stefano osnaghi

We have built a microwave Fabry-Perot resonator made of diamond-machined copper mirrors coated with superconducting niobium. Its damping time (Tc = 130 ms at 51 GHz and 0.8 K) corresponds to a finesse of 4.6 e9, the highest ever reached for a Fabry-Perot in any frequency range. We have tested this resonator by sending across it two circular Rydberg atoms, the first emitting a photon and the second absorbing it after a delay of 1/10 s. This long storage time photon box opens novel perspectives for quantum information. It can be used to perform sequences of hundreds of gate operations on individual atomic qubits. A set-up with one or two photon boxes can store mesoscopic fields made of hundreds of photons for decoherence and non-locality studies.

We present the experimental realisation of a cavity QED atom interferometer based on two consecutive resonant interactions of a circular Rydberg atom with the field mode of a high Q microwave resonator that is initially prepared in the vacuum state. The interferometer scheme is analogous to the Ramsey method with the classical field replaced by the quantum field stored in the cavity mode. We demonstrate the possibility to use this technique to probe, by dispersive interaction, the field stored in a second mode of the resonator.

Following a proposal by two of us [L. G. Lutterbach and L. Davidovich, Phys. Rev. Lett. 78, 2547 (1997)], we have measured the Wigner function at the origin of phase space for a single photon field. Its value is negative, exhibiting the nonclassical nature of this state. The experiment is based on the absorption-free detection of the microwave field stored

We have built a microwave Fabry-Perot resonator made of diamond-machined copper mirrors coated with superconducting niobium. Its damping time (Tc = 130 ms at 51 GHz and 0.8 K) corresponds to a finesse of 4.6 x 109, the highest ever reached for a Fabry-Perot in any frequency range. This result opens novel perspectives for quantum information, decoherence and non-locality studies.

Following a recent proposal by S. B. Zheng and G. C. Guo (Phys. Rev. Lett. 85, 2392 (2000)), we report an experiment in which two Rydberg atoms crossing a non-resonant cavity are entangled by coherent energy exchange. The process, mediated by the virtual emission and absorption of a microwave photon, is characterized by a collision mixing angle four orders of magnitude larger than for atoms colliding in free space with the same impact parameter. The final entangled state is controlled by adjusting the atom-cavity detuning. This procedure, essentially insensitive to thermal fields and to photon decay, opens promising perspectives for complex entanglement manipulations.

To illustrate the quantum mechanical principle of complementarity, Bohr described an interferometer with a microscopic slit that records the particle's path. Recoil of the quantum slit causes it to become entangled with the particle, resulting in a kind of Einstein-Podolsky-Rosen pair. As the motion of the slit can be observed, the ambiguity of the particle's trajectory is lifted, suppressing interference effects. In contrast, the state of a sufficiently massive slit does not depend on the particle's path; hence, interference fringes are visible. Although many experiments illustrating various aspects of complementarity have been proposed and realized, none has addressed the quantum-classical limit in the design of the interferometer. Here we report an experimental investigation of complementarity using an interferometer in which the properties of one of the beam-splitting elements can be tuned continuously from being effectively microscopic to macroscopic. Following a recent proposal, we use an atomic double-pulse Ramsey interferometer, in which microwave pulses act as beam-splitters for the quantum states of the atoms. One of the pulses is a coherent field stored in a cavity, comprising a small, adjustable mean photon number. The visibility of the interference fringes in the final atomic state probability increases with this photon number, illustrating the quantum to classical transition.

We have performed multiparticle entanglement experiments with circular Rydberg atoms crossing one at a time a high Q superconducting microwave cavity. Two-level atoms and a zero or one photon field stored in the cavity act as qubits carrying quantum information. Controlled qubit entanglement is produced by the quantum Rabi oscillation coupling the atom to the cavity field. Qubit state superpositions

Using a single circular Rydberg atom, we have prepared two modes of a superconducting cavity in a maximally entangled state. The two modes share a single photon. This entanglement is revealed by a second atom probing, after a delay, the correlations between the two modes. This experiment opens interesting perspectives for quantum information manipulation and fundamental tests of quantum theory.

Bohr, in his famous discussion with Einstein on complementarity, has stressed that the nature - quantum or classical - of the interferometer parts plays an essential role to account for the fringes visibility. We have performed a Ramsey interferometry experiment with very excited Rydberg atoms, in which one "beam splitting" element is a classical field and the other - a small field stored in a superconducting cavity - continuously evolves from a microscopic quantum device into a macroscopic classical system. When microscopic, it records an unambiguous information on the atomic "path" in the interferometer and no fringes show up. When classical, it is not changed by the interaction with the atom and Ramsey fringes are observed. This experiment illustrates the complementarity concept and its intimate link with the notion of entanglement. It sheds light on the quantum-classical boundary in fundamental measurement processes.