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’.
However, on the quantum scale of humming electrons, the “then” cannot always be predicted. Worse, the “now” often blurs into a fog of uncertainty. A timer just isn’t going to cut it for some scenarios.
A possible 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 wavelike nature of something called a Rydberg state have revealed a new way of measuring time that doesn’t require a precise starting point.
Rydberg atoms are the overinflated balloons of the particle realm. Blown up by lasers instead of air, these atoms contain electrons in extremely high-energy states, orbiting away from the nucleus.
Of course, not every laser pump needs to inflate a person to cartoonish proportions. In fact, lasers are commonly used to tickle electrons into higher energy states for a variety of uses.
In some applications, a second laser can be used to track changes in the electron’s position, including with time. These “pump-probe” techniques can be used to measure the speed of some ultrafast electronics, for example.
Exciting atoms into Rydberg states is a handy trick for engineers, especially when it comes to designing new components for quantum computers. Needless to say, physicists have gathered a considerable amount of information about how electrons move when they are pushed into a Rydberg state.
Being quantum animals, though, their movements look less like beads sliding across a tiny abacus and more like an evening at the roulette table, where every roll and bounce of the ball is compressed into a single game of chance.
The mathematical rulebook behind this wild game of Rydberg electron roulette is referred to as the Rydberg wavepacket.
Just like real waves in a lake, having more than one Rydberg wave packet in a space creates interference, resulting in unique ripple patterns. Drop several Rydberg wave packets into the same atomic pond, and these unique patterns will each represent the separate time it takes for the wave packets to evolve relative to each other.
It is precisely these “fingerprints” of time that the physicists behind this latest set of experiments began to test, showing that they were consistent and reliable enough to serve as a form of quantum timestamping.
Their research involved measuring the effects of laser-excited helium atoms and matching their findings to theoretical predictions to show how their signatures might be maintained over time.
“If you use a counter, you have to set zero. You start counting at some point,” explained physicist Marta Berholz of Uppsala University in Sweden, who led the team. Young Scientist.
“The advantage of this is that you don’t have to start the clock – you just look at the interference structure and say ‘okay, 4 nanoseconds have passed.’
A guide with evolving Rydberg wave packets could be used in conjunction with other forms of pump-probe spectroscopy that measures small-scale events when they are sometimes 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 would be like measuring the race of an unknown sprinter against a number of athletes running at fixed speeds.
By looking for the signature of entangled Rydberg states within a sample of pump-probe atoms, technicians could observe a time stamp for events as fleeting as just 1.7 trillionths of a second.
Future quantum clock experiments could replace helium with other atoms, or even use a laser pulse of different energies, to expand the timestamp driver to fit a wider range of conditions.
This research was published in Physical Review Research.