $$n$$ is the number of fermionic modes. Protection: Detects products of Majorana operators with weight up to $$d-1$$. Physically, protects against dephasing errors caused by coupling of fermion density to the environment and bit-flip errors caused by quasiparticle poisoning processes. Parents: Fermionic code, Qubit stabilizer code. Parent of: Kitaev honeycomb code.
Member of a class of qubit stabilizer codes based on the abelian phase of the Kitaev honeycomb model. Parent of: XYZ$$^2$$ hexagonal stabilizer code. Cousins: Kitaev honeycomb code.
Qubit stabilizer code constructed from a self-orthogonal binary BCH code via the CSS construction, from a Hermitian self-orthogonal quaternary BCH code via the stabilizer-over-$$GF(4)$$ construction, or by taking a Euclidean self-orthogonal BCH code over $$GF(2^m)$$, converting it to a binary code, and applying the CSS construction. Parents: Qubit stabilizer code. Cousin of: Galois-qudit BCH code.
Stub. (see Sec. III E of [29]) Protection: Code exhibiting symmetry-protected self-correction. The energy barrier for symmetry-preserving exhitations outside of the code space grows linearly with the lattice width. When the system is coupled locally to a thermal bath respecting the symmetry and below a critical temperature, the memory time grows exponentially with the lattice width. Parents: Qubit stabilizer code.
An $$[[n,k,d]]$$ stabilizer code constructed from a quaternary classical code using the one-to-one correspondence between the four Pauli matrices $$\{I,X,Y,Z\}$$ and the four elements $$\{0,1,\alpha^2,\alpha\}$$ of the quaternary field $$GF(4)$$. Protection: Detects errors on $$d-1$$ qubits, corrects errors on $$\left\lfloor (d-1)/2 \right\rfloor$$ qubits. Parents: Qubit stabilizer code. Parent of: Five-qubit perfect code. Cousins: Dual additive code, Dual linear code. Cousin of: Qubit BCH code, Stabilizer code over $$GF(q^2)$$.
A 1D translationally invariant stabilizer code whose encoding is a constant-depth circuit of nearest-neighbor gates on alternating even and odd bonds that consist of transverse-field Ising Hamiltonian interactions. The code allows for perfect state transfer of arbitrary distance using local operations and classical communications (LOCC). Protection: Code distance is 1 for open boundary conditions similar to a repetition code, and 3 for periodic boundary conditions with an encoding circuit depth of 4.
A family of stabilizer codes of distance $$3$$ that saturate the asymptotic quantum Hamming bound. Can be obtained from the CSS construction using a first-order $$[2^r,r+1,2^{r-1}]$$ RM code and a $$[2^r,2^r-1,2]$$ even-weight code [24]. Protection: Protects against any single qubit error. Parents: Qubit stabilizer code. Cousin of: Hamming code.
A family of abelian topological CSS stabilizer codes defined on a $$D$$-dimensional lattice which satisfies two properties: The lattice is (1) a homogeneous simplicial $$D$$-complex obtained as a triangulation of the interior of a $$D$$-simplex and (2) is $$D+1$$-colorable. Qubits are placed on the $$D$$-simplices and generators are supported on suitable simplices [33]. For 2-dimensional color code, the lattice must be such that it is 3-valent and has 3-colorable faces, such as a honeycomb lattice. The qubits are placed on the vertices and two stabilizer generators are placed on each face [34]. Protection: As with the surface code, the code distance depends on the specific kind of lattice used to define the code. More precisely, the distance depends on the homology of logical string operators [34].
A family of abelian topological CSS stabilizer codes whose generators are few-body $$X$$-type and $$Z$$-type Pauli strings associated to the stars and plaquettes, respectively, of a cellulation of a two-dimensional surface (with a qubit located at each edge of the cellulation). Toric code often either refers to the construction on the two-dimensional torus or is an alternative name for the general construction. The construction on surfaces with boundaries is often called the planar code [38]. Protection: Toric code on an $$L\times L$$ torus is a $$[[2L^2,2,L]]$$ CSS code, and there exists a planar code with $$[[L^2,1,L]]$$ [39]. More generally, the code distance is related to the homology of the cellulation [40].
