Bose condensation and Bogoliubov excitation in resonator-embedded superconducting qubit network

TL;DR

This paper reports on the experimental observation of Bose-Einstein condensate formation and Bogoliubov excitations in a superconducting qubit network embedded in a resonator, revealing bistability and nonlinear quantum dynamics.

Contribution

It demonstrates the first experimental realization of Bose condensation and Bogoliubov excitations in a superconducting qubit-resonator system, with tunable bistability signatures.

Findings

Observation of Bose-Einstein condensate in a superconducting resonator

Detection of Bogoliubov-like excitations via two-tone spectroscopy

Identification of bistability in photon number at critical pump power

Abstract

Superconducting qubit networks (SQNs) embedded in a low-dissipative resonator is a promising device allowing one not only to establish the collective quantum dynamics on a macroscopic scale but also to greatly enhance the sensitivity of detectors of microwave photons. A quantum ac Stark effect provided by coupling between an SQN and microwave photons of a resonator, leads to a strong nonlinear interaction between photons. Here, we present a two-tone spectroscopy experiment in which a set of 10 superconducting flux qubits is coupled to the input R- resonator and the output T- transmission line. An external microwave pump field close to the resonance frequency populates macroscopically the resonator mode as a Bose-Einstein condensate, while a second probe beam scans the resonances referred also as Bogoliubov-like excitations. The corresponding excitation frequency measured from the transmission coefficient, |S21(f)| displays an abrupt change of the resonant dip position once the power of the pump field overcomes a critical value Pcr. This sharp shift occurs in a narrow region of pump frequencies, and can be tuned by an applied magnetic field. It is a signature of bistability of the photon number inside the resonator, in agreement with theory.