The creation of a photon-atom bound state was first envisaged for the case of an atom in a long-lived excited state inside a high-quality microwave cavity. In practice, however, light forces in the microwave domain are insufficient to support an atom against gravity. Although optical photons can provide forces of the required magnitude, atomic decay rates and cavity losses are larger too, and so the atom-cavity system must be continually excited by an external laser. Such an approach also permits continuous observation of the atom's position, by monitoring the light transmitted through the cavity. The dual role of photons in this system distinguishes it from other single-atom experiments such as those using magneto-optical traps, ion traps or a far-off-resonance optical trap. Here we report high-finesse optical cavity experiments in which the change in transmission induced by a single slow atom approaching the cavity triggers an external feedback switch which traps the atom in a light field containing about one photon on average. The oscillatory motion of the trapped atom induces oscillations in the transmitted light intensity; we attribute periodic structure in intensity-correlation-function data to 'long-distance' flights of the atom between different anti-nodes of the standing-wave in the cavity. The system should facilitate investigations of the dynamics of single quantum objects and may find future applications in quantum information processing.

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.

The Stark interaction of polar molecules with an inhomogeneous electric field is exploited to select slow molecules from a room-temperature reservoir and guide them into an ultrahigh vacuum chamber. A linear electrostatic quadrupole with a curved section selects molecules with small transverse and longitudinal velocities. The source is tested with formaldehyde (H 2 CO) and deuterated ammonia (ND 3 ). With H 2 CO a continuous flux is measured of Ϸ10 9 /s and a longitudinal temperature of a few kelvin. The data are compared with the result of a Monte Carlo simulation.The past years have seen an explosion of activity in the field of cold atomic gases ͓1͔. It is interesting and desirable to extend these investigations to molecules, which have a complex internal structure and can as a consequence possess a permanent electric dipole moment. Trapping cold polar molecules will lead to new physics due to the long range and anisotropy of the dipole-dipole interaction ͓2͔. Slow molecules for precision measurements or interferometry are further motivations behind the ongoing efforts. However, the complexity and density of energy levels in the rotational and vibrational manifolds largely precludes the effective use of laser cooling techniques ͓3͔. Therefore, a number of different approaches have been considered for cooling and trapping molecules. Buffer-gas cooling in a cryogenic environment is one possibility, but requires a rather complex setup ͓4͔. Another method is photoassociation, but this is limited to simple molecules with laser-cooled precursor atoms ͓5͔. A different technique uses deceleration by the Stark effect, where packages of polar molecules are decelerated with time-varying electric fields ͓6-8͔. Other, mostly mechanical methods have also been proposed but remain to be demonstrated ͓9,10͔.It is, however, not necessary to produce slow molecules, as they are present in any thermal gas, even at room temperature. Slow molecules only need to be filtered out. For this reason, already in the 1950s it was attempted to select the slowest atoms from a hot beam using gravity ͓11͔. These attempts failed, mostly because the slow particles were kicked away by the fast ones. Much later, it was demonstrated that slow lithium atoms can be efficiently guided out of a hot beam with strong permanent magnets, providing a robust and cheap source of slow atoms ͓12͔, e.g., for Bose-Einstein condensation experiments. In the same spirit, an efficient and simple filtering technique could play an important role towards the production of a cold molecular gas.In this paper we describe an experiment in which the Stark interaction of polar molecules with an inhomogeneous, electrostatic field is exploited to efficiently select and guide slow molecules out of a room-temperature reservoir into ultrahigh vacuum. Whether a dipolar molecule is weak-field seeking and trapped by an electric-field minimum, or strongfield seeking and expelled, depends on whether the average orientation of the rotating molecular dipole is antiparallel or parallel to t...

No abstract

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 ...

We present a method which delivers a continuous, high-density beam of slow and internally cold polar molecules. In our source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electrostatic quadrupole guide. For ND3 the source produces fluxes up to (7± 7 4 ) × 10 10 molecules/s with peak densities up to (1.0± 1.0 0.6 ) × 10 9 molecules/cm 3 . For H2CO the population of rovibrational states is monitored by depletion spectroscopy, resulting in single-state populations up to (82 ± 10)%.

Single atoms absorb and emit light from a resonant laser beam photon by photon. We show that a single atom strongly coupled to an optical cavity can absorb and emit resonant photons in pairs. The effect is observed in a photon correlation experiment on the light transmitted through the cavity. We find that the atom-cavity system transforms a random stream of input photons into a correlated stream of output photons, thereby acting as a two-photon gateway. The phenomenon has its origin in the quantum anharmonicity of the energy structure of the atom-cavity system. Future applications could include the controlled interaction of two photons by means of one atom.

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...