Marking the passage of time in a world of ticking clocks and swinging pendulums is a simple case of counting down the seconds between then and now. However, on the quantum scale of buzzing electrons, "later" isn't always predictable. Worse, "now" is often blurred in a haze of uncertainty. In some situations, a stopwatch is simply not suitable. According to researchers at Uppsala University in Sweden, a potential solution can be found in the form of the quantum fog itself.
Their experiments with the undulating nature of what is called the Rydberg state revealed a new way of measuring the time that does not require a precise reference point. Rydberg atoms are inflated balloons in the realm of particles. Inflated with lasers instead of air, these atoms contain electrons in extremely high energy states circulating away from the nucleus.
Of course, not every pumping of the laser should inflate the atom to cartoonish proportions. In fact, lasers are regularly used to introduce electrons to higher energy states for a variety of purposes. In some applications, a second laser can be used to monitor changes in electron position, including over time. Such "probe pump" methods can be used, for example, to measure the speed of some ultra-fast electronic devices.
Putting atoms into a Rydberg state is a convenient trick for engineers, especially when designing new components for quantum computers. Needless to say, physicists have accumulated a considerable amount of information about how electrons move after entering the Rydberg state.
The mathematical rule book of this wild game is called the Rydberg Wave Packet. Like waves in a pond, having more than one Rydberg wave packet in space causes interference, resulting in unique ripple patterns. Throw enough Rydberg wavepacks into the same atomic pond, and each of these unique patterns will represent the specific time it takes for the wavepacks to evolve in unison with each other.
It was these "fingerprints" of time that the physicists behind the latest series of experiments set out to see if they were sufficiently consistent and reliable to serve as a form of quantum time record. Their study involved measuring the results of laser excitation of helium atoms and comparing the results with theoretical predictions to show how their signatures can represent the length of time.
"If you're using a counter, you need to define zero. At some point you start counting,” physicist Martha Berholts of Uppsala University in Sweden, who led the team, told New Scientist. "The advantage of this method is that you don't have to wind up the clock - you just look at the interference pattern and say 'OK, it's been 4 nanoseconds.'"
The Rydberg Wavepackage Elaboration Handbook can be used in conjunction with other forms of pump probe spectroscopy that measure small-scale events where this is less distinct from time to time or simply too inconvenient to measure. It is important to note that none of the printouts require "then" and "now" as start and end points. It would be like measuring the run of an unknown sprinter with several runners running at a certain speed.
By searching for a signature of perturbing Rydberg states in a sample of atoms with a pump probe, the researchers were able to observe a timestamp of events occurring in as little as 1.7 trillionths of a second. Future quantum clock experiments could replace helium with other atoms and even use laser pulses of different energies to extend the timestamp guide for a wider range of conditions.
This study was published in the journal Physical Review Research.
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