Description

WHEN: 2018/10/31 , 10:30

WHERE: seminar room of Theoretical Physics Dept.
 

I will report on the experimental demonstration of fast, high-fidelity 
transport of atomic wave packets in spin-dependent optical lattices. 
The basic idea is to transport a cesium atom from a given lattice site 
to a neighboring one in the shortest time allowed by quantum 
mechanics, under only two  conditions: (1) the wave packet, which is 
initially prepared in the motional ground state of the initial 
trapping potential, must end up in the ground state of the final 
trapping potential, (2) during the process, the depth of the optical 
lattice potential cannot exceed a certain value fixed by some natural 
constraints (e.g., finite laser power).

Our transport experiments are carried out using 
polarization-synthesized optical lattices, which relying on a fast 
polarization synthesizer [1], enabling full and independent control of 
the potential for the spin-up and spin-down states. The sub-nanometer 
precision and high bandwidth of our system allows us to test quantum 
optimal control to speed up the transport dynamics. To achieve such a 
speed-up, optimal transport ramps allow several motional excitations 
to be created during the transport process, before these are refocused 
back into the ground state at the end. Optimizing the process for 
various transport times, we clearly observe a minimum time below which 
transport operations unavoidably create motional excitations of the 
atoms in the final trapping potential. This time defines the quantum 
speed limit of the target transformation.

Beyond their fundamental interest, quantum manipulations at the 
quantum speed limit are expected to find numerous applications for 
quantum computing and quantum sensing. In this respect, I will show a 
first application of quantum optimal control to enhance the contrast 
of an atomic Mach-Zehnder interferometer, enabling the measurement of 
external forces with high precision.

[1] C. Robens, S. Brakhane, W. Alt, D. Meschede, J. Zopes and A. 
Alberti, “Fast, High-Precision Optical Polarization Synthesizer for 
Ultracold-Atom Experiments,” Phys. Rev. Appl. 9, 034016 (2018)