Researchers in the Hybrid Quantum Systems Group at the Swiss Federal Institute of Technology in Zurich have put a sapphire crystal weighing 16 micrograms in a quantum-mechanical superposition of two vibrational states. The researchers “excited the crystal into vibrations such that its atoms oscillated back and forth simultaneously and in two opposite directions — putting the entire crystal in what is known as a state of quantum superposition,” reports Scientific American. From the report:
As the research group reports in Science, this condition is much like that of the cat in the famous thought experiment of physicist Erwin Schrodinger. In Schrodinger’s quantum-mechanical scenario, a cat is simultaneously alive and dead, depending on the decay of an atom that releases a vial of poison. The sapphire crystal in the new experiment has been put in the macroscopic equivalent of that “cat state.” Such states can help scientists fathom how and why the laws of the quantum world transition into the rules of classical physics for larger objects.
To get the sapphire, which consists of about 10^17 atoms, to behave like a quantum-mechanical object, the research group set it to oscillate and coupled it to a superconducting circuit. (In the terms of the original thought experiment, the sapphire was the cat, and the superconducting circuit was the decaying atom.) The circuit was used as a qubit, or bit of quantum information that is simultaneously in the states “0” and “1.” The circuit’s superposition was then transferred to the oscillation of the crystal. Thus, the atoms in the crystal could move in two directions at the same time — for example, up and down — just as Schrodinger’s cat is dead and alive at the same time. Importantly, the distance between these two states (alive and dead or up and down) had to be greater than the distance ascribed to the quantum uncertainty principle, which the ETH Zurich scientists confirmed. Using the superconducting qubit, the researchers succeeded in determining the distance between the crystal’s two vibrational states. At about two billionths of a nanometer, it’s tiny — but still large enough to distinguish those two states from each other beyond doubt.
These findings have “pushed the envelope on what can be considered quantum mechanical in an actual lab experiment,” says Shlomi Kotler, a physicist who studies quantum mechanical circuits at the Hebrew University of Jerusalem. Kotler did not participate in the study. […] Kotler notes that finding larger cat states is a way of “stretching the limit” of observed quantum-mechanical objects — in this case, by demonstrating that something as massive as 16 micrograms can exist in this state. (Though, to be clear, 16 micrograms is still microscopic.)