Scientists have invented a trippy new way to measure time by searching for eerie “fingerprints” in the quantum realm, which governs the universe at very small scales, reports a new study. The novel technique differs from the most familiar ways of keeping time because it is not anchored to a “time zero” that marks the start of a recorded period.
Our human compulsion to tell time has manifested in ingenious mechanisms over the millennia, including sundials, stopwatches, and hyper-precise atomic clocks. All of these devices measure time by clocking the period between two intervals, whether that is the back-and-forth swing of a pendulum in a grandfather clock, or the time between the starting shot of a race and the winner crossing the finish line.
Now, a team led by Marta Berholts, an experimental physicist at Uppsala University, have created a very different type of “quantum watch” that does not require an initial “time zero” reference point to make its time measurements, according to a recent study in Physical Review Research.
“Unlike any other clock, this quantum watch does not utilize a counter and is fully quantum mechanical in its nature,” the team said in the study. “The quantum watch has the potential to become an invaluable tool in pump-probe spectroscopy”—which is an technique for studying ultrafast chemical reactions—”due to its simplicity, assurance of accuracy, and ability to provide an absolute timestamp, i.e. there is no need to find time zero.”
Berholts and her colleagues created this quantum watch by shooting lasers at helium atoms until they reached an excited “Rydberg” state with special properties. Scientists use Rydberg atoms to study all kinds of interesting problems in physics, but the new study focuses on the unpredictable signatures, called “wave packets,” produced by excited electrons that orbit the atoms.
These packets emerge from a quantum phenomenon called superposition, in which an object can occupy two states of reality at the same time. In the macro-scale world that humans experience, it’s impossible to be in two places at once, or to simultaneously exist in two different realities, but such mind-boggling feats are possible under the weird quantum rules that exist on the very small scales of atoms and subatomic particles.
When the wave packets of multiple Rydberg atoms interact, complex “fingerprints” emerge that are somewhat analogous to the choppy waters of colliding ripples in a pond. These patterns, known as quasiunique beat signatures (QUBS), are so idiosyncratic that they can be used as timestamps that measure the evolution of the wave packets relative to each other.
“We show that the oscillations resulting from an ensemble of highly excited Rydberg states” can “give rise to a unique interference pattern that does not repeat during the lifetime of the wave packet,” the team explained in their study.
“These fingerprints determine how much time has passed since the wave packet was formed and provide an assurance that the measured time is correct,” the researchers noted. “Unlike other clocks such as mechanical, quartz crystal, or atomic, which operate by counting the number of oscillations from a well-defined frequency, the QUBS-based timer does not utilize a counter. Instead, it provides a fingerprint representing a specific time and hence only requires interaction when initiating and reading out the time.”
In other words, the patterns created by the interacting atoms are so distinct that they carry an innate record of their lifespans. In addition to the novelty of this timekeeping approach, the team notes that the watch is highly accurate, with a margin of error of just eight femtoseconds (one femtosecond is a millionth of a billionth of a second).
“The main advantage of the quantum watch,” the team said, “is that it effectively has no sources of systematic error…If there were any unknown external forces that affected the energy levels, giving a systematic error in the derived QUBS-time, then it would result in deviations between the measured and calculated QUBS.”
“The fact that we have a quantitative agreement, indicates that there are no systematic errors and that the derived time is correct, or in other words, we have proof that the derived time is accurate,” the researchers continued. “Such a quantum watch offers a unique opportunity to have an absolute timestamp without the necessity to measure time zero.”
To that end, the researchers suggest many tweaks to their technique that could make it widely applicable for a host of experiments that use different atomic elements or photon energies. This future research could provide new glimpses of the bizarre quantum domain.
“If you’re using a counter, you have to define zero. You start counting at some point,” Berholts told 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.'”
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