Optical Clocks Could Reveal the Quantum Nature of Time

Although life contains countless variables, there is one measure that strictly defines our existence: time.

We tend to imagine it as fixed, seamless and one-way – the arrow of time travels directly forwards, and we simply follow its course.

Yet what if time is rather less rigid than our everyday experience implies? What if it possesses a concealed quantum character?

Optical clocks and the quantum nature of time

A new paper by a group of physicists outlines how optical clocks – an exceptionally accurate form of atomic clock that relies on optical light frequencies rather than microwave signals – could reveal time’s quantum nature. This, in turn, may help explain the wider and still-enigmatic nature of time.

"It turns out, there are deeper facets of time that no one has ever experienced, and that have never been measured either," physicist Igor Pikovski of Stevens Institute of Technology in the US told ScienceAlert.

"According to quantum theory, there can be instances where time does not simply change steadily at one rate. Instead, there are 'many times in superposition', i.e., it passes at different rates at the same time.

"This means in practice that a single clock would record several different times, not just a single one as we are usually used to. This has never been observed before, but we show that this is something that modern ion-clocks could now detect."

For hundreds of years, time was regarded as absolute, following Sir Isaac Newton’s definition. He portrayed it as a universal constant: an independent component of objective reality that outside forces could not alter.

Albert Einstein then put the disruptive force of relativity into Newtonian physics. His new theoretical frameworks demonstrated that time is relative, capable of passing more quickly or slowly according to motion and gravity.

"There is no universal time, and only what we call 'proper time': Each observer records their own time, and it can differ," Pikovski explained.

"This is what we work with, namely that the flow of time changes with velocity and position. The 'twin paradox' is a typical example of relativistic time according to Einstein, where a twin takes a round-trip in a rocket, and when he comes back he is younger than his other twin who aged more staying on Earth."

Time dilation is a consequence of relativity and is consequently already well understood.

Where relativity meets quantum theory

What has yet to be tested experimentally is time’s possible behaviour in the quantum regime: at scales where relativity on its own can no longer account for the Universe’s behaviour and quantum theory becomes relevant.

Even within quantum theory, though, time is generally still handled as a classical process, steadily ticking away in a linear background.

"One of the most important challenges of modern physics is to find a quantum theory of gravity," Pikovski explained.

"In such a theory, we expect many of the otherwise classical concepts like time and gravity to be described by something fundamentally quantum. So we know that time as we describe it today cannot be the final story – something is missing when quantum theory comes into the picture."

In their study, Pikovski and his colleagues set out methods by which ultra-precise optical clocks, which tick with the oscillations of laser-excited atoms, might investigate quantum temporal phenomena.

These phenomena include temporal superposition, in which overlapping times may coexist, and entanglement, through which time and motion can be connected and affect one another’s behaviour.

"Entanglement and superposition are hallmarks of quantum behavior," Pikovski told ScienceAlert.

"Our work shows that even time itself could have such quantum hallmarks, which is not what is typically assumed in quantum physics."

In practical terms, one clock might record multiple times simultaneously, divided by unimaginably tiny differences – intervals of roughly tens of attoseconds, measurable only because an optical atomic clock is sufficiently precise.

Atomic clocks can already detect minute relativistic effects, including time dilation. For instance, raising one clock by only a few centimetres above another changes the Earth’s gravitational pull by enough to produce a dilation effect that the clocks can register.

Pikovski and his colleagues’ work suggests that optical clocks could also achieve the precision needed to observe quantum effects.

Squeezing quantum effects into view

The researchers suggest applying a quantum method called "squeezing", which amplifies very small fluctuations within a system. Here, it could strengthen the quantum behaviour of atoms inside a clock, allowing time’s unusual effects to become more apparent.

Certain effects may be observable using existing technology, whereas others remain too delicate and too slight. Those that can be reached, however, are worth investigating.

The methods put forward by the team could provide the first experimental proof that time itself is capable of quantum-mechanical behaviour.

This would offer physicists another route to explore the meeting point between relativity and the quantum realm, while also providing fresh insight into time’s fundamental nature.

"I think it can give us hints, and experimental input, on how our everyday notions of reality are misleading. Quantum theory is not just bizarre, it also implies a very different fundamental structure of the universe that is at odds with everyday experience," Pikovski said.

"Einstein famously remarked: 'Is the Moon there when nobody looks?' He made this remark to highlight the bizarre predictions of quantum mechanics.

"If time itself inherits these quantum features, i.e., time can be in superposition when nobody looks, this to me would be a fascinating glimpse into the strange inner workings of nature, and give hints towards new frontiers of fundamental physics."

The paper was published in Physical Review Letters.