All conventional methods to laser-cool atoms rely on repeated cycles of optical pumping and spontaneous emission of a photon by the atom. Spontaneous emission in a random direction provides the dissipative mechanism required to remove entropy from the atom. However, alternative cooling methods have been proposed for a single atom strongly coupled to a high-finesse cavity; the role of spontaneous emission is replaced by the escape of a photon from the cavity. Application of such cooling schemes would improve the performance of atom-cavity systems for quantum information processing. Furthermore, as cavity cooling does not rely on spontaneous emission, it can be applied to systems that cannot be laser-cooled by conventional methods; these include molecules (which do not have a closed transition) and collective excitations of Bose condensates, which are destroyed by randomly directed recoil kicks. Here we demonstrate cavity cooling of single rubidium atoms stored in an intracavity dipole trap. The cooling mechanism results in extended storage times and improved localization of atoms. We estimate that the observed cooling rate is at least five times larger than that produced by free-space cooling methods, for comparable excitation of the atom.

Optical nonlinearities typically require macroscopic media, thereby making their implementation at the quantum level an outstanding challenge. Here, we demonstrate a nonlinearity for one atom enclosed by two highly reflecting mirrors 1 . We send laser light through the input mirror and record the light from the output mirror of the cavity. For weak laser intensity, we find the vacuum-Rabi resonances 2-11 . But for higher intensities, we observe an extra resonance 12 , which originates from the fact that the cavity can accommodate only an integer number of photons and that this photon number determines the characteristic frequencies of the coupled atom-cavity system 13-15 . We selectively excite such a frequency by depositing two photons at once into the system and find a transmission that increases with the laser intensity squared. The nonlinearity differs from classical saturation nonlinearities [16][17][18][19] and is direct spectroscopic proof of the quantum nature of the atom-cavity system. It provides a photon-photon interaction by means of one atom, and constitutes a step towards a two-photon gateway or a singlephoton transistor 20 . The quantum nonlinearity has its origin in the fact that under the condition of strong coupling, a system composed of a single atom and a single cavity mode has properties that are distinctively different from those of the bare atom (without the cavity), or the bare cavity (without the atom), or just the sum of the two (Fig. 1a). In fact, the composite system forms a new quantum entity, the so-called atom-cavity molecule, made of matter and light, with its own characteristic energy spectrum. This spectrum consists of an infinite ladder of pairs of states, the dressed states (Fig. 1b). The first doublet contains one quantum of energy and can be probed by laser spectroscopy. For weak probing, the resulting spectrum is independent of the laser intensity and has been dubbed the vacuum-Rabi or normal-mode spectrum, consisting of a pair of resonances symmetrically split around the bare atomic and cavity resonances. This spectrum was first observed with atomic beams 1 , and has been explored recently with single dipole-trapped atoms 2-4 . It constitutes a benchmark for strong atom-cavity coupling and is central to most cavity quantum electrodynamics (QED) experiments, including those outside atomic physics [5][6][7][8][9][10][11] . Note that the normal-mode spectrum on its own can equally well be described classically, by linear dispersion theory or a coupled oscillator model (atomic dipole and cavity field).The next-higher-lying doublet contains two quanta of energy and lacks a classical explanation 12,21,22 . The corresponding dressed states have been observed (together with a few higher-order states) in microwave cavity QED [13][14][15] and even ion trapping, where phonons play the role of photons 23 . At optical frequencies, evidence for these states has indirectly been obtained in two-photon correlation experiments where the conditional response of the system on detection of an ...

The energy-level structure of a single atom strongly coupled to the mode of a high-finesse optical cavity is investigated. The atom is stored in an intracavity dipole trap and cavity cooling is used to compensate for inevitable heating. Two well-resolved normal modes are observed both in the cavity transmission and the trap lifetime. The experiment is in good agreement with a Monte Carlo simulation, demonstrating our ability to localize the atom to within λ/10 at a cavity antinode. 32.80.Pj, Experimental research in quantum information science with atoms and ions [1] is based on the ability to control individual particles in a truly deterministic manner. While spectacular advances have recently been achieved with trapped ions interacting via phonons [2,3], the precise control of the motion of atoms exchanging photons inside an optical cavity [4] or emitting single photons on demand [5,6] is still a challenge. Although very successful, experiments in cavity quantum electrodynamics with single laser-cooled atoms [7,8,9] are complicated by the motion of the atom in the standing-wave mode of the optical cavity [10,11]. The lack of control over the atomic motion is mainly due to the heating effects of the various laser fields employed to trap and excite the atom inside the cavity in combination with the limited ability to cool the atom between two highly reflecting mirrors facing each other at a microscopic distance [12,13]. Only recently, good localization of the atom at an antinode of the cavity mode has been achieved by applying optical molasses [14] or a novel cavity cooling force [15] to a trapped atom.In this Letter, we go one step further and employ cavity cooling to probe the energy spectrum of a single trapped atom strongly coupled to a high-finesse resonator [16,17]. In previous experiments using thermal beams, the spectrum was explored only for many atoms [18,19], one atom on average [20,21], or single cold atoms transiting the cavity [22]. Our experiment is the first in which the normal-mode (or vacuum-Rabi) splitting of a single atom trapped inside a cavity is observed. Both the cavity transmission and the trapping time are investigated. The results agree with a Monte Carlo simulation and demonstrate that remarkably good control can be obtained over this fundamental quantum system.The cavity used in the experiment (Fig. 1) has a finesse F = 4.4 × 10 5 , a mode waist w 0 = 29 µm and a length l = 122 µm [15]. A single TEM 00 mode of the cavity is near-resonant with the 5 2 S 1/2 F = 3, m F = 3 ↔ 5 2 P 3/2 F = 4, m F = 4 transition of 85 Rb at λ = 780.2 nm. The atom-cavity coupling at an antinode of the standing wave, g/2π = 16 MHz, is large compared to the amplitude decay rates of the atomic excitation, γ/2π = 3 MHz, and the cavity field, κ/2π = 1.4 MHz. Strong coupling is reached, resulting in critical photon and atom numbers n 0 = γ 2 /2g 2 ≈ 1/60 and N 0 = 2γκ/g 2 ≈ 1/30, respectively. This strongly coupled atom-cavity system is probed by a weak near-resonant beam impinging on the cavity. The probe bea...

A single atom strongly coupled to a cavity mode is stored by three-dimensional confinement in bluedetuned cavity modes of different longitudinal and transverse order. The vanishing light intensity at the trap center reduces the light shift of all atomic energy levels. This is exploited to detect a single atom by means of a dispersive measurement with 95% confidence in 10 s, limited by the photon-detection efficiency. As the atom switches resonant cavity transmission into cavity reflection, the atom can be detected while scattering about one photon.