A sapphire crystal weighing 16 micrograms is the largest object ever to exist in a quantum-mechanical superposition of two vibrational states. Researchers at the Hybrid Quantum Systems Group at the Swiss Federal Institute of Technology in Zurich (ETH Zurich) 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.
As the research group reports in Science, this condition is much like that of the cat in the famous thought experiment of physicist Erwin Schrödinger. In Schrödinger’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.
[Read more about “cat state” experiments in physics]
To get the sapphire, which consists of about 1017 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 Schrödinger’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.
For quantum-mechanical objects—existing at the scale of atoms and subatomic particles—such superpositions of classically incompatible states are common. Macroscopic objects made of very many atoms, on the other hand, normally obey classical mechanics: they cannot assume two contradictory states simultaneously. Just as a cat cannot be alive and dead at the same time, a crystal cannot vibrate up and down at the same time. The great puzzle here, however, is why it usually cannot. After all, no matter how large an object is, it is composed of atoms and subatomic particles that obey the rules of quantum physics.
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.)
There are several possible explanations for why larger objects do not follow quantum mechanics. For example, as the number of atoms increases, perhaps more and more influences cause quantum-mechanical states to decay. Another possibility is that gravity plays a role. The hope is that ever larger cat states can help to eventually solve the Schrödinger’s cat puzzle.
Indeed, Matteo Fadel of ETH Zurich, who co-led the study with Marius Bild and Yu Yang, hopes to build on the team’s success with the sapphire and superconductor to test some of these possibilities. “I am interested in exploring the potential of our devices to investigate fundamental physics, including low-energy quantum gravity phenomenology,” Fadel says.
Stable, controllable macroscopic quantum states—such as those created in this study—are also of technical interest. For example, they can be used in error correction methods within increasingly complex quantum computers. Kotler explains that quantum computing could rely on devices that link electrical components for processing and mechanical objects for memory—much as the authors of this paper coupled a superconducting cubit to the sapphire crystal.
This article originally appeared in Spektrum der Wissenschaft and was reproduced with permission.
Editor’s Note (5/16/23): This article was updated after posting to clarify that Matteo Fadel co-led the study and to add additional information about the research group.