Self-correcting quantum code

Self-correcting quantum code[1][2]

Description

Also called a self-correcting quantum memory or thermally stable encoding. A ground-state encoding of an \(n\)-body geometrically local Hamiltonian whose logical information is recoverable for arbitrary long times in the \(n\to\infty\) limit after interaction with a sufficiently cold thermal environment. Typically, one also requires a decoder whose decoding time scales polynomially with \(n\) and a finite energy density. The original criteria for a self-correcting quantum memory, informally known as the Caltech rules [3][4], also required finite-spin Hamiltonians.

The effect of a Markovian thermal environment consists of a Lindbladian in Davies form admitting a Gibbs steady state at some temperature \(T\) [5]. To test whether a system is self-correcting, an initial codeword \(\rho(0)\) is evolved under the Davies Lindbladian and the code Hamiltonian (or, if we are to allow extra passive protection, the code Lindbladian) to the state \(\rho(t)\) at time \(t\), after which it is decoded via decoding map \(\cal{D}\). The memory time \(\tau\) is defined to be \begin{align} \tau=\sup\left\{ t>0\,|\left\Vert {\cal D}(\rho(t))-\rho(0)\right\Vert _{1}<\epsilon\right\} \tag*{(1)}\end{align} for some fixed \(\epsilon\). For a self-correcting memory, there exists a critical temperature \(T_\star>0\) such that \(\tau\to\infty\) (typically, exponentially with \(n\)) as \(n\to\infty\) for any temperature \(T<T_{\star}\) and any codeword \(\rho(0)\). A memory is partially self-correcting if \(\tau\) scales polynomially with \(n\) up to some cutoff \(n_{max}\). A self-correcting memory is typically associated with a (stable) phase of quantum matter.

Protection

Self-correcting classical memories exist in two and higher dimensions, with the canonical example being the classical Ising model. In that model, a classical bit is stored in the overall magnetization. The magnetization is thermally stable due to the fact that there is an \(n\)-dependent (i.e., macroscopic) energy cost of flipping a contiguous region of physical bits [6][5]. This cost scales with the surface area of the region, and the surface area is \(n\)-dependent for dimensions greater than one.

Self-correcting quantum memories currently exist in four and higher dimensions, with their existence in three dimensions being an open question. For similar reasons as the classical Ising model, the four-dimensional toric code is a self-correcting quantum memory due to an order \(O(n)\) energy cost of creating a logical error [1][2]. On the other hand, the 2D toric code is not thermally stable [7][8][9][10] because its string-like logical operators anti-commite with stabilizer generators supported only at their ends, and thus have a constant energy cost of creation. There is a general upper bound on the relaxation rate of a qubit stabilizer or qubit subsystem stabilizer quantum memory interacting with a Markovian environment [11].

An \(n\)-dependent energy barrier to creating all logical errors is likely necessary for a thermally stable memory, having been shown as such for a large class of 2D topological phases [12][13][14]. Two-dimensional stabilizer codes [15] and encodings of frustration-free code Hamiltonians [16] admit only constant-energy excitations, and so do not have admit such a barrier. No-go theorems for 3D models are much more restrictive, and there exist several candidates for self-correction as well as several partially self-correcting memories (see cousins below).

Parent

Hamiltonian-based code

Child

Symmetry-protected self-correcting quantum code

Cousins

Translationally invariant stabilizer code — 3D translationally-invariant qubit stabilizer code families with constant \(k\) support logical string operators and thus cannot be self-correcting [17]. For non-constant \(k\), such families can support at most a logarithmic energy barrier [18].
Higher-dimensional surface code — The 4D toric code is a self-correcting quantum memory [1][2].
3D surface code — The 3D welded solid code is partially self-correcting with a power-law energy barrier [19]. The 3D toric code is a classical self-correcting memory, whose protected bit admits a membrane-like logical operator [4].
Color code — The 6D color code is a self-correcting quantum memory [20].
Haah cubic code — Cubic code 1 is partially self-correcting with a logarithmic energy barrier [21].
Quantum repetition code — The bit-flip repetition code associated with the 2D classical Ising model is a self-correcting classical memory [5; Sec. V.A].
Repetition code — The repetition code associated with the 2D classical Ising model is a self-correcting classical memory [5; Sec. V.A].
Bacon-Shor code — 3D Bacon-Shor codes were conjectured to be self-correcting [22], but there remain issues to be resolved in order to validate this conjecture (see [5; Sec. IX.B]).
Matrix-model code — Matrix-model codes are similar to self-correcting memories in the sense that memory time becomes infinite in the thermodynamic limit, but with corrections being polynomial in \(N\).
Quantum locally testable code (QLTC) — The notion of an energy barrier in a self-correcting memory is intimately related to the soundness of a QLTC.

