For more than a century, physicists have relied on two separate rulebooks to describe nature. Quantum mechanics excels at explaining atoms, photons, and every jiggle inside laboratories. Einstein’s general relativity, meanwhile, accounts for the way planets and stars bend the very fabric of space-time.
Each framework is staggeringly accurate inside its own arena, yet they cannot be combined in a single equation.
Closing that rift has been a dream project for generations. Now a fresh proposal points to an unexpectedly practical route: link a trio of ultrafine atomic clocks with the same quantum technology that will one day support a globe-spanning quantum internet, then scatter those clocks along the side of a mountain.
If everything works, the experiment will watch quantum superpositions ride Earth’s curved space-time and reveal whether the two great theories can truly coexist.
Quantum clocks on the hillside
The idea comes from Igor Pikovski at Stevens Institute of Technology, Jacob Covey at the University of Illinois Urbana-Champaign, and Johannes Borregaard at Harvard University.
Their paper, “Probing Curved Spacetime with a Distributed Atomic Processor Clock,” just published in PRX Quantum, lays out a detailed plan to marry quantum networking with precision metrology.
“The interplay between quantum theory and gravity is one of the most challenging problems in physics today, but also fascinating,” says Pikovski. “Quantum networks will help us test this interplay for the first time in actual experiments.”
The team’s blueprint treats a mountain slope as a ready-made laboratory, using natural height differences to amplify the tiny time shifts predicted by relativity.
Atomic clocks as quantum probes
Atomic clocks already underpin GPS satellites and global timekeeping because they lose only a second every billion years. Each tick comes from the regular light absorbed and emitted by atoms such as ytterbium-171.
In the new scheme, three such clocks sit at elevations separated by hundreds of feet – enough for gravity to make the topmost clock run a hair faster than the one at the base.
Because the clocks are quantum devices, they can be placed in superposition, meaning they effectively tick in several places at once.
Physicists have already confirmed gravitational time dilation with aircraft and satellites, but those tests rely on classical synchronization.
Using a truly quantum clock raises the bar by letting the same single tick sample multiple altitudes at once, squeezing uncertainty far below the part-per-quadrillion level.
Entanglement as the secret sauce
To keep the trio operating as one coherent device, the researchers rely on a highly resilient entangled W-state.
Only one of the three nodes carries the active “excitation,” yet all share responsibility for it. If one station suffers a hiccup, the overall state survives, a feature prized in quantum communication.
Entanglement also enables quantum teleportation of timing information, letting the scientists recombine the separated pieces without physically hauling hardware up and down the slope.
Network links often rely on photons coursing through buried fiber or free-space laser connections. Either route can ferry entanglement over miles, as recent field trials between Chicago and Boston have shown.
The mountainside layout taps that same infrastructure, so every tweak made for the gravity test feeds directly back into the playbook for tomorrow’s secure data highways.
Following quantum clock beats
As the entangled clock ticks, each node accumulates a slightly different phase because the local flow of time is not identical.
When those phases are brought back together through teleportation, the interference pattern should display three distinct beat notes.
Their spacing encodes the altitude differences measured in feet as well as the combined effect of quantum superposition and curved space-time.
If either theory – quantum mechanics or general relativity – fails to describe reality at this overlap, the rhythm will stray from expectations.
Counting those beat notes requires detectors that resolve differences smaller than a billionth of a second. Modern frequency combs let researchers compare optical clock signals with that level of finesse, turning what once seemed a sci-fi feat into standard lab practice.
Finding terra firma
Beyond a proof of principle, the test would put long-standing speculations on firmer footing.
“We assume that quantum theory holds everywhere – but we really don’t know if this is true,” says Pikovski.
“It might be that gravity changes how quantum mechanics works. In fact, some theories suggest such modifications, and quantum technology will be able to test that.”
A measurable deviation might hint at new physics, while a perfect match would still tighten the bounds on any would-be quantum-gravity adjustment by orders of magnitude.
Quantum internet with benefits
Tools developed for the curved-space-time experiment mirror the hardware racing toward a secure quantum internet.
Entangled Bell pairs, teleportation channels, and error-tolerant W-states are exactly what future data links will need to move qubits between city-scale processors.
By turning those same tricks to fundamental research, the community gains a double dividend: technology gets a demanding field test, and basic science gains reach far beyond a conventional lab bench.
Organizations planning intercity quantum links already mount repeaters on towers and rooftops, where elevation shifts come for free.
Embedding fundamental tests into those rollouts might transform mundane network maintenance into a new branch of precision geodesy.
Quantum clocks and future tech
Building the network will still be an engineering lift. Optical fibers must carry entanglement hundreds of feet with minimal loss, while laser systems keep the ytterbium atoms chilled to microkelvin temperatures.
Yet none of those tasks lie outside today’s state of the art.
If the experiment runs and the beats line up with theory, physicists will have brought two rival descriptions of the universe a little closer together.
And if the beats slip, an even bigger adventure will begin – one that may finally show how quantum science and gravity influence each other.
Either result – agreement or surprise – will refine blueprints for space-based missions aiming to stretch entangled clocks between satellites.
Those projects could bring the same test into stronger gravitational gradients near massive bodies, extending the quest well beyond any earthly mountain ridge.
The full study was published in the journal Physical Review.
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