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A cat acting out Schrödinger’s famous thought experiment. Hope he doesn’t die!
Photo: Dwight Sipler (Wikimedia Commons)

One of the hallmark predictions of quantum mechanics is that particles behave unpredictably—but a new experiment seems to complicate some of those core ideas.

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Researchers were able to predict a kind of atomic behavior called a quantum jump and even reverse the jump in a new experiment on an artificial atom. Such research could bring up bigger questions about the nature of physics and could have important implications for improving quantum computers that rely on the rules of quantum mechanics in order to function.

“Our experiment shows that there’s more to the story” of how quantum mechanics works, study author Zlatko Minev, a researcher at IBM’s Thomas J. Watson Research Center, told Gizmodo.


Quantum mechanics’ core assumption is that on the smallest scales, atomic properties are quantized, meaning that particles take on discrete, rather than continuous states—their properties exist along a staircase rather than a ramp. For example, an electron can be in a lowest-energy state, but if you add a little more energy, it doesn’t slowly transition into the new higher-energy state. Rather, it unpredictably snaps into the new state. If you’re not looking at it, the atom can take on intermediate states—but these aren’t midway points. The atom would be in both states at the same time, and then once you observed it, it would immediately snap into one state or the other.

But researchers wondered if they could predict these jumps and stop them from happening, according to the paper published in Nature.


The team’s artificial atom is an experimental apparatus composed of a circuit made from wire that carries charge without resistance with a special kind of insulating fence, called a Josephson junction, placed in the middle of the wire. In regular atoms, “states” are represented by the location of the electron around the atom’s nucleus, but in this artificial atom, the state is represented by a quantized property whose value changes as electrons pass the insulating fence. This is a quantum system (it’s technically a two-qubit quantum computer) and follows the same rules as other quantum systems, including electrons around atoms.

The researchers apply two specially tuned microwave signals. One signal supplies just the right amount of energy for the atom to transition between the ground state and the excited state, while the other signal indirectly measures the energy of the circuit during this transition.


Detectors measure a bright flashing photon signal—reflections from the second microwave pulse—when the artificial atom is in the ground state. When their atom is in the excited state, the researchers observe no flashes. The sensitive detectors were able to measure every last photon until the signal went dark—the sign that the transition was about to occur. When researchers sent another pulse at just the right time, they were able to stop and reverse the transition.

You might be familiar with Schrödinger’s cat. It’s a thought experiment in which a cat’s life depends on some two-state quantum process, and according to the rules of quantum mechanics, once the experiment is set in motion, the cat is alive and dead simultaneously until you open the box. In this case, the cat being alive is the ground state, and the cat being dead is the excited state. The implication of this research is that the scientists can indirectly watch the “cat” move from the alive state to the alive-and-dead simultaneously state, and intervene to save the cat.


Other researchers were impressed by the paper’s results. “It’s a great experiment,” Klaus Mølmer, professor at Aarhus University in Denmark, told Gizmodo. Mølmer pointed out that by no means does this mean that any quantum process, like radioactive decay, be undone and returned to its initial state. But he said that the researchers carefully point out the limits to their experiment.

In this experiment, the researchers only had a moment’s notice before the transition between states occurred; they can’t predict the exact day and time of the transition. But this level of foresight could be useful for quantum computers. Today, computers based on the rules of quantum mechanics are rudimentary and can succumb to random errors. Technology based on this experiment might allow quantum computing researchers to identify errors right as they’re occurring. And indeed, some of the study’s authors are working for quantum computing companies; Minev himself holds a permanent research position at IBM.


There’s more work to go before this research is integrated into existing quantum computers. But for now, it’s pretty radical that we can watch quantum mechanics unfold in real-time.


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