Direct lifetime measurements and interactions of charged defect states in submicron Josephson junctions

Abstract

We have measured the emission and capture times of individual electron traps residing within the tunneling barrier of very small-area (<0.05 //m 2 ) Josephson junctions. The voltage-bias dependence of the times is consistent with a simple nonequilibrium model in which the bias enhances the rate for electrons to tunnel into the trap from one side of the barrier and exit out the other. Some junctions show clear evidence of interactions between traps, and for certain bias conditions the noise displays predominantly series kinetics.PACS numbers: 85.25.+k, 73.20.-r, 74.50.-fr Small-area tunnel junctions have recently been the focus of a number of low-frequency noise studies. 1 " 4 Unlike larger systems where the low-frequency noise exhibits a rather featureless \/f power spectrum, tunnel junctions can be made so small that the discrete nature of the underlying microscopic processes becomes apparent in the noise. This was first demonstrated by Rogers and Buhrman x who showed that the noise-power spectra of their junctions were dominated by a small number of Lorentzian features arising from the trapping and untrapping of single electrons into localized defect states within the tunneling barrier. The trapped electron alters the junction conductance by charging a small region about the trapping site, thereby blocking conduction through this channel. The voltage noise contributed by one such trap displays a series of discrete switching events, resembling a random telegraph signal, characterized by electron emission and capture times. Because the trapping couples to the junction voltage in such a simple, distinctive fashion, it is possible under certain favorable conditions to observe directly the switching behavior of one or several of these traps. In this Letter we report lifetime measurements of charged defect states in very small-area Josephson junctions under various temperature and voltage-bias conditions. The lifetimes show only a weak temperature dependence below 4 K, consistent with previous findings 2 ' 3 that the trapping process displays tunneling kinetics at low temperatures. In contrast, the emission and capture rates are both enhanced significantly by the application of a voltage bias, regardless of polarity. We propose a simple model to explain this behavior. In addition, the noise does not always exhibit a simple superposition of random telegraph switching when several traps are active at the same time; instead, interactions between the traps can produce a voltage noise that displays series kinetics. These observations show that the low-frequency noise of this system cannot always be described by a simple parallel-kinetics model composed of independent fluctuators.Using electron-beam lithography and a PblnAu-In2C>3-Pb edge-junction technology, we fabricate tunnel junctions with areas of <0.05 /im 2 having normal-state resistances on the order of 1 kn. When the junction is biased at voltages greater than 2A/e, charge trapping produces a voltage noise which scales with the normal resistance and the bias cu...

Decoherence in Josephson-junction qubits due to critical-current fluctuations

“…There is strong evidence from the voltage dependence that the dominant charges enter the barrier from one electrode and exit to the other, and that the fluctuators exhibit a crossover from thermal activation to tunneling behavior at about 15 K. In the tunneling regime, the fluctuating entity has been shown to involve an atomic mass, suggesting that ionic reconfiguration plays an important role in the tunneling process. Interactions between traps resulting in multiple level hierarchical kinetics have been observed, 19 but usually the traps can be considered to be local and non-interacting. In this limit, the coexisting traps produce a distribution of Lorentzian features that superimpose to give a 1/f -like spectrum.…”

Dynamical control of qubit coherence: Random versus deterministic schemes

“…While a detailed analysis of randomized control of genuine 1/f noise would be interesting on its own, we begin here with the case of a single fluctuator. This provides an accurate approximation for mesoscopic devices where noise is dominated by a few fluctuators spatially close to the system [23,53,54,55]. Let the time-dependent Hamiltonian describing the noisy qubit be given by Eqs.…”

Model for1/fFlux Noise in SQUIDs and Qubits

“…Wellstood et al [4] reported values of S Φ 1/2 (1 Hz) of (4-10) μΦ 0 Hz -1/2 at temperatures below 0.1 K in 12 Nb, Pb and PbIn devices. Recently, Yoshihara et al [5] showed that l/f flux noise with Critical current fluctuations in Josephson junctions have been widely studied, for example [6][7][8], and are understood to arise from the trapping and release of electrons in traps in the tunnel barrier. In the case of high transition temperatures (T c ) SQUIDs at 77 K, l/f flux noise is ascribed to thermal activation of vortices among pinning sites [9].…”