Physical and logical qubits: Difference between revisions

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{{Short description|Types of quantum information}}

{{Use American English|date=January 2019}}

{{Excessive citations|date=November 2018|details=No need for multiple citations per claim}}

In [[quantum computing]], a ''[[qubit]]'' is a unit of information analogous to a [[bit]] (binary digit) in [[classical computing]], but it is affected by [[quantum mechanical properties]] such as [[superposition (quantum mechanics)|superposition]] and [[quantum entanglement|entanglement]] which allow qubits to be in some ways more powerful than classical bits for some [[task (computing)|task]]s. Qubits are used in [[quantum circuit]]s and [[quantum algorithm]]s composed of [[quantum logic gates]] to solve [[computational problem]]s, where they are used for [[input/output]] and intermediate computations.

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|issue=1

|pages=94|doi=10.1038/s41467-017-00045-1|pmid=28733580|pmc=5522494|issn=2041-1723

}}</ref> subject to [[unitary transformation (quantum mechanics)|unitary transformation]]s, has a long enough [[coherence time]] to be usable by quantum logic gates (c.fcf. [[propagation delay#Electronics|propagation delay]] for classical logic gates).<ref name="SixPhysicalQubits" /><ref>{{Cite web|url=https://www.iarpa.gov/index.php/research-programs/logiq/logical-qubits|title=Logical Qubits (LogiQ)|website=Intelligence Advanced Research Projects Activity|language=en-us|access-date=2018-09-18}}</ref><ref>{{Cite web|url=https://www.iarpa.gov/index.php/research-programs/logiq/logical-qubits|title=Logical Qubits (LogiQ)|website=www.iarpa.gov|language=en-us|access-date=2018-10-04}}</ref>

{{AsSince the development of|September 2018}}the first quantum computer in 1998, most technologies used to implement qubits face issues of stability, [[quantum decoherence|decoherence]],<ref name="Detecting bit-flip errors">{{Cite journal|last1=Ristè|first1=D.|last2=Poletto|first2=S.|last3=Huang|first3=M.-Z.|last4=Bruno|first4=A.|last5=Vesterinen|first5=V.|last6=Saira|first6=O.-P.|last7=DiCarlo|first7=L.|date=2014-10-20|title=Detecting bit-flip errors in a logical qubit using stabilizer measurements|arxiv = 1411.5542

|journal=Nature Communications|volume=6|issue=1|pages=6983|doi=10.1038/ncomms7983|pmid=25923318|pmc=4421804|issn=2041-1723}}</ref><ref name="A Very Small Logical Qubit">{{Cite journal|last=Kapit|first=Eliot|date=2016-04-12

|title=A Very Small Logical Qubit

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|arxiv = 1608.06335

|journal=Physical Review X|volume=8|issue=2|pages=021058|doi=10.1103/PhysRevX.8.021058|bibcode=2018PhRvX...8b1058J|s2cid=119108989|issn=2160-3308}}</ref> Thus, contemporary logical qubits [[Qubit#Physical implementations|typically consist of]] many physical qubits to provide stability, error-correction and fault tolerance needed to perform useful computations.<ref name="SixPhysicalQubits" /><ref name="A Very Small Logical Qubit" /><ref name=":4" />

In 2023, Google researchers showed how quantum error correction can improve logical qubit performance by increasing the physical qubit count.<ref name="Suppressing quantum errors">{{Cite journal|last=Acharya|first=Rajeev|date=2023-02-22

|title=Suppressing quantum errors by scaling a surface code logical qubit

|arxiv = 2207.06431

|journal=Nature |volume=614 |issue=7949 |pages=676–681|doi=10.1038/s41586-022-05434-1|pmid=36813892 |pmc=9946823|bibcode=2023Natur.614..676G |issn=1476-4687}}</ref> These results found that a larger logical qubit (49 physical qubits) had a lower error rate, about 2.9 percent per round of error correction, compared to a rate of about 3.0 percent for the smaller logical qubit (17 physical qubits).<ref>{{Cite web |last=Conover |first=Emily |date=2023-02-22 |title=Google's quantum computer reached an error-correcting milestone |website=ScienceNews |language=en-US |url=https://www.sciencenews.org/article/google-quantum-computer-sycamore-milestone |access-date=2024-07-09}}</ref>

