Schrödinger’s cat, a thought experiment envisioned by the Austrian physicist Erwin Schrödinger in 1935, is a paradox that applies the concept of superposition in quantum physics to objects encountered in everyday life. The idea is that a cat is placed in a sealed box with a radioactive source and a poison that will be triggered if an atom of the radioactive substance decays. Quantum physics suggests that the cat is both alive and dead, until someone opens the box and, in doing so, changes the quantum state. Now a team of physicists from Yale University and the University of Auckland has figured out how to catch and save Schrödinger’s famous cat by anticipating its jumps and acting in real time to save it from proverbial doom. The discovery enables physicists to set up an early warning system for imminent jumps of artificial atoms containing quantum information.
For a tiny object such as an electron, molecule, or an artificial atom containing quantum information (a qubit), a quantum jump is the sudden transition from one of its discrete energy states to another.
In developing quantum computers, researchers crucially must deal with the jumps of the qubits, which are the manifestations of errors in calculations.
The quantum jumps were theorized by Danish physicist Niels Bohr a century ago, but not observed until the 1980s, in atoms.
The new experiments peer into the actual workings of a quantum jump for the first time.
The results reveal a surprising finding that contradicts Bohr’s established view — the jumps are neither abrupt nor as random as previously thought.
“These jumps occur every time we measure a qubit. Quantum jumps are known to be unpredictable in the long run,” said Professor Michel Devoret, from Yale University and the Yale Quantum Institute.
“Despite that, we wanted to know if it would be possible to get an advance warning signal that a jump is about to occur imminently,” added Yale University’s Dr. Zlatko Minev.
The scientists used a special approach to indirectly monitor a superconducting artificial atom, with three microwave generators irradiating the atom enclosed in a 3D cavity made of aluminum.
The doubly indirect monitoring method, developed by the team for superconducting circuits, allows the researchers to observe the atom with unprecedented efficiency.
Microwave radiation stirs the artificial atom as it is simultaneously being observed, resulting in quantum jumps.
The tiny quantum signal of these jumps can be amplified without loss to room temperature. Here, their signal can be monitored in real time.
This enabled the physicists to see a sudden absence of detection photons (photons emitted by an ancillary state of the atom excited by the microwaves); this tiny absence is the advance warning of a quantum jump.
“The beautiful effect displayed by this experiment is the increase of coherence during the jump, despite its observation,” Professor Devoret said.
“You can leverage this to not only catch the jump, but also reverse it,” Dr. Minev added.
This is a crucial point. While quantum jumps appear discrete and random in the long run, reversing a quantum jump means the evolution of the quantum state possesses, in part, a deterministic and not random character; the jump always occurs in the same, predictable manner from its random starting point.
“Quantum jumps of an atom are somewhat analogous to the eruption of a volcano,” Dr. 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.”
The team’s paper appears in the journal Nature.
Z.K. Minev et al. To catch and reverse a quantum jump mid-flight. Nature, published online June 3, 2019; doi: 10.1038/s41586-019-1287-z