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AIP conference proceedings

Small Josephson Junction As Detector Of Non-Gaussian Noise

Rene K. Lindell

Published Jan 1, 2005• 0 citations

Abstract

The current in a a Coulomb blockaded Josephson junction is sensitive to voltage or phase fluctuations in its electromagnetical environment. This property can be used to detect noise also from non‐Gaussian noise sources. We have experimentally studied IV‐curves of as a function of the shot noise current in a nearby tunnel junction. We observe a linear enhancement of zero‐bias conductance of the Josephson junction as a function of the shot noise power. Furthermore, with increasing shot noise power, the IV‐curves becomes asymmetric with a shift of minimum conductance and the appearance of a local conductance maximum. A quantitative explanation for the asymmetric features due to the non‐Gaussian noise sources is still an open question. There is also motivation to pursue modifications of the current scheme in order to detect individual statistical moments of noise and to connect different noise sources to the Josephson junction.

DOI: 10.1063/1.2138669

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The current in a a Coulomb blockaded Josephson junction is sensitive to voltage or phase fluctuations in its electromagnetical environment. This property can be used to detect noise also from non‐Gaussian noise sources. We have experimentally studied IV‐curves of as a function of the shot noise current in a nearby tunnel junction. We observe a linear enhancement of zero‐bias conductance of the Josephson junction as a function of the shot noise power. Furthermore, with increasing shot noise power, the IV‐curves becomes asymmetric with a shift of minimum conductance and the appearance of a local conductance maximum. A quantitative explanation for the asymmetric features due to the non‐Gaussian noise sources is still an open question. There is also motivation to pursue modifications of the current scheme in order to detect individual statistical moments of noise and to connect different noise sources to the Josephson junction.

We have investigated the influence of shot noise on the IV-curves of a single mesoscopic Josephson junction. We find that the blockade of the Cooper pair current is strongly suppressed in the presence of shot noise due to tunnelling in an adjacent SIN junction. Our experimental findings can be accounted for by an extension of the phase correlation theory. Shot-noise effect in a resistive environment R can be characterized by the effective noise temperature T_N = eIR /2 k_B, which means that a Josephson junction can easily detect shot noise from a current well below 1 pA.

Non-Gaussian fluctuations of the electrical current can be detected with a Josephson junction placed on-chip with the noise source. We present preliminary measurements with an NIS junction as a noise source, and a Josephson junction in the thermal escape regime as a noise detector. It is shown that the Josephson junction detects not only the average noise, which manifests itself as an increased effective temperature, but also the noise asymmetry. A theoretical description of the thermal escape of a Josephson junction in presence of noise with a non-zero third cumulant is presented, together with numerical simulations when the noise source is a tunnel junction with Poisson noise. Comparison between experiment and theory is discussed.

We investigate theoretically and experimentally the environment-induced voltage shot noise in current biased Josephson junctions induced by phase particle tunneling. Quantum mechanical treatment based on the Caldeira-Leggett model with tight-binding formulation in local Wannier bases gives a clear picture of the voltage shot noise. A universal form of the zero-frequency noise spectrum is obtained, which exhibits a quadratic dependence on the mean voltage in small bias region. The quadratic dependence is verified experimentally on junctions covering a wide range of parameters, and is found also in junction arrays of various array sizes.

Intrinsic noise is known to be ubiquitous in Josephson junctions. We introduce a rough tunnel junction model characterized by a very small number of pinholes---transport channels possessing a transmission coefficient close to unity and analyze its transport properties. Although it may still have just a small total leakage current, it can lead to enormous current fluctuations in the low-voltage regime. We show that even fully transparent transport channels between superconductors contribute to shot noise due to the uncertainty in the number of Andreev cycles. We discuss shot-noise enhancement by multiple Andreev reflection in such a junction and investigate whether pinholes might contribute as a microscopic mechanism of two-level current fluctuators. We discuss the connection of these results to the junction resonators observed in Josephson phase qubits.

We study a Josephson junction (JJ) in the regime of incoherent Cooper-pair tunneling, capacitively coupled to a nonequilibrium noise source. The current-voltage (I-V) characteristics of the JJ are sensitive to the excess voltage fluctuations in the source, and can thus be used for wideband noise detection. Under weak driving, the odd part of the I-V can be related to the second cumulant of noise, whereas the even part is due to the third cumulant. After calibration, one can measure the Fano factors for the noise source, and get information about the frequency dependence of the noise.