References

[1]: E. Dennis et al., “Topological quantum memory”, Journal of Mathematical Physics 43, 4452 (2002) arXiv:quant-ph/0110143 DOI
[2]: R. Alicki et al., “On thermal stability of topological qubit in Kitaev’s 4D model”, (2008) arXiv:0811.0033
[3]: C. G. Brell, “A proposal for self-correcting stabilizer quantum memories in 3 dimensions (or slightly less)”, New Journal of Physics 18, 013050 (2016) arXiv:1411.7046 DOI
[4]: O. Landon-Cardinal et al., “Perturbative instability of quantum memory based on effective long-range interactions”, Physical Review A 91, (2015) arXiv:1501.04112 DOI
[5]: B. J. Brown et al., “Quantum memories at finite temperature”, Reviews of Modern Physics 88, (2016) arXiv:1411.6643 DOI
[6]: R. Peierls, “On Ising’s model of ferromagnetism”, Mathematical Proceedings of the Cambridge Philosophical Society 32, 477 (1936) DOI
[7]: R. Alicki, M. Fannes, and M. Horodecki, “A statistical mechanics view on Kitaev’s proposal for quantum memories”, Journal of Physics A: Mathematical and Theoretical 40, 6451 (2007) arXiv:quant-ph/0702102 DOI
[8]: Z. Nussinov and G. Ortiz, “Autocorrelations and thermal fragility of anyonic loops in topologically quantum ordered systems”, Physical Review B 77, (2008) arXiv:0709.2717 DOI
[9]: R. Alicki, M. Fannes, and M. Horodecki, “On thermalization in Kitaev’s 2D model”, Journal of Physics A: Mathematical and Theoretical 42, 065303 (2009) arXiv:0810.4584 DOI
[10]: F. Pastawski et al., “Limitations of Passive Protection of Quantum Information”, (2009) arXiv:0911.3843
[11]: S. Chesi et al., “Thermodynamic stability criteria for a quantum memory based on stabilizer and subsystem codes”, New Journal of Physics 12, 025013 (2010) arXiv:0907.2807 DOI
[12]: K. Temme, “Thermalization time bounds for Pauli stabilizer Hamiltonians”, (2016) arXiv:1412.2858
[13]: A. Kómár, O. Landon-Cardinal, and K. Temme, “Necessity of an energy barrier for self-correction of Abelian quantum doubles”, Physical Review A 93, (2016) arXiv:1601.01324 DOI
[14]: A. Lucia, D. Pérez-García, and A. Pérez-Hernández, “Thermalization in Kitaev’s quantum double models via Tensor Network techniques”, (2021) arXiv:2107.01628
[15]: S. Bravyi and B. Terhal, “A no-go theorem for a two-dimensional self-correcting quantum memory based on stabilizer codes”, New Journal of Physics 11, 043029 (2009) arXiv:0810.1983 DOI
[16]: O. Landon-Cardinal and D. Poulin, “Local Topological Order Inhibits Thermal Stability in 2D”, Physical Review Letters 110, (2013) arXiv:1209.5750 DOI
[17]: B. Yoshida, “Feasibility of self-correcting quantum memory and thermal stability of topological order”, Annals of Physics 326, 2566 (2011) arXiv:1103.1885 DOI
[18]: J. Haah, “Local stabilizer codes in three dimensions without string logical operators”, Physical Review A 83, (2011) arXiv:1101.1962 DOI
[19]: K. P. Michnicki, “3D Topological Quantum Memory with a Power-Law Energy Barrier”, Physical Review Letters 113, (2014) arXiv:1406.4227 DOI
[20]: H. Bombin et al., “Self-Correcting Quantum Computers”, (2012) arXiv:0907.5228
[21]: S. Bravyi and J. Haah, “Quantum Self-Correction in the 3D Cubic Code Model”, Physical Review Letters 111, (2013) arXiv:1112.3252 DOI
[22]: D. Bacon, “Operator quantum error-correcting subsystems for self-correcting quantum memories”, Physical Review A 73, (2006) arXiv:quant-ph/0506023 DOI

Page edit log

Victor V. Albert (2022-09-28) — most recent
Victor V. Albert (2022-05-18)
Yi-Ting (Rick) Tu (2022-05-02)

Cite as:

“Self-correcting quantum code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2022. https://errorcorrectionzoo.org/c/self_correct

Github: https://github.com/errorcorrectionzoo/eczoo_data/tree/main/codes/quantum/properties/self_correct.yml.