Refrigeration of a 1D gas of microwave photons

TL;DR

This paper proposes a method to cool a one-dimensional microwave photon gas in a superconducting transmission line using a nonlinear Josephson element, enabling sub-millikelvin temperatures and revealing a novel condensation transition.

Contribution

It introduces a new cooling scheme for microwave photon gases that balances engineered photon transfer with thermalization, enabling low-temperature states and novel phase transitions.

Findings

Photon gas can be cooled to sub-millikelvin temperatures.

A new condensation transition occurs in the non-equilibrium photon gas.

The scheme is feasible with realistic experimental parameters.

Abstract

We discuss a conceptually simple scheme for cooling a one dimensional gas of microwave photons in a superconducting transmission line. By shunting one end of the transmission line by a nonlinear Josephson element, we show how a cooling mechanism can be engineered that transfers photons from high- into low-frequency modes, while preserving their total number. We evaluate the resulting nonequilibrium steady state of the photon gas, which arises from a competition between this engineered cooling process and the natural, number non-conserving thermalization with the surrounding bath. Our analysis predicts that for realistic experimental parameters, this mechanism can be used to prepare photonic gases at sub-millikelvin temperatures, considerably below the typical base temperature of a dilution refrigerator. In addition, the system exhibits a new type of condensation transition that does not occur in the corresponding equilibrium scenario. As an outlook, we discuss potential applications of this cooling approach for quantum simulation schemes with interacting microwave photons.