July 18, 2005

Publication: SQUID with frequency-dependent damping: Readout of flux qubits

Posted by Arcane Gazebo at July 18, 2005 11:40 AM

We published the same week as Harry Potter—that's going to cut into our readership for sure.

A quantum computer needs a mechanism to read out the state of each quantum bit (qubit) at the end of the calculation; in our experiments we use a superconducting quantum interference device (SQUID) for this purpose. (SQUIDs are very sensitive devices for measuring magnetic fields.) In quantum mechanics, any measurement affects the state of the object being measured, and this is true for our qubit and SQUID. In the paper we look at ways to maximize the sensitivity of the measurement while minimizing the corresponding back-action on the qubit.

Phys. Rev. B 72, 024513 (2005)

Superconducting quantum interference device with frequency-dependent damping: Readout of flux qubits

T. L. Robertson,1 B. L. T. Plourde,1 T. Hime,1 S. Linzen,1 P. A. Reichardt,1 F. K. Wilhelm,2 and John Clarke1

1Department of Physics, University of California, Berkeley, California 94720-7300, USA
2Physics Department, Arnold Sommerfeld Center for Theoretical Physics and Center for NanoScience, Ludwig-Maximilians-Universität München, 80333 München, Germany

(Received 7 February 2005; published 11 July 2005)

Recent experiments on superconducting flux qubits, consisting of a superconducting loop interrupted by Josephson junctions, have demonstrated quantum coherence between two different quantum states. The state of the qubit is measured with a superconducting quantum interference device (SQUID). Such measurements require the SQUID to have high resolution while exerting minimal backaction on the qubit. By designing shunts across the SQUID junctions appropriately, one can improve the measurement resolution without increasing the backaction significantly. Using a path-integral approach to analyze the Caldeira-Leggett model, we calculate the narrowing of the distribution of the switching events from the zero-voltage state of the SQUID for arbitrary shunt admittances, focusing on shunts consisting of a capacitance Cs and resistance Rs in series. To test this model, we fabricated a dc SQUID in which each junction is shunted with a thin-film interdigitated capacitor in series with a resistor, and measured the switching distribution as a function of temperature and applied magnetic flux. After accounting for the damping due to the SQUID leads, we found good agreement between the measured escape rates and the predictions of our model. We analyze the backaction of a shunted symmetric SQUID on a flux qubit. For the given parameters of our SQUID and realistic parameters for a flux qubit, at the degeneracy point we find a relaxation time of 113 µs, which limits the decoherence time to 226 µs. Based on our analysis of the escape process, we determine that a SQUID with purely capacitive shunts should have narrow switching distributions and no dissipation.

©2005 The American Physical Society

URL: http://link.aps.org/abstract/PRB/v72/e024513


PACS: 03.67.Lx, 85.25.Cp, 85.25.Dq



And I think "The Degeneracy Point" might make a good book title.

Posted by: Dad | July 18, 2005 12:42 PM

I unfortunately have no idea what the abstract to your paper means, but congrats on the publication!

On more frightening note, while I was checking my Hotmail account, I noticed a MSN ad that said, "Help build the ultimate playlist," and at first glance I read this as, "Help build the ultimate physicist."

Posted by: Jolene | July 18, 2005 5:46 PM

Hey! No building synthetic physicists! The job market's bad enough as it is. :)

Posted by: Arcane Gazebo | July 18, 2005 9:53 PM

Here here on Travis's job market comment! (And you're making these comments without having gone through it multiple times!)

I wonder how much overlap there is in the Phys Rev B readership and Harry Potter...?

I recently got an expository paper accepted in a (relatively speaking, of course) well-read mathematics journal/magazine. (It's the American Mathematical Society's answer to Physics Today.) The only reason I mention this is that the article title has a David Bowie reference.

Posted by: Mason | July 18, 2005 10:39 PM
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