May 29 2019
Quantum physics scientists of the Schliesser Lab at the Niels Bohr Institute, University of Copenhagen, have achieved a new regime of precision with regards to force and position measurements. Their experiment is the first to exceed the so-called “Standard Quantum Limit” (SQL), which occurs in the most universal (and successful) optical methods for ultra-precise position measurements.
PhD students Junxin Chen and Massimiliano Rossi on either side of David Mason, first author on the article in Nature Physics. David is holding the silicon nitride membrane in his hand in a pair of tweezers. (Image credit: University of Copenhagen)
For over five decades, experimentalists have raced to surpass the SQL using a range of methods, but have not been successful. In their latest work, the scientists at the Niels Bohr Institute have achieved the trick with a simple alteration of the standard method, which allows the required termination of quantum noise in the measurement. The result and fundamental experiment have possible implications for gravitational wave astronomy methods, in addition to force microscopy with biological applications. The study is currently published in the esteemed scientific magazine, Nature Physics.
The trouble with quantum noise
Quantum actions have quantum consequences. In the setting of measurements, this frequently means that the very act of measuring a quantum system disrupts it. This effect is termed as “backaction”, and is a consequence of fundamental quantum ambiguities, first conceived by Werner Heisenberg during his time at Niels Bohr’s Copenhagen Institute in the 1920s. In a number of instances, this fixes a limit to how exact a measurement can get.
Gravitational wave telescopes like the Laser Interferometer Gravitational-Wave Observatory (LIGO), whose discoveries were bestowed the 2017 Physics Nobel prize, bounce laser light off a mirror to measure its position, in an optical configuration called an interferometer. The “imprecision” of this measurement can be enhanced by increasing the laser power, but ultimately the haphazard kicks of the laser photons will disrupt the mirror position, resulting in a less-sensitive measurement which leaves faint or distant astronomical objects unnoticed. By ideally balancing the imprecision noise and backaction, one can achieve a minimum amount of extra noise, fixing the “Standard Quantum Limit” (SQL). This minimum noise level fixes the best precision possible by all types of conventional interferometer.
To bypass this limit, one must alter the interferometer in a certain way to prevent these quantum noise sources. In the five decades since the SQL was determined, different suggestions have been presented, and recent years have delivered several proof-of-concept experimental demonstrations. Thus far, no experiment has truly measured the position of an object with a precision which surpasses the SQL. But this is precisely what the Copenhagen team has realized, thanks to innovative optical and nanomechanical methods.
Better than the gold standard
“The SQL is something of a gold standard for the quality of a measurement. It is nothing that can’t fundamentally be overcome, but as far as force and position measurements are concerned, it turned out to be very hard. Even LIGO isn’t there yet. But with our system we thought we should stand a chance,” explains Prof. Schliesser, who headed the team. This system is an experimental platform created in Schliesser’s group in the last few years. Similar to LIGO, it uses a laser-powered interferometer to measure a position, here a membrane composed of the ceramic silicon nitride. While very thin about 20 nm, the membrane is more than a few millimeters wide and simply visible by the naked eye. The “trick” used by the scientists to surpass the SQL involves making a distinctive measurement of the light reflected off the membrane. In this configuration, the detector is able to concurrently measure both the inaccuracy and backaction in a way that allows these noise sources annul each other out. Simply put, what remains is a “clean” measurement.
30 % improvement is very good news for practical applications
Once we knew we could get very close to the SQL, the modifications required to beat it were actually rather straightforward. We are using quantum effects that arise in the measurement setup itself, so the extra technological effort is actually limited. That is good news for potential practical applications.
Dr. David Mason, US Postdoc and Study Lead Author, University of Copenhagen
Using this method, the group at NBI could measure the location of their membrane with a precision approximately 30% better than what the SQL would enable. This signifies a turning point moment for quantum measurements of mechanical objects, emphasizing how far the state-of-the-art has been advanced, and indicating an optimistic path ahead. Opto-mechanical systems like the one explored here are poised to continue helping the development of methods associated with gravitational wave astronomy, while also applying their high level of sensitivity in other domains. Devices from the Schliesser Lab are already being incorporated into advanced force-sensing applications, where they may facilitate MRI-like images at a nanometer scale, possibly imaging individual HI or influenza viruses.