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Exploring Photon-photon Interactions with Cold Rydberg Atoms

Periodic Reporting for period 1 - EPICA (Exploring Photon-photon Interactions with Cold Rydberg Atoms)

Reporting period: 2015-09-01 to 2017-08-31

"The requirements for the basic building blocks of Quantum Information Processing (QIP) are sometimes contradictory: one must seek components that are immune to interactions to preserve their fragile quantum properties, and, at the same time, to have strong, controllable interactions to perform operations on the quantum information they carry.

Different implementations using atomic, solid state or photonic systems have different strengths and weaknesses, so there has been a growing interest in the study of hybrid systems, that combine different technologies to implement those basic building blocks. Photons are the best system to transfer quantum information, because they travel very fast (as fast as it is possible) and are virtually immune to losing their quantum properties; however, photon-photon interactions are very weak, and most materials only show nonlinearities at high photon numbers.

Recently, it has been shown that Rydberg atoms can mediate strong, controllable interactions between individual photons, thus being an ideal system for QIP tasks. ""Exploring Photon-photon Interactions using Cold Rydberg Atoms"" (EPICA) is a project aimed at exploring the effective photon-photon interactions mediated by Rydberg atoms. The aims of this project were three:

– To combine Rydberg atoms with externally generated single-photons.
– To describe in a fundamental way dipole-dipole interactions in the context of multiple stored photons in a highly interacting medium (Rydberg atoms).
– To apply this fundamental knowledge to implement basic building blocks for Quantum Information Processing, such as two-photon gates, that could eventually work with single-photons that can be entangled; or others such as a Fock state discriminator or a Fock state emitter.

As a result of our work on EPICA, we have obtained two key results in line with the objectives of the proposal:

- We have shown that the nonlinear properties of this cold cloud of Rydberg atoms are enhanced when the state of the photons are stored in the cloud, compared to the case when the photons just propagate through it. By using this mechanism, we were able to show nonlinearities at the level of tens of photons in a simple magneto-optical trap.
- We have coupled a single photon from a correlated pair inside this highly-nonlinear medium, and we have checked that the correlations in the initial pair still persist. The combination of the photon pair source and the highly-nonlinear Rydberg medium (when the nonlinearity reached the single-photon level) would implement a building block for quantum information processing; with this combination, one could for example construct two-photon gates -universal for quantum computation-, or implement deterministic Bell-state measurements.

The work on this hybrid system continues in the group of Hugues de Riedmatten at ICFO and elsewhere, but we have provided some key stones towards the use of Rydberg atoms for photonic quantum information processing and quantum communication.

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Let us remember that the aim of project EPICA is to study photon-photon interactions. Since these interactions are almost inexistent in vacuum for visible photons, we need to mediate these interactions by interfacing them with another material. In our project, this strongly interacting material is a cloud of cold Rubidium atoms excited to a Rydberg state. In preparation of the experiments performed for EPICA, we have improved a magneto-optical trap to generate a cold cloud of atoms.

To couple states of light to Rydberg states of the cold cloud of atoms we used an auxiliary (“coupling”) laser. This coupling laser helps us to interface the state of the light and that of the atoms. By controlling this coupling laser we can affect the dynamics of the pulses of light of interest: we can control the speed of the pulses and how much of the state of the pulse is transferred to the cloud as a Rydberg excitation. This interface is often called “electromagnetically induced transparency”, EIT, and since we address with it Rydberg states, we shall call it Rydberg EIT.

The work carried out in EPICA can be separated in two different experiments, which correspond to the two main publications arising from this project:

- In a first experiment, we have coupled pulses of light using Rydberg EIT to the cloud of cold atoms. By changing the number of input photons and measuring the number of output photons, we can study the nonlinear behaviour of the system. We performed the mapping between the pulses of light and the Rydberg excitations in two different ways: in the first, we let the pulses propagate slowly through the cloud (by leaving coupling laser unchanged during the process); in the second, we stored the state of the pulses in the medium, and retrieved them after a controllable time (by turning off and on again the coupling laser).

Our results, published in Distante et al. Phys. Rev. Lett. 117, 113001 (2016), show that the second process (storage) shows a stronger nonlinearity than the first one. This indicates that storage can be more effective at performing certain QIP tasks.

- In a second experiment, coupled the Rydberg machine to a source of correlated photon pairs. In this experiment, we stored and retrieved using Rydberg EIT the state of one of the photons of the pair to a Rydberg ensemble which presented a nonlinearity at the level of tens of photons.

This experiment, published in Distante et al. Nature Comms. 8, 14072 (2017), showed that the correlations between the photons in the pair were preserved even after mapping one of them into this highly-nonlinear medium.
The two results that have previously been described (enhanced nonlinearity and coupling with a single photon) show experiments beyond the state of the art.
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