LKB - Atom chips group

Rubidium Cavity QED

Rubidium Cavity QED


An exciting and fast‐growing research field has emerged at the interface of fundamental physics and technology, where the nonclassical features of quantum mechanics are employed to engineer powerful, radically new technologies. Apart from quantum cryptography – for which commercial systems are already available – quantum metrology is perhaps the one closest to real applications. High-fidelity control over light-matter coupling at the single-quantum level plays a prominent role in all quantum technologies.

Only a small number of mechanisms are available so far to create the multiparticle entangled states that are the crucial resource of all quantum technologies. A “massively parallel” approach has given spectacular results in the last few years. Its idea is to use a high-finesse optical cavity to create an effective interaction that entangles the atoms. Entanglement experiments with such atomic ensemble CQED systems have enabled the production of spin-squeezed states, and have enabled observation of the Dicke quantum phase transition. By combining atom chip technology with the novel FFP microcavity – both developed in our group – we have been able to realize a miniature and robust platform for such experiments. This has enabled us to produce W states (a fundamental class of entangled states) and other multiparticle-entangled states enabling metrological gain. Using the exceptionally strong coupling of our FFP cavities, we have also developed one of very few quantum state tomography methods applicable to atomic qubits, which moreover provides single-qubit resolution in mesoscopic ensembles.



Single Atom Register in an Optical Cavity

Motivated by applications in quantum-enhanced metrology, quantum simulation and quantum information processing, we are now building a new cold atom experimental platform devoted to multi-particle entanglement creation and characterization.
The goal of this project is to extend the generation of multi-particle entanglement to “mesoscopic” ensembles of neutral atoms (~100 particles), combining optical Cavity Quantum Electro-Dynamics (CQED) with quantum gas microscope techniques developed in the field of optical lattices.



Our setup will realize a single-atom qubit register inside an optical cavity, where each lattice site is strongly and identically coupled to the mode of a high-finesse optical cavity. The cavity allows creation of entangled states by the collective interaction of the atoms with the cavity mode, while a high-resolution microscope adds the capability to perform local operations on each atom of the register, opening the way for the generation and analysis of entangled states beyond the symmetric ones.

This system provides an ideal test-bed to investigate different methods for multi-particle entanglement generation and to study their fundamental limits. Quantum-enhanced metrology schemes can be explored and caracterized in this setup, in particular dissipative preparation of spin squeezed atomic ensembles [1]. This system can also realise an effective Dicke model, as proposed in [2] which exhibits a quantum phase transition, already observed for large numbers of atoms [3] [4]. With smaller atom number (10 to 50 atoms) we will be able to use quantum tomography techniques [5] to study the role of entanglement in the vicinity of the quantum phase transition as well as its scaling laws.

Experimental setup

An atom elevator

A single atom register inside an optical cavity
Our first objective is to realize a single-atom qubit register in a 1D intra-cavity optical lattice.



To reach this goal, we will assemble the following components :

  • At the heart of this experimental system is a new type of miniature fiber-based Fabry-Pérot (FFP) cavity developed in our group. This high finesse optical cavity is realized between two curved mirrors on the tips of two optical fibers facing each other (grey cylinders) and operates in the strong coupling regime of cavity QED. An intra-cavity 1D lattice (in yellow) at twice the resonant probe wavelength (1560 nm) ensures the trapping of no more than one atom in each site by collisional blockade, with an equal and maximal coupling of the atoms to the cavity probe field (in red).
  • A high-resolution microscope, able to detect and address single atoms in the lattice. At the beginning of each experiment cycle, it provides us with an accurate measurement of the initial atom number. At the end of the cycle, it represents a powerful source of information for state analysis by detecting simultaneously the fluorescence of each atom. A focused laser combined with microwave transition will allow single-site resolved addressing.
  • Transverse Raman beams (red arrows) will be added for cooling the atoms in the cavity to the vibrational ground-state. They will be used to perform two-photon Raman transitions assisted by the cavity between two stable atomic states. This will reduce the contribution to the losses of spontaneous emission, improving the quality of the states produced, as well as opening interesting new physical situations for creating multi-particle entangled states.

A macroscopic cavity with a high-NA lens

We have assembled a macroscopic Fabry-Pérot cavity with a high-NA lens.


An atom elevator

We transport cold atoms from a magneto-optical trap inside the macroscopic cavity with an atom elevator. A dipole beam is vertically translated by using an acousto-optical deflector at the focus of a lens.

Atom elevator to transport atoms inside the cavity (a fiber cavity in the drawing).

New optical fiber cavity with tailored birefringence and optimal atom-cavity mode overlap

We produce fiber cavity with double wavelength coating at 780nm for the probe light and at 1560nm for the trapping light, to get atoms maximally and equally coupled to the cavity field. Optimizing the shape on the Mirror allows to tailor the birefringence of the cavity.

anr-fb6f2This work is supported by the ANR SAROCEMA project, grant ANR-14-CE32-0002 of the French Agence Nationale de la Recherche.

[1] E.G. Dalla Torre , Physical Review Letters 110 120402 (2013)[2] F. Dimer , Physical Review A 75 013804 (2007)[3] K. Baumann , Nature, 464 09009 (2010)[4] M. P. Baden , arXiv:1404.0512v1 (2014)[5] F. Haas , Science 344 180 (2014)