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Quantum technologies

Dieter Meschede's research group
People - Digital quantum simulators
MSc. Richard Winkelmann
Last position
in our group:
PhD student
Field of research
in our group:
Digital quantum simulators

Publications(up to 2022)

  • F. G. Winkelmann, C. A. Weidner, G. Ramola, W. Alt, D. Meschede and A. Alberti
    Direct measurement of the Wigner function of atoms in an optical trap, arXiv:2205.15248 [quant-ph] accepted in J. Phys. B: At. Mol. Phys., (2022)arXivBibTeXPDF

    We present a scheme to directly probe the Wigner function of the motional state of a neutral atom confined in an optical trap. The proposed scheme relies on the well-established fact that the Wigner function at a given point (x,p) in phase space is proportional to the expectation value of the parity operator relative to that point. In this work, we show that the expectation value of the parity operator can be directly measured using two auxiliary internal states of the atom: parity-even and parity-odd motional states are mapped to the two internal states of the atom through a Ramsey interferometry scheme. The Wigner function can thus be measured point-by-point in phase space with a single, direct measurement of the internal state population. Numerical simulations show that the scheme is robust in that it applies not only to deep, harmonic potentials but also to shallower, anharmonic traps.

  • G. Ramola, R. Winkelmann, K. Chandrashekara, W. Alt, X. Peng, D. Meschede and A. Alberti
    Ramsey imaging of optical traps, Phys. Rev. Appl. 16, 024041 (2021)arXivBibTeXPDF

    The mapping of the potential landscape with high spatial resolution is crucial for quantum technologies based on ultracold atoms. However, the imaging of optical dipole traps is challenging because purely optical methods, commonly used to profile laser beams in free space, are not applicable in a vacuum. In this work, we demonstrate precise in situ imaging of optical dipole traps by probing a hyperfine transition with Ramsey interferometry. Thereby, we obtain an absolute map of the potential landscape with micrometer resolution and shot-noise-limited spectral precision. The idea of the technique is to control the polarization ellipticity of the trap laser beam to induce a differential light shift proportional to the trap potential. By studying the response to polarization ellipticity, we uncover a small but significant nonlinearity in addition to a dominant linear behavior, which is explained by the geometric distribution of the atomic ensemble. Our technique for imaging of optical traps can find wide application in quantum technologies based on ultracold atoms, as it applies to multiple atomic species and is not limited to a particular wavelength or trap geometry.

  • F. G. H. Winkelmann
    Optical plane selection in a dipole trap, (2021), PhD thesisBibTeXPDF
    Quantum technology has advanced considerably within the last decades [1, 2]. Quantum simulators are among the primary goals of this ongoing „quantum revolution“ [3]. They promise insight into many-particle phenomena that are too complex to study on classical machines [4].
    In this thesis, I present my contribution to the discrete-time quantum walk simulator (DQSIM) experiment. We trap neutral cesium atom in a two dimensional state-dependent optical lattice [5], with the goal of realizing two-dimensional discrete-time quantum walks [6] and multi-particle entanglement [7].
    The atoms are imaged using a high numerical objective lens [8] that allows us to resolve the spatial distribution inside the lattice. An additional retro-reflected beam provides state-independent confinement along the imaging axis. To measure multi-particle interference, we have to confine the atomic ensemble to a single layer along the imaging axis. I propose a novel way of plane selection with neutral cesium atoms in an optical dipole trap utilizing artificial magnetic fields created by a gradient of polarization. The preparation of thin volumes is demonstrated. With further careful adjustment of the experimental parameters, this technique will enable the selection of single planes.
    We have to apply a magnetic guiding field to enable state-dependent transport of atoms. I designed a current stealing circuit to enable the long coherence times required for quantum simulations. The magnetic guiding field is stabilized to the level of 1 ppm. We measure a coherence time in free fall of T 2 =1.7 (1.4|2.1) ms. Vertical magnetic field gradients appear to be the limiting factor. With plane selection, coherence times of several tens of ms appear possible. This will allow for quantum walks with several hundred steps. The state-dependent potential of the DQSIM experiment can also be used to reconstruct the vibrational state of neutral atoms. I numerically investigate a novel scheme to probe the Wigner function by directly measuring the expectation value of the displaced parity operator. Measuring the parity operator requires us to tune the lattice depth dynamically. Displacing the atoms purely in position space without transferring momentum requires fast modulation of the lattice position. I demonstrate that we can use the processing capabilities of our digital intensity and phase control to achieve this. Stable operation over a large dynamical range is realized by linearizing the system response. Feed-forward control of the lattice position in conjunction with internal model control increases the modulation bandwidth from 230 kHz to 3.3 MHz.
    Precise control over the vibrational degree of freedom is a prerequisite to preparing arbitrary states of motion, such as Fock states. I demonstrate Raman sideband cooling along the vertical direction using the D1 transition of cesium. This complements the microwave mediated sideband cooling that we use to cool horizontally.
    Finally, I discuss possible future experiments such as the release-retrap technique to enhance the filling factor in the center of the trap [9, 10], magnetic quantum walks [11], and direct measurement of the exchange phase of indistinguishable particles [12].