Researchers have created the heaviest system put into superposition yet. The crystal weighs 16 micrograms, or about one-fifth of an eyelash, and has been placed in a quantum state that has been popularized by a famous thought experiment: Schrӧdinger’s cat.
In the experiment, an imaginary cat is placed in a box. Inside the box, there is a vial of poison activated by the decay of a radioactive element, something that is governed by the laws of quantum mechanics. Until the box is opened, the cat is simultaneously dead and alive. In quantum mechanics, it is possible for a quantum state to be in two ways at once, and only a measurement can lead to it settling on one or the other.
In this experiment, the team combined an oscillating crystal with a superconducting circuit. The circuit is a qubit or quantum bit – it can be in a state of 0 or 1 as well as both states in superposition. The link between the crystal and the circuit is a piezoelectric material: when the crystal oscillates there’s an electric field. So the superposition of the circuit with its electric field can be linked to the crystal, placing its oscillations in superposition.
“By putting the two oscillation states of the crystal in a superposition, we have effectively created a Schrödinger cat weighing 16 micrograms,” senior author Professor Yiwen Chu, from the ETH in Zurich, said in a statement.
The separation between the two states was tiny. It was only a billionth of a billionth of a meter; that is displacement smaller than an atom. And yet, we can measure such separation and the states were distinguishable.
Being the size of a grain of sand might not seem a really heavy object but when it comes to quantum mechanics, it is huge. We do not understand why over a certain point, quantum effects seem to disappear. We do not experience it in the macroscopic world. The team is interested in going even heavier and further probing the limits of quantum mechanics.
“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”, Chu added.
This heavy superposition might have applications in quantum computers, where qubits are usually made by single atoms. Such a sensitive crystal might also be used to detect cosmic events such as gravitational waves.
The research is published in Science.