Future Directions: Atomic Clocks Meet Quantum Entanglement

Quantum theory — developed in the early 1900s — describes the universe at its smallest and most fundamental scales. Atomic clocks were one of the first technologies to put quantum’s counterintuitive predictions to practical use. A new generation of quantum technologies is now giving scientists unprecedented capabilities and opening possibilities that previously seemed like science fiction. Unsurprisingly, atomic clocks are playing a starring role.

Quantum networking

One of quantum theory’s most profound and startling predictions is entanglement: the idea that multiple objects separated in space can be intimately connected through a shared quantum state. Recently, entanglement has evolved from a scientific curiosity to a foundation of practical technologies, including nascent quantum computers and quantum sensors.

Some scientists believe another technology that entanglement could enable is the quantum network. Such networks would distribute quantum entanglement via optical fibers over a metropolitan area or even between continents, using satellites. Quantum networks could enable new forms of secure communication, distributed quantum computing and sensing, and new ways to study astronomical signals and gravitational waves. Rudimentary quantum networks are being built in several metro areas, including Washington, D.C., Chicago and Delft, Netherlands.

Quantum networks require ultraprecise timing. To establish entanglement between distant nodes, identical particles of light known as photons sent from the nodes must arrive at a central location at exactly the same time. Photons move at light speed, so a delay of even a billionth of a second can ruin the possibility of entanglement.

That’s why quantum network builders are turning to atomic clocks. For example, in the Washington, D.C.-area quantum network known as DC-QNet, which includes NIST, NASA and several defense labs, scientists plan to use atomic clocks to reduce the effects of noise in the optical fibers that make up the network and ensure that photons arrive at their destinations at just the right time.

Entangled clocks

As much as quantum entanglement may benefit from atomic clocks, atomic clocks could benefit just as much from entanglement. The precision of ordinary atomic clocks is limited by quantum physics, which places strict constraints on how precisely a quantity such as the ticking rate of a clock can be measured. This fundamental uncertainty is known as the “standard quantum limit.”

Entanglement offers a possible way forward. When particles such as atoms are entangled with each other, what happens to one is “felt” by the whole group. This allows scientists to reduce the noise in their measurements and surpass the standard quantum limit — something a number of research groups have accomplished in recent years. So far none of these entangled clocks has beat the best conventional atomic clock, but it may be just a matter of time before one of them does.

Taking this idea to the next level, JILA’s Jun Ye has envisioned a global network of entangled space clocks. Such a network could provide a time standard far more accurate than present-day GPS and a way to do geodesy and underground sensing with unrivaled accuracy. While such a network is still years away and must overcome numerous technical challenges, ambitious visions such as these may guide the future of timekeeping.

Read more about how NIST and JILA scientists are using entanglement to beat the standard quantum limit.

Putting Einstein to the ultimate test

As clocks get more and more precise, they will start to bump into a regime where the predictions of relativity and quantum collide.

Einstein’s relativity, for all its success, is incompatible with quantum mechanics, the theory that describes the universe at its smallest and most fundamental scales. That’s because in relativity, the universe is continuous, whereas in quantum theory, it’s quantized — organized in discrete, indivisible chunks.

Einstein spent much of the latter part of his career trying unsuccessfully to reconcile the two theories, and no one else has succeeded either. Many physicists believe that quantum theory provides the ultimate description of reality, but to prove that, someone will need to find a crack in relativity.

The next generation of atomic clocks could start to plunge into this realm. If they become precise enough, they will be able to measure gravity’s effects on the ticking rate at a length scale comparable to the size of an atom's quantum wave function. (According to quantum theory, objects such as atoms are both particles and waves; the wave function is the mathematical description of that wave.)

Nobody knows what such an experiment will find — but there is no doubt it would be, as JILA’s Jun Ye puts it, “very interesting.”

Einstein, Time, and Very Small Things - with Jun Ye

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