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Time Domain Analysis of Dynamical Switching in a Josephson Junction
KTH, Superseded Departments, Physics.
KTH, Superseded Departments, Physics.ORCID iD: 0000-0001-8534-6577
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2004 (English)Manuscript (preprint) (Other academic)
Abstract [en]

We have studied the switching behaviour of a small capacitance Josephson junction both in experiment,and by numerical simulation of a model circuit. The switching is a complex process involvingthe transition between two dynamical states of the non-linear circuit, arising from a frequency dependentdamping of the Josephson junction. We show how a specific type of bias pulse-and-hold,can result in a fast detection of switching, even when the measurement bandwidth of the junctionvoltage is severely limited, and/or the level of the switching current is rather low.

Place, publisher, year, edition, pages
2004.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-6277OAI: oai:DiVA.org:kth-6277DiVA: diva2:10950
Note

QC 20100924

Available from: 2006-10-23 Created: 2006-10-23 Last updated: 2014-10-29Bibliographically approved
In thesis
1. Pulse and hold switching current readout of superconducting quantum circuits
Open this publication in new window or tab >>Pulse and hold switching current readout of superconducting quantum circuits
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Josephson junction qubits are promising candidates for a scalable quantum processor. Such qubits are commonly manipulated by means of sequences of rf-pulses and different methods are used to determine their quantum state. The readout should be able to distinguish the two qubit states with high accuracy and be faster than the relaxation time of the qubit. We discuss and experiment with a readout method based on the switching of a Josephson junction from the zero voltage state to a finite voltage state.

The Josephson junction circuit has a non-linear dynamics and when it is brought to a bifurcation point, it can be made arbitrarily sensitive to small perturbations. This extreme sensitivity at a bifurcation point can be used to distinguish the two quantum states if the topology of the phase space of the circuit leads to a quick separation into the final states where re-crossings of the bifurcation point are negligible. We optimize a switching current detector by analyzing the phase space of a Josephson junction circuit with frequency dependent damping.

A pulse and hold technique is used where an initial current pulse brings the junction close to its bifurcation point and the subsequent hold level is used to give the circuit enough time to evolve until the two states can be distinguished by the measuring instrument. We generate the pulse and hold waveform by a new technique where a voltage step with following linear voltage rise is applied to a bias capacitor. The frequency dependent damping is realized by an on-chip RC-environment fabricated with optical lithography. Josephson junction circuits are added on by means of e-beam lithography.

Measurements show that switching currents can be detected with pulses as short as 5 ns and a resolution of 2.5% for a sample directly connected to the measurement leads of the cryostat. Detailed analysis of the switching currents in the RC-environment show that pulses with a duration of 20 us can be explained by a generalization of Kramers' escape theory, whereas switching the same sample with 25 ns pulses occurs out of thermal equilibrium, with sensitivity and speed adequate for qubit readout.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. viii, 82 p.
Series
Trita-FYS, ISSN 0280-316X ; 2006:61
Keyword
Josephson junction, readout, qubit, detector, frequency dependent damping, quantronium, phase-space, correlation analysis
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-4156 (URN)91-7178-462-4 (ISBN)
Public defence
2006-11-10, FA32, AlbaNova Main Building, Roslagstullsbacken 21, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100924Available from: 2006-10-23 Created: 2006-10-23 Last updated: 2010-09-24Bibliographically approved

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Haviland, David B.

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