Tag Archives: atomic clocks

How Atomic Clocks Don't Work

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I was listening to a podcast recently that delved into timekeeping and atomic clocks, and was surprised that they got a couple of details wrong. I haven’t done a post explaining how atomic clocks work, because that’s something easily found on the intertubes, and so I’m not particularly motivated to recreate Wikipedia or HowStuffWorks.

But someone was wrong on the internet, and the basis of that “wrongness” has some physics behind it. The claim was made in explaining clocks that when electrons absorb energy they jump up a level, and then radiate it when they jump back down. And while that’s true, it’s not the basis for a Cesium or Rubidium clock. The thing is that you don’t want the atom to radiate on its own if you are going to make a clock out of it. Transitions between atomic states are not infinitely narrow, i.e. there is an uncertainty in the energy of the emitted photon. This is known as the linewidth of the transition, and for a good clock you want a really narrow transition so that you know what the frequency is. While there are several factors that can increase that linewidth, the fundamental width is due to the Heisenberg Uncertainty relation between energy (or frequency) and time.

The uncertainty of the frequency and the lifetime of the transition are inversely related, and \(Delta omega Delta t = 1 \) (that should be greater than or equals, but latex is choking on that for some reason)

In order to get a narrow transition, you want a long-lived state. So you don’t want something that radiates readily on its own, and atomic clocks don’t. Cesium and Rubidium devices are passive: you shine radiation on them, and then read out whether or not your radiation was on resonance by looking at which state the atoms are in. Active masers do radiate, but as the acronym tells us, the radiation is stimulated, rather than being spontaneous. (Left on its own, the lifetime of the Hydrogen atom state is about 10 million years) The search for long-lived states becomes even more important for optical clocks, since the larger energy differences tend to lead to shorter lifetimes. What is generally done is to search for so-called forbidden transitions, in which the strong coupling of electric dipole transitions aren’t present, and you are left with other types of transitions or ones that must couple through other states and end up taking much longer.

This May Not Bother Anybody Else

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… but it bothers a geek like me. Yeah, another leap second story. I suspect these will propagate, but like a game of “whisper” the errors will compound. Three…Two…One…One…Happy New Year!

They mention THE atomic clock (ha! there are many atomic clocks) and cesium clock/standard, but the picture is of a mercury ion clock. Not that anyone else would notice. Then there’s the mortal sin of the US Naval Observatory hyperlink going to the NIST cesium fountain wikipedia entry. (Don’t get me wrong — the folks at NIST do fantastic things and I have a lot of respect for them. And they’re fun at conferences. But get your links straight)

The blurb about miniature clocks goes to a link talking about optical clocks, which are nowhere near deployment as miniature devices. That’s purely conceptual at this point — full-sized optical lattice devices are cutting-edge at the moment, and require a fair amount of care and feeding. Miniaturization and making them robust enough to be portable, and work as true clocks as opposed to a frequency standard (a true clock runs continuously), is a long way off.

That's Dr. Time to You, Pal!

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Meet the world’s director of time

An interview with Dennis McCarthy, who is the Director of the Directorate of Time (or was at one point; I’m not sure how his retirement and subsequent resurrection affected the job title)

Though the BBC filmed in the lab, none of that footage made it into the embedded clip. Perhaps there’s footage in the show that’s airing on BBC 2, as I type this. I’ll have to check it out.

More than anyone, Dr McCarthy appreciates the need for the world’s population to be synchronised. But for those who don’t spend their working day checking atomic clocks, why is knowing the time so important? Think for a moment about how the GPS satellite navigation system works.

There is a network of over 30 satellites orbiting earth that broadcast a high-precision time-stamp down to the GPS system in your car.

These signals travel at the speed of light, which is very nearly one foot every thousand-millionth of a second – or one nanosecond (for the more metrically minded, that’s around 30cm, which is far less elegant. If there is a God, he built the universe using imperial measurements).

If that last part is true, God has a hell of a sense of humor.

The was one part of the embedded video that made me cringe, and that was the depiction of the Bohr-ish atom (with wavy orbit lines — is that supposed to make it all better?) and the electron making a transition between them. But in that representation, those are the levels described by the principle quantum number, and the transition of microwave clocks is in the spin state of the electrons, oscillating between spin-up and spin-down (whose energy degeneracy is broken because of interactions with the nucleus, which also has spin, and thus a magnetic moment) And the notion that you’re looking at radiation emitted by the atom is true in an active maser but not a passive standard like a cesium or rubidium clock — in those you make a separate measurement of the atom to tell you what state the electron is in.

(I don’t know if it’s a permanent link, but in the “In Today’s Magazine” column there’s Call him Mr Time . Hence the title, though I can’t actually envision Dennis saying that to anyone)

Hear Here

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Ran across this while Googling for something else. NPR interviewed some of my colleagues about the master clock a while back. This aired in January 2007.

The Atomic Secrets of Accurate Time Keeping

I keep forgetting to use the bell ringing analogy when I explain how clocks work. (oh, and when the interviewer says “and his colleagues” near the end, she’s referring, in part, to me. Better than lackey or minion, I suppose.)