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Quantum technologies

Dieter Meschede's research group
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People - Few-atom quantum systems
MSc. Manolo Rivera Lam
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Last position
in our group: 		PhD student
Field of research
in our group: 		Few-atom quantum systems
 

Publications(up to 2021)

  • G. Ness, M. R. Lam, W. Alt, D. Meschede, Y. Sagi and A. Alberti
    Observing crossover between quantum speed limits, Sci. Adv. 7, abj9119 (2021)arXivBibTeXPDF
    ABSTRACT »

    Quantum mechanics sets fundamental limits on how fast quantum states can be transformed in time. Two well-known quantum speed limits are the Mandelstam-Tamm and the Margolus-Levitin bounds, which relate the maximum speed of evolution to the system’s energy uncertainty and mean energy, respectively. Here, we test concurrently both limits in a multilevel system by following the motion of a single atom in an optical trap using fast matter wave interferometry. We find two different regimes: one where the Mandelstam-Tamm limit constrains the evolution at all times, and a second where a crossover to the Margolus-Levitin limit occurs at longer times. We take a geometric approach to quantify the deviation from the speed limit, measuring how much the quantum evolution deviates from the geodesic path in the Hilbert space of the multilevel system. Our results are important to understand the ultimate performance of quantum computing devices and related advanced quantum technologies.

  • M. R. Lam
    Probing the quantum speed limit of atomic matter waves in optical lattices, (2021), PhD thesisBibTeXPDF
    ABSTRACT »
    The development of quantum technologies requires a deep knowledge of quantum systems and a high level of control of quantum states. In this thesis I report on my contribution to three areas that are important to quantum technologies: (i) Imaging of quantum states (ii) Fast transport of matter wave packets (iii) Estimation of the speed limit of quantum evolution. The platform here used consists of single neutral 133 Cs atoms trapped in a state dependent optical lattice potential. The control over the internal state of the atoms and the potential landscape is used as a tool to study the atomic wave packet dynamics.
    In the first part of the thesis I present the experimental setup as well as various experimental techniques that are required for the measurements presented in the following chapters. Two new implementations have been done in order to realize the desired measurements. One of them is a technique to measure the motional ground state population fraction, with an accuracy that is robust over a wide range of temperatures of the thermal ensemble. The second one is a pair of Raman beams to couple two hyperfine states with Rabi frequencies of around 6.5 MHz. Much faster than the observed wave packet dynamics.
    In chapter 3, I present a new technique to obtain time-resolved single-pixel images of quantum wave packets using Ramsey interferometry. The technique shares a clear analogy to classical optical imaging and can be potentially extended to obtain multi-pixel images that contain the same information as the full wave function. Even though the measurements presented in this thesis are restricted to single-pixel images, important information is extracted from them, including the Hamiltonian moments, the energy spectrum of the Hamiltonian and the population probabilities in the basis of motional eigenstates.
    In the last part of the thesis, the quantum speed limit of two different processes are studied. In chapter 4, the Mandelstam-Tamm and the Margolus-Levitin bounds are verified for atomic wave packets in a static optical lattice potential. The bounds impose a limit to the maximum rate of change of a quantum state. Two different regimes are covered: one where the Mandelstam-Tamm bound is more restrictive and one where the Margolus-Levitin bound is more restrictive. Moreover, it has been observed that the atomic wave packets evolve at a rate very close to the limit imposed by the Mandelstam-Tamm bound. In chapter 5, the speed limit of a different quantum process is studied, namely, fast atom transport without motional excitations over distances much longer than the width of the atomic wave packet. The transport trajectories are obtained with optimal quantum control, making possible to realize transport operations down to the shortest fundamental duration - the quantum speed limit. The Mandelstam-Tamm bound is found to predict an absurdly small estimate of the minimum transport duration, but a meaningful bound consistent with the measured speed limit is obtained based on geometric arguments.
  • M. R. Lam, N. Peter, T. Groh, W. Alt, C. Robens, D. Meschede, A. Negretti, S. Montangero, T. Calarco and A. Alberti
    Demonstration of quantum brachistochrones between distant states of an atom, Phys. Rev. X (Featured in Physics) 11, 011035 (2021)arXivBibTeXPDF
    ABSTRACT »

    Transforming an initial quantum state into a target state through the fastest possible route—a quantum brachistochrone—is a fundamental challenge for many technologies based on quantum mechanics. Here, we demonstrate fast coherent transport of an atomic wave packet over a distance of 15 times its size—a paradigmatic case of quantum processes where the target state cannot be reached through a local transformation. Our measurements of the transport fidelity reveal the existence of a minimum duration—a quantum speed limit—for the coherent splitting and recombination of matter waves. We obtain physical insight into this limit by relying on a geometric interpretation of quantum state dynamics. These results shed light upon a fundamental limit of quantum state dynamics and are expected to find relevant applications in quantum sensing and quantum computing.


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