下一代原子钟有多精确？| Nature Podcast

又到了每周一次的 Nature Podcast 时间了！欢迎收听本周由Benjamin Thompson和 Shamini Bundell带来的一周科学故事，本期播客片段讨论原子钟。欢迎前往iTunes或你喜欢的其他播客..下载完整版，随时随地收听一周科研新鲜事。 [本文来自：www.11jj.com]

[本文来自：www.11jj.com]

音频文本：

Interviewer: Benjamin Thompson

Listeners, if you were to ask my friends what my timekeeping is like, they would happily tell you that it is terrible. The old phrase, ‘He’d be late for his own funeral,’ pretty much sums up my ability to meet someone when I say I will. But apparently, I’m not the only one who needs to improve their timekeeping. Being able to accurately tell the time is really important for loads of technologies. Here’s Andrew Ludlow from the National Institute of Standards and Technology in the US with a few examples.

Interviewee: Andrew Ludlow

I think some of the more ubiquitous examples like navigational systems and communication systems often require very stringent timing requirements. You know, Global Navigation Satellite Systems basically exploit atomic clocks, on-board satellites and distributing those timing signals in order to deduce relative position. The ability to kind of keep time very well enables other types of technologies as well.

Interviewer: Benjamin Thompson

In this week’s Nature, Andrew and his colleagues describe a pair of atomic clocks that keep time with astonishing accuracy. They even suggest that these clocks could, in the future, not just be used to measure time, but to measure the strength of gravity on different parts of the Earth. But more on that in a bit. First, let’s talk about atomic clocks. Rather than relying on the swinging of a pendulum to count the passing of time, atomic clocks use something else.

Interviewee: Andrew Ludlow

Here, the time base really is the atom. Mostly it’s electronic oscillations in an atom, and it turns out that, you know, nature has given us some atoms in particular who give us extremely stable oscillations where they don’t change for many reasons. They’re quite fixed in time and therefore they can be very good at making an atomic clock.

Interviewer: Benjamin Thompson

Andrew’s clocks are optical lattice clocks, considered to be the next generation of atomic clocks and in this case use atoms of the element ytterbium. By using a finely-tuned laser, it’s possible to excite electrons within these ytterbium atoms into a different energy state. By measuring the oscillations in the electrons as they move between these states, it’s possible to measure the passage of time. And these transitions happen very, very quickly.

Interviewee: Andrew Ludlow

It’s at about 518 terahertz, and so that’s 5 x 10^-14 times per second, and I should say that that’s a big number. And that’s one of the reasons why it’s useful for a clock is each oscillation is dividing time up into extremely fine intervals, and so that gives us a lot of precision in trying to make measurements of time.

Interviewer: Benjamin Thompson

Andrew and his colleagues have been working on their clocks for a few years now, trying to hone the mechanisms and get them to work as accurately as possible. This is what they’ve detailed in their new paper.

Interviewee: Andrew Ludlow

We showed that we were able to make big advances in the three most important figures of merit for atomic clocks, so these are called systematic uncertainty, instability and reproducibility. And basically, each one of these three are important details that determine how good these clocks are, how useful they are. At the end of the day, all of them together contribute to what you might consider as accuracy – how accurate is the clock. And so, we were able to show advances in each one of these areas, ultimately showing that these systems are capable of making measurements at the level of, you know, one part in 10^−18

or even better than that.

Interviewer: Benjamin Thompson

So, when it comes to measurements, the team are working to 18 digits of precision. But what can these clocks be used for?

Interviewee: Andrew Ludlow

In the article, we highlighted especially one application that there’s been a fair bit of anticipation for these clocks being useful for, and this is what’s kind of been known as relativistic geodesy.

Interviewer: Benjamin Thompson

Relativistic geodesy is the idea that you can use a pair of atomic clocks to measure the gravitational strength of a particular location. As you move further away from the centre of the Earth, gravitational strength decreases. So, the strength of gravity on top of a mountain is less than it is at sea level, for example. And this change in the strength of gravity does some peculiar things to time, put forward by Einstein in his theory of general relativity. To put it very simply, as gravity strength decreases time moves faster, although the effects seen here on Earth are very subtle. However, if you were to have, say, a pair of super-accurate optical lattice clocks, you could in principle send one up a mountain and leave one at sea level. By measuring the tiny differences in the ticking rate of the clocks, you could get a super-accurate measurement of the gravitational strength at the different locations. As well as being able to tell you things like precisely how high a mountain is above sea level, Andrew thinks there could be other benefits to these new measurements.

Interviewee: Andrew Ludlow

In its first go, it will improve geodetic models that, you know, the models that say what the gravitational shape of the Earth is. And then ultimately, those models could have impact in a lot of different areas, including surveying, including water and ice flow – that could be related to climate studies. There’s really a handful of reasons why these models are used quite extensively now.

Interviewer: Benjamin Thompson

While this all might seem a little hypothetical at the moment, efforts to send optical clocks out into the field have been made. You might remember, for instance, us talking here on the podcast about a group who sent an optical lattice clock into the Alps here in Europe. Andrew’s clocks aren’t ready to go anywhere just yet. While that might prevent them being used to measure gravitational strength at different locations, he says there are plenty of things they can be used for in the meantime.

Interviewee: Andrew Ludlow

We’re using these clocks right now in the lab to try to look for new physics, to better understand our Universe. And the idea is pretty straightforward – if you have a device that’s able to measure some quantity up to 18 digits, it’s sensitive to very subtle effects. And basically, as these clocks keep getting better and better, they become more sensitive probes for exploring these new physics and kind of constraining the possibility of deviations from the existing physical theories that we have.

Host: Benjamin Thompson

That was Andrew Ludlow. You can read his paper over at nature.com/nature.ⓝ

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