Marking the passage of
time in a world of ticking clocks and swinging pendulums is a simple case of
counting the seconds between 'then' and 'now'.
Down at the quantum
scale of buzzing electrons, however, 'then' can't always be anticipated. Worse
still, 'now' often blurs into a haze of uncertainty. A stopwatch simply isn't
going to cut it for some scenarios.
A potential solution
could be found in the very shape of the quantum fog itself, according to
researchers from Uppsala University in Sweden.
Their experiments on
the wave-like nature of something called a Rydberg state have revealed a novel
way to measure time that doesn't require a precise starting point.
Rydberg atoms are the
over-inflated balloons of the particle kingdom. Puffed-up with lasers instead
of air, these atoms contain electrons in extremely high energy states, orbiting
far from the nucleus.
Of course, not every pump
of a laser needs to puff an atom up to cartoonish proportions. In fact, lasers
are routinely used to tickle electrons into higher energy states for a variety
of uses.
In some applications, a
second laser can be used to monitor the changes in the electron's position,
including the passing of time. These 'pump-probe' techniques can be used to
measure the speed of certain ultrafast electronics, for instance.
Inducing atoms into
Rydberg states is a handy trick for engineers, not least when it comes to designing
novel components for quantum computers. Needless to say, physicists have
amassed a significant amount of information about the way electrons move about
when nudged into a Rydberg state.
Being quantum animals,
though, their movements are less like beads sliding about on a tiny abacus, and
more like an evening at the roulette table, where every roll and jump of the
ball is squeezed into a single game of chance.
The mathematical rule
book behind this wild game of Rydberg electron roulette is referred to as a
Rydberg wave packet.
Just like actual waves
in a pond, having more than one Rydberg wave packet rippling about in a space
creates interference, resulting in unique patterns of ripples. Throw enough
Rydberg wave packets into the same atomic pond, and those unique patterns will
each represent the distinct time it takes for the wave packets to evolve in
accordance with one another.
It was these very
'fingerprints' of time that the physicists behind this latest set of
experiments set out to test, showing they were consistent and reliable enough
to serve as a form of quantum timestamping.
Their research involved
measuring the results of laser-excited helium atoms and matching their findings
with theoretical predictions to show how their signature results could stand in
for a duration of time.
"If you're using a
counter, you have to define zero. You start counting at some point,"
physicist Marta Berholts from the University of Uppsala in Sweden, who led the
team, explained to New Scientist.
"The benefit of
this is that you don't have to start the clock – you just look at the
interference structure and say 'okay, it's been 4 nanoseconds.'"
A guide book of
evolving Rydberg wave packets could be used in combination with other forms of
pump-probe spectroscopy that measure events on a tiny scale, when now and then
are less clear, or simply too inconvenient to measure.
Importantly, none of
the fingerprints require a then and now to serve as a starting and stopping
point for time. It'd be like measuring an unknown sprinter's race against a
number of competitors running at set speeds.
By looking for the
signature of interfering Rydberg states amid a sample of pump-probe atoms,
technicians could observe a timestamp for events as fleeting as just 1.7
trillionths of a second.
Future quantum watch
experiments could replace the helium with other atoms, or even use laser pulse
of different energies, to broaden the guide book of timestamps to suit a broader
range of conditions.
Reference: Physical Review Research.
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