Project B6

Quantum Simulations with Ultracold Gases in an Optical Lattice

PIs: Prof. G. Rempe, Dr. S. Dürr

Quantum simulations can solve certain problems from several different fields of physics by tailoring the Hamiltonian of another quantum system to match the Hamiltonian of the system of interest. We perform experiments with ultracold gases in optical lattices. These systems have the potential to address topics of solid-state systems.

Atom-Molecule Oscillations

We use a magnetic Feshbach resonance to associate ultracold atom pairs into molecules. The particles are confined to the sites of a deep optical lattice potential, so that after association each site of the lattice is occupied by exactly one molecule. This quantum state closely resembles a Mott insulator. We demonstrated experimentally that the chemical reaction from atom pairs to molecules is fully reversible and can display Rabi-type oscillations which are typical for a quantum mechanical two-level system. This shows that the chemical reaction is controlled at the full quantum level [Phys. Rev. Lett. 99, 033201 (2007)].

Dissipation-Induced Strongly-Correlated Many-Body Systems

The strongly-correlated regime is a particularly interesting regime for many-body systems, especially in solid state physics. Usually the strong correlations result from strong elastic interactions between the particles. We were able to show that this regime can alternatively be reached using strong inelastic interactions. These interactions are usually undesired because they lead to loss of particles. If the inelastic interaction is strong enough, however, the system becomes strongly correlated and the particle loss is suppressed. The suppression can become so large that a further increase of the inelastic interaction strength leads to an even further reduction of the loss. We demonstrated this effect experimentally by studying the example of a dissipative analog of the Tonks-Girardeau gas [Science 320, 1329 (2008)]. We supported this work by detailed theoretical studies [New J. Phys. 11, 013053 (2009) and Phys. Rev. A 79, 023614 (2009)]. This work was carried out in close collaboration with the Cirac group (projects A6 and C7).

Localization of Matter Waves in a Disorder Potential

We studied a system in which localization of matter waves is predicted to occur [Phys. Rev. Lett. 95, 020401 (2005) and Phys. Rev. Lett. 98, 190402 (2007)]. The system is a realization of a Falicov Kimball model. Two hyperfine ground states of rubidium are loaded into a one-dimensional optical lattice which is deep for one species and shallow for the other. Hence, the position distribution of one species is pinned by the lattice whereas the other species can tunnel between lattice sites. The loading is performed such that the position distribution of the pinned species is random. The interspecies on-site interaction thus gives rise to a disorder potential for the mobile species. This is predicted to lead to localization of matter waves somewhat similar to Anderson localization. A known problem when searching for Anderson localization is that interactions between mobile particles can suppress the localization due to screening of the disorder. To circumvent this problem, we used a Feshbach resonance to selectively reduce the interaction strength among mobile particles to near zero. Our experimental efforts showed that, whenever the magnetic field is ramped near the Feshbach resonance, then a disastrously fast loss of particles occurred if the lattice light is on. We attribute this to the fact that near the Feshbach resonance the molecular state that causes the Feshbach resonance is admixed to the atomic state. The lattice light usually causes a weak loss of atoms due to a large number of off-resonant photoassociation lines. When the molecular state is admixed, the Franck-Condon factor for these transitions increases by orders of magnitude, which is probably responsible for the disastrous particle loss observed here. This loss forced us to conclude that we are unable to observe localization with our scheme. We performed this work in close collaboration with the Cirac group (projects A6 and C7) and wrote a joint publication [Phys. Rev. Lett. 105, 160402 (2010)].

Optical Control of a Magnetic Feshbach Resonance

During our experimental attempts towards localization described above, we discovered that a magnetic Feshbach resonance can be shifted using laser light. This effect offers perspectives for the manipulation of the scattering properties on short lengths scales which will offer additional flexibility in future quantum simulation experiments [Nature Phys. 5, 339-342 (2009) and Phys. Rev. A 79, 062713 (2009)].

Quantum memory and remote entanglement

Entanglement between stationary systems at remote locations is a key resource for quantum networks. We experimentally generated such remote entanglement between a single atom inside an optical cavity and a BEC. To this end, two laboratories joined forces. In one lab, a triggered single photon was created in an atom-cavity system, thereby generating entanglement between the internal state of the single atom and the polarization of the photon. The photon was transported in a 30 meter long optical fiber to the BEC in the other lab. Electromagnetically induced transparency was used to store this photon in the BEC in the form of a collective excitation. Here, the BEC served as a quantum memory for the photonic polarization qubit. The storage in the BEC established matter-matter entanglement. After a variable delay, the single photon was retrieved from the quantum memory. In addition, a second photon was generated from the single atom in the cavity, thereby mapping the internal state of the single atom onto the polarization of the single photon. These steps converted the entanglement onto the polarization states of two single photons. Finally the polarization of both photons was detected to determine how well the entanglement survived the storage in the quantum memory. In the experiment, we found a total fidelity of all concatenated operations of 95 %. We observed a lifetime of the matter-matter entanglement of 100 µs, exceeding the photon duration by two orders of magnitude. The fidelity of the entangled state is limited by the properties of the single atom inside the cavity. The performance of the memory was independently measured using attenuated laser pulses, yielding an average process fidelity of unity with an error of only 0.004. [Physical Review Letters 106, 210503 (2011) and http://arxiv.org/abs/1112.4733]

Link: http://www.mpq.mpg.de/cms/mpq/en/departments/quanten/homepage_cms/projects/bec/index.html

TOF

Atom pairs confined in an optical lattice can undergo a chemical reaction to a bound molecular state. Under appropriate conditions, the population oscillates between the bound and the unbound configuration.