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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Two-photon gateway in one-atom cavity quantum electrodynamics

Alexander Kubanek, Alexei Ourjoumtsev, I. Schuster, M. Koch, Pepijn W. H. Pinkse, Karim Murr, Gerhard Rempe

Published Jun 1, 2009• 13 citations
Publisher

Abstract

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.

DOI
10.1109/cleoe-eqec.2009.5192317

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Annual Meeting Optical Society of America•Jan 1, 1990•0 citations
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.
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.
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.
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.
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.
We investigate the cavity excitation spectrum and the photon number distribution in a cavity QED system driven by a broadband squeezed vacuum. In an empty cavity, we show that only states with even number of photons can be measured under resonant condition since the squeezed vacuum consists of states with even number of photons only. When a single atom is trapped in the cavity, the strong coupling between the atom and cavity results in energy splittings of the system, and there exist two peaks in the cavity excitation spectrum at two-photon transition frequencies. At the central frequency, however, all photon states can be detected because of the interaction between the atom and cavity. Therefore, it can be used to detect whether a single atom is trapped in the cavity. We also show that the squeezed vacuum can promote multiphoton excitations in the cavity. Using a coherent probe field, it is possible to explore higher Jaynes-Cummings doublet even if the probe field intensity is very weak.
When a single atom interacts with one mode of an optical cavity and the coupling to the cavity mode is sufficiently large, the spectroscopic response of the composite system to a weak probe is a doublet, with peaks located at the eigenfrequencies of the one-quantum excited states of the Jaynes–Cummings Hamiltonian. So far as these one-quantum states are concerned the spectroscopy of the Jaynes–Cummings model is the same as that of coupled, damped harmonic oscillators. This equivalence has been used to observe a doublet spectrum (the so-called "vacuum" Rabi splitting) by exciting a collective dipole oscillator inside an optical cavity.1,2 At higher levels of excitation the spectroscopy of the Jaynes–Cummings model differs markedly from that of coupled harmonic oscillators. Coupled harmonic oscillators give a doublet at all excitation levels; because of its noncommensurate excited state energies the Jaynes–Cummings model gives a multitude of additional peaks. In this paper we show how the excited states of the Jaynes–Cummings Hamiltonian can be probed by driving a cavity containing a single atom with broadband chaotic light. For a sufficiently large dipole coupling strength the spectrum of the light scattered out of the cavity mode shows peaks at the many resonance frequencies of the Jaynes–Cummings Hamiltonian. A similar multipeaked spectrum is seen in transmission. The spectrum of the light reflected from the cavity shows corresponding absorption dips superimposed on the broadband input.