Superlinear code distance

We have shown that in a process of isolating a charged cluster, there is a logarithmic energy barrier.

The following theorem quantifies how long the process must be. The proof again makes use of renormalization group, and shows that there is a subset of ‘fractal dimension’γ >1 in the support ofE, the operator that makes two separated clusters from the vacuum of which one is charged. We assume thatE has weight minimum possible.

Letwbe an odd positive number. We say a set of sitesC⊂Λ is alevel-pchunkif diam(C)< w^p. A path in the lattice is a finite sequence of sites (u₁, u₂, . . . , u_n) such that d(u_i, u_i+1) = 1. (Recall that we use thel_∞ metricd.) Using paths, we can say whether a set is connected.

Definition 6.4. A connected level-p chunkC ⊆ S is maximal with respect to a set of sites S if there exist a connected subsetC^◦⊆C and a pathζ= (u1, . . . , m, . . . , un)⊆C^◦satisfying

(i) d(u1, un) =w^p−w^p−1, (ii) d(u₁, m), d(u_n, m)≥ ^wp−w₂^p−1,

(iii) C^◦ contains the connected component ofm inBwp−wp−1 2

(m)∩S, and (iv) C contains the connected component ofC^◦ in Bwp−1

2

(C^◦)∩S.

The last two conditions restricts the position ofζ in C such that ζlies sufficiently far from the boundary of C. The sitem will be referred to as a midpointof C. LetS be the support of the Pauli operatorE, any restriction of which obeys the no-strings rule.

Lemma 6.2.1. Given a path ζ in S joining u₁ and u_n such thatd(u₁, u_n) =lw^p−1, there arel disjoint maximal chunks of level pwhose midpoints are onζ.

Proof. For convenience, we assume that thez-coordinates ofu1andunare 0 andlw^p−1, respectively.

Considerl+1 planesPiperpendicular to thez-axis, whosez-coordinates areiw^pfori= 0,1, . . . , l. In each region between the two consecutive planesPi−1andPi, there is a subpathζi= (uji−1, . . . , uj_i) such that d(uji−1, uji) =w^p−1. Choosemi ∈ζi such that d(uji−1, mi), d(mi, uji)≥^wp₂⁻¹. LetCi

be the connected component ofm_i withinS∩ Bwp 2

(m_i). Add, if necessary, some points ofζ_i toC_i to get a maximally connectedC_i⁰. ThisC_i⁰is a maximal chunk of sites with midpoint beingm_i. Any twoC_i⁰’s are disjoint since each of them lies in a unique region enclosed byP_i−1 andP_i.

Lemma 6.2.2. For sufficiently largew, a maximal level (p+ 1) chunkC with respect to S admits a decomposition into w+ 1 or more maximal chunks of level pwith respect toS.

Proof. Recall thatSis the support of the Pauli operatorE, any restriction of which obeys no-strings rule. Define theboundary of a subset U of S to be ∂U =B1(U)∩U^c∩S. Then, any subset U of sites with boundary enclosed in a two disjoint regions can be regarded as a string segment.

By the definition of the maximal chunk, there exists a path (u₁, . . . , m, . . . , u_n) inC^◦⊆C such that d(u₁, u_n) =w^p+1−w^p. We assume that thez-coordinates ofu₁, u_n differ byw^p+1−w^p. We will show that there are sufficiently long and separated paths inC^◦, to which we apply Lemma6.2.1 to findw+ 1 maximal chunks of levelp. They will lie inBwp

2 (C^◦), and hence inC.

LetM (N) be the subset ofS consisted of sites whosez-coordinates differ from that ofu1 (un) by at most ηw^p. First, suppose∂C^◦ is not contained inM ∪N. Sinceu1∈M and un ∈N, there is a site s ∈C^◦ adjacent (of distance 1) to ∂C^◦ such that d(s, u1), d(s, un) > ηw^p. Furthermore, d(s, m)≥ ^wp+1₂^−wp −1; otherwise, C^◦ contains a site in the boundary, which is a contradiction.

Consider the shortest network N of paths inC^◦ connecting four sitesu₁, m, u_n, s. (The length of a network of paths is the number of sites in the union of the paths.) Letζbe the shortest path in N fromu₁tou_n. Ifsis not contained inB_3wp(ζ), thenζ⁰ ⊆ N joiningsto a site onζhas a subpath ζ⁰⁰ ⊆ζ⁰ of diameter at least 2w^p such thatζ⁰⁰ is separated from ζ byw^p. Applying Lemma 6.2.1 to ζ and ζ⁰⁰, we find at least w+ 1 maximal chunk of level p. Ifmis not contained in B3w^p(ζ), a similar argument reveals at leastw+ 1 maximal chunk of levelp.

