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arXiv:1608.06725 · New Journal of Physics

Collective strong coupling of cold potassium atoms in a ring cavity

Robert Culver, Andreas Lampis, Balázs Megyeri, K Pahwa, Lawrence Mudarikwa, Michael Holynski, Ph. W. Courteille, J. Goldwin

Published Nov 24, 2016• 23 citations
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Abstract

We present experiments on ensemble cavity quantum electrodynamics with cold potassium atoms in a high-finesse ring cavity.Potassium-39 atoms are cooled in a two-dimensional magneto-optical trap and transferred to a three-dimensional trap which intersects the cavity mode.The apparatus is described in detail and the first observations of strong coupling with potassium atoms are presented.Collective strong coupling of atoms and light is demonstrated via the splitting of the cavity transmission spectrum and the avoided crossing of the normal modes.

DOI
10.1088/1367-2630/18/11/113043

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Collective strong coupling of cold potassium atoms in an optical ring cavity

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This thesis describes an experiment which studies a cloud of magneto-optically trapped potassium-39 atoms inside an optical ring cavity. The potassium atoms are firstly cooled in a two-dimensional magneto-optical trap (MOT) and are then transferred into a three dimensional MOT which overlaps with the cavity mode. In this thesis, the optimisation of the MOT systems and the characterisation and construction of the optical ring cavity are discussed. After exploring these two systems independently, the two systems are then coupled together to provide an atom-cavity system, which exhibits collective strong coupling. The system is shown to exhibit normal-mode splitting, with a collective Rabi splitting of G = 2π (6.25±0.50)MHz, corresponding to (4.7±0.6) x 10³ atoms in the cavity. Whilst collective strong coupling has been achieved before in other experiments, we believe this is the first experiment to achieve this with potassium. \n \nThe next goal in the experiment is to control the group refractive index of the atoms inside the cavity. The modification of the group index using electromagnetically-induced transparency (EIT) on hot atoms inside a vapour cell has already been previously demonstrated by this experiment; and using the same laser system we aim to demonstrate EIT on the cold intra-cavity atoms as well. Gain mechanisms could also be used to create a ring laser with a controllable group index. These techniques could lead to a range of applications such as enhanced-sensitivity laser ring gyroscopes and active optical clocks. \n \n

•Jul 1, 2017•1 citation

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This thesis focuses on the construction and development of an experiment to study cold potassium atoms in an optical ring cavity. Firstly we load a potassium-39 magneto optical trap (MOT) inside the ring cavity. To achieve this a laser system, rectangular magnetic coils system and vacuum system are designed and constructed. To stabilise the laser system, a detailed study of various potassium spectroscopy techniques is undertaken and the reference laser is suitably locked to the magnetically induced spectroscopy. We load an ensemble of 10\\(^5\\) potassium atoms inside the ring cavity mode and study their collective strong coupling with the cavity field. In the collective strong coupling regime, the photons are coherently exchanged between the atomic ensemble and cavity field. This rate of exchange is determined by the singlephoton Rabi frequency `2\\(g\\)' which must be larger than the cavity field decay rate `2\\(k\\)' and atomic spontaneous emission rate `2\\(ᵧ\\)'. As a result of this coupling, the peak in the cavity transmission signal undergoes a splitting known as `vacuum Rabi splitting' (or `normal mode splitting'). The vacuum Rabi splitting has been realised and measured to be ~ 2\\(π\\). 18 MHz in our system.

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Strong Light-Matter Coupling with Cold Potassium Atoms in a Ring Cavity

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Collective strong coupling of cold potassium atoms in a high-finesse ring cavity is demonstrated. The results imply a large effective optical susceptibility, which sets the foundation for studies of cavity quantum electrodynamics with large dispersion.

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Coherent light-matter interactions with potassium atoms

