The capability to tune the strength of the elastic interparticle interaction is crucial for many experiments with ultracold gases. Magnetic Feshbach resonances 1,2 are widely harnessed for this purpose, but future experiments 3-8 would benefit from extra flexibility, in particular from the capability to spatially modulate the interaction strength on short length scales. Optical Feshbach resonances 9-15 do offer this possibility in principle, but in alkali atoms they induce rapid loss of particles due to light-induced inelastic collisions. Here, we report experiments that demonstrate that light near-resonant with a molecular bound-to-bound transition in 87 Rb can be used to shift the magnetic field at which a magnetic Feshbach resonance occurs. This enables us to tune the interaction strength with laser light, but with considerably less loss than using an optical Feshbach resonance.Using light to change the s-wave scattering length a in ultracold gases offers more flexibility than a magnetic Feshbach resonance because it is possible to apply an almost arbitrary spatial pattern of light using holographic masks. The light intensity can vary on a length scale of typically one optical wavelength and the pattern can also be varied rapidly in time. This could be used for a variety of applications, such as the simulation of the physics of black holes 3,4 , the controlled creation of solitons 5 , studies of the collapse of a Bose-Einstein condensate (BEC) in an unusual regime 6 and the simulation of certain Hamiltonians in which the scattering length needs to be different at different sites of an optical lattice 7,8 . Moreover, if each lattice site contains exactly two atoms 16 and a is varied only on every second lattice site, one could associate molecules at every second lattice site by ramping the magnetic field across the Feshbach resonance, thus producing a quantum state that resembles that of a supersolid. Another possible application for the manipulation of a with light exists in gases consisting of a mixture of different species or spin states. It would be desirable to tune the various scattering lengths in such systems independently, but for that purpose more control parameters than just the magnetic field are needed. Furthermore, if a spatially random light intensity pattern is applied, the scattering length would vary randomly with position, which might give rise to new quantum phases of the atomic gas.A known scheme to manipulate a using light uses a photoassociation resonance, sometimes also called an optical Feshbach resonance. But so far, photoassociation resonances have rarely been used to tune a because they induce rapid loss of particles. The experiments in refs 12, 13 both demonstrated a change of Re(a)/a bg − 1 ∼ ±1 in 87 Rb, where a bg is the background value of a. For these parameters, both experiments incurred losses characterized by a two-body rate coefficient K 2 with an estimated value of ∼10 −10 cm 3 s −1 . Typical densities of the order of 10 14 cm −3 result in lifetimes of the order of 100 µs...

Entanglement between stationary systems at remote locations is a key resource for quantum networks. We report on the experimental generation of remote entanglement between a single atom inside an optical cavity and a Bose-Einstein condensate (BEC). To produce this, a single photon is created in the atom-cavity system, thereby generating atom-photon entanglement. The photon is transported to the BEC and converted into a collective excitation in the BEC, thus establishing matter-matter entanglement. After a variable delay, this entanglement is converted into photon-photon entanglement. The matter-matter entanglement lifetime of 100 μs exceeds the photon duration by 2 orders of magnitude. The total fidelity of all concatenated operations is 95%. This hybrid system opens up promising perspectives in the field of quantum information.

We report on progress towards a measurement of the fine structure constant α to an accuracy of 5 × 10 −10 or better by measuring the ratio h/mCs of the Planck constant h to the mass of the cesium atom mCs. Compared to similar experiments, ours is improved in three significant ways: (i) simultaneous conjugate interferometers, (ii) multi-photon Bragg diffraction between same internal states, and (iii) an about 1000 fold reduction of laser phase noise to -138 dBc/Hz. Combining that with a new method to simultaneously stabilize the phases of four frequencies, we achieve 0.2 mrad effective phase noise at the location of the atoms. In addition, we use active stabilization to suppress systematic effects due to beam misalignment.

A scheme based on electromagnetically induced transparency is used to store light in a Bose-Einstein condensate. In this process, a photonic polarization qubit is stored in atomic Zeeman states. The performance of the storage process is characterized and optimized. The average process fidelity is 1.000 ± 0.004. For long storage times, temporal fluctuations of the magnetic field reduce this value, yielding a lifetime of the fidelity of (1.1 ± 0.2) ms. The write-read efficiency of the pulse energy can reach 0.53 ± 0.05.

We use laser light near resonant with an optical bound-to-bound transition to shift the magnetic field at which a Feshbach resonance occurs. We operate in a regime of large detuning and large laser intensity. This reduces the light-induced atom-loss rate by 1 order of magnitude compared to our previous experiments ͓D. M. Bauer et al., Nat. Phys. 5, 339 ͑2009͔͒. The experiments are performed in an optical lattice and include high-resolution spectroscopy of excited molecular states reported here. In addition, we give a detailed account of a theoretical model that describes our experimental data.