We report on the nondestructive observation of Rabi oscillations on the Cs clock transition. The internal atomic state evolution of a dipole-trapped ensemble of cold atoms is inferred from the phase shift of a probe laser beam as measured using a Mach-Zehnder interferometer. We describe a single color as well as a two-color probing scheme. Using the latter, measurements of the collective pseudospin projection of atoms in a superposition of the clock states are performed and the observed spin fluctuations are shown to be close to the standard quantum limit.

We investigate theoretically and experimentally a nondestructive interferometric measurement of the state population of an ensemble of laser cooled and trapped atoms. This study is a step towards generation of (pseudo-) spin squeezing of cold atoms targeted at the improvement of the Caesium clock performance beyond the limit set by the quantum projection noise of atoms. We calculate the phase shift and the quantum noise of a near resonant optical probe pulse propagating through a cloud of cold 133 Cs atoms. We analyze the figure of merit for a quantum non-demolition (QND) measurement of the collective pseudo-spin and show that it can be expressed simply as a product of the ensemble optical density and the pulse integrated rate of the spontaneous emission caused by the off-resonant probe light. Based on this, we propose a protocol for the sequence of operations required to generate and utilize spin squeezing for the improved atomic clock performance via a QND measurement on the probe light. In the experimental part we demonstrate that the interferometric measurement of the atomic population can reach the sensitivity of the order of √ Nat in a cloud of Nat cold atoms, which is an important benchmark towards the experimental realisation of the theoretically analyzed protocol.

Nondegenerate forward four-wave mixing in hot atomic vapors has been shown to produce strong quantum correlations between twin beams of light [McCormick et al., Opt. Lett. 32, 178 (2007)], in a configuration which minimizes losses by absorption. In this paper, we look at the role of the phase-matching condition in the trade-off that occurs between the efficiency of the nonlinear process and the absorption of the twin beams. To this effect, we develop a semiclassical model by deriving the atomic susceptibilities in the relevant doubleconfiguration and by solving the classical propagation of the twin-beam fields for parameters close to those found in typical experiments. These theoretical results are confirmed by a simple experimental study of the nonlinear gain experienced by the twin beams as a function of the phase mismatch. The model shows that the amount of phase mismatch is key to the realization of the physical conditions in which the absorption of the twin beams is minimized while the cross coupling between the twin beams is maintained at the level required for the generation of strong quantum correlations. The optimum is reached when the four-wave mixing process is not phase matched for fully resonant four-wave mixing.

[1,2] efficient. In this letter we present the first waveguide chip designed to address a BEC along a row of independent junctions, which are separated by only 10 µm and have large atom-photon coupling. We describe the fully integrated, scalable design and demonstrate 11 junctions working as intended, using a low density cold atom cloud with as little as one atom on average in any one junction. Our device opens new possibilities for engineering quantum states of matter and light on a microscopic scale.Micro-fabricated chips are widely used to control clouds of ultra-cold atoms and BoseEinstein condensates [3,4]. Recently, the idea has been extended to the control of ions [5] and similar possibilities exist for molecules [6]. This atom-chip technology provides a way to miniaturise existing atomic physics devices. In addition, it promises new devices that take advantage of the elementary quantum nature of atoms [7][8][9] [16][17][18] or otherwise attached [19] to a chip. A pair of these fibres looking into each other can be used to detect an atom cloud and can reach close to single atom sensitivity [17]. When reflective coatings are added, the gap between two fibres [1,20] or between one fibre and a micro-fabricated mirror [21] becomes a Fabry-Perot resonator. Similarly, a fibre can be coupled to a micro-disk resonator [22,23]. These devices can achieve strong atom-photon coupling for applications in quantum information processing.This letter reports a further order of magnitude scale reduction, in which the 125 µm-diameter optical fibre is replaced by an integrated waveguide only 10 µm across, with a 4 µm square core. Since a BEC is typically ∼ 100 µm long, this size reduction opens the new 3 possibility of intersecting a BEC with many closely spaced atom-photon junctions of high coupling strength. In our device, illustrated in Figure 1, a trench containing the cold atoms cuts through an array of 12 waveguides spaced by 10 µm. This design is a significant advance in the way that photons can be coupled to ultracold atoms.In order to characterise the chip, we have released cold atoms into a junction to measure its sensitivity and to demonstrate the basics of its operation. Every few seconds, 87 Rb atoms are cooled and collected from a room-temperature vapour by a Low-Velocity Intense Source (LVIS), then transferred to a magneto-optical trap (MOT) about 4 mm from the chip surface, where the atom density is up to 4 × 10 −2 atoms/µm 3 and the temperature is ∼ 100µK. We push this cloud towards the chip just before switching off the MOT light and magnetic field, thereby launching the atoms at 40 cm/s into the trench. The light beams from the waveguides diverge only slightly as they cross the trench, with w -the 1/e radius of the field -growing from 2.2 µm to 2.8 µm. Since the width of the trench is L = 16 µm, any given beam interacts with roughly one to four atoms of the cloud as they pass through.Each atom crosses the light in ∼ 7 µs, scattering up to 130 photons (the fully saturated rate is Γ/2 = 1.9 × 10 7 s −1 ).Wi...

Quantum states of light can improve imaging whenever the image quality and resolution are limited by the quantum noise of the illumination. In the case of a bright illumination, quantum enhancement is obtained for a light field composed of many squeezed transverse modes. A possible realization of such a multi-spatial-mode squeezed state is a field which contains a transverse plane in which the local electric field displays reduced quantum fluctuations at all locations, on any one quadrature. Using a travelling-wave amplifier, we have generated a multi-spatial-mode squeezed state and showed that it exhibits localised quadrature squeezing at any point of its transverse profile, in regions much smaller than its size. We observe 75 independently squeezed regions. The amplification relies on nondegenerate four-wave mixing in a hot vapor and produces a bichromatic squeezed state. The result confirms the potential of this technique for producing illumination suitable for practical quantum imaging. arXiv:1409.6561v2 [quant-ph] 8 Jun 2015