Traces of Vacuum Squeezing: A Superconducting Transmission-Line Analogue of Cosmological Particle Creation

Particle creation in time-dependent quantum fields is a central phenomenon in early-universe cosmology, yet it remains experimentally inaccessible in its original context. We show theoretically that superconducting circuits based on a waveguide-like transmission line loaded with dc-SQUIDs can provide a direct analogue of a scalar field in a radiation-dominated Friedmann–Robertson–Walker universe in conformal time. The mode equation of the circuit reduces to the Weber equation, the same equation governing cosmological particle creation in a radiation-dominated background, admitting an exact solution in terms of parabolic cylinder functions and an exponentially suppressed particle spectrum. We analyse the conditions under which the analogy holds, which require a compromise between the duration of the pulse, the uniformity of the modulation and the symmetry of the SQUIDs, and show that dissipation does not erase the cosmological signal. We use the anomalous moment of each mode as a witness of the Bogoliubov mixing: it vanishes for any thermal state and survives dissipation, and, together with the photon number, it certifies squeezing, and with the purity of the state, it identifies whether the pairs created from the vacuum dominate.