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Nonlinear spectroscopy of a single-atom-cavity-system

I. Schuster, Garching (Germany) Max-Planck-Institut fuer Quantenoptik (MPQ)

Published Oct 1, 2008• 0 citations
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Abstract

In this thesis, the properties of a strongly-coupled single-atom-cavity system are studied. Here, the atom predominantly interacts with only one mode of the electromagnetic field. Special attention was paid to the energy spectrum of the system, which consists of an anharmonic ladder of doublets. Developing a new laser cooling method for the preparation of the atom, it was possible for the first time to observe not only the first doublet, but also a state belonging to the second doublet in the optical spectrum. The quantization of light-matter interaction is confirmed by this observation, as the state can only be explained by a bonding of one atom to exactly two photons. Its excitation by a two-photon transition contains a nonlinearity at the level of single quanta. This leads to a dependency of the spectrum on the intensity of the impinging light.

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In this thesis, the properties of a strongly-coupled single-atom-cavity system are studied. Here, the atom predominantly interacts with only one mode of the electromagnetic field. Special attention was paid to the energy spectrum of the system, which consists of an anharmonic ladder of doublets. Developing a new laser cooling method for the preparation of the atom, it was possible for the first time to observe not only the first doublet, but also a state belonging to the second doublet in the optical spectrum. The quantization of light-matter interaction is confirmed by this observation, as the state can only be explained by a bonding of one atom to exactly two photons. Its excitation by a two-photon transition contains a nonlinearity at the level of single quanta. This leads to a dependency of the spectrum on the intensity of the impinging light.
This thesis reports on experiments with single atoms strongly coupled to a single mode of a high-finesse optical cavity. In the first part, it is shown that the motion of the atom in an intracavity dipole trap can be observed with high spatial and temporal resolution. Depending on the atom's current position, the dipole trap is switched to control the motion of the atom. The feedback acts in real time, faster than one oscillation period of the atom in the trap. In the second part the anharmonic energy level structure of the system is investigated. This anharmonicity allows one to selectively populate states containing two energy quanta in a two-photon process, resulting in a nonlinear response of the system with respect to the drive laser intensity. The photon statistics of the light transmitted through the cavity shows photon bunching and super-Poissonian fluctuations, indicating the transmission of photon pairs typical for a two-photon gateway.
Cooling is indispensable for trapping and observing single free atoms.In atomic physics, this can be achieved efficiently by laser cooling.Conventional methods to laser cool atoms are based on repeated optical pumping cycles followed by spontaneous emission of a photon from the atom.Such pumping schemes can only be applied to certain atomic species, which have a closed optical transition.Drawbacks caused by photons spontaneously emitted from the atom can be avoided in the strongly coupled atom-cavity system in which energy and entropy can be removed by photons lost from the cavity.This thesis reports on the first observation of this cooling mechanism which does not rely on spontaneous emission.In the experiment, a single atom is captured in an intracavity dipole trap.Illuminating the system with a weak, slightly blue-detuned light beam extends the storage time of the atom in the trap.The observed cooling force is at least five times larger than the force achieved by free-space cooling methods at comparable excitation of a two-level atom.Utilising cavity cooling, single atoms are prepared strongly-coupled to the mode of a high-finesse cavity.This allows to observe two well-resolved normal-mode peaks in both the cavity transmission as well as in the trap lifetime.The experiment is in agreement with a Monte Carlo simulation, demonstrating the localisation of the atom to within a tenth of a wavelength.The ability to individually excite the normal modes opens new possibilities to use this system for applications in quantum information science, including the realisation of quantum-logic gates.In the future cavity cooling might be applied to systems which cannot be cooled by conventional methods as for instance molecules or an atom carrying a quantum bit.
The experiments discussed in this thesis focus on the interaction of a single trapped atom with the single mode of a high-finesse optical cavity, in the regime of strong coupling. Chapter 1 gives a brief introduction, after which Chapter 2 describes our recent measurements of the transmission spectrum of the atom-cavity system. The spectrum exhibits a clearly resolved vacuum-Rabi splitting, in good quantitative agreement with theoretical predictions. A new Raman scheme for cooling atomic motion along the cavity axis enables a complete spectrum to be recorded for an individual atom trapped within the cavity mode, in contrast to all previous measurements of this type that have required averaging over 10^3-10^5 atoms. Chapter 3 discusses our observations of photon blockade for the transmitted light in the presence of one trapped atom. Excitation of the atom-cavity system by a first photon blocks the transmission of a second one, thereby converting an incident Poissonian stream of photons into a sub-Poissonian, anti-bunched stream, as confirmed by measurements of the photon statistics of the transmitted field. The intensity correlations of the cavity transmission also reveal the energy distribution for oscillatory motion of the trapped atom. Chapter 4 details a set of simple but necessary measurements of relevant experimental parameters such as cavity geometry, linewidth, mirror properties, birefringence, and detection efficiency. The thesis concludes with Appendix A, describing the efficient laser setup we use for our magneto-optical traps.
Atoms absorb and emit resonant light photon by photon. However, an atom strongly coupled to the light field of a high-finesse optical resonator forms a new system with new characteristics, enabling the emission and absorption of photon pairs. The energy eigenstates of the coupled system form an anharmonic ladder of doublets, the so-called Jaynes-Cummings ladder. The first doublet, consisting of the two so-called normal modes, has been observed in various cavity QED systems. It can be explained classically using Maxwell equations as arising from the coupling of a classical dipole to a classical electromagnetic field. In contrast, the higher-order doublet has so far no classical analogue. It requires both the quantization of the atomic internal state as well as the quantization of light.
Light and matter can be unified under the strong coupling regime, creating superpositions of both, called dressed states or polaritons. After initially being demonstrated in bulk semiconductors and atomic systems ,strong coupling phenomena have been realized in solid state optical microcavities. They form an essential ingredient in the exciting physics spanning from many-body quantum coherence phenomena, like Bose-Einstein condensation and superfluidity, to cavity quantum electrodynamics (cQED). A widely used approach within cQED is the Jaynes-Cummings (JC) model that describes the interaction of a single fermionic two-level system with a single bosonic photon mode. For a photon number larger than one, known as quantum strong coupling (QSC), a significant anharmonicity is predicted for the ladder-like spectrum of dressed states. For optical transitions in semiconductor nanostructures, first signatures of the quantum strong coupling were recently published. In our latest report we applied advanced coherent nonlinear spectroscopy to explore a strongly coupled exciton-cavity system. Specifically, we measured and simulated its four-wave mixing (FWM) response, granting direct access to the first two rungs of the JC ladder. This paper summarizes the main results of Ref. 15 and adds FWM experiments obtained on a micropillar cavity in which a doublet of quantum dot (QD) excitons interacts with the cavity mode in the limit of weak to strong coupling.
Optical photons do not interact in vacuum. Therefore, we employ a single atom strongly coupled to a cavity to mediate interactions between photons, first, within a single light field and, second, in two different light fields. For the former, we demonstrate a two-photon blockade whereby the absorption of two photons blocks further excitation. For the latter, we realize strong nonlinear coupling between photons in different cavity modes which can either mutually block each other or only transit the system conjunctly.