Keeping Schrödinger’s Cat Alive: Researchers Find a Way to Predict Changes in Quantum Superposition States

A team of quantum physicists at Yale University has announced that they have resolved one of the defining characteristics in the field of quantum mechanics, in that they have not only made the unpredictable behavior of particles at the quantum level predictable, but the team has also found a way to reverse the seemingly random “quantum jumps” that particles in superposition make, and in the process, save the life of Schrödinger’s cat.

Schrödinger’s cat, a thought experiment designed by Austrian physicist Erwin Schrödinger, is an illustration of the concept of quantum superposition, where a single particle, be it an electron, photon or atom, can exist in two or more energy states at the same time, until the particle is observed, at which point the particle picks a single state to exist in.

In Schrödinger’s thought experiment, a cat is sealed in a box with a radioactive material, along with a Geiger counter that is attached to a sealed vial of poison. Due to the nature of radioactive material, the radiation source may or may not, at random, have one of its atoms decay to release a particle that would trigger the Geiger counter, in turn releasing the poison contained in the vial, killing the cat.

Because the box is sealed, we have no way to determine if the contraption, of which has a fifty-fifty chance of being triggered, has caused the cat to expire or not—at least until we open the box to satisfy our own curiosity. And up until the point that the box is opened, the cat is said to be in a state of superposition, both alive and dead at the same time.

As an aside, it is still a mystery as to why Schrödinger chose to use a cat in his famous thought experiment. Did Edwin particularly dislike cats, or was he poking fun at the mysterious and dualistic nature of our feline friends? Or would it be that the cat in question would have another eight lives following the experiment? Regardless of what Schrödinger was thinking, cats certainly do seem to like boxes…

Nonetheless, Schrödinger didn’t intend this to be a real experiment, just a thought exercise to illustrate the concept of superposition. In the real world, or at least the quantum world, particles exist in this superposition state until observed, at which point they perform a “quantum jump” to whatever random energy state they wind up taking on. This seeming randomness causes problems in fields like quantum computing, where the stability of the superposition state of the computer’s qubits, analogous to the measurement of a “bit” in a traditional computer, are paramount: if the qubits jump to a random “exited” state, they need to be reset to their more useful “ground” state to proceed with their quantum computations.

This sudden jump of the qubits’ energy states occurs at random, and this instability can hamper the usefulness of a quantum computer. These quantum jumps also happen at random times—it could occur in three minutes, three hours, or even three days. Lead by Yale physicist Zlatko Minev, his team of researchers set out to determine if some sort of predictability could be applied to these otherwise random jumps.

“We wanted to know if it would be possible to get an advance warning signal that a jump is about to occur imminently,” Minev explains.

Their experiment involved a single superconducting qubit isolated in an aluminum enclosure, accompanied by three microwave generators that were set up so they could irradiate the qubit. The first microwave generator was used to excite the qubit, to prompt it to switch energy states from its desired “ground” state, although exactly when it would do so would still be random.

The second microwave generator was used as an indirect method of monitoring the qubit’s energy state—remember, direct measurement would result in the qubit “jumping” to a determined state, rendering the point of the experiment moot. This generator monitored the enclosure for the presence of electrons being given off by the qubit while in the ancillary state caused by the first microwave source.

When the second microwave generator detected an absence of photons in the enclosure, that indicated that the qubit was about to make its quantum jump, providing the researchers with an advance warning of the change in the qubit’s state. This method of predicting quantum jumps proved to be extremely reliable, with the pattern remaining consistent across the 6.8 million quantum jumps observed by the team.

“The beautiful effect displayed by this experiment is the increase of coherence during the jump, despite its observation,” explains Yale professor Michel Devoret, of whom participated in the experiment.

“You can leverage this to not only catch the jump, but also reverse it,” according to Minev. This reversal comes about courtesy of the third microwave generator, used to deliver a perfectly-timed energy pulse to the affected qubit, sending it back to its grounded state. Or, to return to Schrödinger’s thought experiment, the researchers used the warning given that the poison trap was about to be sprung, and intervened so as to keep our encapsulated kitty perpetually purring.

Although this new method allows researchers to predict imminent jumps in quantum systems, their long-term predictability currently remains out of reach. “Quantum jumps of an atom are somewhat analogous to the eruption of a volcano,” Minev said. “They are completely unpredictable in the long term. Nonetheless, with the correct monitoring we can with certainty detect an advance warning of an imminent disaster and act on it before it has occurred.”