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    The Best Clock In the Universe Is Made of Pure Light

    Written by

    Michael Byrne


    What is a second? You could do the whole “one one thousand” thing and not be terribly far off, and that might be OK for calculating things like how fast you’re driving on the highway or how much you should get paid for a certain amount of time worked, but having time down to the accuracy of many, many zeros is important for a lot of other things, like doing measurements at the quantum level and using GPS to find very precise locations. And, just generally speaking, as humans in 21st century, we like to know as much as possible about everything, and that means having a whole lot of zeros to the right of the decimal point.

    So we come up with clocks that base their measurements off of the most fundamental thing: light. Time flexible after all — or, better, stretchable. This is because the speed of light in a vacuum is not flexible. That’s Einstein’s relativity, or a quick sniff of it anyhow. If you go really, really fast you will be able to slow down time for yourself because of this. If you’re racing a beam of light, then, from your perspective, the speed of that light will be the same at the starting line as it is in the middle of the race, if you’re not moving or moving very fast. “Your” time will be different, however, for everything to work out right. The point is that you don’t measure the speed of light based on time, you measure time based on the speed of light. OK? Good.

    So, a group of scientists led by Dr. Stefan Droste from the Max Planck Institute of Quantum Optics and the Federal Institute of Physical and Technical Affairs in South and North Germany has been working on a new super-clock that, uh, does that. It uses optical wavelengths of light. We have clocks that use microwave frequencies to tell time (microwaves are the same stuff, photons, as optical light waves; light is a broad spectrum that includes some stuff we don’t normally think of as light), and we have clocks that use light — OK, electromagnetic radiation, correctly — in the ultraviolet spectrum.

    We’ve been working on clocks on that use electromagnetic radiation in the optical frequency range — between ultraviolet and microwave — for a while, but it’s only with Dr. Droste and company’s work that we’ve been able to transmit that signal undistorted. It might seem like an easy thing, like just sending a number. But think about what that number is and how you’re sending it. The number is a very fine measurement of extraordinarily stable electromagnetic waves, and you’re sending it via more electromagnetic waves along fiber optic cables very long distances. Disruptions that wouldn’t make a difference in most any other thing being transmitted can make a time signal this precise worthless.

    Over at Motherboard’s big sister Vice, Ben Majoy has a piece on the new work and talked with Dr. Droste.

    “We do not question the accuracy of time,” Dr. Droste says. “We know that it’s only accurate to a certain extent. Time, as we know it, is based on cesium atomic clocks. These kinds of clocks have accuracies to the order of 1e-15. The confidence in the time that clocks show is increased by comparing hundreds of clocks around the world with one another, and this is important since with only one single clock, you cannot know whether your clock is showing the correct time or not.”

    He explained that modern celcium clocks have tiny oscillating quartz crystals built in that make for a “fairly accurate clock signal.” Conversely, atomic clocks have atoms that get excited and return a less excited, very stable frequency in the microwave region. Now the optical clock, which is the crux of this experiment, is basically the same as the atomic clock, but instead of microwaves, it generates optical frequencies that have about 50,000 to 100,000 times higher frequencies. Oh OK. Gotcha. So those are clocks.

    “The accuracy is a property of a clock and can therefore not be off,” Dr. Droste continues. “The clock itself can be off and the degree of how far off it is defines its accuracy. State-of-the-art optical clocks reach uncertainties of 1e-18. This is the relative accuracy, and from this you can calculate how many nanoseconds the clock will be off after a day, or how many seconds it will be off after a million years.”

    However, Dr. Droste also made it clear that optical clocks are currently in an experimental state and “cannot be transported.” The principle idea behind what they tested then, was the ability to get the stable frequencies out of the lab where the clock lives, sending them with accuracy over distance.

    Dr. Droste and his crew “won,” so to speak. They transferred a signal 10 times as accurate as needed 920 km using 10 signal amplifiers. And now you can all go back to pretending time is anything more than an illusion that has no real basis in physical laws outside of probability. But we’ll talk about that later.


    Reach this writer at michaelb@motherboard.tv.