Coherent response of a low T_c Josephson junction to an ultrafast laser pulse

TL;DR

This paper investigates how ultrafast laser pulses can rapidly alter the state of a low T_c Josephson junction by driving quasiparticles out of equilibrium, enabling fast control of superconducting devices without degrading coherence.

Contribution

It introduces a theoretical framework for understanding ultrafast laser-induced dynamics in Josephson junctions, demonstrating potential for rapid switching and control of superconducting qubits.

Findings

Ultrafast laser pulses can switch the junction from Josephson to voltage state.

The order parameter decreases but coherence remains intact with fast perturbations.

Switching can be controlled by laser intensity and pulse duration.

Abstract

By irradiating with a single ultrafast laser pulse a superconducting electrode of a Josephson junction it is possible to drive the quasiparticles (qp's) distribution strongly out of equilibrium. The behavior of the Josephson device can, thus, be modified on a fast time scale, shorter than the qp's relaxation time. This could be very useful, in that it allows fast control of Josephson charge qubits and, in general, of all Josephson devices. If the energy released to the top layer contact $S1$ of the junction is of the order of $\sim \mu J$, the coherence is not degradated, because the perturbation is very fast. Within the framework of the quasiclassical Keldysh Green's function theory, we find that the order parameter of $S1$ decreases. We study the perturbed dynamics of the junction, when the current bias is close to the critical current, by integrating numerically its classical equation of motion. The optical ultrafast pulse can produce switchings of the junction from the Josephson state to the voltage state. The switches can be controlled by tuning the laser light intensity and the pulse duration of the Josephson junction.