Atomic Clocks Can Now Tick Faster and Slower at the Same Time

At the National Institute of Standards and Technology in Colorado, a single aluminum ion hangs suspended in a trap, cooled to fractions of a degree above absolute zero. It is, in practical terms, as close to nothing as physics permits. And yet it is doing something. The ion is vibrating, jittering through space with the restlessness that quantum mechanics demands of everything, everywhere, always. Those vibrations are tiny, almost unimaginably so. But they are enough. According to a paper published today in Physical Review Letters, they are enough to make time itself go quantum.

Einstein told us, more than a century ago, that time is not a fixed backdrop against which events unfold. It bends. It stretches. A clock moving quickly ticks more slowly than one at rest; a clock deeper in a gravitational field runs slow compared to one higher up. These effects are real and measurable: the GPS satellites orbiting overhead need relativistic corrections or their position calculations would drift by kilometres each day. So far, so counterintuitive but experimentally verified. What Igor Pikovski’s group at Stevens Institute of Technology is now proposing is something weirder still. When a clock’s motion obeys quantum mechanics rather than classical physics, the clock can, in principle, experience two different passages of time simultaneously. The flow of time itself enters superposition.

This is the quantum twin paradox. In the classical version, two identical twins age differently if one takes a high-speed trip while the other stays home. The quantum version asks: can a single clock become both the travelling twin and the stationary one at once?

According to quantum theory, the answer is yes. The problem, until now, has been getting an experiment to agree. The effects involved are extraordinarily small. A clock moving at 10 metres per second for roughly 57 million years would lag behind a stationary clock by just one second, and the quantum corrections on top of that are smaller still. For decades, the effects sat comfortably beyond what any instrument could resolve. But atomic clocks have improved at a pace that continues to embarrass earlier estimates of their limits. The aluminum-ion clocks at NIST are now sensitive enough to detect the tiny frequency shifts caused by thermal vibrations at temperatures just above absolute zero. The new paper shows those same instruments, with some clever additions, may be sufficient to detect the quantum signatures of time dilation for the first time.

“Atomic clocks are now so sensitive, they can detect tiny differences in time caused by just the thermal vibrations at miniscule temperatures,” says Gabriel Sorci, a PhD candidate at Stevens and co-author of the paper. “But even at the absolute zero temperature, the ground state, the ticking rate will still be affected by just the quantum fluctuations alone.”

Squeezing the Vacuum

That last point deserves a moment. At absolute zero, a classical object stops moving entirely. There is no thermal energy left to drive vibrations. But a quantum object cannot be completely still, because pinning down both its position and momentum with perfect precision would violate Heisenberg’s uncertainty principle. The ion still jitters. And because relativistic time dilation depends on velocity, even those vacuum fluctuations shift the clock’s ticking rate. This “vacuum-induced second-order Doppler shift,” as the paper terms it, is a measurable consequence of the quantum vacuum acting on time itself, a kind of ghost vibration that has no classical equivalent. The trouble is, it looks, at least in terms of frequency shift, like something a semiclassical model could reproduce. To get at something genuinely quantum, something that cannot be explained without quantizing time itself, the team had to go further.

The crucial step involves a technique called squeezing, borrowed from quantum information. Rather than simply cooling the ion, the researchers propose manipulating its quantum state in a way that compresses uncertainty in one direction while stretching it in another. A squeezed motional state is one where the position and momentum of the ion are not symmetrically uncertain but lopsided in a precise, engineered way. When such a state is prepared, the paper shows, the coupling between the ion’s internal clock and its external motion generates entanglement: the clock states and the motional states become correlated in a way that has no classical analogue. That entanglement shows up as a reduction in the interferometric visibility of the clock, a slight blurring of its signal that cannot be explained by any model that treats proper time as a fixed classical parameter.

“Time plays very different roles in quantum theory and in relativity,” says Pikovski. “What we show is that bringing these two concepts together can reveal hidden quantum signatures of time-flow that can no longer be described by classical physics.”

A Route to the Lab

The good news, if you’ve been wondering whether this will remain a thought experiment, is that the technology required seems within reach. Squeezed states of the required kind have already been generated in ion traps. The coherence times needed, the period over which the clock’s quantum state must remain undisturbed for the signal to accumulate, are demanding but not fantastical given recent progress. Christian Sanner at Colorado State University, one of the experimental collaborators on the paper, is direct about it: “We have the technology to generate the required squeezing and a path to reach the clock precision needed in ion clocks to observe such effects for the first time.”

What It Would Mean

Assuming the experiment works, what would it actually tell us? The observation that time dilation can generate quantum entanglement would be, arguably, the first direct test of what physicists sometimes call the “interface” between general relativity and quantum mechanics, the zone where both theories are supposed to be relevant simultaneously, and where they stubbornly refuse to fit together. Atomic clocks have tested relativistic time dilation before, with great precision. They have tested quantum mechanics, with even greater precision. They have never before been used to probe a regime where both are genuinely necessary to explain what is happening. The entanglement-induced visibility drop that Sorci and Pikovski compute is precisely that: a signal that requires both frameworks, and that classical physics, or even semiclassical approximations, simply cannot account for.

“Physics is still full of mysteries at the most fundamental level,” says Pikovski, whose recent work also includes proposals for detecting individual gravitons using quantum sensors. “Quantum technologies are now giving us new tools to shed light on them.” That might be the understatement of the year. An ion that ticks both faster and slower at the same time is not merely an odd corner case of quantum foundations. It is time itself, behaving the way quantum mechanics says everything must behave, and only now, after more than a century of atomic physics, do we have the tools to catch it in the act.

DOI: https://doi.org/10.1103/qhj9-pc2b

Frequently Asked Questions

What does it actually mean for time to be in quantum superposition?

In ordinary physics, a clock always records a single, definite amount of elapsed time. Quantum mechanics allows physical systems to exist in superpositions of different states simultaneously, and when a clock’s motion obeys quantum rather than classical rules, the time it registers can in principle exist in superposition too: the clock is effectively both the twin who stayed home and the one who took the relativistic trip. The new research shows this is not just a theoretical curiosity but something measurable in today’s ion trap laboratories.

Why do atomic clocks need to be cooled to near absolute zero for this experiment?

At higher temperatures, thermal vibrations swamp the tiny quantum effects the researchers are looking for. Even at absolute zero, though, quantum mechanics forbids the ion from being completely still: vacuum fluctuations persist, and these turn out to shift the clock’s ticking rate through a mechanism with no classical equivalent. Getting close to absolute zero strips away the thermal noise and lets the purely quantum contributions become visible.

Could this research help unify quantum mechanics and general relativity?

That is part of the appeal. Physicists have long known that quantum mechanics and general relativity give contradictory pictures of the universe at their extremes, but experiments probing the overlap are rare. Observing time-dilation-induced entanglement would be the first direct measurement of a regime where both theories are simultaneously necessary, rather than one or the other being a good-enough approximation. It would not solve the unification problem on its own, but it would give theorists real data from a part of physics that has so far been inaccessible.

Is the squeezing technique something new?

Squeezed states in ion traps have already been demonstrated experimentally, so the researchers are not asking for untested technology. What is new is using squeezed motional states specifically to amplify the quantum signature of relativistic time dilation and generate the clock-motion entanglement that would serve as direct evidence. The paper effectively shows that tools already built for quantum computing and quantum metrology can be repurposed to probe something far more fundamental.