Multiple-tunneling noise in superconducting tunnel junctions from partial current integration

Abstract

Superconducting tunnel junctions can be used as high-resolution particle or photon energy spectrometers. A photon absorbed in a superconductor breaks Cooper pairs into quasiparticles. These quasiparticles tunnel through the junction barrier and are detected as a pulse of excess current. Many junction designs allow the quasiparticles to tunnel more than once, an exponentially mixed Poisson process. However, multiple tunneling increases the fluctuation in the measured charge. We calculate the significance of these fluctuations algebraically as a function of time during the current pulse. We also calculate the finite integration window that minimizes the contribution of this noise. In addition, we calculate the effects of a low-pass amplifier and a Gaussian-shaping amplifier on the tunneling noise. With certain filtering time constants, the tunneling noise can be reduced while still providing some gain.

Dynamics of nonequilibrium quasiparticles in narrow-gap superconducting tunnel junctions

“…While, as expected, the new devices exhibit significantly improved detector characteristics, several totally new phenomena have also emerged, including internal amplification due to qp back tunneling, 7 enhanced tunneling and phonon noise, 8,9 and time-dependent tunneling statistics. 10,11 However, the most exciting discovery has been that of a whole new class of phenomena related to the formation of a nonequilibrium, coupled qp-phonon state due to multiple tunneling under bias.…”

Integration time dependence of tunnel noise and energy resolution of superconducting tunnel junctions

“…However, as Hiller et al [8] have pointed out, the situation changes for finite integration times. This is most obvious for larger n: after a time t which is short compared to the average tunnel time Γ t −1 it is unlikely to find any sequence of events consisting of n tunnelings, while at a t of the order of several Γ t −1 this probability will be much larger.…”

“…Figure 3 illustrates the application of Eq. 9 to a practical example of analogue pulse amplification and shaping using standard methods [8]. The current pulse from an (asymmetric) STJ is pre-amplified and then detected by a two-channel shaping stage, which is the equivalent of a (CR-RC) 2 bipolar semi-Gaussian filter, pole-zero compensated for the preamp.…”

“…In previous work, we have reported intrinsic energy resolutions below the limit set by the combined tunnel and Fano noise, measured with STJs that do exhibit multiple quasiparticle tunnelling [5,7]. Hiller et al [8] recently pointed out how a careful choice of the pulseintegration time could reduce the tunnel noise. In this paper we present experiments, which support the explanation in terms of the time dependence of the statistics of the tunnel process and we demonstrate how the tunnel noise can be controlled via the pulse integration time.…”

Optical photon detection in Al superconducting tunnel junctions

“…where ∆ g is the energy gap of the absorbing electrode, E is the photon energy, F is the Fano factor [1,2], G is the statistical broadening due to multiple tunnelling of the quasiparticles [3,4,5,6] and H is the factor due to cancellation tunnel events [7,8], which become important for junctions with a low energy gap. In view of the better predicted energy resolution compared to Ta-based STJs (∆ g =700µeV) we have successfully fabricated high-quality Al-based (∆ g =180µeV) junctions and tested their abilities as NIR to soft-UV photon detectors.…”