title: Observing coherence effects in an overdamped quantum system.
abstract: It is usually considered that the spectrum of an optical cavity coupled to an atomic medium does not exhibit a normal-mode splitting unless the system satisfies the strong coupling condition, meaning the Rabi frequency of the coherent coupling exceeds the decay rates of atom and cavity excitations. Here we show that this need not be the case, but depends on the way in which the coupled system is probed. Measurements of the reflection of a probe laser from the input mirror of an overdamped cavity reveal an avoided crossing in the spectrum that is not observed when driving the atoms directly and measuring the Purcell-enhanced cavity emission. We understand these observations by noting a formal correspondence with electromagnetically induced transparency of a three-level atom in free space, where our cavity acts as the absorbing medium and the coupled atoms play the role of the control field.
authors.affiliation: Centre for Cold Matter, Imperial College, Prince Consort Road, London SW7 2BW, UK.|School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
authors.forename: Y-H
authors.initials: YH
authors.lastname: Lien
authors.affiliation: Centre for Cold Matter, Imperial College, Prince Consort Road, London SW7 2BW, UK.|School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
authors.forename: G
authors.initials: G
authors.lastname: Barontini
authors.affiliation: Centre for Cold Matter, Imperial College, Prince Consort Road, London SW7 2BW, UK.|Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien, 1020 Vienna, Austria.
authors.forename: M
authors.initials: M
authors.lastname: Scheucher
authors.affiliation: Centre for Cold Matter, Imperial College, Prince Consort Road, London SW7 2BW, UK.|Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK.
authors.forename: M
authors.initials: M
authors.lastname: Mergenthaler
authors.affiliation: School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
authors.forename: J
authors.initials: J
authors.lastname: Goldwin
authors.affiliation: Centre for Cold Matter, Imperial College, Prince Consort Road, London SW7 2BW, UK.
authors.forename: E A
authors.initials: EA
authors.lastname: Hinds