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Quantum mechanics has traditionally described the behaviour of elementary particles, atoms and molecules. However, there is no known reason why larger objects, inhabiting the realm of classical mechanics, do not follow the same rules.
This would mean these objects could display the same ‘spooky’ quantum behaviour, to use Einstein’s phrase. This includes entanglement, when two distant objects become intertwined in a manner that defies classical physics and our common-sense understanding of reality.
“The most archetypal classical systems in physics are ‘massive moving objects’ and so are a prime place to look for quantum potential in the macroscopic world,” says Mika Sillanpää, the coordinator of the EU-funded CAVITYQPD project, supported by the European Research Council. “While this has been a long-held ambition, the difficulties of the experiments have presented significant barriers.”
Using specially developed techniques capable of sensitive quantum measurements, alongside another group, the team demonstrated for the first time that small mechanical resonators – only 15 microns across, but massive on the atomic scale – can be put into an entangled quantum state.
“Entanglement is extremely fragile and has previously been mostly observed in microscopic systems such as light or atoms, and more recently in superconducting electric circuits,” explains Sillanpää, from Aalto University, the project host.
Quantum relationships in the classical world
In CAVITYQPD, micromechanical resonators (MRs), in this case tiny vibrating drumheads made from metallic aluminium on a silicon chip, represented massive moving objects. The movement of these objects typically behaves according to the rules of classical physics; the state of an object, such as a pendulum, makes its movement in space and time predictable.
“Modern society relies on micromechanical resonators because they are in all our electronics, where their dependable, predictable and precise vibrations provide a ‘clock’ for processors,” adds Sillanpää.
The project set out to determine if entanglement between two MRs could be induced and detected in the way it can between atomic systems.
While there are many different types of MRs, CAVITYQPD developed their own versions, testing designs in cleanroom settings, complemented by computer simulations.
Entanglement was induced by placing the vibrating MRs in a small device known as a cavity resonator. Here, the MRs are manipulated into an entangled state using a superconducting microwave circuit. “When entanglement is achieved, the MRs vibrate in sync which is detectable within the microwave emissions, giving us direct evidence of the successful entanglement,” explains Sillanpää.
For this to work, the experiments are conducted at a temperature of near absolute zero, at -273 °C.
A quantum leap for engineering?
Many companies and research institutes are currently investing in quantum technology, and entanglement is a key component of quantum information processing.
MRs could offer a quantum interface between the everyday world and the solid-state qubit processors of quantum computers, offering computational power well beyond current capabilities. Indeed, the CAVITYQPD team were amongst the first to couple an MR to a superconducting qubit – the basic unit of quantum information.
A feasible path forward could now be the conversion of qubit information into photons using a mechanical resonator. This would enable long-distance quantum information networking over optical fibre networks. “An interface connecting solid-state qubits to optical signals offers a range of applications beyond quantum computing, including hypersensitive quantum sensors which could, for example, measure gravitational waves,” says Sillanpää.
The team now plans to investigate highly ambitious, but still poorly understood, topics of concern to quantum physics, such as the precise nature of the interplay between quantum mechanics and gravity, and Einstein’s theory of general relativity.
“Yes, it is highly ambitious, but research worth doing should seem daunting beforehand,” concludes Sillanpää.