Also called a generalized Shor code [44]. A $$[[m_1 m_2,1,\min(m_1,m_2)]]$$ CSS code family obtained from concatenating an $$m_1$$-qubit phase-flip repetition code with an $$m_2$$-qubit bit-flip repetition code. Logical codewords are \begin{align} \begin{split} |\overline{0}\rangle&=\frac{1}{2^{m_2/2}}\left(|0\rangle^{\otimes m_1}+|1\rangle^{\otimes m_1}\right)^{\otimes m_2}\\ |\overline{1}\rangle&=\frac{1}{2^{m_2/2}}\left(|0\rangle^{\otimes m_1}-|1\rangle^{\otimes m_1}\right)^{\otimes m_2}~. \end{split} \end{align} Protection: Has distance $$d=\min(m_1,m_2)$$. Parent of: Quantum repetition code, $$[[9,1,3]]$$ Shor code. Cousin of: $$[[4,2,2]]$$ CSS code.
Also known as the iceberg code. CSS stabilizer code for $$m\geq 2$$ with generators $$\{XX\cdots X, ZZ\cdots Z\}$$ acting on all $$2m$$ physical qubits. Admits a basis such that each codeword is a superposition of a computational basis state labeled by a bitstring $$b$$ and a state labeled by the negation of $$b$$. Such states generalize the two-qubit Bell states and three-qubit GHz states and are often called cat states. Protection: Detects a single-qubit error. Parent of: $$[[4,2,2]]$$ CSS code.
Five-qubit stabilizer code with generators that are symmetric under cyclic permutation of qubits, \begin{align} \begin{split} S_1 &= IXZZX \\ S_2 &= XZZXI \\ S_3 &= ZZXIX \\ S_4 &= ZXIXZ~. \end{split} \end{align} Protection: Smallest stabilizer code that protects against a single error on any one qubit. Detects two-qubit errors.
Dynamically-generated stabilizer-based code whose logical qubits are generated through a particular sequence of check-operator measurements such that the number of logical qubits is larger than when the code is viewed as a static subsystem stabilizer code. After each measurement in the sequence, the codespace is a joint $$+1$$ eigenspace of an instantaneous stabilizer group (ISG), i.e., a particular stabilizer group corresponding to the measurement. The ISG specifies the state of the system as a Pauli stabilizer state at a particular round of measurement, and it evolves into a (potentially) different ISG depending on the check operators measured in the next step in the sequence. As opposed to subsystem codes, only specific measurement sequences maintain the codespace. Protection: Protects against single-qubit Pauli noise and check operator measurement errors. Parents: Crystalline-circuit code. Parent of: Floquet color code, Honeycomb Floquet code.
Entanglement-assisted CSS code utilized in a quantum polar coding scheme producing entangled pairs of qubits between sender and receiver. In such a scheme, the amplitude and phase information of a quantum state is handled in complementary fashion [49] using an encoding based on classical polar codes. Variants of the initial scheme have been developed for degradable channels [50] and extended to arbitrary channels [51]. Protection: Protects against Pauli noise and erasures. Parents: EA qubit stabilizer code. Cousins: Polar code.
Code whose logical subspace is labeled by different fusion outcomes of Ising anyons present in the non-Abelian topological phase of the Kitaev honeycomb model [52]. Each logical qubit is constructed out of four Majorana operators, which admit braiding-based gates due to their non-Abelian statistics and which can be used for topological quantum computation. Parents: Majorana stabilizer code, Topological code. Cousin of: Honeycomb Floquet code, Matching code.
A CSS code constructed by separately constructing the $$X$$ and $$Z$$ check matrices using product constructions from classical codes. A particular $$[[512,174,8]]$$ code performed well [55] against erasure and depolarizing noise when compared to other notable CSS codes, such as the asymptotically good quantum Tanner codes.
