A new study proposes using ultra-precise atomic clocks to test whether the flow of time can exist in a quantum superposition. The researchers hope to measure for the first time a single clock “ticks” at multiple rates simultaneously.
Time is one of the most basic concepts in everyday life, but also one of the most mysterious in physics. In Einstein's theory of relativity, time is not absolute. It depends on the speed of movement and the gravitational field. A clock moving at high speed or in a different gravitational field will measure time slightly differently than another clock. Quantum mechanics adds an even stranger layer to this: if an object can be in several states at the same time, it is possible that the time measured by a quantum clock could also be in several states at the same time.
A new study published in the journal Physical Review Letters offers an experimental way to test this possibility. The study, led by Igor Pikovsky, a professor of theoretical physics at Stevens Institute of Technology, and collaborating with experimental teams at Colorado State University and the National Institute of Standards and Technology (NIST), examined how atomic clocks, some of the most accurate timekeeping systems ever built, could also serve as a laboratory for testing the connection between relativity and quantum mechanics.
The main idea is simple to describe, but very difficult to test. If a clock itself is a quantum system, and its motion can be in a superposition, that is, in several states of motion at the same time, then the time it measures may also be in a superposition. In other words, the same clock may “experience” several different time rates at the same time. The researchers compare this to the well-known Schrödinger thought experiment, in which a cat can be seemingly alive and dead until a measurement is made. In the new case, it is not the cat that is in two states, but the flow of time of the clock itself.
When relativity meets quantum
The theory of relativity states that each clock measures time slightly differently depending on its speed and location. For example, a clock moving at 10 meters per second for 57 million years will be only one second behind a stationary clock. This difference is very small, but experiments with precise atomic clocks have already confirmed such phenomena. One famous expression of this is the “twin paradox,” in which a twin who sets off on a high-speed journey through space returns slightly younger than his twin who remains on Earth.
The quantum version of the question is even stranger: Can a single clock move in multiple quantum paths at the same time, and therefore measure multiple “lifetimes”? Previous theoretical work has suggested that this is possible, but until now the effects have been too weak to measure directly. Now, researchers say, advances in atomic clocks and trapped-ion technologies could make the idea experimentally testable.
Atomic clocks enter the quantum regime
The systems the researchers are talking about are based on single ions, such as aluminum or ytterbium, trapped in electric fields and cooled to temperatures very close to absolute zero. With the help of lasers, their quantum state can be controlled and the frequency of atomic transitions can be measured with great precision. These are the same principles used in both advanced atomic clocks and quantum computing based on trapped ions.
One of the researchers, Gabriele Sorci, explained that atomic clocks have become so sensitive that they can detect tiny time differences resulting from thermal fluctuations at extremely low temperatures. But even at absolute zero, when the system is in its ground state, quantum fluctuations still exist. According to the study, these fluctuations alone can affect the ticking rate of the clock.
The team also suggests using special quantum states called "squeezed states." In such states, the uncertainty in one physical quantity, such as position, is reduced at the cost of increasing the uncertainty in another quantity, such as momentum. Using such states could highlight the quantum signatures of the flow of time and allow them to be measured in the laboratory.
This does not mean that time will “disappear” or that the clock will show two different times to the human eye. These are tiny effects, at the level of individual quantum systems, that require extremely precise instrumentation. But if measured, they would provide rare evidence that the concept of time, as measured by a physical clock, is also affected by quantum rules.
Pikowski concludes that time plays very different roles in relativity and quantum mechanics. The new study shows that combining the two theories may reveal hidden quantum signatures of the flow of time that cannot be explained within the framework of classical physics. Beyond its philosophical significance, the proposed experiment demonstrates how quantum technologies developed for precise clocks and quantum computing are becoming fundamental research tools for the deepest questions of physics.
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