S. Osnaghi

Coherent Control of an Atomic Collision in a Cavity

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 nonresonant 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 4 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.

Seeing a single photon without destroying it

Light detection is usually a destructive process, in that detectors annihilate photons and convert them into electrical signals, making it impossible to see a single photon twice. But this limitation is not fundamental—quantum non-demolition strategies permit repeated measurements of physically observable quantities, yielding identical results. For example, quantum non-demolition measurements of light intensity have been demonstrated, suggesting possibilities for detecting weak forces and gravitational waves. But such experiments, based on nonlinear optics, are sensitive only to macroscopic photon fluxes. The non-destructive measurement of a single photon requires an extremely strong matter–radiation coupling; this can be realized in cavity quantum electrodynamics, where the strength of the interaction between an atom and a photon can overwhelm all dissipative couplings to the environment. Here we report a cavity quantum electrodynamics experiment in which we detect a single photon non-destructively. We use atomic interferometry to measure the phase shift in an atomic wavefunction, caused by a cycle of photon absorption and emission. Our method amounts to a restricted quantum non-demolition measurement which can be applied only to states containing one or zero photons. It may lead to quantum logic gates based on cavity quantum electrodynamics, and multi-atom entanglement.

Ultrahigh finesse Fabry-Perot superconducting resonator

The authors acknowledge support by the DGA, by the Japan Science and Technology Agency JST, by the EU under the IP projects “QGATES” and “SCALA,” and by a Marie-Curie fellowship of the European Community to one of the authors S.K.

Step by step engineered entanglement with atoms and photons in a cavity

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 are produced and analyzed by classical microwave pulses before and after the atom cross the high Q cavity, using a Ramsey interferometer arrangement. We have demonstrated the coherent operation of a quantum phase gate and used it to perform for the first time a quantum nondestructive measurement of a single photon. Combining this gate with quantum Rabi oscillations of various durations, we have entangled step by step three subsystems—two atom and one field mode—by a controlled succession of one and two qubit operations. Once some limitations of our experiment are overcome, it will be generalized to larger numb...

Ramsey Interferometry with a Single Photon Field in Cavity QED

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.

Deterministic entanglement between three quantum systems in cavity quantum electrodynamics

We have generated and tested entanglement between the electronic states of two circular Rydberg atoms and a zero or one photon field stored in a high finesse superconducting microwave resonator. The preparation scheme uses the strong resonant coupling between the atoms and a single field mode of the resonator. This coupling results in a reversible and cyclic energy exchange between the cavity field and the atoms (Rabi oscillations).

Single-photon entanglement between two modes of the electromagnetic field in a cavity

Summary form only given. We have prepared a single-photon entangled state involving two field modes of a high finesse microwave superconducting cavity, and we have probed its coherence after storing it for a variable time in the cavity. We use the resonant interaction between circular Rydberg atoms and the field. This coupling is stronger than any dissipation rate so that the energy exchange between the atoms and the cavity is periodic (Rabi oscillation). A first atom prepares the entangled field state (writer atom) and a second one probes its coherence (reader atom).