Detection of signals in presence of noise through Josephson junction switching currents.

Josephson junctions can be employed to reveal a sinusoidal signal in presence of Gaussian noise. To mimic realistic setups, the detection is performed linearly ramping the bias current until a switch to the finite voltage occurs; the analysis of the resulting switching currents can be exploited to decide about the presence of the harmonic drive. The signal is applied in two conditions: with an unknown initial phase (incoherent strategy) and with a known initial phase (coherent strategy). In both conditions, the analysis of the efficiency of the detection, performed through the signal-to-noise ratio, as estimated by the Kumar-Carrol index, shows that the dependence upon the Josephson junction ramp rate is beneficial, especially for relatively fast speed. One can conclude that the collection of the switching currents is a robust technique, and thus it is possible to exploit the advantages of a predetermined finite time to collect the data.

Noise and disorder effects in a series of birhythmic Josephson junctions coupled to a resonator.

The effects of uncorrelated white noise on a series of Josephson junctions coupled to a linear RLC resonator are investigated. Both the cases of identical and nonidentical parameters are considered. The junctions are hysteretic, and hence can be considered birhythmic; that is, capable of oscillating at different frequencies for the same set of parameters. It is also found that, in the presence of noise, a uniform array behaves similarly to a single Josephson junction. However, the magnitude of the effective energy that characterizes the response to noise becomes smaller as the number of elements of the array increases, making the resonator less stable. Disorder in the parameters drastically changes the physics of the array. The disordered array of Josephson junctions misses the birhythmic properties for large values of the variance of the disorder parameter and remains birhythmic only for low values of the disorder parameter. Finally, disorder makes it difficult to locate the separatrix, hinting to a more complex structure of the effective energy landscape.