Discover - USA (2020-01 & 2020-02)

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Quantum weirdness struck again in 2019. An experiment described

in Nature in June settled a passionate debate that’s divided physicists

for over a century, while also raising new questions. Researchers

announced they’d tracked a quantum leap in unprecedented detail, show-

ing that it’s possible not only to predict when a particle might jump, but

also — bizarrely — reverse it mid-hop.

“There’s more to the story of quantum physics than we thought,” says

physicist Zlatko Minev, a research scientist at IBM who led the experiment

while at Yale University.

The notion of a quantum jump originated in 1913, when Danish physicist

Niels Bohr introduced the revolutionary idea that electrons only circle the

nucleus of atoms in discrete orbits, or energy levels. Electrons jump from

one level to another, Bohr hypothesized, by absorbing or emitting a packet of

energy, called a quantum. The particles can exist on one level or another, but

never in between. According to this idea, quantum leaps are instantaneous

and random.

Other physicists have railed against the idea that a particle jumps so

abruptly. “How does an [electron] transition without ever having been in

the middle?” asks Minev. To probe the mid-jump mysteries, Minev and

his collaborators used an “artificial atom,” an experimental setup that can

effectively mimic electron behaviors, including a quantum jump.

Quantum states change when measured directly, so to avoid that pitfall

Minev and his team instead observed a proxy: the level of photons reflected

or absorbed as the system changed states and energy levels. They collected

and analyzed data on the scale of microseconds, which allowed them to look

for behaviors not visible at longer time intervals. Yale physicist and co-senior

author Michel Devoret compares it to watching a movie in slow motion. “Like

in cinema, you can see things you cannot see at fast speed.”

At such fine scales, the quantum jump appeared less like an abrupt jerk and

more like a smooth, continuous transition from one energy state to another.

The researchers also noted that the system sent out a subtle signal before a

leap, and that with a carefully calibrated pulse of light, they could reverse

jumps already in progress. Manipulating quantum states in this way, says

Minev, may be useful in error-correction for quantum computers.

The experiment confirms that during a quantum jump, the particle really

does exist in two states at once. “In a typical quantum fashion, Bohr was right

and wrong at the same time,” says Minev.

A Quantum Jump

Caught in Slo-Mo

BY STEPHEN ORNES

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