It might not look like much, but this tiny levitating particle could be the key to a new generation of quantum sensors. Using a carefully designed magnetic trap, physicists in Sweden and Austria succeeded in levitating a 48-μm-diameter sphere of superconducting material and keeping it stable enough to characterize its motion – an achievement they describe as a “critical first step” towards using the sphere’s position to create quantum states. Such position-based quantum states could have applications in several areas, including metrology and searches for the mysterious dark matter thought to make up 85% of the universe’s mass.
To levitate their microsphere, the team needed to overcome both gravity and the attractive van der Waals force that would otherwise keep the microsphere glued to the surface. They did this by constructing a chip-based magnetic trap from wires made of niobium, which becomes a superconductor at low temperatures. This trap creates the magnetic field “landscape” needed to levitate the superconducting microsphere via the mechanism known as Meissner-state field expulsion, in which currents that arise in the superconductor completely oppose the external magnetic field.
“Key to our success was achieving a magnetic field strength high enough to initiate levitation and to keep it stable,” explains team leader Wilef Wieczorek of the Chalmers Institute of Technology in Sweden. “For that, we had to carry 0.5 A of current at millikelvin temperature through the setup without heating up the experiment.”
The levitation remained stable over a period of days. During this time, researchers from Chalmers and the Institute for Quantum Optics and Quantum Information (IQOQI) in the Austrian Academy of Sciences measured the particle’s centre-of mass motion using an integrated DC superconducting quantum interference device (SQUID) magnetometer. They did this while continuously tuning the frequency of the magnetic trapping potential between 30 and 160 Hz, which enabled them to characterize the amplitude of the particle’s motion as a function of these frequency shifts.
More sensitive force and acceleration sensors
Wieczorek and colleagues say their experiment could make it possible to develop better sensors for force and acceleration. “Our work is critical first step to creating quantum states in the position of the micrometre-sized particle,” Wieczorek tells Physics World. “It paves the way to coupling the motion of the particle to superconducting quantum circuits, which would facilitate quantum state generation of the particles’ motion.”
In the long term, Wieczorek says that the team’s platform could be developed into a precise force and acceleration sensor with applications in dark matter searches. The instruments used in such searches must be highly sensitive to have any hope of detecting shifts due to dark matter, which is believed to interact with normal matter only weakly, via the force of gravity.
Wieczorek and colleagues, who report their new technique in Physical Review Applied, say they will now try to reduce the motional amplitude of their microspheres by improving several technical aspects of their experiments. This might include installing passive cryogenic isolation and using feedback-based cooling techniques routinely employed in the field of cavity optomechanics.
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