Editing Quantum decoherence (section)

== Prevention ==

===Concept===
[[Quantum computing#Decoherence|Decoherence]] causes the system to lose its quantumness, which invalidates the superposition principle and turns 'quantum' to 'classical'.<ref>{{Citation |last=Zurek |first=Wojciech H. |title=Decoherence and the transition from quantum to classical -- REVISITED |date=2003-06-10 |arxiv=quant-ph/0306072 |bibcode=2003quant.ph..6072Z }}</ref> It is a major challenge in quantum computing.

A real quantum system inevitably meets the surrounding environment, the interaction shows up as noise in physical process.  It's extremely sensitive to environmental noise. Electromagnetic fields, temperature fluctuations, and other external perturbations, as well as measurement, lead to decoherence.

Decoherence is a challenge for the practical realization of [[Quantum computer|quantum computers]], since such machines are expected to rely heavily on the undisturbed evolution of quantum coherences. They require that the coherence of states be preserved and that decoherence be managed, in order to actually perform quantum computation. Because of decoherence, it is necessary to finish the quantum process before the qubit state is decayed.<ref>{{Cite book |last1=Zhang |first1=Yu |last2=Deng |first2=Haowei |last3=Li |first3=Quanxi |last4=Song |first4=Haoze |last5=Nie |first5=Leihai |chapter=Optimizing Quantum Programs Against Decoherence: Delaying Qubits into Quantum Superposition |date=July 2019 |title=2019 International Symposium on Theoretical Aspects of Software Engineering (TASE) |pages=184–191 |doi=10.1109/TASE.2019.000-2|arxiv=1904.09041 |isbn=978-1-7281-3342-3 }}</ref>

The physical quantity coherence time is defined as the time that the quantum state holds its superposition principle.

Preventing decoherence and thus extending the coherence time of quantum systems serves to improve the stability of the computation.<ref>{{Cite book |last1=Zhang |first1=Yu |last2=Deng |first2=Haowei |last3=Li |first3=Quanxi |last4=Song |first4=Haoze |last5=Nie |first5=Leihai |chapter=Optimizing Quantum Programs Against Decoherence: Delaying Qubits into Quantum Superposition |date=July 2019 |title=2019 International Symposium on Theoretical Aspects of Software Engineering (TASE) |pages=184–191 |doi=10.1109/TASE.2019.000-2|arxiv=1904.09041 |isbn=978-1-7281-3342-3 }}</ref>

=== Methods and Tools ===
Researchers have developed many methods and tools to mitigate or eliminate the negative influences from decoherence. Several typical ways are listed below.

==== Isolation from Environment ====
The most basic and direct way to reduce decoherence is to prevent the quantum system from interacting with the environment by any type of isolation. Here are some typical examples of isolation methods.

