By the mid-twentieth century, quantum mechanics was the most contentious debate in physics, with its revolutionary descriptions of the interaction of particles in terms of probability, uncertainty, and discrete values. Albert Einstein infamously disagreed with one of its defining principles that reasoned the shorter the duration of a measurement on a particle, the less precisely the particle’s frequency can be known. This tradeoff, known as the Heisenberg uncertainty principle, turned out to be correct — and it had a major implication: it placed a fundamental restriction on how precisely time could be tracked.
Every clock in human history has operated conceptually the same: it references some naturally occurring frequency as the basis for its “tick,” which defines the passage of a unit of time. Clocks have historically been far too imprecise for the Heisenberg limit to affect uncertainty in their frequencies — until December of 2020 when a group of MIT physicists announced they had broken past a major hurdle in the pursuit of ultra-precise timekeeping, putting the Heisenberg limit in sight.
Humans have practiced timekeeping since the dawn of humanity. From following the periodic movement of the stars and sun to mechanical clocks based on the swing of a pendulum to modern electrical crystal clocks that use the natural vibrational frequency of quartz, humans have constantly battled the universe’s inherent uncertainty and chaos to increase the precision of timekeeping. By the time Einstein was alive and debating quantum mechanics, humans had whittled down uncertainty in timekeeping to the drift of one second every other day. Yet this is not precise enough for many modern applications, including space missions, GPS technology, the keeping of the international time standard, and cutting-edge physics experiments. This precision came with the atomic clock.
From following the periodic movement of the stars and sun to mechanical clocks based on the swing of a pendulum to modern electrical crystal clocks that use the natural vibrational frequency of quartz, humans have constantly battled the universe’s inherent uncertainty and chaos to increase the precision of timekeeping.
When atoms transition from an excited higher-energy state to a lower energy state, they emit a particle of light with a frequency tied solely to fundamental universal constants. For this reason, physicists refer to this atomic light frequency as the “ideal standard” for timekeeping. The motivating goal of atomic clocks is to utilize this frequency as its standard. They achieve this by probing a group of atoms using a laser with a frequency close to that of the atom’s light frequency. The atoms react by releasing this light, which the clock then references to guess the frequency again with the laser, repeating the cycle. The atomic frequency drives the tick of the clock.
However, when probing a group of atoms, uncertainty soars above the theoretical Heisenberg limit as all the atoms act independently of each other. Known as the “standard quantum limit,” the MIT physicists have overcome this final hurdle by successfully implementing a proposed technique that squeezes the atomic states. When performed on the entire group, the atoms become dependent on one another and entangled, collapsing uncertainty down to the fundamental Heisenberg limit.
The group reported uncertainty measurements only 1.9 times greater than the Heisenberg limit, a figure signifying an unprecedented precision in the tracking of time. As measurement sophistication continues to increase and non-fundamental uncertainties in measurement methods no longer dominate, the precision of time measurements asymptotically approaches the theoretical Heisenberg limit as these clocks are left running for longer periods.
The applications for this new technology are far-reaching. This level of precision is encouraging for some of physics’ most notorious modern-day challenges, including the search for gravitational waves at large observatories like MIT and Caltech’s Laser Interferometer Gravitational-Wave Observatory, or LIGO. Northeastern physics professor and former LIGO researcher Dr. Alessandra Di Credico explains, atomic clocks allow gravitational-wave scientists to “combine the arrival times of different signals coming from independent interferometers placed on the Earth’s surface” creating the capability to triangulate these readings and pinpoint their origin. She adds, researchers can then “compare the arrival time of such a signal with other experiments such as space-based telescopes, detectors of EM radiation, or Earth-based neutrino detectors.”
Through comparing atomic clock measurements, LIGO and Virgo succeeded in triangulating the location of the event in the sky and confirming the observation with dozens of Earth and space telescopes that witnessed the merger.
This technique was first demonstrated in 2017 by two independent gravitational wave groups, LIGO in the United States and Virgo in Italy, with the detection of waves from two stars death-spiraling into an energetic collision. Through comparing atomic clock measurements, they succeeded in triangulating the location of the event in the sky and confirming the observation with dozens of Earth and space telescopes that witnessed the merger. Similar applications for ultra-precise atomic clocks exist throughout physics — from “dark matter” to fundamental particle physics research — bestowing physicists with an arsenal of new tools to probe the mysteries of the cosmos further than ever before.
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