A CSS code formed from a classical Reed-Muller code (or its punctured versions) in which polynomials over finite fields encode data. This is done by transforming these polynomials into the stabilizer generator matrices. Protection: Detects errors on $$d-1$$ qubits, corrects errors on $$\left\lfloor (d-1)/2 \right\rfloor$$ qubits.
Consider a CSS code whose $$Z$$-stabilizers are determined by the dual of a classical $$[n, k_1]$$ linear binary code $$C_1$$, and whose $$X$$-stabilizers are determined by a classical $$[n, k_2]$$ binary code $$C_2 \subset C_1$$. This code is quantum divisible if all weights in $$C_2$$ share a common divisor $$\Delta > 1$$, and all weights in each coset of $$C_2$$ in $$C_1$$ are congruent to $$\Delta$$. Protection: Distance $$d$$ is upper bounded by the two classical codes that determine the CSS code.
A triorthogonal $$m \times n$$ binary matrix is triorthogonal if its rows $$r_1, \ldots, r_m$$ satisfy $$|r_i \cdot r_j| = 0$$ and $$|r_i \cdot r_j \cdot r_k| = 0$$, where addition and multiplication are done on $$\mathbb{Z}_2$$. The triorthogonal code associated with the matrix is constructed by mapping non-zero entries in even-weight rows to $$X$$ operators, and $$Z$$ operators for each row in the orthogonal complement. Protection: Weight $$t$$ Pauli errors, where $$t$$ depends on the family. For example, Ref. [58] provides a family of distance $$2$$ codes. It also presents a $$[[49, 1, 5]]$$ code. Parent of: $$[[15,1,3]]$$ quantum Reed-Muller code. Cousins: Quantum Reed-Muller code. Cousin of: Color code, Quantum divisible code.
Family of $$[[k+4,k,2]]$$ CSS codes with transversal Hadamard gates; relevant to magic state distillation. Four stablizer generators are $$X_1X_2X_3X_4$$, $$Z_1Z_2Z_3Z_4$$, $$X_1X_2X_5X_6...X_{k+4}$$, and $$Z_1Z_2Z_5Z_6...Z_{k+4}$$. Protection: Detects weight-1 Pauli errors. The $$r$$-level contatenated H code detects weight Pauli errors up to weight $$2^r-1$$.
$$[[15,1,3]]$$ quantum Reed-Muller code $$[[15,1,3]]$$ CSS code that is most easily thought of as a tetrahedral 3D color code. This code contains 15 qubits, represented by four vertices, four face centers, six edge centers, and one body center. The tetrahedron is cellulated into four identical polyhedron cells by connecting the body center to all four face centers, where each face center is then connected by three adjacent edge centers. Each colored cell corresponds to a weight-8 $$X$$-check, and each face corresponds to a weight-4 $$Z$$-check. A logical $$Z$$ is any weight-3 $$Z$$-string along an edge of the entire tetrahedron. The logical $$X$$ is any weight-7 $$X$$-face of the entire tetrahedron. Cousin of: $$[[8,3,2]]$$ code.
A $$[[7,1,3]]$$ CSS code that uses the classical binary $$[7,4,3]$$ Hamming code for protecting against both $$X$$ and $$Z$$ errors. The parity-check matrix for the $$[7,4,3]$$ Hamming code is \begin{align} H = \left(\begin{matrix} 1&0&0&1&0&1&1\\ 0&1&0&1&1&0&1\\ 0&0&1&0&1&1&1 \end{matrix}\right), \end{align} and the check matrix for the Steane code is therefore \begin{align} \left(\begin{matrix} 0&H\\ H&0 \end{matrix}\right). \end{align} The stabilizer group for the Steane code has six generators. Logical codewords are \begin{align} \begin{split} |\overline{0}\rangle&=\frac{1}{\sqrt{8}}\Big(|0000000\rangle+|1010101\rangle+|0110011\rangle+|1100110\rangle\\&\,\,\,\,\,\,\,\,+|0001111\rangle+|1011010\rangle+|0111100\rangle+|1101001\rangle\Big)\\|\overline{1}\rangle&=\frac{1}{\sqrt{8}}\Big(|1111111\rangle+|0101010\rangle+|1001100\rangle+|0011001\rangle\\&\,\,\,\,\,\,\,\,+|1110000\rangle+|0100101\rangle+|1000011\rangle+|0010110\rangle\Big)~. \end{split} \end{align} The automorphism group of the code is $$PGL(3,2)$$ [61]. Protection: The Steane code is a distance 3 code. It detects errors on 2 qubits, corrects errors on 1 qubit. Cousin of: Quantum divisible code, $$[7,4,3]$$ Hamming code.