* [[Ultra-high vacuum|High Vacuum]]: Placing qubits in an ultra-high vacuum environment to minimize interaction with air molecules.{{cn|date=December 2024}}
* [[Cryogenic cooling|Cryogenic Cooling]]: Operating quantum systems at extremely low temperatures to reduce thermal vibrations and noise.{{cn|date=December 2024}}
* [[Electromagnetic shielding|Electromagnetic Shielding]]: Enclosing quantum systems in materials that block external electromagnetic fields - such as mu-metal or superconducting materials - reduces decoherence caused by unwanted electromagnetic interference.{{cn|date=December 2024}}
* Shielding [[Cosmic ray|Cosmic Rays]]: In August 2020 scientists reported that ionizing radiation from environmental radioactive materials and cosmic rays may substantially limit the coherence times of [[Qubit|qubits]] if they aren't shielded adequately which may be critical for realizing fault-tolerant superconducting quantum computers in the future.<ref>{{cite news |title=Quantum computers may be destroyed by high-energy particles from space |url=https://www.newscientist.com/article/2252933-quantum-computers-may-be-destroyed-by-high-energy-particles-from-space/ |access-date=7 September 2020 |work=New Scientist}}</ref><ref>{{cite news |title=Cosmic rays may soon stymie quantum computing |url=https://phys.org/news/2020-08-cosmic-rays-stymie-quantum.html |access-date=7 September 2020 |work=phys.org |language=en}}</ref><ref>{{cite journal |last1=Vepsäläinen |first1=Antti P. |last2=Karamlou |first2=Amir H. |last3=Orrell |first3=John L. |last4=Dogra |first4=Akshunna S. |last5=Loer |first5=Ben |last6=Vasconcelos |first6=Francisca |last7=Kim |first7=David K. |last8=Melville |first8=Alexander J. |last9=Niedzielski |first9=Bethany M. |last10=Yoder |first10=Jonilyn L. |last11=Gustavsson |first11=Simon |last12=Formaggio |first12=Joseph A. |last13=VanDevender |first13=Brent A. |last14=Oliver |first14=William D. |date=August 2020 |title=Impact of ionizing radiation on superconducting qubit coherence |url=https://www.nature.com/articles/s41586-020-2619-8 |journal=Nature |language=en |volume=584 |issue=7822 |pages=551–556 |arxiv=2001.09190 |bibcode=2020Natur.584..551V |doi=10.1038/s41586-020-2619-8 |issn=1476-4687 |pmid=32848227 |s2cid=210920566 |access-date=7 September 2020}}</ref>
* Better Materials: Fabricating qubits from special materials, like highly pure or isotopically enriched ones, to minimize intrinsic noise of the material, including noise from defects or nuclear spins.{{cn|date=December 2024}}
* Circuit Design: Optimizing the coherence ability when designing the construction of quantum circuits, similar to the concern in classical circuits.{{cn|date=December 2024}}
* Mechanical and Optical Isolation: Using equipment like vibration isolation tables and acoustic isolation materials, reducing sources of mechanical noise, and shielding against external light—common in physical experiments.{{cn|date=December 2024}}

==== Quantum Error Correction (QEC) ====
{{Only primary sources|date=December 2024}}
One of the most powerful tools for combating quantum decoherence is '''[[Quantum error correction|Quantum Error Correction]]''' (QEC). QEC schemes encode quantum information redundantly across multiple physical qubits, allowing for the detection and correction of errors without directly measuring the quantum state. These QEC protocols rely on the assumption that errors affect only a small fraction of qubits at any given time, enabling the detection and correction of errors through redundant encoding. Here are some representative QEC protocols.

* [[Shor code|Shor Code]]:<ref>{{Cite journal |last=Shor |first=Peter W. |date=1995-10-01 |title=Scheme for reducing decoherence in quantum computer memory |url=http://dx.doi.org/10.1103/physreva.52.r2493 |journal=Physical Review A |volume=52 |issue=4 |pages=R2493–R2496 |doi=10.1103/physreva.52.r2493 |bibcode=1995PhRvA..52.2493S |issn=1050-2947}}</ref> One of the first quantum error correction codes, it encodes a single qubit into nine physical qubits to protect against both bit-flip and phase-flip errors.

* [[Steane code|Steane Code]]:<ref>{{Cite journal |last=Steane |first=A. M. |date=1996-07-29 |title=Error Correcting Codes in Quantum Theory |url=http://dx.doi.org/10.1103/physrevlett.77.793 |journal=Physical Review Letters |volume=77 |issue=5 |pages=793–797 |doi=10.1103/physrevlett.77.793 |pmid=10062908 |bibcode=1996PhRvL..77..793S |issn=0031-9007}}</ref> A 7-qubit code that provides error correction for arbitrary errors.

* Surface Codes:<ref>{{Cite journal |last1=Dennis |first1=Eric |last2=Kitaev |first2=Alexei |last3=Landahl |first3=Andrew |last4=Preskill |first4=John |date=2002-09-01 |title=Topological quantum memory |url=http://dx.doi.org/10.1063/1.1499754 |journal=Journal of Mathematical Physics |volume=43 |issue=9 |pages=4452–4505 |doi=10.1063/1.1499754 |arxiv=quant-ph/0110143 |bibcode=2002JMP....43.4452D |issn=0022-2488}}</ref> A more scalable error correction code that uses a 2D lattice of qubits with high threshold for errors.

* [[Quantum error correction#Bosonic codes|Bosonic Codes]]: A type of quantum error-correcting code designed specifically to protect quantum information in continuous-variable systems.{{cn|date=December 2024}}

However, QEC comes at a significant cost: it requires a large number of physical qubits to encode a single logical qubit, and fault-tolerant error correction methods introduce additional computational overhead.