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This thesis presents experimental work undertaken towards realising a system composed of magneto-optically-trapped potassium-39 atoms (39K MOT) enclosed in a ring cavity. The length of the cavity is 9.51 (5) cm and consequently its volume is big compared to cavities used for cavity quantum electrodynamics experiments, operating in the single-atom regime. As such, in order to enter a regime where the light-matter interactions are strong, thousands of atoms have to collectively interact with the mode of the cavity. We achieve strong interactions with potassium atoms for the first time, after 4.36(3) x 104atoms were overlapped with the cavity mode. The signature of our system lying in the collective strong coupling regime was the observation of the splitting and avoided crossing of the normal modes, revealing a coupling strength constant G = 2π x 19.1(1.6) MHz. \n \nThe thesis also discusses the application of the eltectromagnetically-induced transparency (EIT) technique to control the group index of refraction (ng) of atomic ensembles. We firstly apply EIT in the Λ configuration, in a heated vapour cell of potassium. This helped us to better understand the underlying fundamentals of the technique, before ultimately applying it in the (main) cold atoms experiment. Deeply sub-natural EIT features have been achieved, for which we calculate ng as high as 6330(160). Furthermore, by performing a novel measurement of the circular dichroism (and thus birefringence). We observed greater anisotropy when the weak probe and strong coupling beams, which participate in the formation of the EIT system. have lin∥ lin rather than \nlin ⊥ linpolarisation. \n \nBy optimising the temperature and optical depth of the 39K MOT, EIT investigations were made possible with the cold atoms. While a splitting of the absorption line was observed at high coupling beam powers, no EIT resonances were observed when the coupling power was reduced. Our measurements imply that the EIT signal is compromised by decoherence factors, such as the inhomogeneity of the magnetic field across the atomic cloud. Eliminating decoherence and achieving EIT in the strongly interacting atom-cavity system will enable studies with applications in metrology and sensing.

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This thesis describes an experiment with a cloud of magneto-optically trapped Potassium-39 spatially overlapped with the modes of a high-finesse ring cavity. Optical gain was observed with the system in steady state, without any beams additional to the normal cooling and repump beams in a potassium-39 magneto-optical trap. This was found to operate on a Raman gain mechanism between atomic hyperfine ground states, which has not been seen under similar conditions with other atomic species. Lasing into the cavity modes was then studied, with a particular focus on aspects specific to the ring cavity geometry. This showed a directional bistability and non-reciprocity. The bistability can be described by existing theory, but the mechanism of the non-reciprocity is unknown, despite a detailed investigation. The second order coherence time of the system was explored below threshold, and demonstrates a strong dependence on gain, which may have applications in the detection of extremely small gains.

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Cold atoms in a ring cavity

Lawrence Mudarikwa

An ensemble of atoms coupled to a high finesse optical cavity is an ideal test bed for the study of the cooperative behaviour of atom-photon systems. Dicke showed that an ensemble of excited atoms coupled to a light field interacts with the light in a collective and coherent fashion leading to the emission of highly directional spontaneous emission whose intensity scales with the square of the number of atoms, a phenomenon known as superradiance. \nThis thesis describes the build of an experiment to study cooperative atom-photon interactions in a ring cavity. Particular focus is given to the cavities used in the experiment. Firstly a transfer cavity used for transferring stability to off-resonant lasers in the experiment, this was developed with the capability of exploiting Gouy phase degeneracies to produce tightly spaced frequency discriminants to be used as lock points. Secondly the ring cavity for the experiment which is atypical in its design allowing for bidirectional probing of the cavity mode and dynamic manipulation of the intra-cavity optical lattice. An intra-cavity MOT was produced and collective strong coupling was observed in the cavity with an Neff=6400 atoms coupled to the cavity mode.

