Researchers are hoping to improve high-precision clocks by entangling their atoms.

by Patrick L Barry and Dr Tony Phillips

Einstein called it "spooky action at a distance." Now researchers are using an astonishing property of quantum mechanics called "entanglement" to improve atomic clocks - humanity's most precise way to measure time. Entangled clocks could be as much as 1000 times more stable than their non-entangled counterparts.

This improvement would benefit pilots, farmers, hikers - in short, anyone who uses the Global Positioning System (GPS). Each of the 24+ GPS satellites carries four atomic clocks on board. By triangulating time signals broadcast from orbit, GPS receivers on the ground can pinpoint their own location on Earth.

NASA uses atomic clocks for spacecraft navigation. Geologists use them to monitor continental drift and the slowly changing spin of our planet. Physicists use them to check theories of gravity. An entangled atomic clock might keep time precisely enough to test the value of the Fine Structure Constant, one of the fundamental constants of physics.

"The ability to measure time with very high precision is an invaluable tool for scientific research and for technology," says Alex Kuzmich, a physicist at the Georgia Institute of Technology.

Through its office of Biological and Physical Research, NASA recently awarded a grant to Kuzmich and his colleagues to support their research. Kuzmich has studied quantum entanglement for the last 10 years and has recently turned to exploring how it can be applied to atomic clocks.

Einstein never liked entanglement. It seemed to run counter to a central tenet of his theory of relativity: nothing, not even information, can travel faster than the speed of light. In quantum mechanics, all the forces of nature are mediated by the exchange of particles such as photons, and these particles must obey this cosmic speed limit. So an action "here" can cause no effect "over there" any sooner than it would take light to travel there in a vacuum.

Image by Patrick L. Barry Making a measurement on one entangled particle affects the properties of the other instantaneously.

But two entangled particles can appear to influence one another instantaneously, whether they're in the same room or at opposite ends of the Universe. Pretty spooky indeed.

Quantum entanglement occurs when two or more particles interact in a way that causes their fates to become linked: It becomes impossible to consider (or mathematically describe) each particle's condition independently of the others'. Collectively they constitute a single quantum state.

Two entangled particles often must have opposite values for a property - for example, if one is spinning in "up" direction, the other must be spinning in the "down" direction. Suppose you measure one of the entangled particles and, by doing so, you nudge it "up." This causes the entangled partner to spin "down." Making the measurement "here" affected the other particle "over there" instantaneously, even if the other particle was a million miles away.

While physicists and philosophers grapple with the implications for the nature of causation and the structure of the Universe, some physicists are busy putting entanglement to work in applications such as "teleporting" atoms and producing uncrackable encryption.

Atomic clocks also stand to benefit. "Entangling the atoms in an atomic clock reduces the inherent uncertainties in the system," Kuzmich explains.

At the heart of every atomic clock lies a cloud of atoms, usually cesium or rubidium. The natural resonance's of these atoms serve the same purpose as the pendulum in a grandfather clock. Tick-tock-tick-tock. A laser beam piercing the cloud can count the oscillations and use them to keep time. This is how an atomic clock works.

"The best atomic clocks on Earth today are stable to about one part in 10¹⁵," notes Kuzmich. That means an observer would have to watch the clock for 10¹⁵ seconds or 30 million years to see it gain or lose a single second.

Click on the image to learn more. Lasers are a key ingredient of atomic clocks - both the ordinary and entangled variety.

The precision of an atomic clock depends on a few things, including the number of atoms being used. The more atoms, the better. In a normal atomic clock, the precision is proportional to the square-root of the number of atoms. So having, say, 4 times as many atoms would only double the precision. In an entangled atomic clock, however, the improvement is directly proportional to the number of atoms. Four times more atoms makes a 4-times better clock.

Using plenty of atoms, it might be possible to build a "maximally entangled clock stable to about one part in 10¹⁸," says Kuzmich. You would have to watch that clock for 10¹⁸ seconds or 30 billion years to catch it losing a single second.

Kuzmich plans to use the lasers already built-in to atomic clocks to create the entanglement.

"We will measure the phase of the laser light passing through the cloud of atoms," he explains. Measuring the phase "tweaks the laser beam," and if the frequency of the laser has been chosen properly, tweaking the beam causes the atoms to become entangled. Or, as one quantum physicist might say to another, "such a procedure amounts to a quantum non-demolition (QND) measurement on the atoms, and results in preparation of a Squeezed Spin State."

How soon an entangled clock could be built - much less launched into space aboard a hypothetical new generation of GPS satellites - is difficult to predict, cautions Kuzmich. The research is still at the stage of just demonstrating the principle. Building a working prototype is probably several years away.

But thanks to research such as this, having still-better atomic clocks available to benefit science and technology is only a matter of time.