Publications  Cavity QED
2015

R. Reimann, W. Alt, T. Kampschulte, T. Macha, L. Ratschbacher, N. Thau, S. Yoon and D. Meschede
CavityModified Collective Rayleigh Scattering of Two Atoms, Phys. Rev. Lett. 114, 023601 (2015)arXivBibTeXABSTRACT »We report on the observation of cooperative radiation of exactly two neutral atoms strongly coupled to the single mode field of an optical cavity, which is close to the losslesscavity limit. Monitoring the cavity output power, we observe constructive and destructive interference of collective Rayleigh scattering for certain relative distances between the two atoms. Because of cavity backaction onto the atoms, the cavity output power for the constructive twoatom case (N=2) is almost equal to the singleemitter case (N=1), which is in contrast to freespace where one would expect an N^2 scaling of the power. These effects are quantitatively explained by a classical model as well as by a quantum mechanical model based on Dicke states. We extract information on the relative phases of the light fields at the atom positions and employ advanced cooling to reduce the jump rate between the constructive and destructive atom configurations. Thereby we improve the control over the system to a level where the implementation of twoatom entanglement schemes involving optical cavities becomes realistic.
2014

R. Reimann, W. Alt, T. Macha, D. Meschede, N. Thau, S. Yoon and L. Ratschbacher
Carrierfree Raman manipulation of trapped neutral atoms, New J. Phys. 16, 113042 (2014)arXivBibTeXABSTRACT »We experimentally realize an enhanced Raman control scheme for neutral atoms that features an intrinsic suppression of the twophoton carrier transition, but retains the sidebands which couple to the external degrees of freedom of the trapped atoms. This is achieved by trapping the atom at the node of a blue detuned standing wave dipole trap, that acts as one field for the twophoton Raman coupling. The improved ratio between cooling and heating processes in this configuration enables a five times lower fundamental temperature limit for resolved sideband cooling. We apply this method to perform Raman cooling to the twodimensional vibrational ground state and to coherently manipulate the atomic motion. The presented scheme requires minimal additional resources and can be applied to experiments with challenging optical access, as we demonstrate by our implementation for atoms strongly coupled to an optical cavity.

S. Gammelmark, W. Alt, T. Kampschulte, D. Meschede and K. Mølmer
Hidden Markov Model of atomic quantum jump dynamics in an optically probed cavity, Phys. Rev. A 89, 043839 (2014)arXivBibTeXABSTRACT »We analyze the quantum jumps of an atom interacting with a cavity field, where strong coupling makes the cavity transmission depend on the timedependent atomic state. In our analysis we employ a Bayesian approach that conditions the population of the atomic states at time t on the cavity transmission observed both before and after t, and we show that the state assignment by this approach is more decisive than the usual conditional quantum states based on only earlier measurement data. We also provide an iterative protocol which, together with the atomic state populations, simultaneously estimates the atomic jump rates and the transmission signal distributions from the measurement data. Finally, we take into account technical fluctuations in the observed signal, e.g., due to spatial motion of the atom within the cavity, by representing atomic states by several hidden states, thereby significantly improving the state's recovery.

T. Kampschulte, W. Alt, S. Manz, M. MartinezDorantes, R. Reimann, S. Yoon, D. Meschede, M. Bienert and G. Morigi
Electromagneticallyinducedtransparency control of singleatom motion in an optical cavity, Phys. Rev. A 89, 033404 (2014)arXivBibTeXABSTRACT »
We demonstrate cooling of the motion of a single neutral atom confined by a dipole trap inside a highfinesse optical resonator. Cooling of the vibrational motion results from electromagnetically induced transparency (EIT)–like interference in an atomic lambdatype configuration, where one transition is strongly coupled to the cavity mode and the other is driven by an external control laser. Good qualitative agreement with the theoretical predictions is found for the explored parameter ranges. Further, we demonstrate EIT cooling of atoms in the dipole trap in free space, reaching the ground state of axial motion. By means of a direct comparison with the cooling inside the resonator, the role of the cavity becomes evident by an additional cooling resonance. These results pave the way towards a controlled interaction among atomic, photonic, and mechanical degrees of freedom.
2012

S. Brakhane, W. Alt, T. Kampschulte, M. MartinezDorantes, R. Reimann, S. Yoon, A. Widera and D. Meschede
Bayesian Feedback Control of a TwoAtom SpinState in an AtomCavity System, Phys. Rev. Lett. 109, 173601 (2012)arXivBibTeXABSTRACT »We experimentally demonstrate realtime feedback control of the joint spinstate of two neutral Caesium atoms inside a high finesse optical cavity. The quantum states are discriminated by their different cavity transmission levels. A Bayesian update formalism is used to estimate state occupation probabilities as well as transition rates. We stabilize the balanced twoatom mixed state, which is deterministically inaccessible, via feedback control and find very good agreement with MonteCarlo simulations. On average, the feedback loops achieves near optimal conditions by steering the system to the target state marginally exceeding the time to retrieve information about its state.
2010

S. Reick, K. Mølmer, W. Alt, M. Eckstein, T. Kampschulte, L. Kong, R. Reimann, A. Thobe, A. Widera and D. Meschede
Analyzing quantum jumps of one and two atoms strongly coupled to an optical cavity, J. Opt. Soc. Am. B 27, A152 (2010)arXivBibTeXABSTRACT »We induce quantum jumps between the hyperfine ground states of one and two Cesium atoms, strongly coupled to the mode of a highfinesse optical resonator, and analyze the resulting random telegraph signals. We identify experimental parameters to deduce the atomic spin state nondestructively from the stream of photons transmitted through the cavity, achieving a compromise between a good signaltonoise ratio and minimal measurementinduced perturbations. In order to extract optimum information about the spin dynamics from the photon count signal, a Bayesian update formalism is employed, which yields timedependent probabilities for the atoms to be in either hyperfine state. We discuss the effect of superPoissonian photon number distributions caused by atomic motion.

