epl draft Optically detuned light storage in metastable helium: efficiency and stored phase M.-A. Maynard 1 , T. Labidi 1 , M. Mukhtar 1 , S. Kumar 2 , R. Ghosh 2,3 , F. Bretenaker 1 and F. Goldfarb 1 1 Laboratoire Aim´ e Cotton, CNRS - Universit´ e Paris Sud 11 - ENS Cachan, 91405 Orsay Cedex, France 2 School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India 3 School of Natural Sciences, Shiv Nadar University, Gautam Budh Nagar, UP 203207, India PACS 42.50.Gy – Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption. PACS 42.50.Ex – Optical implementations of quantum information processing and transfer. PACS 42.50.Md – Optical transient phenomena: quantum beats, photon echo, free-induction de- cay, dephasings and revivals, optical nutation, and self-induced transparency. Abstract. - Electromagnetically induced transparency (EIT) in metastable helium at room tem- perature is experimentally shown to exhibit remarkable light storage capabilities for intermediate values of the detuning between the coupling and probe beams and the center of the atomic Doppler profiles. For an optical depth lower than 3, an 11% efficiency after a 3 μs storage time has been achieved for a detuning measuring twice the Doppler linewidth. The associated storage lifetime has been measured to be equal to 11 μs. Moreover, an additional phase shift is shown to be imposed to the retrieved pulse of light when the EIT protocol is performed at non-zero optical detunings. The origin and possible applications of this detuning are discussed. Since the discovery of coherent population trapping (CPT) [1,2] and electromagnetically induced transparency (EIT) 15 years later [3], different applications of these phe- nomena were found, ranging from atom cooling [4] to the decrease of group velocities down to a few meters per sec- ond [5,6]. This led to the well-known stored light experi- ments that were performed using EIT in various systems such as cold atoms, gas cells, or doped crystals [7–9]. The EIT-based storage protocol relies on the long-lived Raman coherence between the two fundamental states of a Λ system, where two ground levels are optically coupled to the same excited level. When a strong coupling beam is applied on one of the two transitions, a narrow trans- parency window limited by the Raman coherence decay rate is opened along the other leg of the Λ system. Be- cause of the slow-light effect associated with such a dra- matic change of the absorption properties of the medium, a weak probe pulse that excites the second transition is compressed when it propagates inside the medium. When it is fully inside this medium, the coupling beam can be suddenly switched off and the signal is then mapped onto the Raman coherences that were excited by the two photon process. Finally, the signal pulse can be simply retrieved by switching on the coupling beam again. Atoms at room temperature in a gas cell are particu- larly attractive for light storage because of the simplicity of their implementation. However, the significant Doppler broadening has to be considered and might be expected to place a strong limitation. Its effect can nevertheless be minimized using co-propagating coupling and probe beams, and easy-of-use simple gas cells have thus turned out to be attractive for slow or even stopped light ex- periments [10]. The atoms preferably used for such ex- periments are alkali atoms, mainly rubidium and some- times sodium or caesium. Experimental achievements have shown that squeezing can be preserved after slow- ing down or storage by EIT at optical resonance in an alkali cell at room temperature [11,12], and the same phe- nomenon was used to store and retrieve Laguerre-Gaussian modes [13] and 2D images [14, 15], or to entangle 2D im- ages [16]. Quite good results were also obtained with caesium atoms in the Raman regime [17], [18] – with an optical detuning more than 10 times larger than the Doppler width – and some previous results were obtained with other alkali atoms and smaller detunings of about the Doppler width [12, 14, 19]. However, a detailed study of the effect of the optical detuning on storage efficiency and properties is difficult in alkali atoms because of the p-1