Smallest 3D color code whose physical qubits lie on vertices of a cube and which admits a transversal CCZ gate. Similar constructions exist on $$d$$-dimensional hypercubes and are called hyperoctahedron $$[[2^d,d,2]]$$ codes [64]. Parents: Color code.
Kitaev surface code on a fractal geometry, which is obtained by removing qubits from the surface code on a cubic lattice. Stub. Parents: Kitaev surface code. Cousins: Higher-dimensional surface code.
A family of Kitaev surface codes on planar or toric surfaces of dimension greater than two. Stub. Parents: Kitaev surface code. Cousin of: Fractal surface code, Self-correcting quantum code.
Hyperbolic surface code An extension of the Kitaev surface code construction to hyperbolic manifolds in dimension two or greater. Given a cellulation of a manifold, qubits are put on $$i$$-dimensional faces, $$X$$-type stabilizers are associated with $$(i-1)$$-faces, while $$Z$$-type stabilizers are associated with $$i+1$$-faces. Protection: Constructions (see code children below) have yielded distances scaling favorably with the number of qubits. The use of hyperbolic surfaces allows one to circumvent bounds on surface code parameters that are valid for surfaces with bounded geometry. Parents: Kitaev surface code. Cousins: Holographic code.
A family of Kitaev surface codes on the non-orientable 2-dimensional compact manifold $$\mathbb{R}P^2$$ (in contrast to a genus-$$g$$ surface). Whereas genus-$$g$$ surface codes require $$2g$$ logical qubits, qubit codes on $$\mathbb{R}P^2$$ are made from a single logical qubit. Protection: If $$\mathcal{C}$$ is a cellulation of $$\mathbb{R}P^2$$, then the bit-flip distance $$d_X$$ is the shortest cycle in $$\mathcal{C}$$, and the phase-flip distance $$d_Z$$ is the shortest cycle in the dual cellulation $$\mathcal{C}^*$$. Parents: Kitaev surface code. Cousin of: $$[[9,1,3]]$$ Shor code.
Also known as the $$C_4$$ code. Four-qubit CSS stabilizer code with generators $$\{XXXX, ZZZZ\}$$ and codewords \begin{align} \begin{split} |\overline{00}\rangle = (|0000\rangle + |1111\rangle)/\sqrt{2}~{\phantom{.}}\\ |\overline{01}\rangle = (|0011\rangle + |1100\rangle)/\sqrt{2}~{\phantom{.}}\\ |\overline{10}\rangle = (|0101\rangle + |1010\rangle)/\sqrt{2}~{\phantom{.}}\\ |\overline{11}\rangle = (|0110\rangle + |1001\rangle)/\sqrt{2}~. \end{split} \end{align} This code is the smallest single-qubit error-detecting code. It is also the smallest instance of the toric code, and its various single-qubit subcodes are small planar surface codes. Protection: Detects a single-qubit error [68] or single erasure [25]. Not able to correct arbitrary single-qubit errors because $$\lfloor \frac{d-1}{2} \rfloor =0$$. Approximately corrects a single amplitude damping error [69]. Cousin of: Heavy-hexagon code, $$[[8,3,2]]$$ code.
