Cavity-QED with Ions in a Micro Trap

• Applicant: Professor Dr. Ferdinand Schmidt-Kaler
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2010 to 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 181595191

• Final Report Year: 2015

Final Report Abstract

In the course of the project, we have improved on the design of our fiber-integrated, segmented micro-trap. The integration is modular and exchangeable. We have built a 100µm cavity from laser-machined, mirrored fiber ends, and proved the feasability of fiber based cavities integrated into micro-structured Paul traps. We tested our trap design extensively, leading to a range of results in optimizing ion transport and ion crystal splitting. We’ve achieved a very low heating rate of 0.1 motional exitations during a fast transport duration of 3.6µs, only five motional cycles of the trap, over a distance of 280µm. While splitting, we can adiabatically separate a two-ion crystal to a distance of two segments, 400µm, in 80µs, inducing less than 5 motional excitations in the process. We have also detailed theoretical tools to optimize both transport and separation, paving the way for more improvements. We have also proposed and demonstrated techniques to reduce the decoherence of qubits in the trap. By using decoherence-free logical qubits made up of two physical qubits[A1], for example |1) = |1, 0) and |0) = |0, 1), where the decoherence effects cancel each other out, the coherence time can be improved significantly at the cost of higher (physical) qubit requirements. Further improvements deal with using the ion as a magnetic field probe to measure stray magnetic fields at high precision both in space as well as intensity, making it possible to directly gain knowledge about and counteract one of the biggest contributors to decoherence, and adding interesting applications to studying ionic species along the way. We have developed a qubit readout method that is more robust that previous ones when it comes to thermal excitation of the ion, by modelling the thermal motion and using this information as input to evaluating the gathered data. Finally, we have developed a new method of creating concave fiber ends for fiber-based mirrors, by using a focused ion beam (FIB), which mills any wanted shape into the fiber using a Gallium ion beam. Using a FIB frees us from some of the constraints laser machined fibers have, especially from the high spherical aberration due to the lasers gaussian form being imprinted onto the fiber face, which limited such cavities to a length of 100µm or less. Applying the FIB process to glass fibers proved surpisingly difficult mainly due to the fact that this method was developed for conductors and semiconductors, not isolators, and also due to the very high requirements we had for the smoothness of the fiber ends prior to applying the mirror substrate. We now are in the process of integrating a 250µm length cavity into a trap, with a finesse of 18000, high enough for us to reach the strong coupling regime within this trap.

Publications

• "Quantum gate in the decoherencefree subspace of trapped-ion qubits", Europhysics Letters 92, 30006 (2010)
  P. A. Ivanov, U. G. Poschinger, K. Singer, F. Schmidt-Kaler
  
• "Single ion as a shot-noise-limited magnetic-field-gradient probe", Physical Review A 83, 062329 (2011)
  A. Walther, U. G. Poschinger, F. Ziesel, M. Hettrich, A. Wiens, J. Welzel, F. Schmidt- Kaler
  
• "Controlling fast transport of cold trapped ions", Physical Review Letters 109, 080501 (2012)
  A. Walther, F. Ziesel, T. Ruster, S. T. Dawkins, K. Ott, M. Hettrich, K. Singer, F. Schmidt- Kaler, U. G. Poschinger
  
• "Entangled states of trapped ions allow measuring the magnetic field gradient of a single atomic spin", Europhysics Letters 99, 53001 (2012)
  F. Schmidt-Kaler, R. Gerritsma
  
• "Interaction of a laser with a qubit in thermal motion and its application to robust and efficient readout", Applied Physics B: Lasers and Optics 107, 1159 (2012)
  U. G. Poschinger, A. Walther, M. Hettrich, F. Ziesel, F. Schmidt-Kaler
  
• "Controlling the transport of an ion: Classical and quantum mechanical solutions", New Journal of Physics 16, 075007 (2014)
  H. A. Fürst, M. H. Goerz, U. G. Poschinger, M. Murphy, S. Montangero, T. Calarco, F. Schmidt-Kaler, K. Singer, C. P. Koch
  
• "Dynamics and control of fast ion crystal splitting in segmented Paul traps", New Journal of Physics 16, 073012 (2014)
  H. Kaufmann, T. Ruster, C. T. Schmiegelow, F. Schmidt-Kaler, U. G. Poschinger
  
• "Experimental realization of fast ion separation in segmented Paul traps", Phys. Rev. A 90, 033410 (2014)
  T. Ruster, C. Warschburger, H. Kaufmann, C. T. Schmiegelow, A. Walther, M. Hettrich, A. Pfister, V. Kaushal, F. Schmidt-Kaler, U. G. Poschinger