Suppose bothsandmare contained inB3w^p(ζ). Observe that ζ\(B4w^p(s)∪ B4w^p(m)) consists of three connected components ζ1, ζ2, ζ3, two of which have diameter ≥ ^wp+1₂^−wp −8w^p and the other has diameter ≥(η−4)w^p. Two distinct Bwp

2 (ζi) andBwp

2 (ζj) (i, j = 1,2,3) do not overlap because of the minimality ofζ. Applying Lemma6.2.1, we findw+η−21 maximal chunks of level p. Choosingη >21, we get the desired result.

Next, suppose ∂C^◦ is contained inM ∪N. Let s_M, s_N ∈ C^◦\(M ∪N) be sites adjacent to M and N, respectively. The separation between M and N is (w−1−2η)w^p. If it is greater than η⁰αw^p, there must be az-plane P that containss_M or s_N such that P∩C^◦ has diameter> η⁰w^p; Otherwise, the no-strings rule is violated. Letv1, v2∈P∩C^◦be sites separated byη⁰w^p. The four sites,u1, un, v1, v2are sufficiently separated and connected by some paths inC^◦. Arguing as before, we find (w−1) +η⁰−16 maximal chunks of level p. The choice of η⁰ >17 and w >1 + 2η+η⁰α proves the lemma.

Theorem 6.3. Let E be a Pauli operator creating S, a neutral cluster of defects containing a charged clusterS⁰ ⊆S of diameter rsuch that there are no other defects within distanceRfromS⁰. If r+ 2R < L_tqo, then the weight of E must be≥cR^γ for some constantγ >1 andc.

Proof. The support of the minimal Pauli operatorE in Theorem6.3must admit a path connecting S⁰ and S\S⁰. Otherwise, S⁰ can be regarded as being created locally, and our topological order condition demands the cluster be neutral. Since the path has length ≥ R, Lemma 6.2.1 says we have a maximal chunk of level p where p is such that w^p ≤ R < w^p+1. Lemma 6.2.2 implies any maximal chunk of levelp must contain at least (w+ 1)^p sites. This concludes the proof with γ=^log(w+1)_logw >1.

A similar argument proves the lower bound d= Ω(L^γ) on the code distancedof the cubic code since the minimal logical operator must contain a path of lengthL.

Chapter 7

Renormalization group decoder and error threshold theorem

Any error correcting scheme would be comprised of a chosen code space, an encoding procedure, and a decoding procedure. We have studied a way to choose a code space via additive/stabilizer code formalism. Our focus has been the situation where the code space is realized as a ground space of a local Hamiltonian. The encoding is a process in which one prepares a state that is to be transferred or stored, and then one embeds the state into the designed code space. For a concatenated code the encoding would be hierarchical resembling the very way the code is constructed. Interestingly, a ground state of toric code model can also be prepared in a similarly hierarchical way [105]. In case of the cubic code, for example, the encoding can be done by inverting the real-space renormalization group procedure presented in Section5.4.

The decoder of a quantum code restores a damaged state into the code space. In contrast to its name, the decoder should not reveal any information that is encoded. Rather, it prepares the state appropriate for next information processing step which assumes that the state is in the code space; it detects errors and suggests an operator that would undo the errors. The performance of the decoder is measured by how closely the damaged state is restored to the original encoded state. In this chapter, we explain a decoding algorithm, calledrenormalization group decoder, that is applicable for a family of topological codes including the cubic code. A very similar idea appears in Harrington’s thesis [114]. A decoder for 2D toric code with a similar name was proposed by Duclos-Cianci and Poulin [115]. The two decoders are conceptually similar, and the running times are the same up to a multiplicative constant. Our decoder is however advantageous for its simplicity and applicability. In particular, our decoder is the only decoder so far that has a positive error threshold under stochastic error when used with the cubic code. In fact, our decoder provides a universal positive error threshold for all topological codes in a given number of spatial dimensions, as we prove in Section7.5.

Formally, if we restrict ourselves to local additive codes, the decoder is an association of a Pauli

operatorP to any possible syndrome S such that the Pauli operatorP transformsS to the empty syndrome. Recall that the syndrome measurement reveals locations of defects (flipped stabilizer generators) created by an unknown error. The renormalization group (RG) decoder attempts to annihilate the defects comprising the syndromeS by dividing them into disjoint connected clusters S=C1∪. . .∪Cmand then trying to annihilate each clusterCa individually. More specifically, the decoder checks whetherCa can be annihilated by a Pauli operator Pa supported on a sufficiently small spatial regionb(Ca) enclosingCa. If such a local annihilation operatorPa exists, the decoder updates the syndrome by erasing all the defects comprisingCa, records the operatorPa, and moves on to the next cluster. IfC_acannot be annihilated, the decoder skips it. The annihilation operatorP_ais not unique. However, if the enclosing regionb(C_a) is small enough to ensure that no logical operator can be supported onb(C_a), all annihilation operatorsP_a must be equivalent modulo stabilizers and the choice ofP_a does not matter.