==== Dynamical Decoupling ====
'''[[Dynamical decoupling|Dynamical Decoupling]]''' (DD) is another typical [[quantum control]] technique used against decoherence, especially for systems that are coupled to noisy environments. DD involves applying an external sequence of control pulses to the quantum system at strategically timed intervals to average out environmental interactions. This technique effectively manipulates the irreversible component of quantum systems interact with surrounding environment by the external controllable interactions.<ref>{{Cite journal |last1=Viola |first1=Lorenza |last2=Knill |first2=Emanuel |last3=Lloyd |first3=Seth |date=1999-03-22 |title=Dynamical Decoupling of Open Quantum Systems |url=https://link.aps.org/doi/10.1103/PhysRevLett.82.2417 |journal=Physical Review Letters |language=en |volume=82 |issue=12 |pages=2417–2421 |doi=10.1103/PhysRevLett.82.2417 |arxiv=quant-ph/9809071 |bibcode=1999PhRvL..82.2417V |issn=0031-9007}}</ref> Dynamical decoupling has been experimentally demonstrated in various systems, including trapped ions<ref>{{Cite journal |last1=Biercuk |first1=Michael J. |last2=Uys |first2=Hermann |last3=VanDevender |first3=Aaron P. |last4=Shiga |first4=Nobuyasu |last5=Itano |first5=Wayne M. |last6=Bollinger |first6=John J. |date=2009-06-25 |title=Experimental Uhrig dynamical decoupling using trapped ions |url=https://link.aps.org/doi/10.1103/PhysRevA.79.062324 |journal=Physical Review A |language=en |volume=79 |issue=6 |page=062324 |doi=10.1103/PhysRevA.79.062324 |arxiv=0902.2957 |bibcode=2009PhRvA..79f2324B |issn=1050-2947}}</ref> and superconducting qubits.<ref>{{Cite journal |last1=Jurcevic |first1=Petar |last2=Javadi-Abhari |first2=Ali |last3=Bishop |first3=Lev S |last4=Lauer |first4=Isaac |last5=Bogorin |first5=Daniela F |last6=Brink |first6=Markus |last7=Capelluto |first7=Lauren |last8=Günlük |first8=Oktay |last9=Itoko |first9=Toshinari |last10=Kanazawa |first10=Naoki |last11=Kandala |first11=Abhinav |last12=Keefe |first12=George A |last13=Krsulich |first13=Kevin |last14=Landers |first14=William |last15=Lewandowski |first15=Eric P |date=2021-03-17 |title=Demonstration of quantum volume 64 on a superconducting quantum computing system |url=http://dx.doi.org/10.1088/2058-9565/abe519 |journal=Quantum Science and Technology |volume=6 |issue=2 |pages=025020 |doi=10.1088/2058-9565/abe519 |arxiv=2008.08571 |bibcode=2021QS&T....6b5020J |issn=2058-9565}}</ref> Here are some examples of representative sequences.

* [[Spin echo|Spin Echo]] (SE): SE is the consisting of a single π-pulse, which inverts the state of system.{{cn|date=December 2024}}

* Periodic Dynamical Decoupling (PDD): Applying control pulse periodically, PDD averages out the influence of the environment and decoupling the qubit.<ref>{{Cite journal |last1=Viola |first1=Lorenza |last2=Lloyd |first2=Seth |date=1998-10-01 |title=Dynamical suppression of decoherence in two-state quantum systems |url=https://link.aps.org/doi/10.1103/PhysRevA.58.2733 |journal=Physical Review A |language=en |volume=58 |issue=4 |pages=2733–2744 |doi=10.1103/PhysRevA.58.2733 |arxiv=quant-ph/9803057 |bibcode=1998PhRvA..58.2733V |issn=1050-2947}}</ref>

* Carr-Purcell-Meiboom-Gill (CPMG) Sequence:<ref>{{Cite journal |last1=Meiboom |first1=S. |last2=Gill |first2=D. |date=1958-08-01 |title=Modified Spin-Echo Method for Measuring Nuclear Relaxation Times |url=http://dx.doi.org/10.1063/1.1716296 |journal=Review of Scientific Instruments |volume=29 |issue=8 |pages=688–691 |doi=10.1063/1.1716296 |bibcode=1958RScI...29..688M |issn=0034-6748}}</ref> CPMG is an extension of SE. It applies a series of π-pulses.