Encodes $$1$$ qubit into $$n$$ qubits according to $$|0\rangle\to|\phi_0\rangle^{\otimes n}$$ and $$|1\rangle\to|\phi_1\rangle^{\otimes n}$$. Also known as a bit-flip code when $$|\phi_i\rangle = |i\rangle$$, and a phase-flip code when $$|\phi_0\rangle = |+\rangle$$ and $$|\phi_1\rangle = |-\rangle$$. Protection: Bit-flip code detects bit-flip errors $$X$$ on $$\left\lfloor (n-1)/2\right\rfloor$$ qubits and does not detect any phase-flip errors $$Z$$. Phase-flip code detects phase-flip errors $$Z$$ on $$\left\lfloor (n-1)/2\right\rfloor$$ qubits and does not detect any bit-flip errors $$X$$. Because they protect against only one type of noise, both codes can be thought of as a classical $$[n,1,d]$$ repetition code with classical distance $$d=\left\lfloor (n-1)/2\right\rfloor$$ embedded in a quantum system. Parents: Quantum parity code (QPC). Cousins: Hamiltonian-based code.
Nine-qubit CSS code that is the smallest such code to correct a single-qubit error. Logical codewords are \begin{align} \begin{split} |\overline{0}\rangle&=\frac{1}{2\sqrt{2}}\left(|000\rangle+|111\rangle\right)^{\otimes3}\\ |\overline{1}\rangle&=\frac{1}{2\sqrt{2}}\left(|000\rangle-|111\rangle\right)^{\otimes3}~. \end{split} \end{align} The code works by concatenating each qubit of a phase-flip with a bit-flip repetition code. Therefore, the code can correct both type of errors simultaneously. Protection: The code detects two-qubit errors or corrects an arbitrary single-qubit error. Parents: Quantum parity code (QPC). Cousin of: Lloyd-Slotine nine-mode code.
Also called a checkerboard code. CSS variant of the surface code defined on a square lattice that has been rotated 45 degrees such that qubits are on vertices, and both $$X$$- and $$Z$$-type check operators occupy plaquettes in an alternating checkerboard pattern. Protection: The $$[[L^2,1,L]]$$ variant of this family includes the $$[[9,1,3]]$$ surface-17 code, named as such because 8 ancilla qubits are used for check operator measurements alongside the 9 physical qubits. Parent of: Surface-17 code.
Also called the tailored surface code (TSC). Non-CSS derivative of the surface code whose generators are $$XXXX$$ and $$YYYY$$, obtained by mapping $$Z \to Y$$ in the surface code. Protection: As a stabilizer code, $$[[n=O(d^2), k=O(1), d]]$$. Cousins: Heavy-hexagon code.
Non-CSS variant of the rotated surface code whose generators are $$XZXZ$$ Pauli strings associated, clock-wise, to the vertices of each face of a two-dimensional lattice (with a qubit located at each vertex of the tessellation). Protection: As a stabilizer code, $$[[n=O(d^2), k=O(1), d]]$$. Parent of: Five-qubit perfect code.
Stub. Parents: Floquet code. Cousins: Color code.
Floquet code inspired by the Kitaev honeycomb model [52] whose logical qubits are generated through a particular sequence of measurements. Protection: Protective features similar to the surface code: on a torus geometry, the code protects two logical qubits with a code distance proportional to the linear size of the torus. Properties of the code with open boundaries are discussed in Refs. [82][83], and various other generalizations have been proposed [84]. Parents: Floquet code.
An instance of the matching code based on the Kitaev honeycomb model. It is described on a hexagonal lattice with $$XYZXYZ$$ stabilizers on each hexagonal plaquette. Each vertical pair of qubits has an $$XX$$, $$YY$$, or $$ZZ$$ link stabilizer depending on the orientation of the plaquette stabilizers. Protection: As a stabilizer code with boundaries, protects a single qubit with parameters $$[[2 d^2, 1, d]]$$. Parents: Matching code.
