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{{Short description|Quantum computing implementation}}
'''Superconducting quantum computing''' is a branch of [[Solid-state physics|solid state]] physics and quantum computing that implements [[superconductivity|superconducting]] [[electronic circuit]]s using superconducting qubits as artificial atoms, or [[quantum dot]]s. For superconducting qubits, the two logic states are the [[ground state]] and the [[excited state]], denoted <math>|g\rangle \text{ and } |e\rangle</math> respectively.<ref name="docs.pennylane.ai" /> Research in superconducting quantum computing is conducted by companies such as [[Google]],<ref>{{cite journal |last1=Castelvecchi |first1=Davide |title=Quantum computers ready to leap out of the lab in 2017 |journal=Nature |date=5 January 2017 |volume=541 |issue=7635 |pages=9–10 |doi=10.1038/541009a|pmid=28054624 |bibcode=2017Natur.541....9C |s2cid=4447373 |doi-access=free }}</ref> [[IBM]],<ref name="IBM">{{cite web |title=IBM Makes Quantum Computing Available on IBM Cloud |url=https://www-03.ibm.com/press/us/en/pressrelease/49661.wss |archive-url=https://web.archive.org/web/20160504214945/http://www-03.ibm.com/press/us/en/pressrelease/49661.wss |url-status=dead |archive-date=May 4, 2016 |website=www-03.ibm.com |date=4 May 2016}}</ref> [[IMEC]],<ref>{{Cite web|url=https://www.imec-int.com/en/articles/imec-enters-the-race-to-unleash-quantum-computing-with-silicon-qubits|title=Imec enters the race to unleash quantum computing with silicon qubits|website=www.imec-int.com|language=en|access-date=2019-11-10}}</ref> [[BBN Technologies]],<ref>{{cite journal | arxiv=1704.08314 | doi=10.1063/1.5006525 | title=Hardware for dynamic quantum computing | date=2017 | last1=Ryan | first1=Colm A.
{{As of|2016|May|df=US}}, up to 9 fully controllable [[qubit]]s are demonstrated in the 1D [[Array programming|array]],<ref>{{cite journal |last1=Kelly |first1=J. |last2=Barends |first2=R. |last3=Fowler |first3=A. G. |last4=Megrant |first4=A. |last5=Jeffrey |first5=E. |last6=White |first6=T. C. |last7=Sank |first7=D. |last8=Mutus |first8=J. Y. |last9=Campbell |first9=B. |last10=Chen |first10=Yu |last11=Chen |first11=Z. |last12=Chiaro |first12=B. |last13=Dunsworth |first13=A. |last14=Hoi |first14=I.-C. |last15=Neill |first15=C. |last16=O’Malley |first16=P. J. J. |last17=Quintana |first17=C. |last18=Roushan |first18=P. |last19=Vainsencher |first19=A. |last20=Wenner |first20=J. |last21=Cleland |first21=A. N. |last22=Martinis |first22=John M. |title=State preservation by repetitive error detection in a superconducting quantum circuit |arxiv=1411.7403 |journal=Nature |date=4 March 2015 |volume=519 |issue=7541 |pages=66–69 |doi=10.1038/nature14270|pmid=25739628 |bibcode=2015Natur.519...66K |s2cid=3032369 }}</ref> and up to 16 in 2D architecture.<ref name="IBM" /> In October 2019, the [[John M. Martinis|Martinis]] group, partnered with [[Google]], published an article demonstrating novel [[quantum supremacy]], using a chip composed of 53 superconducting qubits.<ref>{{cite journal |last1=Arute |first1=Frank |last2=Arya |first2=Kunal |last3=Babbush |first3=Ryan |last4=Bacon |first4=Dave |last5=Bardin |first5=Joseph C. |last6=Barends |first6=Rami |last7=Biswas |first7=Rupak |last8=Boixo |first8=Sergio |last9=Brandao |first9=Fernando G. S. L. |last10=Buell |first10=David A. |last11=Burkett |first11=Brian |last12=Chen |first12=Yu |last13=Chen |first13=Zijun |last14=Chiaro |first14=Ben |last15=Collins |first15=Roberto |last16=Courtney |first16=William |last17=Dunsworth |first17=Andrew |last18=Farhi |first18=Edward |last19=Foxen |first19=Brooks |last20=Fowler |first20=Austin |last21=Gidney |first21=Craig |last22=Giustina |first22=Marissa |last23=Graff |first23=Rob |last24=Guerin |first24=Keith |last25=Habegger |first25=Steve |last26=Harrigan |first26=Matthew P. |last27=Hartmann |first27=Michael J. |last28=Ho |first28=Alan |last29=Hoffmann |first29=Markus |last30=Huang |first30=Trent |last31=Humble |first31=Travis S. |last32=Isakov |first32=Sergei V. |last33=Jeffrey |first33=Evan |last34=Jiang |first34=Zhang |last35=Kafri |first35=Dvir |last36=Kechedzhi |first36=Kostyantyn |last37=Kelly |first37=Julian |last38=Klimov |first38=Paul V. |last39=Knysh |first39=Sergey |last40=Korotkov |first40=Alexander |last41=Kostritsa |first41=Fedor |last42=Landhuis |first42=David |last43=Lindmark |first43=Mike |last44=Lucero |first44=Erik |last45=Lyakh |first45=Dmitry |last46=Mandrà |first46=Salvatore |last47=McClean |first47=Jarrod R. |last48=McEwen |first48=Matthew |last49=Megrant |first49=Anthony |last50=Mi |first50=Xiao |last51=Michielsen |first51=Kristel |last52=Mohseni |first52=Masoud |last53=Mutus |first53=Josh |last54=Naaman |first54=Ofer |last55=Neeley |first55=Matthew |last56=Neill |first56=Charles |last57=Niu |first57=Murphy Yuezhen |last58=Ostby |first58=Eric |last59=Petukhov |first59=Andre |last60=Platt |first60=John C. |last61=Quintana |first61=Chris |last62=Rieffel |first62=Eleanor G. |last63=Roushan |first63=Pedram |last64=Rubin |first64=Nicholas C. |last65=Sank |first65=Daniel |last66=Satzinger |first66=Kevin J. |last67=Smelyanskiy |first67=Vadim |last68=Sung |first68=Kevin J. |last69=Trevithick |first69=Matthew D. |last70=Vainsencher |first70=Amit |last71=Villalonga |first71=Benjamin |last72=White |first72=Theodore |last73=Yao |first73=Z. Jamie |last74=Yeh |first74=Ping |last75=Zalcman |first75=Adam |last76=Neven |first76=Hartmut |last77=Martinis |first77=John M. |title=Quantum supremacy using a programmable superconducting processor |journal=Nature |date=October 2019 |volume=574 |issue=7779 |pages=505–510 |doi=10.1038/s41586-019-1666-5 |pmid=31645734 | arxiv=1910.11333|bibcode=2019Natur.574..505A |doi-access=free }}</ref>
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=== Superconductors ===
Unlike typical conductors, superconductors possess a [[critical temperature]] at which resistivity plummets to zero and conductivity is drastically increased. In superconductors, the basic charge carriers are pairs of [[electron]]s (known as [[Cooper pairs]]), rather than single [[fermion]]s as found in typical conductors.<ref>{{Cite web |title=Cooper Pairs |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/coop.html }}</ref> Cooper pairs are loosely bound and have an energy state lower than that of [[Fermi energy
==== Bose–Einstein condensates ====
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== Qubit archetypes ==
The three primary superconducting qubit archetypes are the [[phase qubit|phase]], [[charge qubit|charge]] and [[flux qubit|flux]] qubit. Many hybridizations of these archetypes exist including the fluxonium,<ref>{{cite journal|last1=Manucharyan|first1=V. E.|last2=Koch|first2=J.|last3=Glazman|first3=L. I.|last4=Devoret|first4=M. H.|title=Fluxonium: Single Cooper-Pair Circuit Free of Charge Offsets|arxiv=0906.0831|journal=Science|date=1 October 2009|volume=326|issue=5949|pages=113–116|doi=10.1126/science.1175552|pmid=19797655 |bibcode=2009Sci...326..113M|s2cid=17645288}}</ref> [[transmon]],<ref>{{cite journal |arxiv=0812.1865 |last1=Houck |first1=A. A. |last2=Koch |first2=Jens |last3=Devoret |first3=M. H. |last4=Girvin |first4=S. M. |last5=Schoelkopf |first5=R. J. |title=Life after charge noise: recent results with transmon qubits |journal=Quantum Information Processing |date=11 February 2009 |volume=8 |issue=2–3 |pages=105–115 |doi=10.1007/s11128-009-0100-6|bibcode=2009QuIP....8..105H |s2cid=27305073 }}</ref> Xmon,<ref>{{cite journal |last1=Barends |first1=R. |last2=Kelly |first2=J. |last3=Megrant |first3=A. |last4=Sank |first4=D. |last5=Jeffrey |first5=E. |last6=Chen |first6=Y. |last7=Yin |first7=Y. |last8=Chiaro |first8=B. |last9=Mutus |first9=J. |last10=Neill |first10=C. |last11=O’Malley |first11=P. |last12=Roushan |first12=P. |last13=Wenner |first13=J. |last14=White |first14=T. C. |last15=Cleland |first15=A. N. |last16=Martinis |first16=John M. |title=Coherent Josephson Qubit Suitable for Scalable Quantum Integrated Circuits |arxiv=1304.2322 |journal=Physical Review Letters |date=22 August 2013 |volume=111 |issue=8 |pages=080502 |doi=10.1103/PhysRevLett.111.080502|pmid=24010421 |bibcode=2013PhRvL.111h0502B |s2cid=27081288 }}</ref> and quantronium.<ref>{{cite journal |last1=Metcalfe |first1=M. |last2=Boaknin |first2=E. |last3=Manucharyan |first3=V. |last4=Vijay |first4=R. |last5=Siddiqi |first5=I. |last6=Rigetti |first6=C. |last7=Frunzio |first7=L. |last8=Schoelkopf |first8=R. J. |last9=Devoret |first9=M. H. |title=Measuring the decoherence of a quantronium qubit with the cavity bifurcation amplifier |arxiv=0706.0765 |journal=Physical Review B |date=21 November 2007 |volume=76 |issue=17 |pages=174516 |doi=10.1103/PhysRevB.76.174516|bibcode=2007PhRvB..76q4516M |s2cid=19088840 }}</ref> For any qubit implementation the logical [[quantum states]] <math>\{|0\rangle,|1\rangle\}</math> are [[Map (mathematics)|mapped]] to different states of the physical system (typically to discrete [[energy level]]s or their [[quantum superposition]]s). Each of the three archetypes possess a distinct range of Josephson energy to charging energy ratio. Josephson energy refers to the energy stored in Josephson junctions when current passes through, and charging energy is the energy required for one Cooper pair to charge the junction's total capacitance.<ref name="Martinis-2004">{{Cite
