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Nanofiber-Trapped Cold Atoms and Applications

Periodic Reporting for period 1 - NANOFI (Nanofiber-Trapped Cold Atoms and Applications)

Reporting period: 2015-04-01 to 2017-03-31

In quantum information science, two important challenges are the development of efficient optical quantum memories and reliable sources of single-photons. A promising category of such devices is based on ensembles of cold neutral atoms. The motivation comes from the fact that collective effects related to large number of atoms make it much easier to achieve a strong and controllable coupling between the medium and the light.

The NANOFI project realised at Laboratoire Kastler Brossel in Paris aimed at addressing those problems, by performing experiments based on an ensemble of cold atoms trapped in the vicinity of a nanofiber, which will enable to obtain a larger optical thickness, a better coupling between collective excitations and light modes, and thus a larger efficiency than previous ensemble-based implementations. The project investigated this platform for light-matter interfacing and its possible applications.

Three overall objectives were pursued and obtained. The first one was the implementation of an all-fibered trap for atoms around a nanofiber where not only large optical thicknes was observed but also atomic ordering was achieved. Using this trap, and in particular, the atomic ordering, it was possible to create conditions for a Bragg reflection from the trapped atoms, where reflections as high as 75% were observed for pulses at the single-photon level. Finally, using this setting, heralding and retrieval of single excitations into atomic chains were observed, and the generation of high-purity heralded single photons was achieved.
The objectives of the project were the generation of an all-fibered dipole trap for atoms in the vicinity of the nanofiber and the realization of different quantum optics experiments using this setting.

All-fibered dipole trap.
The first experimental project was the realization of an optical lattice for atoms around the nanofiber. This lattice was formed using an all-fibered dipole trap: four beams traveling inside the nanofiber with well-selected detunings and powers provided a trapping potential for atoms nearby. Once the atoms were loaded in the dipole trap, they didn’t travel away from the nanofiber and their interaction with the nanofiber field was maximized.

Using this all-fibered dipole trap, an optical depth as high as 100 was obtained with only 4000 trapped atoms. The lifetime of the dipole trap was higher that 50 ms. In addition, by carefully selecting the wavelength of two trapping beams; it was possible to engineer the distance between the atoms in the optical lattice.

One important improvement was the implementation of a filtering system. In order to filter out all the beams involved in the dipole trapping process and work at the single-photon level, two filtering system with an isolation of 140 dBm and transmission of 75 % were implemented (one on each arm of the nanofiber). Each filtering system consisted of a pair of volume Bragg gratings and dichroic mirrors. In addition, each filtering system was later upgraded to filter out the different control beams used in the atomic experiments. This upgrade reduced the total transmission to 50% but increased the isolation particularly for beams close to the signal wavelength, as closed as 1 GHz.

Bragg Mirror.
One of the first experiments using this all-fibered dipole trap was the generation of conditions to observe Bragg reflection out of the trapped atoms, by engineering the distance between the atoms and making the optical lattice period nearly commensurate with the resonant wavelength. In this configuration, each atom behaved as a partially reflecting mirror, and due to the strong coupling in the nanofiber, an ordered chain of around 2000 atoms was sufficient to realise an efficient Bragg mirror with up to 75% reflection of the guided mode. This result was a significant improvement from the best experimental result using free-space trapped atoms where a maximum Bragg reflection of 80% was realized but with 10 millions atoms.
In addition, we observed the chiral properties of the nanofiber in the Bragg reflection. The experimental results of this experiment are published in Phys. Rev. Lett. 117, 133603.

Emissive Optical Memory.
The second experiment done with the nanofiber setting was the heralding and retrieval of a single excitation in the atomic chains, and the subsequent generation of strongly correlated pairs of photons. The protocol used was DLCZ (Duan-Lukin-Cirac-Zoller), and had been implemented in ensembles of atoms in free space. The results represented the first time such a protocol has been implemented in a nanoscale waveguide.
Using these correlated photons, it was possible to generate high-purity heralded single photons into the nanofiber mode that traveled outside the system into a commercial fiber. The high quality of the heralded single photon was verified.

The different results obtained during the fellowship have been presented in various international conferences in the field of atomic physics, quantum physics and quantum information, such as:

• The 25th International Conference on Atomic Physics (ICAP 2016, Seoul, Korea).
• The International Conference on Quantum Communication, Measurements and Computing (QCMC 2016, Singapore).
• Quantum Information and Measurement IV (QIM 2017, Paris, France).
• Workshop on Quantum Light-Matter Interaction in Low Dimensions (2017, Barcelona Spain).
• Optical Nanofiber Applications (ONNA 2017, Okinawa, Japan).
• Conference on Lasers and Electro-Optics 2017 Europe (CLEO 2017, Munich, Germany)

The published paper is in a top international peer-review journal (PRL), but it has also being available on arXiv for free access ( The Bragg reflection result was picked up by different popularization science magazines and newspaper like La Recherche (N. 518 December 2016 pag. 31), Nature Photonics Research Highlights (10, 691 2016), Optics and Photonics News (26 September 2016), and

The next paper will be submitted in a top international peer-review journal shortly, but it has been already presented in the international conferences mentioned before. It will be available on arXiv as well.
The fellowship explored different quantum optics experiments based on ensembles of cold neutral atoms trapped around an optical nanofiber.

Neil Corzo has performed different experiments with the novel setting. The results obtained during the fellowship (Bragg mirror reflection, and storage and retrieval of a single excitation) represent pioneer outcomes in the field of waveguide QEC, quantum communication and quantum information.

The large Bragg reflection obtained in the nanofiber setting, 75 % with 2000 atoms, represents an improvement over the best Bragg reflection observed in a free-space configuration, 80 % but with 10 millions atoms. The experimental result also represents one of the first implementations where both the atomic ordering and optical depth play an important role in this setting. Previous experiments relied manly on the optical depth of the medium and not on the ordering of the atoms.

The second experimental result, the storage and retrieval of a single excitation, shows the great capabilities of the nanofiber setting to perform experiments in the quantum regime. It also shows for the first time the generation of on-demand pure single photons from atoms close to a nanoscale waveguide.