Editing Charge qubit (section)

{{Short description|Superconducting qubit implementation}}
[[Image:Cooper_pair_box_circuit.png|thumb|Circuit diagram of a charge qubit circuit.  The island (dotted line) is formed by the superconducting electrode between the gate capacitor and the junction capacitance.]]
In [[quantum computing]], a '''charge qubit''' (also known as '''Cooper-pair box''') is a [[qubit]] whose basis [[quantum state|states]] are [[electric charge|charge]] states (i.e. states which represent the presence or absence of excess [[Cooper pair]]s in the island).<ref>{{cite journal | last1=Bouchiat | first1=V. | last2=Vion | first2=D. | last3=Joyez | first3=P. | last4=Esteve | first4=D. | last5=Devoret | first5=M. H. |author-link5=M. H. Devoret| title=Quantum Coherence with a Single Cooper Pair | journal=Physica Scripta | publisher=IOP Publishing | volume=T76 | issue=1 | year=1998 | issn=0031-8949 | doi=10.1238/physica.topical.076a00165 | page=165-170| bibcode=1998PhST...76..165B | s2cid=250887469 | doi-access= }}</ref><ref>{{cite journal | last1=Nakamura | first1=Y. |author-link=Yasunobu Nakamura| last2=Pashkin | first2=Yu. A. | last3=Tsai | first3=J. S. |author-link3=Jaw-Shen Tsai| title=Coherent control of macroscopic quantum states in a single-Cooper-pair box | journal=Nature | publisher=Springer Science and Business Media LLC | volume=398 | issue=6730 | year=1999 | issn=0028-0836 | doi=10.1038/19718 | pages=786–788|arxiv=cond-mat/9904003| bibcode=1999Natur.398..786N | s2cid=4392755 }}</ref><ref>{{cite journal | last1=Lehnert | first1=K. W. | last2=Bladh | first2=K. | last3=Spietz | first3=L. F. | last4=Gunnarsson | first4=D. | last5=Schuster | first5=D. I. | last6=Delsing | first6=P. | last7=Schoelkopf | first7=R. J. |display-authors=5| title=Measurement of the Excited-State Lifetime of a Microelectronic Circuit | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=90 | issue=2 | date=2003-01-17 | issn=0031-9007 | doi=10.1103/physrevlett.90.027002 | page=027002| pmid=12570573 | bibcode=2003PhRvL..90b7002L }}</ref> In [[superconducting quantum computing]], a charge qubit<ref name=":0">{{Cite journal|last1=Makhlin|first1=Yuriy|last2=Schoen|first2=Gerd|last3=Shnirman|first3=Alexander|date=2001-05-08|title=Quantum state engineering with Josephson-junction devices|journal=Reviews of Modern Physics|volume=73|issue=2|pages=357–400|doi=10.1103/RevModPhys.73.357|issn=0034-6861|arxiv=cond-mat/0011269|bibcode=2001RvMP...73..357M|s2cid=6687697 }}</ref> is formed by a tiny [[superconductor|superconducting]] island coupled by a [[Josephson junction]] (or practically, [[superconducting tunnel junction]]) to a superconducting reservoir (see figure).  The state of the qubit is determined by the number of Cooper pairs that have tunneled across the junction. In contrast with the charge state of an atomic or molecular ion, the charge states of such an "island" involve a macroscopic number of conduction electrons of the island. The quantum superposition of charge states can be achieved by tuning the gate voltage ''U'' that controls the chemical potential of the island. The charge qubit is typically read-out by electrostatically coupling the island to an extremely sensitive [[electrometer]] such as the [[radio-frequency]] [[single-electron transistor]].

Typical [[spin–spin relaxation|''T''<sub>2</sub> coherence times]] for a charge qubit are on the order of 1–2&nbsp;μs.<ref name=Houck>{{cite journal | 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 | volume=8 | issue=2–3 | date=2009-02-11 | issn=1570-0755 | doi=10.1007/s11128-009-0100-6 | pages=105–115|arxiv=0812.1865| bibcode=2009QuIP....8..105H | s2cid=27305073 }}</ref> Recent work has shown ''T''<sub>2</sub> times approaching 100 μs using a type of charge qubit known as a [[transmon]] inside a three-dimensional superconducting cavity.<ref name=Paik>{{cite journal | last1=Paik | first1=Hanhee | last2=Schuster | first2=D. I. | last3=Bishop | first3=Lev S. | last4=Kirchmair | first4=G. | last5=Catelani | first5=G. | last6=Sears | first6=A. P. | last7=Johnson | first7=B. R. | last8=Reagor | first8=M. J. | last9=Frunzio | first9=L. | last10=Glazman | first10=L. I. | last11=Girvin | first11=S. M. | last12=Devoret | first12=M. H. | last13=Schoelkopf | first13=R. J. | title=Observation of High Coherence in Josephson Junction Qubits Measured in a Three-Dimensional Circuit QED Architecture | journal=Physical Review Letters | volume=107 | issue=24 | date=2011-12-05 | issn=0031-9007 | doi=10.1103/physrevlett.107.240501 | page=240501| pmid=22242979 |arxiv=1105.4652| bibcode=2011PhRvL.107x0501P | s2cid=19296685 }}</ref><ref name=Rigetti>C. Rigetti ''et al.'', "Superconducting qubit in waveguide cavity with coherence time approaching 0.1 ms," [https://arxiv.org/abs/1202.5533 arXiv:1202.5533] (2012)</ref> Understanding the limits of ''T''<sub>2</sub> is an active area of research in the field of [[superconducting quantum computing]].