Charge insensitive qubit design derived from the Cooper pair box
Jens Koch · Terri M. Yu · Jay Gambetta · A. A. Houck · D. I. Schuster · J. Majer · Alexandre Blais · M. H. Devoret · S. M. Girvin · R. J. Schoelkopf
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Na jednej stronie
This is the paper that made the Josephson junction into a component you can build a machine out of. A Cooper pair box stores quantum information in whether one extra pair of electrons has tunnelled onto a small island, which makes it exquisitely sensitive to any stray charge drifting nearby, and stray charge is everywhere. The Yale group's move is deceptively simple: shunt the junction with a large capacitor so that the Josephson energy dwarfs the charging energy, by a factor of order a hundred rather than of order one. They then show the pay-off is lopsided in the designer's favour. Sensitivity to stray charge falls off exponentially as that ratio grows, while the anharmonicity you need to address one transition without exciting the next falls off only as a slow power law. The result is a qubit that no longer has to be tuned to a charge sweet spot, has no direct-current connections at all, and couples more strongly to the microwave resonator used to read it out. They call it the transmon.
Dlaczego ma tu znaczenieChapter 11 treats macroscopic quantum phase as an engineering variable, and this is the design that made that variable stable enough to build with: essentially every superconducting quantum processor in service, including the hardware behind the quantum energy teleportation demonstration on this site's shelf, is a descendant of it. It is also a clean worked example of the site's recurring lesson that a quantum device is designed by choosing which noise channel to expose yourself to.
Co twierdzi
01The transmon is a Cooper pair box operated at a much larger ratio of Josephson energy to charging energy, of order a hundred, achieved by shunting the junction with a large capacitance. The design rests on an asymmetry in how the two quantities that matter scale with that ratio: the charge dispersion, which sets sensitivity to stray charge, falls exponentially, while the anharmonicity, which is what allows one transition to be addressed selectively, falls only as a weak power law.Abstract; Section I, Introduction; Sections II B and II C
Settled physics02The practical consequences the authors claim are that the qubit becomes effectively insensitive to charge, which removes the need for individual electrostatic gates and for locking each qubit to a charge degeneracy point, and which removes the vulnerability to quasiparticle poisoning shifting a box off its optimal point. Both make scaling to many qubits easier rather than harder.Section I, Introduction; Section VI, Summary and conclusions
Published and peer-reviewed03The predicted dephasing times set the design against its parent. At a ratio of 85 the transmon is given 400,000 nanoseconds against charge noise where a Cooper pair box at a ratio of 1 is given 1,000; 3,600,000 nanoseconds against flux noise against 1,000,000; and 35,000 nanoseconds against critical-current noise against 17,000. The conclusion drawn is that transmon dephasing should be suppressed far enough that coherence is limited only by relaxation.Table I, comparison of dephasing times for the transmon and Cooper pair box qubits
Published and peer-reviewed04The exponential insensitivity to charge noise is bought without a penalty elsewhere: the paper argues the transmon is in fact less sensitive to flux and to critical-current noise than the Cooper pair box, which is unusual among Josephson qubit designs, where suppressing one channel typically exposes another. Flux qubits work at a similar energy ratio and reach comparable charge insensitivity but are markedly more exposed to flux noise away from their sweet spot.Section VI, Summary and conclusions
Published and peer-reviewed05The qubit gains rather than loses coupling to its readout: the transmon couples more strongly to a transmission-line cavity acting as a bus, and in the dispersive regime the coupled system is described by an alternating-current Stark shift Hamiltonian in complete analogy with the Cooper pair box, so existing control and readout protocols transfer to it unchanged. The device is operated with no direct-current connections to the rest of the circuit.Section I, Introduction; Section III on circuit quantum electrodynamics; Section VI
Published and peer-reviewed06What to watch, as the paper itself frames it: every number in it is a prediction from the authors' own noise analysis rather than a measurement, and the paper closes by saying it expects the transmon to belong to a new generation of superconducting qubits. The measurement that settles it is a transmon operated at the flux sweet spot showing coherence limited by relaxation alone.Table I caption; Section VI, Summary and conclusions
What to watch
Droga do źródła
https://doi.org/10.1103/PhysRevA.76.042319SOURCE READ IN FULL, TEXT NOT REPRODUCED. The paper was read on 2026-09-11 from the arXiv record cond-mat/0703002, version 2 of 26 September 2007, which carries the journal reference Physical Review A 76, 042319. The arXiv record declares no open licence: it carries arXiv's own non-exclusive licence to distribute, which lets arXiv serve the file and grants a reader nothing beyond reading it, so no sentence of the paper is carried here. The summary and the claims are written in this site's own words from the paper read in full, and each locator names the section or table the statement comes from. The predicted dephasing times in the claims are the numbers printed in the paper's Table I.
Jak cytować
Jens Koch, Terri M. Yu, Jay Gambetta, A. A. Houck, D. I. Schuster, J. Majer, Alexandre Blais, M. H. Devoret, S. M. Girvin, R. J. Schoelkopf (2007) Charge insensitive qubit design derived from the Cooper pair box. doi:10.1103/PhysRevA.76.042319
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