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MALICIA Work Packages

WP1 – Rydberg blockaded ensembles

WP leader: Prof. Tilman Pfau (USTUTT)

 Objectives. Single and multi-photon sources based on the Rydberg blockade for thermal alkali ensembles (Rb and Cs).

 Description. USTUTT will focus on the first demonstration of a working single photon source based on thermal vapor cells. Besides doing experiments with Rubidium vapor based on readily available technology we will extend the technique to Cesium which is the best candidate for a long lived quantum memory and quantum repeaters. This will allow for lower working temperatures and higher optical thickness as well as larger bandwidth. We will also demonstrate the scalability of the concept by Hong-Ou-Mandel correlations between two emitters and fabricate larger arrays which are individually addressable.

AU will carry out theoretical analyses and calculations of the field emitted collectively by the atomic ensembles. Particular emphasis will be put on the role of randomly positioned and moving atoms. We will investigate Raman type schemes, and try to control the emitted photon wave packets by the temporal shape of the classical control fields, and we will suggest and analyze means to eliminate decoherence and error mechanisms due to inhomogeneities and motion within the atomic sample. We will also investigate the range of light states made available by deterministic emission by several microcells, and by the detection of interfering components from cells in scalable architectures. UniUlm will build on the investigations described above to develop efficient numerical simulations of the dynamics of light coupled to thermal vapor cells, and it will apply quantum optimal control theory to tailor the pulse shaping of the different schemes in order to optimize for fidelity in the output of the devices and to yield robustness against dephasing effects.

 

WP2 – Optically thick samples

 WP leader: Prof. Eugene Polzik (UCPH)

Objectives. Building blocks for quantum networks based on room-temperature atomic gases in microstructured cells.

 Description. UCPH will work on the design and fabrication of microcells, including a suitable spin-protection coating to achieve long lifetimes of the collective atomic spin state. A design with an integrated low-finesse cavity would give the possibility to increase the optical depth and thus optimize the efficiency of the light atom coupling. A major technical challenge is the manufacturing microcells with anti-reflection coated windows coated with the spin protecting alkene layer coupled to the Cs reservoir via a valve. An alternative design with Brewster windows instead of antireflection coated windows will be also explored. The cells will be incorporated into a low finesse cavity and positioned in the magnetically shielded environment. In the first step we will test the possibility to entangle two such cells via either a projective measurement or dissipation driven method. At the same time we will study decoherence processes in such microcells. A major work task will be the characterization of the constructed atom light interface, including the evaluation of the feasibility of different quantum communication protocols and quantum state engineering beyond Gaussian states. By incorporating a single photon counter we will generate a single Fock excitation in a microcell via probabilistic photon detection. The generated state will be characterized by atomic tomography. The next logical step is to identify (and implement) possible two or three cell protocols, demonstrating the path to a multi-node microcell network.

MPG will theoretically analyse the light-matter interaction for small atomic ensembles and explore their possibilites to generate non-Gaussian states and operations. We will develop new efficient interface protocols adapted to the microscopic dimensions of the cell using stroboscopic interactions to engineer effective Hamiltonians and employing adiabatic passage techniques. We will combine dissipative approaches for entanglement generation with linear passive operations on the atoms and feedback operations.

 LUH will develop more realistic models for decoherence in light-matter interfaces. We will investigate the effects of the true multi-level structure of atoms in the off-resonant Faraday interaction, thereby extending existing models based on simple, idealized level schemes. This will allow us to identify optimized geometries for this light-matter interface.

 

 WP3 – Quantum gases

 WP leader: Prof. Francesco Saverio Cataliotti (LENS)

Objectives. To increase the efficiency of information transfer between the radiation and the atomic ensemble

 Description. LENS will work on the transfer of the quantum state of light to collective atomic excitations and vice versa. Our aim is to realize such a system by working with cold and ultra-cold atomic samples, studying their properties reducing down the temperature all the way to the condensation limit. Experimentally the main problem is to increase the efficiency of information transfer between the radiation and the atomic ensemble. We will also explore a tomographic method for the characterization of atomic quantum states. The two experiments will proceed in parallel as their respective demonstration is independent of each other. Depending on the success of the proposed experiments we will then explore the possibility of using the tomography method for the detection of the non-classical states created with optical quantum interference.

MPG will develop a theoretical framework to describe the mapping between collective excitations and photons including dipole-dipole interaction and multiple light scattering under different trapping conditions. Using these tools, we will investigate how the control on the states of trapped atoms can be used to generate useful and interesting quantum states of light. We propose a method to measure quantum dynamical correlations in strongly correlated states of ultracold atoms. The scheme combines the quantum non-demolition detection of strongly correlated systems and quantum memories. The former is used to probe (without demolishing the system) an observable that does not commute with the many-body Hamiltonian. The latter stores the information of the first measurement until a second beam probes the strongly correlated system at a later time and reads the previous measurement stored in the quantum memory.

UULM will apply its own recently developed CRAB (Chopped RAndom Basis) algorithm to the case of coupled light-matter excitations in atom-chip nanostructures. The main idea is to optimize the coupling efficiency in a dynamical way, by varying the amplitude of suitably chosen frequency components in the time dependence of the control field. In a first stage this iterative procedure will rely on a numerical simulation of light propagation in the atomic medium, taking into account experimental knowledge about the real parameters but within an open-loop approach. Subsequently, the CRAB method will be developed into a closed-loop procedure replacing the simulation engine with interrogation of theexperimental system itself. This will allow for direct feedback into the experimental procedure, by iteratively guiding the optimization via measurement results, thereby including automatically the imperfections unavoidably present in a real situation.

 

WP4 – Quantum interfaces

 WP leader: Prof. Tommaso Calarco (UULM)

Objectives.  To theoretically investigate future quantum interfaces based on atoms trapped in optical lattices, and to devise a efficient and feasible quantum repeater architecture based on the technological platforms available within MALICIA.

 Description. While current experiments on light-matter interfaces are concerned mainly with atomic ensembles, a promising direction allowing for greater control in this sense is that of atoms trapped in optical lattices. The UULM group will theoretically investigate this system based on its expertise in exact simulation methods like the Density Matrix Renormalization Group and other Tensor Network approaches, combining them with its recently developed CRAB optimization method (which is the only efficient algorithm available to date for coherent optimized control of many-body quantum systems) and applying their combination for the first time to light-matter interaction processes. This more forward-looking line of research will complement the more applied tasks of UULM in WP1 and WP3, and it will aim at incorporating classical and quantum noise in the description, to open the way to optimal control of many-atom open systems, of crucial importance for future quantum interfaces.

Partner LUH, LENS and MPG will investigate possibilities to combine the experimental platforms available within the research project to devise an efficient new quantum repeater architecture for long distance communication. We will analyse how the systems at hand, namely hot and cold atomic ensembles, and the readily accessible quantum operations i.e., absorption, emission, and detection of photons, can be used to build quantum networks. We will also explore ways to use Rydberg-blockaded ensembles at room temperature as a nonlinear optical medium for two-photon gates, which can be employed in a quantum repeater protocol as quantum logic gate for entanglement swapping and purification. We will study the more theoretical aspects of combination of weak optical nonlinearities with photo detection, as well asthe spatiotemporal aspects of mode distortion for efficient photon gates. Finally the problem of quantum noise in light-matter interfaces will be investigated further with a focus on both, the development of realistic noise models, and to develop a better understanding of fundamental limitations.