J.-M. Raimond

Field locked to a Fock state by quantum feedback with single photon corrections.

Fock states with photon numbers n up to 7 are prepared on demand in a microwave superconducting cavity by a quantum feedback procedure that reverses decoherence-induced quantum jumps. Circular Rydberg atoms are used as quantum nondemolition sensors or as single-photon emitter or absorber actuators. The quantum nature of these actuators matches the correction of single-photon quantum jumps due to relaxation. The flexibility of this method is suited to the generation of arbitrary sequences of Fock states.

Measurement of a microwave field amplitude beyond the standard quantum limit

We report a quantum measurement beyond the standard quantum limit (SQL) for the amplitude of a small displacement acting on a cavity field. This measurement uses as a resource an entangled mesoscopic state, prepared by the resonant interaction of a circular Rydberg atom with a field stored in a superconducting cavity. We analyze the measurement process in terms of Fisher information and prove that it is, in principle, optimal. The experimental precision achieved, 2.4 dB below the SQL, is well understood in terms of experimental imperfections. This method could be transposed to other systems, particularly to circuit QED, for the precise measurement of weak forces acting on oscillators.

Measuring the photon number parity in a cavity: from light quantum jumps to the tomography of non-classical field states

The photon number parity is a binary observable playing an important role in the description of the non-classical features of light. In microwave experiments, this observable can be measured by using a single Rydberg atom evolving in a two dimension space and interacting dispersively with a field stored in one or two cavities. The procedure, based on atomic interferometry, maps the field parity (+ 1 or − 1) onto the states of the two-level atom. By performing repeated measurements of the field parity, it is possible to observe field quantum jumps, to reconstruct the complete Wigner function of an arbitrary field and to study how it evolves due to decoherence.

Quantum field state measurement and reconstruction in a cavity by quantum nondemolition photon counting

Quantum nondemolition photon (QND) counting in a high Q cavity is performed by using circular Rydberg atoms. The atoms behave as clocks whose ticking rate is affected by light shifts induced by the cavity field. Measurning the atoms projects the field on non-classical states such as number states or Schrodinger cat states. We also use the QND measurement method for reconstructing the Wigner function of these states and to monitor their decoherence. These field manipulation methods can be applied to state preparation by quantum feedback and to demonstrate non locality with two fields located in separated cavities.