4He Versus 3He Josephson Effect: Vibration Decoherence

Several on-going experiments searching for the Josephson effect in 4He close to the λ-transition employ experimental cells with a weak link in the form of an array of submicron holes, with the size of the array and the cell dimensions very close to those used for 3He-B. In the same environment, the 4He experiment is more prone to decoherence by mechanical vibrations. The problem is due to the shift of the maximum of the vibration response of the experiment to low frequencies (0.1 Hz or less) and to the increase of the power spectrum density of the seismic velocities with decreasing frequency in this frequency range. To avoid decoherence, one needs to lower the cut-off frequency of the vibration isolation or to use an array with a larger open area. The latter option is briefly discussed