Up until now, quantum entanglement has only been demonstrated in minuscule systems, such as atoms and light. Two teams of researchers have now shown how to generate and detect entanglement of macroscopic objects, they entangled a pair of jiggling objects big enough to be seen with a magnifying glass or even the naked eye.

A team led by physicist Mika Sillanpää entangled the motion of two vibrating aluminium sheets shaped into drumheads, each with a diameter of 15 micrometers, which is equivalent to the thickness of a few strands of spider silk. Another team led by physicist Sungkun Hong achieved a similar result with a pair of 15 micrometer long silicon beams, which contract and expand in width along part of the beam. The results have been reported in the April 26 Nature.

Quantum entanglement is a weird, counter-intuitive concept from quantum theory. In entanglement, the quantum states of two or more objects(such as particles) are intertwined, even if the objects are separated by huge distances. The state of each object cannot be described independently of the state of the others, a quantum state for the whole system can only be described. If you measure the state of one object, the state of the other object is immediately revealed, regardless of how far apart the objects are.

Albert Einstein called entanglement “spooky action at a distance.” He thought it was impossible because one particle of an entangled pair seems to ‘know’ what measurement was made on the other, and this information would need to be transmitted faster than the speed of light over arbitrarily large distances. Entanglement, however, has been verified experimentally, and demonstrated over various distances. Recently, a Chinese satellite entangled particles over a distance of 1,200 km.

In the experiment done by Sillanpää at Aalto University in Finland , two tiny aluminium sheets, made up of about a trillion atoms and just barely possible to see with the naked eye, vibrate like drumheads. The drumheads interacted with microwaves bouncing back and forth in a cavity, causing the motions of the drumheads to synchronize. The magnetic fields of the microwave circuit absorbed all the thermal vibrations, removing noise, and leaving only the quantum mechanical vibrations. The experiment was conducted at a temperature near absolute zero, -273 °C, to stop almost all of the molecular motion. The entanglement between the large bodies lasted for almost half an hour.

In the work done by Hong’s team, entanglement was shown with two silicon beams, which were big enough to see with a magnifying glass. Inside a region of each beam, in a 1-micrometer-long section made up of about 10 billion atoms, the beams expanded and contracted when hit with infrared light. The infrared light used is the wavelength transmitted in optical fibre telecommunications networks, which means this technology could be used in a future quantum network.

It wasn’t easy to prove that the synchronised motions of the vibrating structures were due to entanglement, a valid theoretical model of the system needed to be developed. Sillanpää says: “The biggest challenge was the theoretical understanding of the data. We measured the data exactly two years ago, but it is published not until now! This is because it was so demanding to develop the proper theoretical model. When we took the data, we had no idea if we are entangled or not. It was a great moment later to find out that all features of the data were explained by the new modelling.”

This research is very important for the coming quantum technology revolution.“It’s a first demonstration of entanglement over these artificial mechanical systems,” says Hong, of the University of Vienna. If entanglement can be achieved in macroscopic structures designed by humans, the structures can be made for specific technological requirements. To build a quantum internet connecting distant quantum computers, entangled particles or objects are required. The vibrating drumheads could be used to convert the quantum bits in a processor to quantum information sent to other computers.

The research is also interesting from a fundamental point of view, to find out how big a ‘quantum object’ can be. “One of our motivations is to keep on testing how far we can push quantum mechanics,” says Sillanpää. “There might be some fundamental limit for how big objects can be” and still be quantum.

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