In the group Hybrid atom-ion quantum systems, we study mixtures of cold neutral atoms interacting with trapped ions.
Ultracold atom-ion mixtures
Experiments with ultracold atoms and ions allow us to study quantum phenomena in the laboratory, such as Bose-Einstein condensation, quantum entanglement and quantum phase transitions. Although a lot of work has been done on the systems of ions and atoms separately, combining the two and studying their interactions in the quantum regime is a relatively new field [1-3]. In our experiment, we study the interactions between Yb+ ions and an ultracold gas of fermionic Li atoms. Although interactions between single ions and atoms have been studied before, our experiment is new in that it combines two species of atoms and ions that have a very large mass ratio (~29). It has been shown theoretically  that such a combination should allow reaching lower temperatures such that quantum phenomena can be studied.
Fig: In our experimental setup, we trap crystals of Yb+ ions (top) in a Paul trap (middle). An atomic cloud is prepared a few cm away from the ions in a magneto-optical trap. Next we transport the atoms up by switching magnetic fields and let the atoms interact with the ions. In the near future, we will upgrade our atom trap to a purely optical one, allowing us to reach much lower temperatures.
We also aim at studying ultracold atoms interacting with crystals of trapped ions. Since our atoms are indeed fermionic, the system has stunning similarities to a natural crystalline solid (with now the atoms playing the role of the electrons) and should feature solid state phenomena such as fermion-phonon coupling and quantum phase transitions [I]. This could allow us to use the system as a quantum simulator of solids and to study the dynamics of many-body atom-ion systems.
Quantum simulation was proposed by Feynman in the 80s as a means to solve the problem of simulating quantum mechanics on classical computers. The problem is that the complexity of quantum problems grows exponentially with system size, such that numerical simulations of hard problems on computers become utterly impossible for systems larger than a few tens of particles. Instead Feynman proposed to employ well-controlled quantum laboratory systems (that inherently follow the laws of quantum mechanics) to mimic the dynamics of quantum model Hamiltonians of interest. This may sound a bit complicated, but actually simulators of classical physics are very common in science and technology: You can think of analogue versions such as crash test dummies or wave simulators, or digital versions such as weather forecast software, or software for predicting the movement of planets. A quantum simulator is the quantum analogue of such devices.
To assess the suitability of our system for such fancy quantum technology as quantum simulation, we first have to perform a thorough characterization. The first thing we did, was checking whether our Li and Yb+ ions would react to form molecules. This turned out to be not the case, which is a good thing since otherwise our quantum system would collapse into deeply bound states after a while. Some of our first results can be found here.
We are now studying the collisions between the atoms and ions and how these depend on the internal states. Once this project is finished, we will commence a race to the bottom – the bottom of the thermometer that is. To reach the regime where quantum effects dominate, we have to cool the system down to about 10 microKelvin. To reach this temperature, we have at our disposal the full range of techniques from atomic physics, such as laser cooling and evaporative cooling. For the atoms, it should be no problem to reach these temperatures, but for the ions it is known to be harder. Analysis of trap and numerical simulations suggest however that we will be successful.
Figure: A string of ions in a Paul trap is overlapped with a cloud of ultra-cold Li atoms trapped by a laser beam.
For an overview of the field of hybrid atom-ion systems you can have a look at this nice review paperfrom the group of Johannes Hecker Denschlag.
 A. Grier, M. Cetina, F. Orucevic and V. Vuletic, Phys. Rev. Lett. 102, 223201 (2009).
 Christoph Zipkes, Stefan Palzer, Carlo Sias & Michael Köhl, Nature 464, 388-391 (2010).
 Stefan Schmidt, Arne Härter and Johannes Hecker Denschlag, Phys. Rev. Lett. 105, 133202 (2010).
 M. Cetina, A. Grier and V. Vuletic, Phys. Rev. Lett. 109, 253201 (2012).
Our funding is provided by the European Research Counsil via the ERC Starting Grant Hybrid atom-ion Quantum Systems and a Vidi Grant of the Dutch Science Foundation.