Direct observation of the quantum-fluctuation driven amplitude mode in a microcavity polariton condensate

TL;DR

This paper reports the first direct observation of the quantum-fluctuation driven amplitude mode in an exciton-polariton condensate, achieved without external perturbations, revealing quantum depletion effects through high-precision spectroscopic measurements.

Contribution

It demonstrates the intrinsic quantum-fluctuation driven amplitude mode in a microcavity polariton condensate, a phenomenon not previously observed without external excitation.

Findings

Detection of weak photoluminescence below the condensate energy

Identification of ghost branches related to quantum depletion

Spectral features match theoretical predictions

Abstract

The Higgs amplitude mode is a collective excitation studied and observed in a broad class of matter, including superconductors, charge density waves, antiferromagnets, 3He p-wave superfluid, and ultracold atomic condensates. In all the observations reported thus far, the amplitude mode was excited by perturbing the condensate out of equilibrium. Studying an exciton-polariton condensate, here we report the first observation of this mode purely driven by intrinsic quantum fluctuations without such perturbations. By using an ultrahigh quality microcavity and a Raman spectrometer to maximally reject photoluminescence from the condensate, we observe weak but distinct photoluminescence at energies below the condensate emission. We identify this as the so-called ghost branches of the amplitude mode arising from quantum depletion of the condensate into this mode. These energies, as well as the overall structure of the photoluminescence spectra, are in good agreement with our theoretical analysis.