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<p><b>New page</b></p><div>{{Short description|Methods for protecting quantum information from noise}}<br />
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|text='''Quantum error correction''' is a ScholarlyWiki page in the Quantum Collection about protecting fragile quantum information from noise, decoherence, and imperfect operations.<br />
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== Overview ==<br />
'''Quantum error correction''' is the theory and practice of protecting [[Quantum information|quantum information]] from errors caused by noise, imperfect gates, measurement faults, and [[Physics:Quantum decoherence|decoherence]]. It is a central ingredient in scalable [[Quantum computing|quantum computing]], because physical qubits are never perfectly isolated from their environment.<br />
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The basic strategy is to encode one logical qubit into a larger system of physical qubits. The encoded state is arranged so that certain errors can be detected by measuring auxiliary observables called syndromes. These syndrome measurements reveal information about the error without directly revealing the unknown logical state.<br />
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== Key ideas ==<br />
Quantum error correction is constrained by the [[Physics:Quantum No-cloning theorem|no-cloning theorem]] and by measurement disturbance. A quantum code cannot simply copy an unknown qubit several times and vote on the result. Instead, it stores information nonlocally across an entangled subspace.<br />
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A simple schematic encoding has the form<br />
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<math display="block"><br />
|\psi\rangle = \alpha |0\rangle + \beta |1\rangle<br />
\quad \longmapsto \quad<br />
|\psi_L\rangle = \alpha |0_L\rangle + \beta |1_L\rangle,<br />
</math><br />
<br />
where <math>|0_L\rangle</math> and <math>|1_L\rangle</math> are logical code states made from several physical qubits. If a correctable error occurs, the state leaves the code space in a structured way. Syndrome extraction identifies which correction should be applied.<br />
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== Error syndromes ==<br />
An error syndrome is a set of measurement outcomes that distinguishes different error patterns while preserving the encoded quantum information. In stabilizer codes, the syndrome is obtained by measuring commuting stabilizer operators. These measurements indicate whether the state remains in the code space or has been displaced by an error.<br />
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For example, bit-flip, phase-flip, and combined Pauli errors can be treated as basic components of more general noise. A code is designed so that these components move the logical state into distinguishable syndrome subspaces. A recovery operation then maps the damaged state back toward the code space.<br />
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== Stabilizer codes ==<br />
Many important quantum codes are [[Stabilizer code|stabilizer codes]]. A stabilizer code is defined by a group of commuting Pauli operators that leave every valid code state unchanged. The logical information is stored in the simultaneous eigenspace of these operators.<br />
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The stabilizer framework gives a compact language for describing codes, syndrome circuits, logical gates, and fault-tolerant operations. Examples include the Shor code, Steane code, color codes, and surface codes. Surface-code families are especially important in modern architecture studies because they use mostly local checks on a two-dimensional layout.<br />
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== Fault tolerance ==<br />
Quantum error correction must itself be implemented using imperfect hardware. '''Fault tolerance''' is the design principle that errors introduced during correction should not spread uncontrollably into logical failures. This requires careful syndrome extraction, verified or repeated measurements, protected logical gates, and decoding algorithms that infer the most likely correction from noisy data.<br />
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The threshold theorem states, in broad terms, that arbitrarily long quantum computation is possible if physical error rates are below a suitable threshold and if enough overhead is used. The actual threshold depends on the noise model, code family, connectivity, gate set, measurement fidelity, and decoder.<br />
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== Relation to noise ==<br />
Quantum error correction connects directly to [[Physics:Quantum noise|quantum noise]], [[Physics:Quantum channel|quantum channels]], and [[Physics:Quantum Open systems|open quantum systems]]. A physical error process can be modeled as a channel, while a code and recovery map are designed so that the combined logical channel is much closer to the identity operation.<br />
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In near-term devices, error correction is limited by qubit counts, gate fidelities, leakage, crosstalk, and measurement latency. Nevertheless, smaller codes, repetition codes, and error-detection experiments are used to test the principles that underlie larger fault-tolerant machines.<br />
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== See also ==<br />
{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also}}<br />
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== References ==<br />
{{reflist|3}}<br />
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* {{Cite journal |last1=Shor |first1=Peter W. |title=Scheme for reducing decoherence in quantum computer memory |journal=Physical Review A |volume=52 |issue=4 |pages=R2493-R2496 |year=1995 |doi=10.1103/PhysRevA.52.R2493}}<br />
* {{Cite journal |last1=Steane |first1=Andrew M. |title=Error Correcting Codes in Quantum Theory |journal=Physical Review Letters |volume=77 |issue=5 |pages=793-797 |year=1996 |doi=10.1103/PhysRevLett.77.793}}<br />
* {{Cite journal |last1=Gottesman |first1=Daniel |title=Stabilizer Codes and Quantum Error Correction |journal=arXiv |year=1997 |eprint=quant-ph/9705052}}<br />
* {{Cite journal |last1=Kitaev |first1=A. Yu. |title=Fault-tolerant quantum computation by anyons |journal=Annals of Physics |volume=303 |issue=1 |pages=2-30 |year=2003 |doi=10.1016/S0003-4916(02)00018-0 |arxiv=quant-ph/9707021}}<br />
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{{Author|Harold Foppele}}<br />
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