How quantum research makes uncertainty part of the equation

Do you have a freshly sharpened pencil at hand? Try placing the pencil on its tip so that it stays upright. Your pencil probably did not stay upright by itself. But is this an impossible task in principle?

This experiment takes us into the astonishing world of quantum mechanics and quantum sensors – and into our laboratory at the University of Vienna. 

What would Isaac Newton have to say about this?

If the pencil is not exactly straight and steady, it will fall due to gravity. It is in an unstable position. The slightest disturbance, such as slight air movements or minimal vibrations, can cause the pencil to fall. But what makes our experiment fail can actually be helpful in practice: Some seismometers use a similar principle to measure movements of the ground with a high degree of sensitivity, which can be used to predict earthquakes, for example.

We can greatly reduce the above-mentioned disturbances in a laboratory. According to Newton's classical mechanics, the pencil should remain upright for any period of time if there are no disturbances and it is in the ideal initial position. For centuries, classical mechanics has been a pillar of our physical worldview ‒ and we still use it successfully to build complex machines today. But when it comes to describing some phenomena, especially with regard to the smallest building blocks in the world, classical mechanics fails.

Using quantum mechanics for new descriptions of the world

At the beginning of the 20th century, scientists were forced to radically rethink their approach and develop a new description of the world in the form of quantum mechanics due to experimental observations. This has since been confirmed by countless experiments, forming the basis of many established and emerging technologies ranging from lasers to quantum computers.

Quantum mechanics offers fascinating insights that often differ from our everyday experience. For example, we are forced to accept that we can never know or determine the exact location and momentum of an object, in this case our pencil, at the same time. This is ruled out by the Heisenberg uncertainty principle. This limitation is not of a practical but of a fundamental nature, because quantum physics does not describe the pencil's position as a dot, but as an inkblot.

How does the pencil fall?

According to this description, the pencil does not simply fall. Instead, the uncertainty of its position spreads rapidly: The "inkblot" gets bigger. However, this also means that asking whether the pencil will fall to the left or right is meaningless. Only an observation can determine where the pencil is located (also read When there is no reality without observation). This thought takes some getting used to. However, what may sound philosophical has practical implications: For example, sensors cannot, in principle, measure disturbances that are smaller than the original uncertainty.

Is it really conceivable that not only do small atoms behave like this, but an everyday object like our pencil does too? So far, experiments have demonstrated that very light objects are subject to large uncertainties, whereas massive objects are subject to very small uncertainties. But only an experiment can show whether an object the size of our pencil falls to the left and right "at the same time" as described above. If it does not, we would have to improve our worldview even further.

Quantum balance in the laboratory

In our laboratory at the University of Vienna, we use a simplified version of the pencil experiment. We are not yet able to control such large objects well enough at the quantum level. Instead of a pencil, we use a tiny glass sphere – just 50 nanometres in size, which is about a thousandth of the diameter of a hair. Consisting of around one trillion atoms, it is already a huge object from the perspective of quantum mechanics.

The sphere balances on a "mountain top", which is shaped using laser light. This technique is based on the so-called optical tweezers, for which Arthur Ashkin was awarded the Nobel Prize in 2018: A highly focussed beam of light draws a glass sphere into its centre, which corresponds to our "valley" into which the sphere falls.

If we position two such valleys of light next to each other, an unstable point is created between them – our "mountain top". We have already built a demonstration experiment and a few years ago we also succeeded in preparing the location and momentum with the necessary precision of Heisenberg's uncertainty. We are now working on placing the sphere precisely on the "mountain top" to observe how the quantum uncertainty spreads in both directions, in line with our pencil experiment.

Sensors at the quantum limit

The experimental challenges that we encounter in basic research also inspire new technological ideas. Experiments that react so sensitively to even the slightest disturbance are ideal for detecting the faintest of signals. Floating nanoparticles and microparticles are being developed as sensors for electromagnetic fields, seismometers and as gyroscopes for navigation. Their sensitivity is so high that even collisions with individual gas molecules can be detected - a promising approach for pressure measurements in an ultra-high vacuum. The recoil of radioactive isotopes decaying in microspheres has also been demonstrated, which could lead to advances in particle and astrophysics. The unstable configuration described here - as with the seismometer mentioned at the beginning - could be the key to even better sensors.

So, the pencil cannot simply stay upright. But even after 100 years of quantum physics, the question remains fascinating: Where does the pencil fall when nobody is looking?