OSA Levitated Optomechanics Incubator
James Millen
Two of the applications of optomechanical systems are testing quantum physics and the creation of ultrasensitive sensors. For the first application, a serious limitation for solid-state experiments is decoherence, which means that the environment around the optomechanical system destroys the quantum state. At this meeting Hendrick Ulbricht from the University of Southampton, UK and Markus Arndt, from the University of Vienna in Austria, discussed how we could test quantum physics using particles that are levitated, or freely moving through space. Once your mechanical object is not physically connected to your experiment, like in the solid state, many of the sources of decoherence vanish. This gives us the opportunity to answer the question of why we don’t see quantum physics in the world around us, to see if there’s a mass limit to quantum physics. Levitated optomechanical systems also give us the opportunity to explore extensions to quantum physics as we now know it.
The opportunity for ultra-sensitive force sensing is incredibly topical in this climate of funding quantum technologies. One way to improve force sensitivity is to increase the quality factor of a vibrating sensor; as a macroscopic example, this would be like making a single bell stroke last for hours or days, instead of seconds. Freeing an oscillator from clamping forces by using levitated optomechanical systems, as discussed by Andrew Geraci from the University of Reno in the US, allows a massive increase in quality factor, creating levitated microspheres that would take years to ring down after you give them a kick! His group has reached zeptoNewton / Hz1/2 force sensitivity, which is equivalent to being able to measure mass changes on the level of single viruses.
How do we create levitated optomechanical systems? Romain Quidant, from ICFO in Spain, introduced us to his experiment where silica nanospheres are trapped in optical tweezers and cooled to milliKelvin temperatures using feedback cooling. He is able to trap particles at the end of an optical fiber and manipulate them under vacuum conditions. Nikolai Kiesel from the University of Vienna, Austria, discussed how his group uses an optical cavity to levitate and cool nanospheres, and is working towards doing this in high vacuum by transporting nanospheres using a hollow core fiber.
These two experiments highlight the growing expertise at exquisitely manipulating nanoscale objects in vacuum conditions. Kishan Dholakia from the University of St. Andrews in the UK pointed out that not only can you trap and control the position of the nanoparticles, but also control their rotation. His group uses birefringent Vaterite particles and spins them using light beams with orbital or spin angular momentum. Markus Arndt, from the University of Vienna, Austria, introduced us to the control of silica nanorods using linearly polarized light. Both of these groups see micron scale objects rotating at speeds of tens of MHz! Rotation could be exploited to create micro-gyroscopes, as pressure sensors, or to stabilize the rotating particle.
Finally, we have been introduced to novel control techniques, which may shape the future of the field. Romain Quidant mentioned the possibility of trapping small particles using plasmonic tweezers, which allow control on a sub-wavelength level. Oriol Romero-Isart, from the University of Innsbruck, Austria, imagines a radically different system, where superconducting microspheres at cryogenic temperatures are used as an optomechanical system. He proposes that this is a way of greatly extending the potential for optomechanical systems to explore the mass-limits of quantum physics.
Stay tuned for more tomorrow when we explore Classical and Quantum Thermodynamics, and Nonlinear Dynamics and MilliKelvin Cavity-Cooling of Levitated Nanoparticles.
James Millen is a Marie Curie fellow specializing in levitated optomechanical systems in the Quantum Nanophysics Group at the University of Vienna. Twitter: @quantumworkshop
