We report the experimental realization of a new kind of optical lattice for ultracold atoms where arbitrarily large separation between the sites can be achieved without renouncing to the stability of ordinary lattices. Two collinear lasers, with slightly different commensurate wavelengths and retroreflected on a mirror, generate a superlattice potential with a periodic "beat-note" profile where the regions with large amplitude modulation provide the effective potential minima for the atoms. To prove the analogy with a standard large spacing optical lattice we study Bloch oscillations of a Bose Einstein condensate with negligible interactions in the presence of a small force. The observed dynamics between sites separated by ten microns for times exceeding one second proves the high stability of the potential. This novel lattice is the ideal candidate for the coherent manipulation of atomic samples at large spatial separations and might find direct application in atom-based technologies like trapped-atom interferometers and quantum simulators.
We report the experimental realization of a new kind of optical lattice for ultracold atoms where arbitrarily large separation between the sites can be achieved without renouncing to the stability of ordinary lattices. Two collinear lasers, with slightly different commensurate wavelengths and retroreflected on a mirror, generate a superlattice potential with a periodic "beat-note" profile where the regions with large amplitude modulation provide the effective potential minima for the atoms. To prove the analogy with a standard large spacing optical lattice we study Bloch oscillations of a Bose Einstein condensate with negligible interactions in the presence of a small force. The observed dynamics between sites separated by ten microns for times exceeding one second proves the high stability of the potential. This novel lattice is the ideal candidate for the coherent manipulation of atomic samples at large spatial separations and might find direct application in atom-based technologies like trapped-atom interferometers and quantum simulators.
In this thesis I report the realization of a large spacing optical lattice exploiting the beat-note of two retro-reflected laser beams with slightly different wavelengths i.e 1064 nm and 1013 nm. The resulting potential, in addition to a very fast modulation, exhibits a slow periodicity that provides an optical potential for the atoms that behaves like an effective lattice with 10 micrometers of separation between lattice sites. \nI studied theoretically the band structure of the new lattice and I performed numerical simulations in order to obtain predictions of the energy gap as a function of the lattice depth in different configurations. \nRegarding my experimental contribution, I have complitely designed and assembled the setup for the stabilization of the intensity and the frequency of the laser sources. In particular, for the stabilization of the relative frequencies of the two lasers, I develope a technique that allows to lock the two frequencies to the same optical cavity. \nThanks to the high control of the residual forces on the sample and the capability to cancel the interatomic interactions using magnetic Feshbach resonaces, we have managed to observe spatial Bloch oscillations of condensates, measuring the atom number in each well. The observed dynamics exhibits a coherence up to 1 s that correponds to a sensitivity of the order of 5 times10^-5 g with a spatial resolution of ten microns. We characterized experimentally the frequency and the amplitude of the oscillations for different value of the force and the results show a very good agreement with theoretical simulations. \nTo our knowledge the sensitivity acquired in this work is the higher reported in litterature in the measurement of small forces (<< g) using a trapped atom interferometer. The main limitations to the coherence time and the precision of the measurements are related to the limited control we have in the way we tune the external force, i.e, optically. \nThe simplicity of the Bloch oscillation interferometer and the creation of a large spacing periodic potential open interesting perspective in the measurements of small and local forces with trapped condensates and can be useful for the investigation of Casimir-Polder forces or deviations of the \ngravitational law at short distances. \nIn the first half of my PhD I participated to another project: we studied the behaviour of an ultra-cold mixture of two different internal states of potassium. In particular regimes of interaction the cloud tends to collapse but, at high density, many-body effects occur and stabilize the mixture to a fixed volume. In free space it doesn't collapse or expand and it behaves like a liquid droplet. We have characterized the time evolution and the equilibrium properties of the droplets and we have peformed experimental studies on the collisional dynamics between two droplets. Good agreement with theoretical analysis confirm our control on the system and open interesting perspective for the studies of their excitation spectrum and superfluid behaviour.
