Credit: Kat Stockton

Predicting the leaps of Schrödinger’s Cat

Researchers have deciphered one of the key mysteries of quantum mechanics — predicting sudden ‘leaps’ in a system’s state. Thus devising a method to finally rescue the most famous moggy in science history.

Yale researchers have figured out how to catch and save Schrödinger’s famous cat, the symbol of quantum superposition and unpredictability, by anticipating its jumps and acting in real time to save it from proverbial doom. In the process, they overturn years of cornerstone dogma in quantum physics.

The discovery enables researchers to set up an early warning system for imminent jumps of artificial atoms containing quantum information.

Yale researchers have found a way to catch and save Schrödinger’s famous cat, the symbol of quantum superposition and unpredictability. (Kat Stockton)

Schrödinger’s cat is a well-known and paradoxical analogy used to illustrate the concept of superposition — the ability for two opposite states to exist simultaneously — and unpredictability in quantum physics.

The idea as presented by Erwin Schrödinger 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. The superposition theory of quantum physics suggests that until someone opens the box, the cat is both alive and dead — a superposition of states. Opening the box to observe the cat causes it to abruptly change its quantum state randomly.

Thus forcing our hypothetical feline to be either dead or alive.

Don’t jump! Predicting quantum leaps

The quantum jump or leap refers to a discrete — non-continuous — and random change in the state when it is observed.

This new experiment — performed in the lab of Yale professor Michel Devoret and proposed by lead author Zlatko Minev — peers into the actual workings of a quantum jump for the first time. A study announcing the discovery appears in the June 3rd online edition of the journal Nature.

The results reveal a surprising finding that contradicts Danish physicist Niels Bohr’s established view — these jumps, say the researchers, are neither abrupt nor as random as previously thought.

For a tiny object such as an electron, molecule, or an artificial atom containing quantum information (known as a qubit), a quantum jump is a sudden transition from one discrete energy states to another. A key element of developing quantum computers is dealing with the jumps of the qubits — which are the manifestations of errors in calculations.

The enigmatic quantum jumps were theorized by Bohr a century ago, but not observed until the 1980s, in atoms.

Devoret, the F.W. Beinecke Professor of Applied Physics and Physics at Yale and member of the Yale Quantum Institute, explains: “These jumps occur every time we measure a qubit.

“Quantum jumps are known to be unpredictable in the long run.”

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

The experiment was inspired by a theoretical prediction by professor Howard Carmichael of the University of Auckland, a pioneer of quantum trajectory theory and a co-author of the study.

Researchers say reliably managing quantum data and correcting errors as they occur is a key challenge in the development of fully useful quantum computers.

The Yale team used a special approach to indirectly monitor a superconducting artificial atom — three microwave generators irradiating the atom enclosed in a 3D cavity made of aluminium. This doubly indirect monitoring method — developed by Minev 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 which results can be amplified without loss to room temperature. Thus allowing the signal to be monitored in real time.

This enables the researchers to see a sudden absence of detection photons. This tiny absence alerting researchers to an imminent quantum jump.

Devoret continues: “The beautiful effect displayed by this experiment is the increase of coherence during the jump, despite its observation.

“You can leverage this to not only catch the jump — but also reverse it.”

Why is this so significant?

The crucial point, the researchers say, is that 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.

Minev says: “Quantum jumps of an atom are somewhat analogous to the eruption of a volcano.

“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.

In addition to its fundamental impact, the discovery is a potentially major advance in understanding and controlling quantum information. One of the major hurdles with controlling quantum systems is their inherent randomness.

Whilst this development doesn’t remove that non-deterministic nature — nothing can, it’s intrinsic — the ability to predict this randomness is invaluable.

Original research: DOI: 10.1038/s41586–019–1287-z