Probing quantum physics with correlated atomic pairs

This bi-national joint research project concerns the creation, detection, and characterization of highly correlated quantum states of ultracold atomic ensembles. In close analogy to correlated photon pairs and quantum optics in general, we aim to establish quantum-atom-optics with massive particles as a resource for fundamental investigation towards entanglement and interferometry using non-classical input states. In particular, we will develop efficient sources of moment-correlated twin atom pairs. Both project partners are among the world leaders in producing and characterizing exotic quantum states, following mostly complementary approaches. The Austrian partner will draw advantage of the atom chip technology in order to accurately control the production of correlated atom pairs using controlled parametric excitation and down- conversion-like relaxation into well defined momentum states. Raman transitions using internal atomic states may be used to more efficiently "pump" the pair-correlating process and characterize the dynamics using internal state labeling. Using one-dimensional (1D) Bose-Einstein condensates as a starting point, the obtained high-momentum particles may serve as a high-energy probe for low-dimensional physics. The French Partner will use optical superradiance in an elongated (1D) geometry or quantum depletion in a 2D optical lattice geometry to generate correlated pairs. The tremendous progress of quantum optics was strongly connected to the availability of single photon counters. In quantum-atom-optics, single atom detectors will play an equally important role. Here again, the two project partners are among the world leading groups, having found complementary experimental solutions to the problem. The Austrian partner uses a novel "fluorescence light sheet" detection scheme whereas the French partner makes use of impact ionization using metastable He* atoms and an MCP detector. Both groups have successfully demonstrated their ability to measure (two-point) quantum correlations by repeating the classical Hanbury-Brown- Twiss experiment with ultracold or quantum degenerate atomic Bose gases. This joint project will allow the partners to exchange and share their knowledge and technology to achieve the common research goals. Superior methods for the creation, detection, and characterization of correlated atom states will be identified and adapted by both partners. Existing successful experimental tools will be implemented in both groups; joined PhD projects ensure the transfer of expertise and material. This will allow the partners to go significantly beyond the research goals they would be able to reach on their own in the time frame of the project.

In the frame of the CAP project, both Austrian and French partners developed sources for twin atom beams. These non-classical sources emit atoms by pairs, and the velocity of each atom depends on the velocity of its partner. Such correlations between atoms are forced by energy and momentum conservation and engineered with the help of atom-atom interactions. Correlated states are at the heart of some of the most intriguing manifestations of quantum mechanics. A prime example is pairs of correlated photons, which have been used for fundamental tests of quantum mechanics and for practical applications like quantum cryptography or quantum communications. The twin atoms beams we generated during this project are the basic element required to perform analogous experiments with atoms and are the first step towards the generation of entangled twin atom beams. For such an entangled state, a measure performed on one atom can instantaneously affect its twin. The twin atom beam source of the TU Wien team is based on a quantum degenerate gas of bosons (87Rb atoms) trapped in an elongated micro-trap on an atomchip. All atoms are initially in the lowest possible energy state, forming a Bose-Einstein condensate (BEC). By shaking the micro-trap with a specially optimized sequence, one can transfer the complete BEC to the first transverse excited state of the trap. This is a very highly non-equilibrium state and the only way for atoms to return the lowest energy state is in pairs having opposite momentum. This process can be seen as the matter wave equivalent to down conversion, which is at the heart of the creation of correlated photon pairs. A unique detection system allows measuring the velocities of single atoms, and thus characterizing the correlated state. We demonstrated nearly perfect correlations and close to -10 dB squeezing. In addition we extensively studied the optimal control sequences needed to create the initial excited state of the quantum degenerate gas, and the pair creation process itself. To demonstrate entanglement, one needs to add a second mode and interference. Additional elements are required to produce and characterize entangled twin atom beams This led us to two additional experiments: (1) splitting and interfering a condensate: we demonstrated number squeezing and entanglement when splitting a condensate, and implemented an trapped atom interferometer, (2) we improved the excitation pulses which allowed us to speed up the excitation process by nearly a factor 5 and we demonstrated an interferometer with motional states of a trapped BEC.