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.

Ring trap Linear trap

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).

Mounted optics and trap render

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 [1, 2].

Remote entanglement

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. [3] 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 [4].

Setup for the Cabrillo scheme

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.



[1] G. Araneda, G. Cerchiari, D. B. Higginbottom, P. C. Holz, K. Lakhmanskiy, P. Obšil, Y. Colombe, and R. Blatt, Rev. Sci. Instrum. 91, 113201 (2020)

[2] G. Hétet, L. Slodička, A. Glatzle, M. Hennrich, R. Blatt, Phys. Rev. A 82, 063812 (2010)

[3] C. Cabrillo, J. I. Cirac, P. Garcia-Fernández, P. Zoller, Phys. Rev. A 59, 1025 (1999)

[4] L. Slodička, G. Hétet, N. Röck, P. Schindler, M. Hennrich, R. Blatt, Phys. Rev. Lett. 110, 083603 (2013)

[5] G. Cerchiari, G. Araneda, L. Podhora, L. Slodička, Y. Colombe, R. Blatt, Appl. Phys. Lett. 119, 024003 (2021) 


Position measurement of dipolar scatterers via self-homodyne detection


self homodyne webpage


  The study of levitated dipolar scatterers is a growing field in physics that promises to uncover the connection between quantum mechanics and gravity [2], to access the QED forces exert on atoms by external boundary conditions and to deliver future devices for ultra-precise force sensing. In collaboration with the nanosphere team of the QI group, we investigated how the measure the position of these scatterers by controlling their spontaneous emission (SE) with a spherical mirror. Developing on the experience of atomic experiments [2, 3], we predict that our method, unlike other state-of-the-art techniques, can reach the Heisenberg limit of detection, i.e. when the position measurement is only bounded by the recoil force of the scattered light.
  The setup splits the solid angle surrounding the scatterer into two regions as depicted in the figure. Half of the solid angle is occupied by a mirror and half by a detector. The mirror produces an image of the dipolar scatterer that interferes with the primary emitted field at the detector. This configuration realizes the self-homodyne of the emitted field by self-interfering each emitted photon. Self-interference allows controlling the SE of the scatterer while obtaining superior mode-matching of the radiation fields for homodyne detection. The radiation exists from the scatterer-mirror system only from one side enabling the measurement with only a single one-sided detector.
  The new method is suitable for precision studies of any dipolar scatterer such as atoms and nanoparticles without requiring the existence of a complex electronic structure.The self homodyne configuration for detection can also be extended to sense the motion of an ion chain as depicted in the following figure ions [3].

two ion detection




[1] "Position measurement of a dipolar scatterer via self-homodyne detection", G. Cerchiari, L. Dania, D. S. Bykov, R. Blatt, and T. E. Northup, Phys. Rev. Lett. A 104, 053523 (2021) 

[2] "Measuring Ion Oscillations at the Quantum Level with Fluorescence Light", G. Cerchiari, G. Araneda, L. Podhora, L. Slodička, Y. Colombe, and R. Blatt, Phys. Rev. Lett. 127, 063603 (2021)

[3] "Motion analysis of a trapped ion chain by single photon self-interference" G. Cerchiari, G. Araneda, L. Podhora, L. Slodička, Y. Colombe, and R. Blatt, Appl. Phys. Lett. 119, 024003 (2021)


Project members

Gio 02 800 800 1
Dr. Giovanni Cerchiari (PostDoc)

e-mail: giovanni.cerchiari(at)

room: 4/06


WEISER Yannick web

Yannick Weiser (Ph.D. student)

e-mail: yannick.weiser(at)

room: 4/06


Prof. Rainer Blatt (senior advisor)


Former members

Yves Colombe (senior scientist)

Gabriel Araneda (now at the University of Oxford, UK)

Lukáš Podhora (visiting PhD student from the Palacký University)

Daniel Higginbottom (PhD student, also with the ANU in Canberra, Australia)

Petr Obšil (visiting PhD student from the Palacký University)

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)







  • Gabriel Araneda
    Experiments with single photons emitted by single atoms
    Ph.D. thesis, 2020. Download
  • Daniel Higginbottom
    Atom-light couplers with one, two and ten billion atoms
    Ph.D. thesis, 2018, Australian National University. Download

  • 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