Niels Kjærgaard

Mesoscopic atomic entanglement for precision measurements beyond the standard quantum limit

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 ≳ 105 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.

The kinetics of dissolution of calcium hydroxyapatite in water at constant pH

Abstract The rate of dissolution of microcrystals of pure calcium hydroxyapatite in aqueous suspension was studied at 25°C and at constant pH. The effect of fundamental parameters such as (1) the initial sample mass, (2) the mass fraction remaining at later times, (3) the pH and (4) the concentration of the solute is reported. Specially prepared pyrophosphate-free orthophosphate solution was used in the preparation of the crystals. Carbon dioxide contamination was effectively avoided both in the preparation of crystals and in the kinetic experiments. The pH range studied was 6.6–7.2. An empirical equation representing the dependence of the rate of dissolution on the above-mentioned parameters is determined. The rate is found to be controlled by processes taking place in the surface of the crystals and not by transport processes.

Entanglement-assisted atomic clock beyond the projection noise limit

We use a quantum non-demolition measurement to generate a spin squeezed state and to create entanglement in a cloud of 105 cold cesium atoms. For the first time we operate an atomic clock improved by spin squeezing beyond the projection noise limit in a proof-of-principle experiment. For a clock-interrogation time of 10 μs, the experiments show an improvement of 1.1 dB in the signal-to-noise ratio, compared to the atomic projection noise limit.

Imaging of s and d partial-wave interference in quantum scattering of identical bosonic atoms

We report on the direct imaging of s and d partial-wave interference in cold collisions of atoms. Two ultracold clouds of 87Rb atoms were accelerated by magnetic fields to collide at energies near a d-wave shape resonance. The resulting halos of scattered particles were imaged using laser absorption. By scanning across the resonance we observed a marked evolution of the scattering patterns due to the energy dependent phase shifts for the interfering s and d waves. Since only two partial-wave states are involved in the collision process the scattering yield and angular distributions have a simple interpretation in terms of a theoretical model.

Nondestructive Probing of Rabi Oscillations on the Cesium Clock Transition near the Standard Quantum Limit

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.

Crystalline beam emulations in a pulse-excited linear Paul trap

This paper considers a pulsed voltage excitation of the quadrupole electrodes of a linear Paul ion trap. The transverse dynamics of ions in this time-varying electric field is analogous to that of charged particles in the strong focusing magnetic lattice of a storage ring. By laser cooling ions stored in a pulse-excited linear Paul trap theoretical results on the stability of crystalline ion beams in storage rings can be tested. The stability of ion motion in a pulse-excited trap is derived in (q,a)-parameter formalism and we show where in (q,a) space to expect the formation of Coulomb crystals according to the theory of crystalline ion beams.

Ultra low-noise differential ac-coupled photodetector for sensitive pulse detection applications

We report on the performance of ultra low-noise differential photodetectors especially designed for probing of atomic ensembles with weak light pulses. The working principle of the detectors is described together with the analysis procedures employed to extract the photon shot noise of light pulses with ~1 μs duration. As opposed to frequency response peaked detectors, our approach allows for broadband quantum noise measurements. The equivalent noise charge (ENC) for two different hardware approaches is evaluated to 280 and 340 electrons per pulse, respectively, which corresponds to a dark noise equivalent photon number of n3dB = 0.8 × 105 and n3dB = 1.2 × 105 in the two approaches. Finally, we discuss the possibility of removing classical correlations in the output signal caused by detector imperfection by using double-correlated sampling methods.

Inhomogeneous light shift effects on atomic quantum state evolution in non-destructive measurements

Various parameters of a trapped collection of cold and ultracold atoms can be determined non-destructively by measuring the phase shift of an off-resonant probe beam, caused by the state-dependent index of refraction of the atoms. The dispersive light–atom interaction, however, gives rise to a differential light shift (ac Stark shift) between the atomic states which, for a non-uniform probe intensity distribution, causes an inhomogeneous dephasing between the atoms. In this paper, we investigate the effects of this inhomogeneous light shift in non-destructive measurement schemes in cold caesium. We interpret our experimental data on dispersively probed Rabi oscillations and Ramsey fringes in terms of a simple light shift model which is shown to describe the observed behavior well. Furthermore, we show that by using spin echo techniques, the inhomogeneous phase shift distribution between the two clock levels can be reversed.

Measurements on photo-ionization of 3s3p 1 P1 magnesium atoms

This paper presents experimental studies of resonant two-photon ionization of Mg via the 3s 21 S0 → 3s3p 1 P1 transition using 285.2 nm CW light. Dimensionless ratios of the cross section for ionization into a continuum state of S character relative to ionization into a state of D character are extracted from data obtained in two independent experiments. We obtain the values 0.102±0.003 and 0.096±0.012, respectively, and compare these to current theoretical predictions.

Steerable optical tweezers for ultracold atom studies.

We report on the implementation of an optical tweezer system for controlled transport of ultracold atoms along a narrow, static confinement channel. The tweezer system is based on high-efficiency acousto-optic deflectors and offers two-dimensional control over beam position. This opens up the possibility for tracking the transport channel when shuttling atomic clouds along it, forestalling atom spilling. Multiple clouds can be tracked independently by time-shared tweezer beams addressing individual sites in the channel. The deflectors are controlled using a multichannel direct digital synthesizer, which receives instructions on a submicrosecond time scale from a field-programmable gate array. Using the tweezer system, we demonstrate sequential binary splitting of an ultracold 87Rb cloud into 2(5) clouds.