Electromagnetically Induced Transparency with Single Atoms in a Cavity
Optical nonlinearities offer unique possibilities for the control of light with light. A prominent example is electromagnetically induced transparency (EIT), where the transmission of a probe beam through an optically dense medium is manipulated by means of a control beam 1-3 . Scaling such experiments into the quantum domain with one (or just a few) particles of light and matter will allow for the implementation of quantum computing protocols with atoms and photons 4-7 , or the realization of
... trongly interacting photon gases exhibiting quantum phase transitions of light 8, 9 . Reaching these aims is challenging and requires an enhanced matter-light interaction, as provided by cavity quantum electrodynamics 10-12 . Here we demonstrate EIT with a single atom quasi-permanently trapped inside a high-finesse optical cavity. The atom acts as a quantum-optical transistor with the ability to coherently control 13 the transmission of light through the cavity. We investigate the scaling of EIT when the atom number is increased one-by-one. The measured spectra are in excellent agreement with a theoretical model. Merging EIT with cavity quantum electrodynamics and single quanta of matter is likely to become the cornerstone for novel applications, such as dynamic control of the photon statistics of propagating light fields 14 or the engineering of Fock state superpositions of flying light pulses 15 . Remarkable progress has been achieved towards the optical manipulation of light by means of single quantum emitters. First realizations of optical transistors operating on tightly focused laser beams have been demonstrated with individual molecules, quantum dots or nitrogen vacancies embedded in suitable host materials 13,16-18 . However, the weak light-matter coupling that has been reached in these experiments limits the control capabilities considerably. Moreover, increasing the number of particles is not a straightforward task, owing to the difficulties in the preparation of identical quantum radiators. With this backdrop, a promising avenue is to trap a register of atoms inside an optical cavity 19,20 . High-reflectivity mirrors increase the optical path length and can amplify the matter-light interaction into the strong coupling regime. Optical control has already been achieved in singleatom experiments, including the production of single photons with controlled waveform 21-23 , the generation of polarization-entangled photon pairs 24 , and the state transfer between a faint laser pulse and a single atom 25 . Incorporating EIT will boost the capabilities of cavity quantum electrodynamics (QED) from the production of single photons towards the coherent manipulation of propagating quantum light fields. For a system with many individually addressable atoms, this will ultimately lead to the realization of a quantum network 26 , where the generation, propagation and absorption of light are coherently controlled at the quantum level. In this work, we coherently control the optical properties of a coupled atom-cavity system through the use of EIT. The heart of the apparatus consists of a high-finesse optical cavity with mirrors separated by 495 mm, a TEM 00 -mode waist of 30 mm and a finesse of 56,000. The cavity operates in the intermediate coupling regime with (g 0 , k, c) 5 2p 3 (4.5, 2.9, 3.0) MHz, where g 0 denotes the atomcavity coupling constant at a field antinode for the 87 Rb 5S 1/2 F 5 1 « 5P 3/2 F9 5 1 transition at 780 nm, k is the cavity field decay rate and c the atomic polarization decay rate. The atoms are trapped inside the cavity in a far-detuned standing-wave dipole trap, resulting in an average a.c.-Stark shift of 5 MHz. The cavity is stabilized to the F 5 1 « F9 5 1 transition via a reference laser (l 5 785 nm). This leads to a bare atom-cavity detuning of 2 MHz. Light scattered during cooling intervals is used to obtain images of the trapped atoms with a CCD camera. This allows the precise determination of the number and position of atoms inside the cavity mode during a given experimental run. In order to demonstrate EIT in the regime of single atoms, we record transmission spectra of the atom-cavity system under three distinct physical conditions. The transmission is measured with a weak probe laser near-resonant with the F 5 1 « F9 5 1 transition applied along the cavity axis. In the first step of our experimental protocol, we optically shelve the atom in the hyperfine state F 5 2, therefore effectively decoupling it from the cavity (Fig. 1a) . This yields an empty-cavity transmission spectrum used as a reference. In the second step, the atom is prepared in F 5 1, such that we realize the case of a two-level atom coupled to the cavity (Fig. 1b) . In the third step, we apply an additional control laser transverse to the cavity axis and resonant with the F 5 2 « F9 5 1 transition; this is named the 'cavity EIT' configuration (Fig. 1c) . This forms a L-level scheme suitable for the generation of a coherent dark state. The experimental protocol is continuously repeated at a 25-Hz rate while the probe laser frequency is shifted for every repetition cycle. Thus, we simultaneously measure the three transmission spectra for a given number of trapped atoms. We introduce the main features of cavity EIT by means of transmission spectra obtained with on average 15 atoms trapped inside the cavity (Fig. 1d) . The data points and theory curve given in black correspond to the Lorentzian transmission of the empty cavity. In contrast, the transmission spectrum for the two-state atoms coupled to the cavity (red data and dashed curve) displays the characteristic vacuum-Rabi splitting accompanied by a significant drop in the transmission at the empty cavity resonance (probe-cavity detuning D 5 0). This spectrum is dramatically altered under the conditions of EIT (blue data and theory curve). First, we notice a frequency shift of the vacuum-Rabi resonances due to the 'dressing' of the atom-cavity *These authors contributed equally to this work.