Physicists have set a new record for the heaviest object ever to be recorded in a superposition of locations. The tiny, vibrating crystal weighs 16.2 micrograms – little more than a grain of sand.
Superposition is one of the fundamentals of the whacky world of quantum mechanics. It is a product of the fact that, in the small-scale world of atoms and particles, things are probabilistic. Unlike in our daily experience, you cannot look at a particle’s position and velocity and work out where it will end up at a later time. You can only give a “probability distribution” for where it may end up.
This is because quantum mechanics tells us that particles exist in a “superposition.” Until a measurement is taken, you cannot predict with certainty the particle’s physical properties.
Read more: Schrödinger’s microbe
In 1935, one of the pioneers of quantum mechanics, Erwin Schrödinger, came up with the thought experiment known as “Schrödinger’s cat” to illustrate quantum superposition.
Schrödinger asks us to imagine a cat in a box.
Now imagine that, inside the box, is a vial of toxic gas. Above the vial is a hammer poised to strike the vial, releasing the gas, and killing the cat.
The trigger for the hammer to fall is connected to a Geiger counter which measures radioactive decay coming from a small radioactive sample, also in the box. If the Geiger counter registers a particle emitted due to radioactive decay – itself a random process – then the hammer will fall.
To an outside observer, it is unclear whether the cat is dead or alive inside the box.
It is only when the observer looks inside (takes a measurement) that they can say with certainty if the cat is living or not. Before the measurement is taken, the cat exists in a “superposition” of states: it is both alive and dead. It’s paradoxical in everyday experience, but this is how things really work in the quantum world.
But the effect becomes more and more reduced as the size of the objects being measured increases.
Though the concept of superposition didn’t come until the first decades of the 20th century, the effects of this quantum phenomenon have been measured since Young’s double slit experiment, in 1801 which showed photons would travel through two slits simultaneously.
Now physicists have dwarfed these Schrödinger’s cats and kittens. “Schrödinger’s lion” has been measured experimentally by researchers at the Swiss Federal Institute of Technology (ETH) Zurich.
By coupling a mechanical resonator to a superconducting circuit, they were able to replicate Schrödinger’s paradox at a scale never before seen.
The small oscillating crystal (the big cat) existed in a superposition of two oscillation states when it was connected to the quantum circuit.
“By putting the two oscillation states of the crystal in a superposition, we have effectively created a Schrödinger cat weighing 16 micrograms,” explains lead researcher Professor Yiwen Chu. While not an actual lion, the oscillating crystal is still billions of times heavier than an atom or molecule.
Read more: “Quantum light” manipulation a step closer, with potential in medical imaging and quantum computing
Chu and her colleagues want to test the limits of Schrödinger’s cat even further. “This is interesting because it will allow us to better understand the reason behind the disappearance of quantum effects in the macroscopic world of real cats,” she says.
Such studies may also have applications in quantum technologies.
Quantum bits used in quantum computers could be made more robust by using higher-mass objects rather than single atoms or ions as is currently the norm.
High sensitivity to external noise of massive objects in superposition could also be used for precise measurements of tiny disturbances such as gravitational waves and dark matter detection.
The research is published in Science.