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 on the quantum scale of buzzing electrons, however, “then” is not always predictable. Even worse, the “now” often blurs into a haze of uncertainty. A stopwatch just won’t cut it in some scenarios. According to scientists from Uppsala University in Sweden, a potential solution could be found in the very shape of the quantum fog itself.
Their experiments on the wave nature of something called the Rydberg state revealed a new way of measuring time that does not require a precise starting point. Rydberg atoms are the overblown balloons of the particle realm. Blown up by lasers instead of air, these atoms contain electrons in extremely high energy states, orbiting far from the nucleus. Of course, not every pump laser needs to inflate an atom to cartoonish proportions. In fact, lasers are commonly used to tickle electrons into higher energy states for various purposes.
In some applications, a second laser can be used to track changes in electron position, including the passage of time. These “pump-probe” techniques can be used, for example, to measure the speed of certain ultrafast electronics. Inducing atoms into Rydberg states is a handy trick for engineers, not just when designing new components for quantum computers. Needless to say, physicists have amassed a considerable amount of information about the way electrons move when they are forced into a Rydberg state.
Being quantum animals, their movements are less like beads sliding down a small counter and more like an evening at a roulette table, where every roll and bounce of the ball is squeezed into a single gamble. The mathematical rule book behind this wild game of Rydberg Electron Roulette is referred to as the Rydberg Wave Packet. Just like real waves in a pond, more than one Rydberg wave packet rippling through space creates interference, resulting in unique ripple patterns. Throw enough Rydberg wave packets into the same atomic pond, and each of these unique patterns will represent a different amount of time it takes for the wave packets to evolve in unison.
It is these “fingerprints” of time that the physicists behind this latest set of experiments set out to test, showing that they are consistent and reliable enough to serve as a form of quantum time stamp. Their research involved measuring the results of laser-excited helium atoms and comparing their findings to theoretical predictions to show how their signature results might hold up over time. “If you use a counter, you have to define zero. At some point you start counting,” physicist Marta Berholts of the University of Uppsala in Sweden, who led the team, explained to New Scientist. “The advantage of this is that you don’t have to start the clock – you just look at the interference pattern and say ‘okay, it was 4 nanoseconds’.”
The guide to evolving Rydberg wave packets could be used in combination with other forms of pump-probe spectroscopy to measure small-scale events when they are now and then less clear or simply too inconvenient to measure. Importantly, none of the fingerprints require then and now to serve as the starting and stopping points. It would be like measuring an unknown sprint race with a number of competitors running at a set speed.
By looking for the signature of perturbing Rydberg states in the middle of a sample of pump-probe atoms, the 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 laser pulses of different energies to expand the timestamp guide to suit a wider range of conditions.
