Aalto University: Physicist Mika A. Sillanpää wins a multi-million euro research grant to support work reconciling quantum mechanics and general relativity

The team is trying to solve a hundred-year-old mystery of physics with the help of small gold spheres and extremely low temperatures. The observation of tiny gravitational forces between vibrating spheres may solve the mystery.
The highly competed ERC Advanced Grant, awarded to leading top researchers, is the third ERC grant won by Professor Mika A. Sillanpää. In 2009, he received the ERC Starting Grant targeted at talented young researchers and, in 2013, he was awarded the ERC Consolidator Grant intended for top researchers establishing their careers. Picture: Aalto University.
The highly competed ERC Advanced Grant, awarded to leading top researchers, is the third ERC grant won by Professor Mika A. Sillanpää. In 2009, he received the ERC Starting Grant targeted at talented young researchers and, in 2013, he was awarded the ERC Consolidator Grant intended for top researchers establishing their careers. Photo: Aalto University.

Professor Mika A. Sillanpää at Aalto University has been awarded EUR 2.5 million of ERC Advanced Grant funding from the European Research Council for the GUANTUM project. The goal of the project is to determine the effect of gravity on the quantum-mechanical states and vibrations of two gold spheres on a very small scale, and at extremely low temperatures. This will be the third ERC grant Sillanpää has received.

‘With this research, we are trying to solve a hundred-year-old mystery of physics: The incompatibility of the general relativity and quantum mechanics,’ says Sillanpää.

The theory of general relativity describes the universe, space-time and gravity. Quantum mechanics, on the other hand, is a part of physics that studies particles at the atomic and molecular scale. Gravitational forces have never been detected within a quantum system, and researchers have failed to create a theory that covers both. This is what Sillanpää and his team want to change with their research project.

In the experiment, both gold spheres rest on a very thin membrane so that the spheres are close to each other but free to vibrate. A small gold sphere on a membrane is visible in the center of the image. Picture: Aalto University.
In the experiment, both gold spheres rest on a very thin membrane so that the spheres are close to each other but free to vibrate. A small gold sphere on a membrane is visible in the center of the image. Picture: Aalto University.
In the GUANTUM project, researchers will bring gold spheres, with a diameter of 0.5 mm and a mass of one milligram, acting as sensitive oscillators in a quantum-mechanical state. It is an extremely closed system where phenomena unseen in classical physics may occur. At the same time, they will observe the very small gravitational forces, which cause the gold spheres to be attracted toward each other.

In the experiment, both gold spheres rest on a very thin membrane so that the spheres are close to each other but free to vibrate.

‘We chose gold because it is very dense, which means that the gold sphere exerts a maximised gravitational force in spite of its relatively small size. Based on the preliminary data, the quality factor of the oscillators was incredibly high, which minimises the energy loss in the quantum system,’ Sillanpää says.

Hard work and glorious moments
Sillanpää and his research team are also ready to experiment with elements other than gold. For example, osmium is a very dense but rare element that, as a superconductor, is suitable for minimising electrical energy losses, since there are no electrical losses. Different materials can also be tested in the sample’s thin membrane, and its antennae may utilise superconductors other than aluminium with even smaller energy losses.

‘The preparation of the sample and other parts of the experiment imply hard work. We may encounter totally unexpected problems – which we will not identify before performing the experiments,’ Sillanpää says.

The experiment to observe quantum-mechanical states alongside gravity will progress step by step. In the first phase, the researchers aim to measure the gravitational force itself between the masses weighing a milligram. Gravitational forces have never been detected experimentally between masses even close to such a small size, and it is not clear whether the normal law of gravity applies at such a small scale.

Next, Sillanpää and his team aim to identify gravity in a situation in which the internal quantum indeterminacy of the gold spheres dominates their vibration. In the final stage, they will try to verify a genuinely quantum state, i.e., entanglement, in the sample, alongside gravity. Sillanpää and his team published an article on the entangled quantum state, or “spooky action”, in the journal Nature in 2018.

‘It is a pleasure and privilege to have an opportunity to solve one of the greatest unresolved mysteries of mankind, even though, when working in a laboratory, the glorious moments are rare. Phenomena unknown to modern physics may occur in the experiment when the oscillators are brought to a quantum state and, at the same time, there is a significant gravitational interaction between them. We have almost always succeeded in reaching our goal,’ says Sillanpää.

The project uses the OtaNano national research infrastructure, and part of the ERC funding goes to the purchase of a new cryostat, or dilution refrigerator. The device, manufactured by a Finnish Aalto-spinoff company Bluefors, specialised in delivering instrumentation for quantum technology, is well suited for measuring small vibrations.

‘Precision measurements like these are very sensitive to low-frequency interference that disturbs the phenomena we are looking for. They are very prone to vibrating on their own due to any kind of external tremors’ says Sillanpää.

The field of Quantum mechanics contributes to technological advances such as precision measurements, and quantum information, which will be important for the next generation of computers.

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