A microscopic quantum system under continuous observation exhibits at random times sudden jumps between its states. The detection of this quantum feature requires a quantum non-demolition (QND) measurement repeated many times during the system's evolution. Whereas quantum jumps of trapped massive particles (electrons, ions or molecules) have been observed, this has proved more challenging for light quanta. Standard photodetectors absorb light and are thus unable to detect the same photon twice. It is therefore necessary to use a transparent counter that can 'see' photons without destroying them. Moreover, the light needs to be stored for durations much longer than the QND detection time. Here we report an experiment in which we fulfil these challenging conditions and observe quantum jumps in the photon number. Microwave photons are stored in a superconducting cavity for times up to half a second, and are repeatedly probed by a stream of non-absorbing atoms. An atom interferometer measures the atomic dipole phase shift induced by the non-resonant cavity field, so that the final atom state reveals directly the presence of a single photon in the cavity. Sequences of hundreds of atoms, highly correlated in the same state, are interrupted by sudden state switchings. These telegraphic signals record the birth, life and death of individual photons. Applying a similar QND procedure to mesoscopic fields with tens of photons should open new perspectives for the exploration of the quantum-to-classical boundary.

Squeezing of quantum fluctuations by means of entanglement is a well-recognized goal in the field of quantum information science and precision measurements. In particular, squeezing the fluctuations via entanglement between 2-level atoms can improve the precision of sensing, clocks, metrology, and spectroscopy. Here, we demonstrate 3.4 dB of metrologically relevant squeezing and entanglement for 10 5 cold caesium atoms via a quantum nondemolition (QND) measurement on the atom clock levels. We show that there is an optimal degree of decoherence induced by the quantum measurement which maximizes the generated entanglement. A 2-color QND scheme used in this paper is shown to have a number of advantages for entanglement generation as compared with a single-color QND measurement. N A for the case of independent atoms also referred to as a coherent spin state (CSS). The CSS minimizes the Heisenberg uncertainty product so that, e.g., (δJ z ) 2 (δJ x ) 2 = 1 4| J y | 2 where J y is the expectation value of the spin projection operator. At the expense of an increase in (δJ x ) 2 , it is possible to reduce (δJ z ) 2 (or vice versa) below the projection noise limit while keeping their product constant. This constitutes an example of a spin squeezed state (SSS), for which the atoms need to be correlated. This correlation is ensured to be nonclassical ifwhere ξ defines the squeezing parameter. Under this condition, the atoms are entangled (3) and the prepared state improves the signal-to-noise ratio in spectroscopical and metrological applications (1). Systems of 2 to 3 ions have successfully been used to demonstrate spectroscopic performance with reduced quantum noise and entanglement (4, 5). The situation is somewhat different with macroscopic atomic ensembles where spin squeezing has been an active area of research in the past decade (6-13). To our knowledge, no results reporting ξ < 1 via interatomic entanglement in such ensembles have been reported so far, with a very recent exception of the paper (14) where entanglement in an external motional degree of freedom of 2 · 10 3 atoms via interactions in a Bose-Einstein condensate is demonstrated. Spin Squeezing by Quantum Nondemolition (QND) MeasurementsIn this article, we report on the generation of an SSS fulfilling Eq. 1 in an ensemble of ≈10 5 atoms via a QND measurement (7, 15-17) of J z . We show how to take advantage of the entanglement in this mesoscopic system by using Ramsey spectroscopy (1)-one of the methods of choice for precision measurements of time and frequency (18) (Fig. 1A). The figure presents the evolution of the pseudospin J whose tip is traveling over the Bloch sphere. The Ramsey method allows using the atomic ensemble as a sensor for external fields where the perturbation of the energy difference between the levels ΔE ↑↓ is measured, or as a clock where the frequency of an oscillator is locked to the transition frequency between the two states Ω = ΔE ↑↓ / . Fig. 1 B illustrates how a suitable SSS can improve the precision of the Ramsey measurement pr...

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 × 10 9 , the highest ever reached for a Fabry-Perot in any frequency range. This result opens many perspectives for quantum information processing, decoherence and non-locality studies.PACS numbers: 42.50. Pq, Since Bohr-Einstein's photon box thought experiment, storing a photon for a long time has been a dream of physicists. Cavity quantum electrodynamics (CQED) in the microwave domain comes closest to this goal. Photons are trapped in a superconducting cavity and probed by atoms crossing the field one at a time. Experiments with circular Rydberg atoms and Fabry-Perot resonators have led to fundamental tests of quantum theory and various demonstrations of quantum information procedures [1]. The open geometry of the cavity is essential to allow a perturbation-free propagation of long-lived atomic coherences through the mode. With this cavity structure, however, the field energy damping time T c is very sensitive to geometrical mirror defects, limiting T c to ≃ 1 ms in previous experiments. We report here the realization of a Fabry-Perot resonator at ω/2π = 51 GHz, with T c = 130 ms. The cavity quality factor Q is 4.2 × 10 10 and its finesse 4.6 × 10 9 , the highest ever achieved in any frequency domain for this geometry. This important step opens the way to many CQED experiments. Quantum non-demolition detection of a single photon [2] and generation of mesoscopic non-local quantum superpositions [3] are now accessible. Long term storage of single photon fields opens bright perspectives for quantum information processing. These high-Q cavities are also promising for the stabilization of microwave oscillators or for the search of exotic particles [4].A picture of the cavity C with the top mirror removed is shown in Fig. 1. The mirrors have a diameter D 0 = 50 mm. The distance between their apexes is L = 27.57 mm. Their surface is toroidal (radii of curvature 39.4 and 40.6 mm in two orthogonal planes). The two TEM 900 modes near 51.099 GHz with orthogonal linear polarizations are separated by 1.2 MHz. This large frequency splitting is essential to ensure that atoms are efficiently coupled to a single mode only. The mirrors are electrically insulated. A static electric field parallel to the cavity axis is applied between them to preserve the circular states and to tune the atomic transition via the Stark effect [1]. The 1 cm spacing between mirror edges is partly closed by two guard rings improving the static field homogeneity in C. The atoms of a thermal beam enter and exit the cavity through two large ports (1 cm × 2 cm) so that they never come close to metallic surfaces, preserving them from patch effect stray fields. This ensures a good transmission of atomic coherences through the cavity [2]. Four piezoelectric actuators are employed to translate one of the mirrors and to tune the cavity (within ±5 MHz) wit...