After all clustersCa have been examined, the decoder is left with a new configuration of defects S⁰, which is typically smaller than the original one. If no defects are left, i.e., S⁰ =∅, the decoder stops and returns the product of all recorded Pauli operators Pa. If S⁰ 6= ∅, the decoder applies a scale transformation increasing the unit of length by some constant factor and repeats all the above steps starting from the syndrome S⁰. The scale transformation potentially merges several unerased clustersCa into a single connected cluster whereby giving the decoder one more attempt to annihilate them.

The full decoding algorithm is the iteration of partitioning the defects into the connected clusters and calculating the annihilation operators. It declares failure and aborts if the recorded operator cannot annihilate all the defects before the rescaled unit length is comparable to the lattice size.

A detailed implementation of the RG decoder must be tailored to a specific lattice geometry and a stabilizer code under consideration. It must include a precise definition of the connected clusters of defects Ca and the enclosing regions b(Ca). It must also include an algorithm for choosing the annihilation operatorsPa, a schedule for increasing the unit of length, and clearly stated conditions under which the decoder aborts. In the rest of this chapter we describe an efficient implementation of the RG decoder for arbitrary stabilizer codes satisfying topological order conditions defined in the previous chapter. The only part of this implementation specialized for the 3D cubic code is the “broom algorithm” of Section7.4. As we have noted in Remark6.1, the our topological order conditions are satisfied by every translationally invariant exact code. It turns out that the broom algorithm is also applicable for every translationally invariant code.

7.1 Assumptions and conventions

Let Λ be the regular 3D cubic lattice of linear size L with periodic boundary conditions along all coordinates x, y, z. We shall label sites of Λ by triples of integers (i, j, k) defined modulo Land measure the distance between sites using the`_∞-metric. In other words, the distanced(u, v) between a pair of sitesuand v is the smallest integerr such thatuand v can be enclosed by a cubic box with dimensionsr×r×r. For example, d(u, v) = 1 whenever uand v belong to the same edge, plaquette, or elementary cube of the lattice. Each site of Λ represents one or several physical qubits (two qubits for the 3D Cubic Code). Each elementary cube c represents a spatial location of one or several stabilizer generators For example, there are two generators for the 3D Cubic Code. A generator located at cube c may act only on qubits located at vertices of c. We shall label each elementary cube by coordinates of its center, the triple of half-integers (i, j, k) defined modulo L.

The distance d(c, c⁰) between a pair of cubes c and c⁰ is the distance between their centers. For example,d(c, c⁰) = 1 whenevercandc⁰ share a vertex, an edge, or a plaquette.

Adefectis a stabilizer generator whose eigenvalue has been flipped as a result of the error. We shall use a termcluster of defects, or simplycluster for any set of defects. Define thediameter of a cluster d(C) as the maximum distance d(c, c⁰) where c, c⁰ ∈ C. Here and below the distance between defects is defined as the distance between the cubes occupied by these defects. Given two non-empty clusters C and C⁰, define a distance d(C, C⁰) as the minimum distance d(c, c⁰) where c∈C and c⁰ ∈C⁰. Given an integer r, we shall say that a clusterC is connected at scale r, or simply r-connected, if C cannot be partitioned into two proper subsetsC = C⁰∪C⁰⁰ such that d(C⁰, C⁰⁰)> r. A maximalr-connected subset of a cluster Cis called a r-connected component ofC. Theminimal enclosing boxb(C) of a clusterCis the smallest rectangular boxB enclosing all defects ofC such that all vertices ofB are dual sites of Λ. Note that the minimal enclosing box b(C) is unique as long asd(C)< L/2; if a clusterChas diameterL/2, one may have two boxes with the same dimensions enclosingCthat ‘wrap’ around the lattice in two different ways.

Let G be the abelian group generated by the stabilizer generators. Elements of G are called stabilizers. Let S(P) be the syndrome of a Pauli operator P, that is, the set of all stabilizer generators anticommuting withP. The syndrome can be viewed as a cluster of defects.

We assume that our topological code obeys TQO1 and TQO2 of Defini- tions6.1,6.2throughout the chapter, but not the no-strings rule of Definition5.1.

The 3D cubic code satisfies both of TQO1 and TQO2 withL_tqo= ¹₂L, since it is exact. In order to avoid unnecessary complications due to boundaries, we always assume that L_tqo ≤ ¹₂L. Below we consider only topological stabilizer codes. Continued from the previous chapter, a cluster of defects Cis calledneutralif it can be created from the vacuum by a Pauli operatorP supported on a cube of linear sizeLtqo. Otherwise, the cluster is said to becharged. For example, the 2D toric code [3]

has two types of defects: magnetic charges (flipped plaquette operators) and electric charges (flipped star operators). In this case, a cluster of defectsCis neutral if and only if Ccontains even number of magnetic charges and even number of electric charges. It follows from TQO2 that any neutral cluster of defectsC can be annihilated by a Pauli operator supported on the 1-neighborhood of the minimum enclosing boxb(C).