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Fast Switching Current Detection at low Critical Currents
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0001-8534-6577
2005 (English)In: Realizing Controllable Quantum States - MESOSCOPIC SUPERCONDUCTIVITY AND SPINTRONICS, 2005, 255-262 p.Conference paper, Published paper (Refereed)
Abstract [en]

A pulse-and-hold technique is used to measure the switching of small critical current Josephson junctions. This technique allows one to achieve a good binary detection and therefore measure switching probabilities. The technique overcomes limitations on simple square pulses and allows for the measurement of junctions with critical currents of the order of 10nA with bias pulses of the order of 100ns. A correlation analysis of the switching events is performed to show how the switching probability depends on the wait time between repeated bias pulses.

Place, publisher, year, edition, pages
2005. 255-262 p.
Keyword [en]
tunnel-junctions, circuit, state
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-6278ISI: 000234363700039ISBN: 981-256-468-3 (print)OAI: oai:DiVA.org:kth-6278DiVA: diva2:10951
Conference
3rd International Symposium on Mesoscopic Superconductivity and Spintronics Atsugi City, JAPAN, MAR 01-04, 2004
Note
QC 20100924Available from: 2006-10-23 Created: 2006-10-23 Last updated: 2010-09-28Bibliographically 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
2. Quantum effects in nanoscale Josephson junction circuits
Open this publication in new window or tab >>Quantum effects in nanoscale Josephson junction circuits
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis presents the results of an experimental study on single-charge effects in nanoscale Josephson junctions and Cooper pair transistors (CPTs).

In nanoscale Josephson junctions the charging energy EC becomes significant at sub-Kelvin temperatures and single-charge effects, such as the Coulomb blockade of Cooper pair tunneling, influence the transport properties. In order to observe charging effects in a single Josephson junction, the impedance of the electromagnetic environment surrounding the junction has to be larger than the quantum resistance (RQ=h/4e2≈6.45kΩ).

In this work the high impedance environment is obtained by biasing the sample under test (single Josephson junction or CPT) with four one-dimensional Josephson junction arrays having SQUID geometry. The advantage of this configuration is the possibility of tuning in situ the effective impedance of the electromagnetic environment. By applying a magnetic field perpendicular to the SQUID loops, the Josephson energy EJ of the SQUIDs is suppressed, resulting in an increase of the measured zero bias resistance of the arrays of several orders of magnitude (104< R0 (Ω) <109). This bias method enables the measurement of the same sample in environments with different impedance.

As the impedance of the environment is increased, the current-voltage characteristics (IVCs) of the single Josephson junction and of the CPT show a well defined Coulomb blockade feature with a region of negative differential resistance, signature of the coherent tunneling of single Cooper pairs.

The measured IVCs of a single Josephson junction with SQUID geometry in the high impedance environment show a qualitative agreement with the Bloch band theory as the EJ/EC ratio of the junction is tuned with the magnetic field. We also studied a single nontunable Josephson junction with strong coupling (EJ/EC > 1), where the exact dual of the overdamped Josephson effect is realized, resulting in a dual shape of the IVC, where the roles of current and voltage are exchanged. Here, we make for the first time a detailed quantitative comparison with a theory which includes the effect of fluctuations due to the finite temperature of the environment.

The measurements on CPTs in the high impedance environment showed that the Coulomb blockade voltage is modulated periodically by the gate-induced charge. The gate-voltage dependence of the CPT changes from e-periodic to 2e-periodic as the impedance of the environment is increased. The high impedance environment reduces quasiparticle tunneling rates, thereby restoring the even parity of the CPT island. This behavior suggests that high impedance leads can be used to effectively suppress quasiparticle poisoning.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 103 p.
Series
Trita-FYS, ISSN 0280-316X ; 2006:31
Keyword
Josephson junctions, Josephson junction arrays, electromagnetic environment, Cooper pair transistor, parity effects
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-4004 (URN)91-7178-353-9 (ISBN)
Public defence
2006-06-09, FA32, Albanova University Centrum, Roslagstullsbacken 21, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100928 Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2010-09-28Bibliographically approved

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