The main goal of the Barium experiment is to study the interaction between single trapped ions and free-space photons, i.e. without the enhancement provided by an optical cavity.
Using high numerical aperture optics around single ions, fundamental effects that were inaccessible in the past are now becoming within reach. Phenomena such as the modification of the vacuum mode density around the ion with a single boundary, strong free-space coupling to single ions without a cavity, sub- and super-radiance, quantum feedback control of the ion motion, or distant atomic-entanglement are now investigated.
We trap ions in two separate vacuum setups. The first one contains a ring-electrode Paul trap (depicted below, left) that is best suited for single-ion trapping; the second one has a linear, four blade electrodes Paul trap (depicted on the right) that can trap a string of ions. Both setups feature in-vacuum custom-made lenses with a numerical aperture of 0.40. The ring trap setup is also equipped with an optical microscope, which is not used in experiments but allows our visitors to see the green-blue fluorescence light of a single trapped Barium ion.
We are currently working on a setup with an even higher collection of the fluorescence light of the ion. The new system consists of a hemispherical mirror and a custom high numerical aperture asphere (NA = 0.70) placed on opposite sides of the trapped ion. The hemispherical mirror retro-reflects almost half of the emission of the ion, which is merged with the light emitted in the other direction, and collimated by the aspherical lens (see picture).
In addition to a higher light collection efficiency, this new design will allow us to study phenomena such the strong inhibition of the spontaneous decay, and the enhancement of the emission rate of the ion .
Entanglement is one of the most exciting phenomena in quantum mechanics. In other experiments in our group, entanglement is generated using the common vibration modes of trapped ions that interact electrostatically in the same trap. This is done for example by applying the so-called Molmer-Sorensen gate on the ions. But if the ions are far apart, it is no longer possible to use the common vibration of the ions. A way to circumvent this limitation is to use photons as a bus for generating the entanglement. There are several possible experimental arrangements to this effect; We follow a scheme proposed by Cabrillo et al.  which allows efficient generation of remote entanglement, heralded by the detection of a single photon. We have generated and probed the entanglement of two ions that are effectively separated by a distance of 1 m .
The main limitation in the rate of generation of remote entanglement is the collection of the single photons emitted by the individual ions. With our new trap design we expect our rate to increase by a factor of at least 10.
Toward quantum communication
Different approaches for quantum communication using atoms and photons have been proposed. The main idea is to use trapped atoms or ions as static qubits and photons as flying qubits. The most challenging part is achieving an efficient coupling between photons and static qubits. One approach consists in making use of optical cavities to enhance the interaction between photons and atoms , as is investigated in another project of our group, Cavity QED. We plan to start investigating quantum communication with our new setup, thanks to the improved light collection and light-ion coupling provided by the custom high-NA aspherical lens.
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 L. Slodička, G. Hétet, N. Röck, P. Schindler, M. Hennrich, R. Blatt, Phys. Rev. Lett. 110, 083603 (2013)
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Gabriel Araneda (PhD student)
Daniel Higginbottom (PhD student, also with the ANU in Canberra, Australia)
Petr Obšil (visiting PhD student from the Palacký University)
Rainer Blatt (group leader)
Nicolas Chauvet (master student, ENS Lyon, France)
Lukáš Slodička (now at the Palacký University in Olomouc, Czech Republic)
Gabriel Hétet (now at the Laboratoire Pierre Aigrain, École Normale Supérieure, France)
Nadia Röck (now at the Univ. Klinik für Strahlentherapie-Radioonkologie, Innsbruck)
Sebastian Gerber (now at CERN on the AEgIS experiment)
Daniel Rotter (now at Swarovski Optik)
Yves Colombe, Nicolas Chauvet and Gabriel Araneda in the Barium lab
- "Pure single photons from a trapped atom source"
D. B. Higginbottom, L. Slodička, G. Araneda, L. Lachman, R. Filip , M. Hennrich, R. Blatt
New. J. Phys. 18, 093038 (2016), arXiv:1605.03774
- "Free space interference experiments with single photons and single ions"
L. Slodička , G. Hétet, M. Hennrich, R. Blatt
Chapter in Engineering the Atom-Photon Interaction, Springer, Berlin, 2015
ISBN: 978-3-319-19230-7, arXiv:1502.04302
- "Free-space read-out and control of single-ion dispersion using quantum interference"
G. Hétet, L. Slodička, N. Röck, R. Blatt
Phys. Rev. A 88, 041804(R) (2013), arXiv:1212.0810
- "Atom-atom entanglement by single-photon detection"
L. Slodička, G. Hétet, N. Röck, P. Schindler, M. Hennrich, R. Blatt
Phys. Rev. Lett. 110, 083603 (2013), arXiv:1207.5468
- "Shot-noise limited monitoring and phase locking of the motion of a single trapped ion"
P. Bushev, G. Hétet, L. Slodička, D. Rotter, M. A. Wilson, F. Schmidt-Kaler, J. Eschner, R. Blatt
Phys. Rev. Lett. 110, 133602 (2013), arXiv:1202.2307
- "Interferometric thermometry of a single sub-Doppler-cooled atom"
L. Slodička, G. Hétet, N. Röck, S. Gerber, P. Schindler, M. Kumph, M. Hennrich, R. Blatt
Phys. Rev. A 85, 043401 (2012), arXiv:1202.4559
- "Single atom as a mirror of an optical cavity"
G. Hétet, L. Slodička, M. Hennrich, R. Blatt
Phys. Rev. Lett. 107, 133002 (2011), arXiv:1105.1919
- "Electromagnetically induced transparency from a single atom in free space"
L. Slodička, G. Hétet, S. Gerber, M. Hennrich, R. Blatt
Phys. Rev. Lett. 105, 153604 (2010), arXiv:1005.3289
- "QED with a spherical mirror"
G. Hétet, L. Slodička, A. Glätzle, M. Hennrich, R. Blatt
Phys. Rev. A 82, 063812 (2010), arXiv:1009.2344
- Lukáš Slodička
Single ion - single photon interactions in free space
Ph.D. thesis, 2013. Download
- Nadia Röck
Quantum manipulation on the Barium quadrupolar transition
Master's thesis, 2011. Download
- Sebastian Gerber
Quantum correlation experiments with resonance fluorescence photons of single Barium ions
Ph.D. thesis, 2010. Download
- Daniel Rotter
Quantum feedback and quantum correlation measurements with a single Barium ion
Ph.D. thesis, 2008. Download
- Pavel Bushev
Interference experiments with a single barium ion: from QED towards quantum feedback
Ph.D. thesis, 2004. Download
- Christoph Raab
Interference experiments with the fluorescence light of Ba+ ions
Ph.D. thesis, 2001. Download