FREE SPACE QED · Large QED effects in free space

Large QED effects in free space

The research objectives stated in this Marie Curie project involved fundamental investigations on quantum electrodynamics (QED) and direct coupling to single ions using high-numerical aperture objectives. Most objectives were accomplished and the results published in journals. Specifically: (1) electromagnetically induced transparency (EIT) from a single atom in free space (Phys. Rev. Lett); (2) a single ion as a mirror of an optical Fabry-Perot cavity (Phys. Rev. Lett); (3) modification of atomic level-shift and spontaneous decay with a single spherical mirror (Phys. Rev. A); (4) quantum memory for light using a half cavity (Phys. Rev. A); (5) sideband cooling and thermometry below the Doppler limit (Phys. Rev. A); (6) Faraday rotation and EIT phase shifts from a single trapped ion (in preparation); (7) entanglement between two ions using a single photon detection process (submitted to Phys. Rev. Lett); (8) quantum measurement of the motional state of a trapped ion (submitted to Phys. Rev. Lett); (9) super-radiance and collective level shifts from two ions a metre apart (in preparation). 'EIT from a single atom' Absorption spectroscopy and EIT from a single trapped atom was demonstrated. We focussed a weak light field onto an optically cooled 138Ba ion using a high numerical aperture lens and 3 % extinction of this beam was observed from which we could demonstrate EIT by tuning a control beam over a two-photon resonance. This was published in Physical Review Letters. 'A single ion as a mirror of an optical Fabry-Perot cavity' We investigated the extinction properties of an atom at the focus of a lens mirror system. We could observe a QED effect whereby the ion behaves as the optical mirror of a Fabry-Perot cavity. We showed that the amplitude of the laser field was altered due to a modification of the electromagnetic mode structure around the atom in a regime in which the laser intensity is already changed by the atom alone. This was published in Physical Review Letters. 'Quantum electrodynamics from a single ion in front of a spherical mirror' We investigated theoretically the QED properties of an atomic electron close to the focus of a spherical mirror. We showed that the spontaneous emission and excited-state level shift can be suppressed with mirror-atom distances of many wavelengths. We also found that the Casimir-Polder shift is much larger than for an atom close to an infinite plane mirror. This was published in Physical Review A. 'Quantum memory with a single two-level atom in half cavity' We then proposed a direct application of this system as a quantum memory for single photons and collaborate with the Singapore group at CQT for a complete theoretical characterisation of this system as a memory. This was published in Physical Review A. 'Sideband cooling and thermometry of an ion below the Doppler limit' We also demonstrated the ability to manipulate the long lived 1.76 microns quadrupole transition of our Barium ions. We set up a high finesse (F = 20 000) cavity to lock a fibre laser a this wavelength. We successfully observed Rabi-flopping, Ramsey fringes and achieved ground state cooling of the ion using sideband resolved pumping to the motional ground state. Using the ion in a half cavity, we could then show that the set-up allows thermometry below the ground state of motion. This was recently published in Phys. Rev. A. 'Entanglement between two ions using single photon detection' We demonstrated entanglement between two atoms that are effectively a meter distant, using the single-photon interference process originally propounded by Cabrillo A. and Zoller P. This heralding mechanism allowed us to demonstrate a speedup in entanglement generation by two orders of magnitude over two photon schemes. 'Faraday rotation of a tightly focussed beam from a single trapped atom' Faraday rotation of a laser field induced by a single atom was demonstrated by tightly focussing a linearly polarised laser beam onto a Barium ion. The polarisation rotation signal was further used to measure the phase-shift associated with electromagnetically-induced transparency and to demonstrate read-out of the internal state on the ion quadrupolar transition with a detection fidelity of 98 $\pm$ 1\%. 'Super-radiance and collective level-shifts from two ions a meter-apart' We demonstrated that two meter-distant ions exchanging their field exhibit super/sub radiance and that their excited state energy is also shifted. Our results demonstrate clearly that strong collective effects may be reached using high-numerical aperture objectives and far-distant atoms. 'Quantum measurement of the motional state of a trapped ion' We performed high-resolution real time read-out of the quantum motion of a single and laser cooled Barium ion. Using an interferometric setup we demonstrated shot noise limited measurement of thermal oscillations with a resolution of 4 times the standard quantum limit. We also realised quantum limited phase control of the ion motion, suppressing the photon recoil-induced phase diffusion through a feedback loop. Due to the spectral narrowing in the phase-locked mode, the coherent ion oscillation was measured with a resolution of about 0.3 times the standard quantum limit. All these results have direct implications for single atom magnetometer/dispersive read-out of atomic super positions / studies of quantum electrodynamics and quantum communication. The theoretical and experimental results presented above motivated us to construct a high-optical-access ion trap which will feature a hemispherical mirror that is 4 cm away from the ion and a large objective on the other side. We recently achieved trapping of dust particles in this trap and will soon test it with the ion to investigate these QED effects and to collect photons more efficiently for long distance entanglement.

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