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Quantum control: manipulating and interfacing selected atoms in optical lattices with light

Final Report Summary - QNDLATTICE (Quantum control: manipulating and interfacing selected atoms in optical lattices with light)

The interdisciplinary boundary between the fields of quantum information science, condensed matter, and ultra-cold atomic physics is at the heart of the new field of quantum engineering and will play an important role for the enhancement of our understanding of strongly correlated materials. One of the most important tools in this domain is optical lattices, which are periodic optical potentials created by counter-propagating laser beams. Cold atoms can be trapped in the electric field maxima – forming an artificial “crystal” in analogy to conventional solid state systems. Depending on the depth of the lattice potential, the atoms are either free to tunnel or fixed to individual sites. For ultra-cold atoms the dynamics is very similar to that found in solid state systems. In contrast to solid state systems, however, they have a high degree of purity, regularity, and tunability. They are therefore an ideal test bed for many condensed matter models such as those relevant for high-Tc-superconductivity. In addition these systems constitute a strong candidate for scalable quantum computation.
A fundamental challenge in the field is how to exchange information between atomic states in optical lattices and light using quantum non-demolition (QND) measurements. This would have profound implications on the field of quantum information processing because the highly non-classical states generated in optical lattices could then be used as a resource. In addition, the dynamics of the strongly correlated states in optical lattices could be continuously observed and manipulated.

Fundamental challenge: Manipulate and detect single sites in an optical lattice
Over the past decade numerous experiments have been performed with atoms in optical lattices. A crucial requirement, however, for future applications is the ability to manipulate and detect single sites in a lattice. The possibility to manipulate single atoms will allow the conception of an entirely new generation of experiments in the fields of quantum information and quantum simulation. It will not only become possible to probe and observe the density, spin structure and correlations of fundamental quantum phases at the scale of a lattice site, but also to manipulate the particles on that fundamental microscopic length scale. One could, e.g. flip the spin of a single atom and observe the ensuing dynamics of the many-body system. Since applications rely on tunnelling dynamics, the lattice spacing has to be extremely small (~500nm). The implementation of detection and manipulation at this length scale has proven to be a major technical challenge. Optical imaging of single sites in 0.5um 2D lattices was achieved in during the original Fellowship of the candidate and in a competing project at Harvard. At the start of the Reintegration grant, however, individual sites in optical lattices had not been manipulated.

This led to three project objectives for the grant
• Objective 1: Quantum control of ultra-cold atomic states using measurement and feedback
• Objective 2: QND detection and manipulation in the optical lattice
• Objective 3: Experiments with individual site manipulation and detection in an optical lattice
During the project the candidate has built a versatile independent research activity (group webpage: www.phys.au.dk/qmmg). The project work was distributed among four individual project directions, which will be briefly outlined below. In total progress has been very satisfactory and both fundamental challenges and the three objectives have to a certain extent been addressed and fulfilled.

Collaboration with the group of Prof Immanuel Bloch, MPQ Munich:
The candidate in the first period of the grant participated in additional experiments in the group of his Marie Curie Fellowship, which following an intense period of experimental work led to the realization of addressing of individual atoms in optical lattices. This result, published in Nature in 2011 has become a prominent milestone in the field of ultracold atoms in optical lattices.

Theoretical activities:
Since the topic of the grant dealt with the mixture of two distinct areas of research – ultracold atoms in optical lattices and non-destructive measurements – sufficient theoretical background did not exist. Within the grant period the candidate has established an independent theoretical activity (6 bachelor, 2 master, 1phd during the grant duration) within this field leading to a number of publications. The group has developed their own codes for simulation of various physical phenomena allowing the team to develop and publish: two new architectures for scalable quantum computation in optical lattices and an architecture for the realization of atomtronics. Novel results on quantum control of many-body states using measurements are currently under preparation.

Experimental activities on the existing apparatus:
Here progress has been according to plan. Non-destructive imaging using the Faraday interaction has been successfully implemented on ultra-cold atoms. For this, a new laser system had to be constructed and changes to the light detection after the light-atom interaction implemented. We have demonstrated detection and control of collective oscillations of ultracold atoms and realized the first ever probing of ultracold atoms in optical lattices using the Faraday effect, thus solving fundamental challenge 1 and fulfilling objectives 1 and 2.

Progress towards the setup of the new experimental apparatus:
The long term vision of the project involved the setup of a dedicated experimental apparatus enabling the combination of all objectives simultaneously. According to the timeplan this should be completed around the end of the grant period thus allowing for high profile results in the coming years. We have almost kept the timeplan except for a delay of roughly half a year. This means that whereas the BEC has been achieved, only the laser systems for the optical lattices have been setup but not implemented onto the new apparatus. The reasons for the delay are detailed below, but we anticipate fulfilling these goals within the next half a year, thus allowing us to proceed onto the next high-profile targets.