Right now, about 200 cesium clocks around the world synchronize their time by bouncing microwave radiation off of global positioning system (GPS) satellites. UTC receives monthly readings from the clocks, averages the results, and determines what time it was when the process began. The report is sent back to the clocks, which are adjusted to come into sync with the average. No clock — no matter how fast — can communicate with another clock faster than the speed of light, and all these calculations and adjustments take time. The result is that no one knows what time it "really" is right now.
That may change, thanks to an innovative idea from a research group at Harvard University led by Mikhail Lukin. In the June 15, 2014 issue of Nature Physics, Peter Kómár, Lukin and other members of the group propose a global network of clocks with atoms in their cores linked to each other by quantum entanglement. Such a network would provide faster-than-light coordination and a global standard time.
Fully standardized time could benefit financial markets, physicists and geologists, or even reconnaissance drones. But serious logistical hurdles stand between entangled clocks and a takeover of the world's timekeeping. Still, the idea is intriguing because it combines methods already developed in two fields: atomic clocks and quantum entanglement.
The Tick, Tock of an Atomic Clock
Since their invention in 1949, atomic clocks have been the most accurate clocks on Earth. Yet they operate on the same principle as a pendulum clock: reliance on a steady oscillator. In a pendulum clock, the oscillator is the swinging pendulum. Counting the back-and-forth swings will give you a regular measure of the passage of time. The clock will be as accurate as the pendulum is steady.Far more regular than a swinging pendulum is the frequency of microwave radiation emitted from a cesium atom. At the heart of an atomic clock, a microwave laser tuned to the correct frequency excites the electrons in a cesium atom. Each time the cesium atom absorbs a laser photon, one of its electrons jumps to a higher energy level and then falls back down, re-emitting a photon at a predictable frequency. For Cesium-133, that frequency is 9,192,631,770 cycles per second.
The atomic clock at the National Institute of Standards and Technology in Boulder, Colorado, is so precise that it will not gain or lose a second in 300 million years. And yet, however precise that clock may be, it is always essentially measuring local time, because coordinating with other clocks is imprecise and, well, takes time.
"Spooky Action at a Distance"
Hypothetically, though, that obstacle might be overcome by applying quantum mechanics. Indeed, Lukin's team hopes to use a concept called quantum entanglement to link atomic clocks around the world in a network that communicates faster than the speed of light.Albert Einstein famously called entanglement a theory of "spooky action at a distance." He wasn't far off. At the quantum level, atoms are both waves and particles. Each atom exists in a "superposition of states" until it is measured. A superposition of states dictates that an unmeasured electron exists in many places at once, has a range of momentums, and spins in multiple directions. The probability that, when measured, a particle will be found in any one state is represented by its wave function, mathematically described by the Schrödinger equation. When one aspect of the particle is measured — to find out where it is, for example, or how fast it's moving — the wave function "collapses." The electron now exists in a particular place, and has a particular momentum and a particular spin. In other words, the measured electron loses its wave properties and acts solely as a particle at the moment of measurement. In the next moment, however, the electron's wave properties return.
Under certain circumstances, two particles may become entangled, which means their wave functions combine into one function that cannot be differentiated. Even after the particles are separated in space, this connection is maintained, so that measuring one particle will tell us something about the other particle.
For example, suppose a neutral particle called a pion decays into an electron and a positron. If we assume that the pion was initially at rest, then the electron and positron will fly off in opposite directions. A pion's spin is zero, so in order to conserve angular momentum, one product will be "spin up" and the other will be "spin down." So, if we measure the electron and find it spin up, the positron instantly becomes spin down. Yet before the measurement was taken, the electron and positron were both "spin up" and "spin down" at the same time, with their individual spin being a function of the two particles' relationship. In other words, the particles were entangled.
Recently, physicists have been getting better at creating entanglements experimentally. In 2007, a Viennese research team led by Anton Zeilinger transported the polarization state of one photon to another photon 143 kilometers (88 miles) away. And in 2011, Thomas Monz and his team at the University of Innsbruck entangled 14 calcium atoms by forcing them together with lasers.
Entangling Clocks, Creating Networks
Lukin's group hopes to use quantum entanglement to create a network of atomic clocks. At least one atom in the core of each clock would be entangled with atoms in other networked clocks. But because entanglement is often a fragile connection, redundancies would likely have to be built into the network.In Lukin's proposed mechanism, one clock acts as the "central clock," sending out one photon from an entangled pair to each of the other node clocks via a process called quantum teleportation. Next, the now-entangled photon from the central clock expands the entangled state to other atoms in the node clock. The node clock measures the phase difference between the entangled atoms and its own local timekeeping atoms, which reveals how fast or slow the clock has been running. It then adjusts its own frequency and reports back the results to the central clock. The central clock can average the results from all the clocks and repeat the process to bring the clocks into phase with each other.
What Lukin's group proposes is roughly analogous to a network of pendulum clocks wherein a central clock would coordinate the pendulums for all the other clocks so that no matter where they were, they would swing back and forth in unison. If one pendulum started to slow down slightly, the central clock would give it a little push—just so—and it would come back into phase.
A Global Timekeeping System
An accurate clock network, especially if it could be extended to satellites, would have widespread benefits. For example, time moves slower near massive objects; GPS satellite clocks typically gain about 45 microseconds per day on Earth-bound clocks, since the pull of Earth's gravity is weaker at high altitude. And because an entangled clock network would be exquisitely sensitive to these differences in mass, it could help geologists and physicists with their research. Satellites with networked clocks could detect magma moving underground before volcanic eruptions or detect other subterranean features. The network could also detect small shifts in space and time, helping locate gravitational waves. More fancifully, the military could fly linked clocks over enemy territory to detect secret underground tunnels.The possibilities seem both endless and a very long way off. Entangling 14 atoms in a lab, or two photons at a great distance, is impressive. But Lukin and his team would need to maintain the delicate entanglement across thousands of atoms around the globe — a far more stable connection than Monz or Zeilinger and their teams could manage. The engineering challenges of transporting entangled photons into Earth's orbit are daunting, to say the least.
Yet, in principle, the clock network should work: it uses no new technology, no new methods. It is an idea whose time has come.
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