In 2024, IBM researchers created a quantum error correction code 10 times more efficient than previous research, protecting 12 logical qubits for roughly a million cycles of error checks using 288 qubits.<ref name="High-threshold and low-overhead">{{Cite journal| last=Bravyi |first=Sergei |date=2024-03-27

|title=High-threshold and low-overhead fault-tolerant quantum memory

|arxiv = 2308.07915

|journal=Nature |volume=627 |issue=8005 |pages=778–782|doi=10.1038/s41586-024-07107-7 |pmid=38538939 |pmc=10972743 |bibcode=2024Natur.627..778B |issn=1476-4687}}</ref><ref>{{Cite web |last=Swayne |first=Matt |date=2024-03-28 |title=IBM Reports 10 Times More Efficient Error-Correcting Method Brings Practical Quantum Computers Closer To Reality |website=The Quantum Insider |language=en-US |url=https://thequantuminsider.com/2024/03/28/ibm-reports-10-times-more-efficient-error-correcting-method-brings-practical-quantum-computers-closer-to-reality/ |access-date=2024-07-09}}</ref> The work demonstrates error correction on near-term devices while reducing overhead – the number of physical qubits required to keep errors low.<ref>{{Cite web |last=Crane |first=Leah |date=2023-08-18 |title=IBM has just made error correction easier for quantum computers |website=New Scientist |language=en-US |url=https://www.newscientist.com/article/2388191-ibm-has-just-made-error-correction-easier-for-quantum-computers/ |access-date=2024-07-09}}</ref>

In 2024, Microsoft and Quantinuum announced experimental results that showed logical qubits could be created with significantly fewer physical qubits.<ref>{{Cite web |last=Choi |first=Charles |date=2024-04-03 |title=Microsoft Tests New Path to Reliable Quantum Computers - 1,000 physical qubits for each logical one? Try a dozen, says Redmond |website=IEEE Spectrum |language=en-US |url=https://spectrum.ieee.org/microsoft-quantum-computer-quantinuum |access-date=2024-07-09}}</ref> The team used quantum error correction techniques developed by Microsoft and Quantinuum's [[trapped ion]] hardware to use 30 physical qubits to form four logical qubits. Scientists used a qubit virtualization system and active syndrome extraction—also called repeated error correction to accomplish this.<ref>{{Cite web |last=Timmer |first=John |date=2024-04-03 |title=Quantum error correction used to actually correct errors |website=Ars Technica |language=en-US |url=https://arstechnica.com/science/2024/04/quantum-error-correction-used-to-actually-correct-errors/ |access-date=2024-07-09}}</ref> This work defines how to achieve logical qubits within quantum computation.<ref>{{Cite web |last=Sutor |first=Bob |date=2024-04-05 |title=Quantum in Context: Microsoft & Quantinuum Create Real Logical Qubits |website=The Futurum Group |language=en-US |url=https://futurumgroup.com/insights/quantum-in-context-microsoft-quantinuum-create-real-logical-qubits/ |access-date=2024-07-09}}</ref>

== Overview ==

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== Topological quantum computing ==

The approach of [[topological qubit]]s, which takes advantage of [[topological quantum field theory|topological effects in quantum mechanics]], has been proposed as needing many fewer or even a single physical qubit per logical qubit.<ref name="Quantum Frontiers" /> Topological qubits rely on a class of particles called [[anyon]]s which have [[Spin (physics)|spin]] that is neither [[Half-integer|half-integral]] ([[fermion]]s) nor [[integer|integral]] ([[boson]]s), and therefore obey neither the [[Fermi–Dirac statistics]] nor the [[Bose–Einstein statistics]] of particle behavior.<ref name="Wilczek anyons">{{Cite news|url=https://www.quantamagazine.org/how-anyon-particles-emerge-from-quantum-knots-20170228/|title=How 'Anyon' Particles Emerge From Quantum Knots {{!}} Quanta Magazine|last=Wilczek|first=Frank|date=2018-02-27|work=Quanta Magazine|access-date=2018-09-18}}</ref> Anyons exhibit [[braid symmetry]] in their [[world line]]s, which has desirable properties for the stability of qubits. Notably, anyons must exist in systems constrained to two spatial dimensions or fewer, according to the [[spin–statistics theorem]], which states that in 3 or more spatial dimensions, only fermions and bosons are possible.<ref name="Wilczek anyons" /> In 2025, researchers made progress in topological quantum computing by successfully measuring the state of special particles called [[Majorana fermion|Majorana]] zero modes in a single step.<ref>{{Cite journal |last=Microsoft Azure Quantum |last2=Aghaee |first2=Morteza |last3=Alcaraz Ramirez |first3=Alejandro |last4=Alam |first4=Zulfi |last5=Ali |first5=Rizwan |last6=Andrzejczuk |first6=Mariusz |last7=Antipov |first7=Andrey |last8=Astafev |first8=Mikhail |last9=Barzegar |first9=Amin |last10=Bauer |first10=Bela |last11=Becker |first11=Jonathan |last12=Bhaskar |first12=Umesh Kumar |last13=Bocharov |first13=Alex |last14=Boddapati |first14=Srini |last15=Bohn |first15=David |date=2025-02-20 |title=Interferometric single-shot parity measurement in InAs–Al hybrid devices |url=https://www.nature.com/articles/s41586-024-08445-2 |journal=Nature |language=en |volume=638 |issue=8051 |pages=651–655 |doi=10.1038/s41586-024-08445-2 |issn=0028-0836 |pmc=11839464 |pmid=39972225}}</ref>

== See also ==