Cavity Quantum Electrodynamics
The interaction of a single two-level atom and a single quantized radiation mode is the textbook example of quantum optics. However, it is only since the 1990ies that laboratory experiments can mimic this idealized example. We work on such an experiment combining a small, ultra-high finesse optical cavity with an atomic fountain as a source of ultracold atoms. With this set-up we can clearly see single atoms passing our cavity. Classically this can be explained by the change in the index of refraction caused by the (single-atom !) atomic medium, shifting the cavity resonance. When we detect the presence of an atom, we suddenly increase the depth of an intracavity dipole trap to localize the atom in a region of strong coupling inside the cavity mode. In this way, we prepare a system which allows us to study fundamental aspects of matter-light interaction on the quantum level.
Nonlinear spectroscopy of the atom-cavity systemThe strongly-coupled atom-cavity system forms a new entity with properties that differ from those of the uncoupled atom and the empty cavity. These properties become apparent in the resonance frequencies of the system. Some of these resonances can only be explained by the quantum nature of the light; they are evidence for the single atom interacting with a discrete number of photons in the mode. Apart from the fundamental normal-mode resonances of the system, which are formed by the interaction of a single atom with a single photon, we have observed an additional resonance in the spectrum of the system which is caused by the coupling of a single atom to two photons in the mode. Since this two-photon resonance can only be excited by incoming photon pairs, it shows a nonlinear dependency on the intensity of the exciting light field.
Blue intracavity dipole trapThe standard way of localising an atom in the cavity is a red-detuned standing-wave intracavity dipole trap, formed by exciting a fundamental TEM00 Hermite-Gaussian mode far detuned from the atomic resonance frequency. In this light field, the atom is attracted to regions of high intensity at an antinode of the standing wave, where the atomic resonance frequency is altered by the trap light. This can be avoided by using a blue-detuned dipole trap, which allows localizing the atom in regions of vanishing light intensity. This trap is formed by a combination of modes: One TEM00 blue-detuned mode is used to inhibit motion of an atom along the cavity axis, and a combination of two TEM10 and TEM01 modes forms a donut pattern which closes the trap in radial direction. We have shown in a spectroscopy experiment that the resonance frequency of an atom which is stored in this blue-detuned dipole trap is not altered by the trap light.
Setup of a new cavityCurrently, we are setting up and testing a new cavity, whose conical mirrors will allow us to cool the atoms optically from the sides. This will hopefully increase the trapping time of the atoms and thus enable more sophisticated measurements. So far, we have used the cavity to study the high-order transverse modes of Fabry-Perot resonators. Spectroscopic measurements of these modes revealed that their degeneracy is lifted as it has been predicted by theoretical calculations beyond the paraxial approximation, which have been performed in our group. Now, we are testing the mechanical stability of the new setup and, in the future, we would like to use it to investigate further the nonlinear effects arising from single atoms in the cavity.
Even with a cavity field of on average less than a single photon, the mechanical effects on the ultracold atoms can be substantial. We used this to trap a single atom in the field of on average only a single photon, which is visualized in a short video (800k).
The cavity can not only resonate in its fundamental TEM00 symmetric Gaussian mode, but also in higher-order transverse modes. We could demonstrate experimentally how single atoms pass through these modes. From the theoretical side, together with the Innsbruck group of H. Ritsch, we were able to show how this interaction with the higher-order modes can be used for a single atom detector with more transverse spatial information than a single TEM00 mode would yield. Interestingly, special combinations of the higher-order transverse modes also form the cavity-altered pattern into which a single excited atom in the cavity would emit.
The light forces on an atom in a high-finesse optical cavity are substantially altered with respect to those in free space. We exploit this to demonstrate cavity cooling, extending the lifetime of a single atom in the intracavity dipole trap to an average of ~60 ms. A unique feature of this novel cooling scheme is that is does not rely on spontaneous emission. Instead, the strong coupling of the atom to the cavity allows the dissipation to be transferred to the decay of the cavity field. The cooling also allows measuring the fundamental normal-mode spectrum of the atom-cavity system, which shows the lifting of the atom-cavity degeneracy by the atom-cavity-field coupling.
The high-finesse cavity
The finesse of the cavity is estimated from the measured cavity line width, 1.4 MHz (HWHM), to be better than 4.3x105. This procedure of obtaining the cavity finesse was checked by cavity ring-down measurements. In order to tune the resonance frequencies of the cavity, its length is adjustable by means of a piezo-ceramic tube.
The high-finesse resonator (cavity) consists of two spherical dielectric mirrors of ultrahigh reflectivity. The resonator length is 0.116 mm. Because of the curvature of the mirrors (radius:20 cm), this means that the edges of the mirrors almost touch. But not quite, there is a gap of approximately 0.030 mm left for the atoms to enter.As a consequence of the ultrahigh finesse, the length of the cavity must be very stable. We actively stabilize our cavity to an residual RMS noise less than 0.001 Å. The piezo-ceramic tube encloses the cavity mirrors, creating a very stable configuration. Four holes allow atoms to enter the volume between the mirrors.
The atomic fountain
In our experiment cold Rb atoms are launched upward with velocities comparable to that of a typical garden fountain. Its principle of operation is quite different, though.
In an atomic fountain cold atoms are launched with light forces. First of all we need cold atoms. Cold Rb atoms can routinely be prepared in a magneto-optical trap (MOT), which confines atoms by light pressure from laser beams illuminating the atom cloud from e.g., six directions. We trap and cool typically ten millions of atoms in our MOT, in a few hundred milliseconds from background vapor.
After trapping, a 6 ms long optical molasses cooling is applied to cool the atoms further to about 0.005 mK. At this moment, the frequency of the lower beams is detuned relative to the upper beams. This still cools the atoms, but now in a moving reference frame. By increasing the relative detuning slowly, atoms are accelerated in the vertical direction. The laser beams are detuned with a constant rate to a frequency difference of up to 5.5 MHz, depending on the desired launch velocity. When the molasses light is switched off, the atoms continue to move upwards.
An atom trapped in the cavity
An atom approaches the cavity and tunes the cavity into resonance. Thus, the transmission through the cavity rises, and triggers the feedback switch. The atom is now trapped in the cavity. Its position changes the resonance frequency of the cavity and therefore the transmitted light intensity. Eventually, the atom is heated out of the cavity.