T. Kampschulte, W. Alt, S. Brakhane, M. Eckstein, R. Reimann, A. Widera and D. Meschede
Optical control of the refractive index of a single atom, Phys. Rev. Lett. 105, 153603 (2010)arXivBibTeXABSTRACT »We experimentally demonstrate the elementary case of electromagnetically induced transparency with a single atom inside an optical cavity probed by a weak field. We observe the modification of the dispersive and absorptive properties of the atom by changing the frequency of a control light field. Moreover, a strong cooling effect has been observed at twophoton resonance, increasing the storage time of our atoms twentyfold to about 16 seconds. Our result points towards alloptical switching with single photons.

B. Gadway, D. Pertot, R. Reimann and D. Schneble
Superfluidity of Interacting Bosonic Mixtures in Optical Lattices, Phys. Rev. Lett. 105, 045303 (2010)arXivBibTeXABSTRACT »We report the observation of manybody interaction effects for a homonuclear bosonic mixture in a threedimensional optical lattice with variable state dependence along one axis. Near the superfluidtoMott insulator transition for one component, we find that the presence of a second component can reduce the apparent superfluid coherence, most significantly when the second component either experiences a strongly localizing lattice potential or none at all. We examine this effect by varying the relative populations and lattice depths, and discuss the observed behavior in view of recent proposals for atomicdisorder and polaroninduced localization.
2009

M. Khudaverdyan, W. Alt, T. Kampschulte, S. Reick, A. Thobe, A. Widera and D. Meschede
Quantum jumps and spin dynamics of interacting atoms in a strongly coupled atomcavity system , Phys. Rev. Lett. 103, 123006 (2009)arXivBibTeXABSTRACT »We experimentally investigate the spin dynamics of one and two neutral atoms strongly coupled to a high finesse optical cavity. We observe quantum jumps between hyperfine ground states of a single atom. The interactioninduced normalmode splitting of the atomcavity system is measured via the atomic excitation. Moreover, we observe the mutual influence of two atoms simultaneously coupled to the cavity mode.

B. Gadway, D. Pertot, R. Reimann, M. G. Cohen and D. Schneble
Analysis of KapitzaDirac diffraction patterns beyond the RamanNath regime, Optics Express 17, 1917319180 (2009)arXivBibTeXABSTRACT »We study KapitzaDirac diffraction of a BoseEinstein condensate from a standing light wave for a square pulse with variable pulse length but constant pulse area. We find that for sufficiently weak pulses, the usual analytical shortpulse prediction for the RamanNath regime continues to hold for longer times, albeit with a reduction of the apparent modulation depth of the standing wave. We quantitatively relate this effect to the Fourier width of the pulse, and draw analogies to the Rabi dynamics of a coupled twostate system. Our findings, combined with numerical modeling for stronger pulses, are of practical interest for the calibration of optical lattices in ultracold atomic systems.
2008

M. Khudaverdyan, W. Alt, I. Dotsenko, T. Kampschulte, K. Lenhard, A. Rauschenbeutel, S. Reick, K. Schörner, A. Widera and D. Meschede
Controlled insertion and retrieval of atoms coupled to a highfinesse optical resonator, New J. Phys. 10, 073023 (2008)arXivBibTeXABSTRACT »We experimentally investigate the interaction between one and two atoms and the field of a highfinesse optical resonator. Lasercooled caesium atoms are transported into the cavity using an optical dipole trap. We monitor the interaction dynamics of a single atom strongly coupled to the resonator mode for several hundred milliseconds by observing the cavity transmission. Moreover, we investigate the positiondependent coupling of one and two atoms by shuttling them through the cavity mode. We demonstrate an alternative method, which suppresses heating effects, to analyze the atomfield interaction by retrieving the atom from the cavity and by measuring its final state.
2005

I. Dotsenko, W. Alt, M. Khudaverdyan, S. Kuhr, D. Meschede, Y. Miroshnychenko, D. Schrader and A. Rauschenbeutel
Submicrometer position control of single trapped neutral atoms, Phys. Rev. Lett. 95, 033002 (2005)arXivBibTeXABSTRACT »We optically detect the positions of single neutral cesium atoms stored in a standing wave dipole trap with a subwavelength resolution of 143 nm rms. The distance between two simultaneously trapped atoms is measured with an even higher precision of 36 nm rms. We resolve the discreteness of the interatomic distances due to the 532 nm spatial period of the standing wave potential and infer the exact number of trapping potential wells separating the atoms. Finally, combining an initial position detection with a controlled transport, we place single atoms at a predetermined position along the trap axis to within 300 nm rms.