We present a theoretical analysis of BeatNote Superlattices (BNSLs), a recently demonstrated technique for generating periodic trapping potentials for ultracold atomic clouds, with arbitrarily large lattice spacings while maintaining interferometric stability. By combining two optical lattices with slightly different wavelengths, a beatnote intensity pattern is formed, generating, for low depths, an effective lattice potential with a periodicity equal to the wavelength associated to the difference between the wave vectors of the two lattices. We study the range of lattice depths and wavelengths under which this approximation is valid and investigate its robustness against perturbations. We present a few examples where the use of BNSLs could offer significant advantages in comparison to well-established techniques for the manipulation of ultracold atomic gases. Our results highlight the potential of BNSLs for quantum simulation, atom interferometry, and other applications in quantum technologies.
Abstract We propose a method for separating trapped atoms in optical lattices by large distances. The key idea is the cyclic transfer of atoms between two lattices of variable spacing, known as accordion lattices, each covering at least a factor of two in lattice spacing. By coherently loading atoms between the two superimposed potentials, we can reach, in principle, arbitrarily large atom separations, while requiring only a relatively small numerical aperture. Numerical simulations of our ‘accordion superlattice’ show that the atoms remain localized to one lattice site throughout the separation process, even for moderate lattice depths. In a proof-of-principle experiment, we demonstrate the optical fields required for the accordion superlattice using acousto-optic deflectors. The method can be applied to neutral-atom quantum computing with optical tweezers, as well as quantum simulation of low-entropy many-body states. For instance, a unit-filling atomic Mott insulator can be coherently expanded by a factor of ten in order to load an optical tweezer array with very high filling. In turn, sorted tweezer arrays can be compressed to form high-density states of ultracold atoms in optical lattices. The method can also be applied to biological systems where dynamical separation of particles is required.
We report on experiments studying the transport properties of an atomic Bose-Einstein condensate in an optical lattice of spatial period $\ensuremath{\lambda}∕2n$, where n is an integer, realized with the dispersion of multiphoton Raman transitions. We observe Bloch oscillations, as a clear effect of quantum transport, in a subwavelength-scale periodicity lattice. An unusually large tunneling coupling between lattice sites is evident from the measured effective mass. Future prospects of the different lattice structures are expected in the search for new quantum phases in tailored lattice structures up to quantum computing in optical nanopotentials.
Synthetic dimensions provide a powerful approach for simulating condensed matter physics in cold atoms and photonics, whereby a set of discrete degrees of freedom are coupled together and reinterpreted as lattice sites along an artificial spatial dimension. However, atomic experimental realizations have been limited so far by the number of artificial lattice sites that can be feasibly coupled along the synthetic dimension. Here, we experimentally realize for the first time a very long and controllable synthetic dimension of atomic harmonic trap states. To create this, we couple trap states by dynamically modulating the trapping potential of the atomic cloud with patterned light. By controlling the detuning between the frequency of the driving potential and the trapping frequency, we implement a controllable force in the synthetic dimension. This induces Bloch oscillations in which atoms move periodically up and down tens of atomic trap states. We experimentally observe the key characteristics of this behavior in the real-space dynamics of the cloud, and verify our observations with numerical simulations and semiclassical theory. The Bloch oscillations thus act as a smoking-gun signature of the synthetic dimension, and allow us to characterize the effective band structure. Our methods provide an efficient approach for the manipulation and control of highly excited trap states, and set the stage for the future exploration of topological physics in higher dimensions through the use of a tunable artificial gauge field and finite-range interactions.
We consider a Bose-Einstein condensate of ultracold atoms loaded into a square optical lattice and subject to a static force. For vanishing atom-atom interactions the atoms perform periodic Bloch oscillations for arbitrary direction of the force. We study stability of these oscillations for non-vanishing interactions, which is shown to depend on an alignment of the force vector with respect to the lattice crystallographic axes. If the force is aligned along any of the axes, the mean field approach can be used to identify the stability conditions. On the contrary, for a misaligned force one has to employ the microscopic approach, which predicts periodic modulation of Bloch oscillations in the limit of a large forcing.