8 | New Scientist | 8 June 2019Controlling quantum leaps
FOR over a century, physicists
have been arguing about the
true nature of a quantum leap.
We now have an answer, and in
true quantum form, everyone
was a little bit correct.
The phrase “quantum leap”
has taken a bit of a battering over
the past few decades – for many
people, it calls to mind a cliché for
massive change, or a sci-fi TV
programme starring Scott Bakula.
Scientifically speaking, it describes
one of the core tenets of quantum
physics: that atoms have discrete
energy levels, and electrons within
an atom can jump from one
energy level to the next, but
can’t be observed between those
specific levels.
Titans of physics including
Niels Bohr, who introduced the
idea in 1913, Erwin Schrödinger
and Albert Einstein clashed over
the specifics of these leaps – also
known as quantum jumps. The
questions of whether they were
instantaneous and whether
their timing was random were
particularly contentious.
Now, Zlatko Minev at Yale
University and his colleagues have
settled the debate. “If we zoomin to a very fine scale, the jump is
neither instantaneous nor as fully
random as we thought it was,”
Minev says.
The team achieved this by
building a superconducting
electrical circuit with quantum
behaviour that makes it an
analogue to an atom with three
energy levels: the ground state,
which is the atom’s default state,
a “bright” state connected to the
ground state and a “dark” state
into which the atom can jump.They fired a beam of
microwaves at the artificial
atom to inject energy into the
system. Generally, the atom
rapidly bounced between the
ground state and the bright state,
emitting a photon every time it
did so. But if the atom absorbed
a higher-energy photon from the
beam, it would leap into the dark
state. The dark state was more
stable than the bright state, so
the atom would stay there for
longer without emitting
any photons.
From these signals, the team
was able to tell when a quantum
jump had started by looking for a
flash of light from the bright state,
followed by a lull as the atom
leapt into the dark state. Minev
compares it to predicting a
volcanic eruption. “It’s a random
phenomenon, no one can predict
when the next volcano eruption
will occur, however before the
next eruption does occur, thereare certain signals in the ground
that we can detect and use as a
warning,” he says.
The flash then the lull in light
from the atom are like those
seismic warning signals. On
longer timescales, it is impossible
to predict when the next jump
will occur, as Bohr thought – but
on shorter time spans of just a
few microseconds, it is predictable.
“The fact that such a
quantum jump was seen in a
superconducting circuit ratherthan an atom is indicative of
the fact that we can control this
superconducting circuit in ways
that we cannot control natural
atoms,” says William Oliver at
the Massachusetts Institute of
Technology. We should someday
be able to do the same thing
with real atoms, he says.
This control allowed the team
to do something that Bohr and
his contemporaries would have
deemed impossible: control a
quantum leap.
If, just after a jump had
started, the team hit the atom
with an electrical pulse, they
could intercept it and send the
atom back to the ground state –
something that wouldn’t have
been possible if quantum leaps
were truly instantaneous and
random. Instead, they found
that the leaps took the same
path between the two energy
levels every time, so it was easy
to predict how to bounce them
back (Nature, doi.org/gf3jdc).
This shows that, as Schrödinger
insisted, quantum leaps aren’t
instantaneous. They actually take
about 4 microseconds. “In a sense,
the jumps aren’t jumps,” says
Minev. “If you look at these finer
features, you can do things that
maybe you thought you couldn’t
do because of these little windows
of predictability.”
Eventually, this may be
useful when correcting errors
in quantum computing, Minev
says. An unexpected quantum
jump could mark a mistake in
calculations, and this method
might allow us to spot the start
of the jump and account for the
error, or even reverse it mid-leap.
“This is a very important
scientific result,” says Oliver.
But its relevance to quantum
computers will ultimately
depend on how we build
them, he says. ❚Quantum leaps can be caught in the act and even reversed
“ If we zoom in to a very fine
scale, the jump is neither
instantaneous nor as fully
random as we thought”
PhysicsLeah CraneBOTT
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TV series Quantum Leap
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