Researchers have put a sapphire crystal containing quadrillions of atoms into a superposition of quantum states, bringing quantum effects into the macroscopic world

The sapphire crystal resonator used by ETH researchers to probe the validity of quantum mechanics in objects consisting of trillions of atoms.?(Image: ETH Zurich)

The sapphire crystal resonator used to probe the limits of quantum mechanics

Quantum effects have been demonstrated on one of the largest scales ever, pushing the boundaries of the quantum world. A crystal of 10^16 atoms has been placed in a superposition of two quantum states, breaking the previous record of only 2000 atoms.

When a particle is in a quantum superposition of two states, it occupies both distinct states at the same time – the most famous example is Schrödinger’s cat, in which a theoretical cat in a box is both dead and alive until you open the box to see.

For this experiment, Matteo Fadel at ETH Zürich in Switzerland and his colleagues vibrated a tiny sapphire crystal. They used a superconducting quantum bit, or qubit, to precisely control the crystal’s quantum state. This enabled them to place it in a superposition of two states of motion: vibrating and still.

This is different from a state where the crystal is just vibrating a little bit. Fadel thinks of it like a lamp with a dimmer sending out particles of light, or photons. “You can turn down the luminosity, making it fainter and fainter, and eventually you get to the point where your bulb is either sending out one photon or not sending it – there is no half a particle,” he says. “There’s no such thing as half a vibration here.” Over a few tens of microseconds, the superposition decayed, leaving the crystal still.

The goal of putting so many atoms into a quantum state is to understand whether there is a limit to the scale of quantum effects. “Quantum physics does not put a limit on this in principle – it doesn’t have a problem with me being here and over there at the same time,” says Rainer Kaltenbaek at the University of Ljubljana in Slovenia. “But the more macroscopic these states become, we at some point get into the realm where it might be conceptually challenging for us, and challenge our understanding of space and time and how nature works.”

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We don’t see quantum effects such as superpositions of states in our everyday lives, so it seems likely at some point that these effects start to degrade, perhaps because of the effects of gravity at larger scales. The scale of quantum experiments like this one is denoted by a measure called macroscopicity, which combines factors including the number of atoms in the quantum state, the mass, the degree of difference between the two states in the superposition and the length of time for which the quantum state is maintained.

Fadel and his team calculated a macroscopicity of about 11 for their experiment – much higher than any other test using these kinds of vibrations, but not the highest macroscopicity of any quantum experiment. That record was achieved in 2019 by another team of researchers who held an atom in a spatial superposition of two states 4 micrometres apart, reaching a macroscopicity of 14.

“This is a new and fresh approach that has not been around for long, and even with this new approach they are already near the same level in terms of macroscopicity as we are with other systems that have been around for 20 years or so,” says Kaltenbaek. And this system is potentially relatively easy to scale up, Fadel says.

“It’s another step towards trying to see how macroscopic we can make quantum mechanics. Alternatively, if we discover that there really is some limit to the macroscopicity of superpositions, that would be one of the most exciting discoveries that we could hope to see,” says Tim Kovachy at Northwestern University in Illinois.

If there is a limit, the theory of quantum mechanics is incomplete. “It would have a huge effect on our understanding of physics and the universe,” says Kovachy.

Journal reference: Physical Review LettersDOI: 10.1103/PhysRevLett.130.133604