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charge qubit
In quantum computing, a charge qubit is a qubit whose basis states are charge states (i.e. states which represent the presence or absence of excess Cooper pairs in the island).〔V. Bouchiat, D. Vion, P. Joyez, D. Esteve and M. H. Devoret, "Quantum coherence with a single Cooper pair", ''Physica Scripta'' T76, 165-170 (1998), 〕〔Y. Nakamura, Yu. A. Pashkin and J. S. Tsai, "Coherent control of macroscopic quantum states in a single-Cooper-pair box", ''Nature'' 398, 786-788 (1999), , (arXiv:9904003 )〕〔K. W. Lehnert, B. A. Turek, K. Bladh, L. F. Spietz, D. Gunnarsson, P. Delsing and R. J. Schoelkopf, "Measurement of the excited-state lifetime of a microelectronic circuit", ''Phys. Rev. Lett.'' 90, 027002 (2003), 〕 A charge qubit is formed by a tiny superconducting island (also known as a Cooper-pair box) coupled by a Josephson junction to a superconducting reservoir (see figure). The state of the qubit is determined by the number of Cooper pairs which 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 T2 coherence times for a charge qubit are on the order of 1–2 μs.〔A.A. Houck ''et al.'', "Life after charge noise: recent results with transmon qubits," ''Quant. Inf. Proc.'' 8, 105 (2009), , (arXiv:0812.1865 )〕 Recent work has shown T2 times approaching 100 μs using a type of charge qubit known as a transmon inside a three-dimensional superconducting cavity.〔H. Paik ''et al.'', "Observation of high coherence in Josephson junction qubits measured in a three-dimensional circuit QED architecture," ''Phys. Rev. Lett.'' 107, 240501 (2011), , (arXiv:1105.4652 )〕〔C. Rigetti ''et al.'', "Superconducting qubit in waveguide cavity with coherence time approaching 0.1 ms," (arXiv:1202.5533 ) (2012)〕 Understanding the limits of T2 is an active area of research in the field of superconducting quantum computing.
==References==
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