•Jul 1, 2015•0 citations
We present experiments on ensemble cavity quantum electrodynamics with cold potassium atoms in a high-finesse ring cavity.Potassium-39 atoms are cooled in a two-dimensional magneto-optical trap and transferred to a three-dimensional trap which intersects the cavity mode.The apparatus is described in detail and the first observations of strong coupling with potassium atoms are presented.Collective strong coupling of atoms and light is demonstrated via the splitting of the cavity transmission spectrum and the avoided crossing of the normal modes.
This thesis describes an experiment which studies a cloud of magneto-optically trapped potassium-39 atoms inside an optical ring cavity. The potassium atoms are firstly cooled in a two-dimensional magneto-optical trap (MOT) and are then transferred into a three dimensional MOT which overlaps with the cavity mode. In this thesis, the optimisation of the MOT systems and the characterisation and construction of the optical ring cavity are discussed. After exploring these two systems independently, the two systems are then coupled together to provide an atom-cavity system, which exhibits collective strong coupling. The system is shown to exhibit normal-mode splitting, with a collective Rabi splitting of G = 2π (6.25±0.50)MHz, corresponding to (4.7±0.6) x 10³ atoms in the cavity. Whilst collective strong coupling has been achieved before in other experiments, we believe this is the first experiment to achieve this with potassium. \n \nThe next goal in the experiment is to control the group refractive index of the atoms inside the cavity. The modification of the group index using electromagnetically-induced transparency (EIT) on hot atoms inside a vapour cell has already been previously demonstrated by this experiment; and using the same laser system we aim to demonstrate EIT on the cold intra-cavity atoms as well. Gain mechanisms could also be used to create a ring laser with a controllable group index. These techniques could lead to a range of applications such as enhanced-sensitivity laser ring gyroscopes and active optical clocks. \n \n
This thesis focuses on the construction and development of an experiment to study cold potassium atoms in an optical ring cavity. Firstly we load a potassium-39 magneto optical trap (MOT) inside the ring cavity. To achieve this a laser system, rectangular magnetic coils system and vacuum system are designed and constructed. To stabilise the laser system, a detailed study of various potassium spectroscopy techniques is undertaken and the reference laser is suitably locked to the magnetically induced spectroscopy. We load an ensemble of 10\\(^5\\) potassium atoms inside the ring cavity mode and study their collective strong coupling with the cavity field. In the collective strong coupling regime, the photons are coherently exchanged between the atomic ensemble and cavity field. This rate of exchange is determined by the singlephoton Rabi frequency `2\\(g\\)' which must be larger than the cavity field decay rate `2\\(k\\)' and atomic spontaneous emission rate `2\\(ᵧ\\)'. As a result of this coupling, the peak in the cavity transmission signal undergoes a splitting known as `vacuum Rabi splitting' (or `normal mode splitting'). The vacuum Rabi splitting has been realised and measured to be ~ 2\\(π\\). 18 MHz in our system.
This thesis presents experimental work undertaken towards realising a system composed of magneto-optically-trapped potassium-39 atoms (39K MOT) enclosed in a ring cavity. The length of the cavity is 9.51 (5) cm and consequently its volume is big compared to cavities used for cavity quantum electrodynamics experiments, operating in the single-atom regime. As such, in order to enter a regime where the light-matter interactions are strong, thousands of atoms have to collectively interact with the mode of the cavity. We achieve strong interactions with potassium atoms for the first time, after 4.36(3) x 104atoms were overlapped with the cavity mode. The signature of our system lying in the collective strong coupling regime was the observation of the splitting and avoided crossing of the normal modes, revealing a coupling strength constant G = 2π x 19.1(1.6) MHz. \n \nThe thesis also discusses the application of the eltectromagnetically-induced transparency (EIT) technique to control the group index of refraction (ng) of atomic ensembles. We firstly apply EIT in the Λ configuration, in a heated vapour cell of potassium. This helped us to better understand the underlying fundamentals of the technique, before ultimately applying it in the (main) cold atoms experiment. Deeply sub-natural EIT features have been achieved, for which we calculate ng as high as 6330(160). Furthermore, by performing a novel measurement of the circular dichroism (and thus birefringence). We observed greater anisotropy when the weak probe and strong coupling beams, which participate in the formation of the EIT system. have lin∥ lin rather than \nlin ⊥ linpolarisation. \n \nBy optimising the temperature and optical depth of the 39K MOT, EIT investigations were made possible with the cold atoms. While a splitting of the absorption line was observed at high coupling beam powers, no EIT resonances were observed when the coupling power was reduced. Our measurements imply that the EIT signal is compromised by decoherence factors, such as the inhomogeneity of the magnetic field across the atomic cloud. Eliminating decoherence and achieving EIT in the strongly interacting atom-cavity system will enable studies with applications in metrology and sensing.
This thesis describes an experiment with a cloud of magneto-optically trapped Potassium-39 spatially overlapped with the modes of a high-finesse ring cavity. Optical gain was observed with the system in steady state, without any beams additional to the normal cooling and repump beams in a potassium-39 magneto-optical trap. This was found to operate on a Raman gain mechanism between atomic hyperfine ground states, which has not been seen under similar conditions with other atomic species. Lasing into the cavity modes was then studied, with a particular focus on aspects specific to the ring cavity geometry. This showed a directional bistability and non-reciprocity. The bistability can be described by existing theory, but the mechanism of the non-reciprocity is unknown, despite a detailed investigation. The second order coherence time of the system was explored below threshold, and demonstrates a strong dependence on gain, which may have applications in the detection of extremely small gains.
An ensemble of atoms coupled to a high finesse optical cavity is an ideal test bed for the study of the cooperative behaviour of atom-photon systems. Dicke showed that an ensemble of excited atoms coupled to a light field interacts with the light in a collective and coherent fashion leading to the emission of highly directional spontaneous emission whose intensity scales with the square of the number of atoms, a phenomenon known as superradiance. \nThis thesis describes the build of an experiment to study cooperative atom-photon interactions in a ring cavity. Particular focus is given to the cavities used in the experiment. Firstly a transfer cavity used for transferring stability to off-resonant lasers in the experiment, this was developed with the capability of exploiting Gouy phase degeneracies to produce tightly spaced frequency discriminants to be used as lock points. Secondly the ring cavity for the experiment which is atypical in its design allowing for bidirectional probing of the cavity mode and dynamic manipulation of the intra-cavity optical lattice. An intra-cavity MOT was produced and collective strong coupling was observed in the cavity with an Neff=6400 atoms coupled to the cavity mode.