Compensating fictitious magnetic field gradients in optical microtraps by using elliptically polarized dipole light

Tightly focused optical dipole traps induce vector light shifts ("fictitious magnetic fields") which complicate their use for single-atom trapping and manipulation. The problem can be mitigated by adding a larger, real magnetic field, but this solution is not always applicable; in particular, it precludes fast switching to a field-free configuration. Here we show that this issue can be addressed elegantly by deliberately adding a small elliptical polarization component to the dipole beam. In our experiments with single $^{87}$Rb atoms in a chopped trap, we observe improvements up to a factor 11 of the trap lifetime compared to the standard, seemingly ideal linear polarization. This effect results from a modification of heating processes via spin-state diffusion in state-dependent trapping potentials. We develop Monte-Carlo simulations of the evolution of the atom's internal and motional states and find that they agree quantitatively with the experimental data. The method is general and can be applied in all experiments where the longitudinal polarization component is non-negligible.Comment: 6 pages, 5 figure

Alkali vapor pressure modulation on the 100ms scale in a single-cell vacuum system for cold atom experiments

We describe and characterize a device for alkali vapor pressure modulation on the 100ms timescale in a single-cell cold atom experiment. Its mechanism is based on optimized heat conduction between a current-modulated alkali dispenser and a heat sink at room temperature. We have studied both the short-term behavior during individual pulses and the long-term pressure evolution in the cell. The device combines fast trap loading and relatively long trap lifetime, enabling high repetition rates in a very simple setup. These features make it particularly suitable for portable atomic sensors.Comment: One reference added, one correcte

Overlapping two standing-waves in a microcavity for a multi-atom photon interface

We develop a light-matter interface enabling strong and uniform coupling between a chain of cold atoms and photons of an optical cavity. This interface is a fiber Fabry-Perot cavity, doubly resonant for both the wavelength of the atomic transition and for a geometrically commensurate red-detuned intracavity trapping lattice. Fulfilling the condition of a strong and uniform atom-photon coupling requires optimization of the spatial overlap between the two standing waves in the cavity. In a strong-coupling cavity, where the mode waists and Rayleigh range are small, we derive the expression of the optimal trapping wavelength taking into account the Gouy phase. The main parameter controlling the overlap of the standing waves is the relative phase shift at the reflection on the cavity mirrors between the two wavelengths, for which we derive the optimal value. We have built a microcavity optimized according to these results, employing custom-made mirrors with engineered reflection phase for both wavelengths. We present a method to measure with high precision the relative phase shift at reflection, which allows us to determine the spatial overlap of the two modes in this cavity.Comment: 14 pages, 7 figure

Limits of atomic entanglement by cavity-feedback : from weak to strong coupling

We theoretically investigate the entangled states of an atomic ensemble that can be obtained via cavity-feedback, varying the atom-light coupling from weak to strong, and including a systematic treatment of decoherence. In the strong coupling regime for small atomic ensembles, the system is driven by cavity losses into a long-lived, highly-entangled many-body state that we characterize analytically. In the weak coupling regime for large ensembles, we find analytically the maximum spin squeezing that can be achieved by optimizing both the coupling and the atom number. This squeezing is fundamentally limited by spontaneous emission to a constant value, independent of the atom number. Harnessing entanglement in many-body systems is of fundamental interest [1] and is the key requirement for quantum enhanced technologies, in particular quantum metrology [2]. In this respect, many efforts have been devoted to prepare entangled states in atomic ensembles because of their high degree of coherence and their potential for precision measurement. Spin squeezed states as well as number states have been produced following methods based either on coherent evolution in the presence of a non-linearity in the atomic field [3--5], or on quantum non-demolition measurement [6--8]. Among methods of the first kind, cavity feedback [5, 9] is one of the most promising: it has already allowed for the creation of highly squeezed states [5] and the effective non-linearity introduced by the atom-cavity coupling can be easily switched off, making it very attractive for metrol-ogy applications. In this Letter, we analyze the entangled states that can be produced by cavity feedback in different coupling regimes from weak to strong, and derive the ultimate limits of the metrology gain, extending the optimization of squeezing to unexplored domains of parameters values. After optimization of both the coupling strength and the atom number, we find a maximum squeezing limit that depends only on the atomic structure

Stability of a trapped atom clock on a chip

We present a compact atomic clock interrogating ultracold 87Rb magnetically trapped on an atom chip. Very long coherence times sustained by spin self-rephasing allow us to interrogate the atomic transition with 85% contrast at 5 s Ramsey time. The clock exhibits a fractional frequency stability of $5.8\times 10^{-13}$ at 1 s and is likely to integrate into the $1\times10^{-15}$ range in less than a day. A detailed analysis of 7 noise sources explains the measured frequency stability. Fluctuations in the atom temperature (0.4 nK shot-to-shot) and in the offset magnetic field ($5\times10^{-6}$ relative fluctuations shot-to-shot) are the main noise sources together with the local oscillator, which is degraded by the 30% duty cycle. The analysis suggests technical improvements to be implemented in a future second generation set-up. The results demonstrate the remarkable degree of technical control that can be reached in an atom chip experiment.Comment: 12 pages, 11 figure

Spin waves and Collisional Frequency Shifts of a Trapped-Atom Clock

We excite spin-waves with spatially inhomogeneous pulses and study the resulting frequency shifts of a chip-scale atomic clock of trapped $^{87}$Rb. The density-dependent frequency shifts of the hyperfine transition simulate the s-wave collisional frequency shifts of fermions, including those of optical lattice clocks. As the spin polarizations oscillate in the trap, the frequency shift reverses and it depends on the area of the second Ramsey pulse, exhibiting a predicted beyond mean-field frequency shift. Numerical and analytic models illustrate the observed behaviors.Comment: Will appear soon in Physical Review Letters - Typos correcte