: <math>U_j = - \frac{I_0 \Phi_0}{2 \pi} \cos \delta</math>,
where <math>I_0</math> is the critical current parameter of the Josephson junction, <math>\textstyle \Phi_0 = \frac{h}{2e}</math> is (superconducting) [[Magnetic flux quantum|flux quantum]], and <math>\delta</math> is the [[Phase (waves)|phase difference]] across the junction.<ref name="Martinis-2004" /> Notice that the term <math>cos \delta</math> indicates nonlinearity of the Josephson junction.<ref name="Martinis-2004" /> Charge energy is written as
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==== Gatemon ====
Another variation of the transmon qubit is the Gatemon. Like the Xmon, the Gatemon is a tunable variation of the transmon. The Gatemon is tunable via [[Threshold voltage|gate voltage]]. [[File:Chip unimon.png|thumb|Superconducting circuit consisting of 3 Unimons (blue), each connected to resonators (red), drive lines (green), and joint probe lines (yellow)<ref>
=== Unimon ===
In 2022 researchers from [[IQM
{| class="wikitable"
|+Superconducting Qubit Archetypes
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The journey has been long, arduous and full of breakthroughs but has seen significant advancements in the recent history and has massive potential for revolutionizing computing.
'''Recent Advances in Josephson Junction–Based QPUs'''
A recent paper by Mohebi and Mohseni provides additional insight into the engineering challenges and innovations necessary for advancing superconducting quantum processing units (QPUs):
# '''Decoherence and Noise Mitigation:''' The paper emphasizes that decoherence—primarily due to quasiparticle tunneling—is a major obstacle that limits qubit performance. Improved material innovations and optimized control techniques are essential to reduce noise and enhance qubit coherence.<ref name="MohebiMohseni2025">{{Cite arXiv |title=An Overview of Josephson Junctions Based QPUs |date=3 April 2025 |eprint=2504.02500 |last1=Mohebi |first1=Omid |author2=Alireza Hesam Mohseni |class=cond-mat.supr-con }}</ref>
# '''Fabrication and Reproducibility:''' Achieving consistent and reproducible Josephson junctions is crucial for scaling up superconducting QPUs. The study discusses advanced lithography techniques and control of junction geometry as methods to minimize fluctuations in critical current, thereby enhancing qubit fidelity.<ref name="MohebiMohseni2025" />
# '''Balancing Qubit Parameters:''' The authors highlight the trade-offs between achieving large anharmonicity (to suppress charge noise) and maintaining the nonlinearity required for effective qubit operation. Striking the optimal balance between these factors is pivotal for the development of robust, scalable quantum processors.<ref name="MohebiMohseni2025" />
'''Future of superconducting quantum computing:'''
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* {{cite book |title=Principles of Superconducting Quantum Computers |last1=Stancil |first1=Daniel D. |publisher=John Wiley & Sons|year=2022 |___location=Hoboken, New Jersey |isbn=978-1-119-75072-7 |edition=1st |last2=Byrd |first2=Gregory T. |oclc=1302334194 |id=978-1-119-75074-1 (ebook)}}
* {{Cite book |title=Microwave Techniques in Superconducting Quantum Computers |last=Salari |first=Alan |oclc=1405187817 |id=978-1-63081-988-0 (ebook)| publisher=Artech House| year=2024|isbn=978-1-63081-987-3|type=Unabridged edition |___location=Boston}}
* {{Cite book |title=Single Flux Quantum Integrated Circuit Design |
* {{Cite book |title=Design and Applications of Emerging Computer Systems |
*{{cite book|last1=Awan|first1=Shakil|last2=Kibble|first2=Bryan P.|last3=Schurr|first3=Jürgen|year=2011|title=Coaxial Electrical Circuits for Interference-Free Measurements|series=Electrical Measurement Series 13|___location=Stevenage|publisher=The Institution of Engineering and Technology (IET)|isbn=978-1-84919-069-5|doi=10.1049/PBEL013E |oclc=761013886
}}
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