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Deterministic Logical Photon-Photon Interactions

Final Report Summary - DELPHI (Deterministic Logical Photon-Photon Interactions)

The central objective of the DELPHI project is to design and implement novel schemes for efficient, deterministic, photon-photon interactions, which will be used to implement logical processing and quantum measurement schemes on optical light beams. The applications are aimed towards deterministic quantum engineering of light, and include for instance the operation of photon turnstile devices, the generation of optical Schrödinger's cat states, and the implementation of photonic controlled-phase quantum gates.

The central physical idea to achieve this goal is to convert the photons into Rydberg polaritons, and to design an extremely strong interaction between these polaritons. This idea exploits the very strong interaction through long-range dipole-dipole coupling associated with the so-called “Rydberg dipole blockade” that we observed experimentally just before the start of this project. More precisely, the “signal” light will be first transferred onto a material medium, in practice an ensemble of cold atoms in an optical cavity. One will typically use an atomic 3-level scheme with a strong driving field which couples the incoming signal beam to a storage level, which is a highly excited, long-lived Rydberg state. Then, if several atoms are excited within the ensemble, the very strong dipole-dipole interactions between them will generate a giant optical nonlinearity.

First, we have shown theoretically in 2011 that one can efficiently couple a polariton in and out the optical cavity, on demand, with a high quantum efficiency. We used as a quality criterion the possibility to make the full quantum tomography of a single photon obtained from single polariton, by using an homodyne detection, so that the negative features of the single photon Wigner function can be recovered. The corresponding experiment was carried out and published in 2014, with ground state polaritons in a cold cloud of Rubidium atoms, trapped inside and optical cavity. It has been highlighted by the editors of Physical Review Letters, and represent the state of the art in the high-efficiency (82%) deterministic read-out of a single-photon quantum memory.

In parallel, we have also shown in a series of theoretical papers (2012, 2014, 2015) that Rydberg interactions are able to turn an initial coherent state imprinted on the polaritons into a non-classical state, characterized by a negative Wigner function. We have also introduced various theoretical methods to get an accurate model for the experiments, both in the semi-classical and in the fully quantum regime. Together with this theory work, we have built an entirely new set-up to study the physics of Rydberg interactions. It includes a MOT inside an optical cavity, placed inside a UHV chamber, and loaded by a "Low Velocity Intense Source" (LVIS). As a first experiment with this system, we have measured for the first time the Rydberg-induced dispersive non-linear effect (Kerr effect) seen by a weak probe beam, in presence of a strong coupling beam exciting the Rydberg states. We have also studied resonant non-linear effects due to Electromagnetically Induced Transparency (EIT) with Rydberg states inside a cavity, and especially their strong enhancement due to the presence of this cavity.

The research and technological achievements obtained in this project have created novel methodologies for optical non-linear effects from Rydberg interactions, and the results have been published, and presented in international conferences and seminars. The research work will be pursued after the end of the ERC project, in the framework of several FET EU projects dedicated to quantum information and quantum simulations.

The results of the project have thus contributed to the worldwide effort towards quantum engineering of light, and have emphasized the importance of the physics of polaritons, which have been created here using Rydberg states, but which play also a very important in solid states physics. In order to explore such possible cross displinary developments, we have organized in December 2014 an international workshop called POLARYS (POLAriton blockade effects in RYdberg atoms and Semiconductors), gathering atomic and solid-states communities. The conference has been quite successful, and provided an excellent overview of the worldwide state of the art in this domain.