Interface Between Two Worlds
Ultracold atoms coupled to a micromechanical oscillator
Stephan Camerer, David Hunger, and Philipp Treutlein (left to right)
[This is an invited article based on a recently published work by the authors and their collaborators from Germany and France. -- 2Physics.com]
Authors: Philipp Treutlein, David Hunger, Stephan Camerer
Affiliation: Ludwig-Maximilians-Universität München, Max-Planck-Institut für Quantenoptik, Germany
Link to the Munich atom chip experiments >>
Bose-Einstein condensates of ultracold atoms and micromechanical oscillators are usually thought to belong to different areas of science. The condensates are elusive gaseous objects that display the intriguing phenomena of quantum physics in a very clean way. Mechanical oscillators are tangible tools with widespread technological applications. In a recent experiment , we have coupled the vibrations of a mechanical oscillator to the motion of a Bose-Einstein condensate in a trap. Such a coupling could lead to quantum-level control and readout of mechanical oscillators [2,3], with applications in quantum information processing or precision force sensing.
Bose-Einstein condensates (BECs) of ultracold neutral atoms are quantum systems par excellence. Due to their electric neutrality, the atoms can be very well isolated from the environment. In recent years, a sophisticated experimental toolbox has been developed for cooling, readout and control of the atoms at the quantum level . This has enabled many beautiful experiments in which interesting quantum states of BECs are prepared and studied.
It is an intriguing question to investigate whether this high level of control can be transferred to other systems such as micromechanical oscillators. At low temperatures, the vibrations of mechanical oscillators show quantum behaviour . A sufficiently strong coupling between ultracold atoms and a mechanical oscillator would allow one to create a hybrid quantum system, in which coherent transfer of quantum information or atom-oscillator entanglement could be studied. In applications of micromechanical oscillators as force sensors, it could lead to higher sensitivity.
In our experiment, which is part of the Nanosystems Initiative Munich and involves researchers from the Ludwig-Maximilians-University in Munich, the Max-Planck-Institute of Quantum Optics in Garching, and the Ecole Normale Superieure in Paris, we have made a first step in this direction and coupled a BEC to the vibrations of a micromechanical oscillator. We use an “atom chip” – a chip with microfabricated current-carrying wires – to create magnetic trapping potentials for the atoms. In addition, the chip carries a micromechanical cantilever oscillator, similar to those used in atomic force microscopes. Using the magnetic traps on the atom chip, we prepare a Bose-Einstein condensate and position it close to the cantilever tip.
At small distances of about one micrometer, atom-surface forces such as the Casimir-Polder force result in an attraction between the atoms and the oscillator. This attractive force couples the vibrations of the oscillator and the motion of the atoms in the trap. The system can be thought of as two oscillating pendula with extremely different masses that are coupled with a spring. Through the coupling, the mechanical oscillator excites collective vibrations of the atoms in the trap – in this way we use the atoms to detect the oscillator vibrations.
Figure 1: a) Schematic setup: Micro-cantilever mounted on an atom chip with gold wires. A 87Rb BEC can be trapped and positioned near the cantilever with magnetic fields from wire currents. Cantilever vibrations can be excited with a piezo and independently probed with a readout laser. b) Photograph of the atom chip (scale bar: 1 mm). c) Combined magnetic and surface potential. The surface potential reduces the trap depth to U0. Cantilever oscillations modulate the potential, thereby coupling to atomic motion.
The BEC has a discrete spectrum of vibrational modes, whose frequencies can be tuned by adjusting the magnetic trapping potential. We use this feature to control the coupling and to resonantly couple the oscillator vibrations to selected mechanical modes of the BEC.
The cantilever oscillator that is used in our current experiment has a length of 200 micrometers and is excited to vibrations with several nanometers amplitude, which are then detected with the atoms. By replacing the cantilever with a nanoscale oscillator, such as a carbon nanotube, it could be possible to detect vibrations close to the quantum mechanical ground state motion of the oscillator. To prepare the oscillator close to its ground state, such an experiment has to be performed at very low temperatures in a cryogenic setup. Under these conditions, the atoms could influence the nanotube strongly, opening a path to quantum manipulations.
 David Hunger, Stephan Camerer, Theodor W. Hänsch, Daniel König, Jörg P. Kotthaus, Jakob Reichel, and Philipp Treutlein, “Resonant Coupling of a Bose-Einstein Condensate to a Micromechanical Oscillator”, Phys. Rev. Lett. 104, 143002 (2010). Abstract.
 K. Hammerer, M. Wallquist, C. Genes, M. Ludwig, F. Marquardt, P. Treutlein, P. Zoller, J. Ye, and H. J. Kimble, “Strong coupling of a mechanical oscillator and a single atom”, Phys. Rev. Lett. 103, 063005 (2009). Abstract.
 L. Tian and P. Zoller, “Coupled Ion-Nanomechanical Systems”, Phys. Rev. Lett. 93, 266403 (2004). Abstract.
 S. Chu, “Cold atoms and quantum control”, Nature 416, 206 (2002). Abstract.
 A. D. O’Connell et al., “Quantum ground state and single-phonon control of a mechanical resonator”, Nature 464, 697 (2010). Abstract.