A type of stabilizer code where stabilizer generators are elements of the group $$\{\alpha I, X, \sqrt{Z}]\}^{\otimes n}$$, with $$\sqrt{Z} = \text{diag} (1, i)$$. The codespace is a joint $$+1$$ eigenspace of a set of stabilizer generators, which need not commute to define a valid codespace. Parents: XP stabilizer code. Cousins: Abelian topological code.
A variant of the Kitaev surface code on a 3D lattice. The closely related solid code [88] consists of several 3D surface codes stitched together in a way that the distance scales faster than the linear size of the system. Parents: Higher-dimensional surface code. Cousins: Color code. Cousin of: Self-correcting quantum code.
Hyperbolic surface code constructed using cellulation of a Riemannian Manifold $$M$$ exhibiting systolic freedom [90]. Codes derived from such manifolds can achieve distances scaling better than $$\sqrt{n}$$, something that is impossible using closed 2D surfaces or 2D surfaces with boundaries [91]. Improved codes are obtained by studying a weak family of Riemann metrics on closed 4-dimensional manifolds $$S^2\otimes S^2$$ with the $$Z_2$$-homology. Protection: Four-dimensional manifolds with weak systolic freedom yield $$[[n,2,\Omega(\sqrt{n \sqrt{\log n}})]]$$ surface codes. Cousin of: Ramanujan-complex product code.
Hyperbolic surface code based on cellulations of certain four-dimensional manifolds. The manifolds are shown to have good homology and systolic properties for the purposes of code construction, with corresponding codes exhibiting linear rate. Protection: Protection stems from the relationship between properties of manifolds and CSS codes derived from their cellulation. The number of physical $$k$$ qubits and distance $$d$$ of the code will scale as $$\Omega(n)$$ and $$\Omega(n^\epsilon)$$, respectively. Parent of: Golden code.
Hyperbolic surface codes based on a tessellation of a closed 2D manifold with a hyperbolic geometry (i.e., non-Euclidean geometry, e.g., saddle surfaces when defined on a 2D plane). Protection: Protects against Pauli errors with distance $$d \propto \log(n)$$. Code parameters are $$[[n, (1-2/r - 2/s) n + 2, O(\log n) ]]$$ Parents: Hyperbolic surface code.
Member of CSS code family constructed with a first-order punctured RM$$(1,r)$$ $$[2^r-1,r+1,2^{r-1}-1]$$ code and its even subcode for $$r \geq 3$$. Each code transversally implements a member of an infinite family of diagonal gates from the Clifford hierarchy [98]. Parents: Quantum Reed-Muller code.
CCS code constructed with a classical Hamming code $$[2^r-1,2^r-1-r,3]=C_X=C_Z$$ a.k.a. a first-order punctured Reed-Muller code RM$$(r-2,r)$$. Protection: Protects against any single qubit error. Parents: Quantum Reed-Muller code. Cousins: Hamming code.
Member of CSS code family constructed with a punctured self-dual RM $$[2^r-1,2^{r-1},\sqrt{2}^{r-1}-1]$$ code and its even subcode for $$r \geq 2$$. Parents: Quantum Reed-Muller code. Parent of: $$[[7,1,3]]$$ Steane code.
A $$[[9,1,3]]$$ rotated surface code named for the sum of its 9 data qubits and 8 syndrome qubits. It uses the smallest number of qubits to perform error correction on a surface code with parallel syndrome extraction. Protection: Independent correction of single-qubit $$X$$ and $$Z$$ errors. Correction for some two-qubit $$X$$ and $$Z$$ errors. Parents: Rotated surface code.
Variant of the Guth-Lubotzky hyperbolic surface code that uses regular tessellations for 4-dimensional hyperbolic space. Protection: Nonvanishing rate and asymptotic distance lower bounded by $$n^0.1$$. Parents: Guth-Lubotzky code.
Self-dual Hamming-based CSS code that admits permutation-based CZ logical gates. Cousins: Perfect quantum code.

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