Physicists assess conundrum of quantum theory using levitated nanoparticles

Researchers from the University of Southampton have explored three new techniques that use hovering nanoparticles to test the limits of quantum mechanics.

The experiments, which were part of a major €4.4 million European research programme, generated specialised environments to investigate whether quantum wave functions spontaneously collapse.

Principal Investigator Professor Hendrik Ulbricht and lead researcher Dr Andrea Vinante collaborated with scientists from University College London on the project, which has published its findings in the American Physical Society’s Physical Review A.

“Quantum mechanics is the most successful theory in modern physics, as it describes with incredible accuracy the behaviour of particles, atoms and matter,” Andrea explains. “However, the theory might possibly breakdown when applied to larger systems, as suggested by some theoretical studies.

“To investigate these effects one needs a mechanical system exceptionally isolated from the external world. We are trying to achieve this goal by levitating nano and microparticles in a vacuum at low temperature and studying how to optimise an experiment in this direction.”

Spontaneous wave function collapse has been posed as a possible solution to a puzzle of quantum mechanics where a particle occupying many states is observed as a single state when measured.

According to the model, the collapse motion would cause motion in the particle which could be observed by experiments. This is however profoundly challenging as it is masked by the random motion of atoms in the particle’s environment.

The Southampton experiments are seeking to overcome these limitations by placing the particle in a highly controlled environment, levitating a 200 nanometre-wide ball of silica by using electric fields in a cryogenic vacuum chamber. The research considered using an optical cavity, optical tweezers and a SQUID – a superconducting quantum interference device - to measure the ball’s motion.

Validation of the collapse effect would expand scientists’ understanding of fundamental physics and effect the limits of quantum technologies, however the project concluded that each of the three techniques would need to be further refined to achieve this goal.

These latest experiments were conducted as part of the TEQ programme, funded by the European Union’s Horizon 2020 research and innovation scheme, which has united experts from eight institutions across the UK, the Netherlands, Italy, Austria and Denmark.