Posted on 07/12/2013 10:03:58 AM PDT by ShadowAce
Atoms trapped by light and measured with a CCD camera have the potential to outdo today's most accurate atomic clocks, and although it's early days, a pair of linked optical lattice clocks have yielded accuracy of a second every 300 million years.
In doing so, the group that performed the test, outlined here in Nature and published in full in Nature Communications say their experiment performed at the limit of experimental accuracy, outperforming three linked caesium-based atomic clocks.
Ever since 1967, the time standard has been governed by cesium clocks, in which the transition of atoms between high- and low-energy states is measured with microwaves. Their accuracy is the basis of an awful lot of science, as well as more familiar applications like GPS.
However, the microwave probes are one of the constraints on the accuracy of the devices: the standard clocks use frequencies around 10 GHz, but the atomic oscillations they measure happen trillions of times a second.
So in the competition to devise a more accurate atomic clock, it's no surprise that optics is one of the techniques being investigated, since optical frequencies are far, far higher than microwaves.
The Paris Observatory group hasn't actually broken the record-of-records for accuracy. There is, to pick one example, NIST's quantum logic clock which claims an accuracy of a second every 3.7 billion years.
What they have achieved is to demonstrate that their atomic lattice design has the potential to be the basis of much more accurate atomic clocks in the future. University of Western Australia associate professor and ARC fellow John McFerran, resident atomic clock maker at the institution, explained the significance of the work to The Register.
“They have shown that two optical atomic clocks (based on strontium) agree with each other to within the systematic plus statistical uncertainties, 1.5e-16,” Professor McFerran told Vulture South in an e-mail.
“This heralds a new era: optical clocks agreeing with each other at accuracies exceeding that of the best microwave clocks.”
The optical clock, Professor McFerran explained, has an important theoretical advantage over many other proposed approaches: rather than trying to observe a single atom, the Paris Observatory device works with thousands.
“One can probe tens of thousands of atoms at once, whereas in most ion clocks you are limited to one lonely quantum absorber. So the signal-noise ratio is far superior in lattice clocks, which means assessing the systematic frequency shifts can be carried out much more quickly. And this is a good recipe for perhaps making the best of tomorrow's clocks.”
The optical lattice has another really useful property: it constrains the movement of the atoms to within a half-wavelength of the light you're using for measurement, and constrains the direction of movement rather than randomly. With those constraints, Professor McFarren wrote, “The atoms move in a periodic fashion that is easy to characterise, hence all motional effects can be accounted for in the accuracy estimate.
“In previous neutral atom based clocks the atoms weren't constrained like this: they moved in a random fashion (very slowly though) … a related effect, the second order doppler effect, stopped you dead in your tracks trying make any improvements in accuracy.”
The optical lattice clock uses a variety of what – to the layperson anyhow – look like the more esoteric applications of lasers. One traps the strontium atoms on its half-wavelength nodes, while one or two more lasers use the trick of resonance to cool the atoms down. The cooling and trapping lasers are supplemented by magnetic fields.
Yet another laser – a very high stability, low noise laser – acts as the oscillator to provide the “ticks”.
All of this is held in the hardest vacuum the clock-builder can manage, a CCD camera to capture the signal, and a central computer to switch the magnetic fields and lasers on and off, trigger the camera, change the frequencies of the lasers if required, and record the data.
While a second-in-billions-of-years accuracy might seem an obsessive target, Professor McFarren told Vulture South there are applications both in the physics laboratory and outside it.
Greater accuracy helps test fundamental constants such as the “holier than holy” fine-structure constant, the quantum chromodynamics coupling parameter lambda, and the proton-to-electron mass ratio, he explained, along with helping refine our examinatinos of general relativity via tests of of Einstein's equivalence principle.
However, outside the lab, he said, linked clocks can help monitor changes in altitude at different points on the Earth's surface more accurately. “There are also prospects of studying hydrological flows using atomic clocks,” he noted, “since as the water moves, the strength of gravity changes, and this affects the ticking rate of the clocks.” ®
Do you mean that one, one thousand; two one thousand isn’t accurate?
“Accurate” is such a relative term.....
how many ways do we need to count a second?
Why not just add a day every 29.2 trillion years and be done with it?
How do they know? Did Helen Thomas set the clock?...................
Sure, just ask Dan Rather! :)
In my everyday work, I regularly use time intervals in BILLIONTHS of a SECOND, and sometimes smaller. The accuracy of that time slice is mucho importante in lots of electronics, avionics and nucleonics...........................
I am the opposite. What day is it?
heh
In my everyday work, I regularly use time intervals in BILLIONTHS of a SECOND, and sometimes smaller. The accuracy of that time slice is mucho importante in lots of electronics, avionics and nucleonics...........................
GPS is basically just a bunch of ultra precise clocks that are used to determine the location based off the speed of light and math to determine the intersection of three precisely timed radio signals....
...and people still get lost...............
I was reading somewhere that the GPS satellites are moving so fast relative to us here on the ground that time on those satellites actually passes more slowly (relative to a car on earth).
So the clocks on the satellites have to be adjusted accordingly in order to provide accurate GPS results. WILD!
A man who has two watches doesn’t know what time it is, but a man who has three, does, in the least squares sense.
If you have two time pieces and compare them to each other, you can readily estimate the difference between them, detect drift (which one goes faster or slower), but only mutual instability or random variations. You measure:
D_12(t) = Clock1(t) - Clock2(t)
You can use your observations to correct for the trend, but you cannot impute the variations, the residual after correction, to either one. What you have measured is:
sigma_12(t)^2 = (alpha_1*Clock1(t) - alpha_2*Clock2(t))^2
where alpha_n is the term applied to account for the drift in each clock. With identical clocks alpha_1 = alpha_2 = 1.
In determing variance, you have one equation with two unknowns. If you have reason to believe that both clocks are equally good, you can simply impute have of the variance to each, and assume each varies by half the observe amount with respect to an ideal clock. In other words, you estimate the stability of each clock as:
sigma_1(t)^2 = (sigma_12(t)^2)/2
sigma_2(t)^2 = (sigma_12(t)^2)/2
Once you have at least three clocks, you can make three pairwise measurements and can estimate the relative stability of each with simple algebra.
Once you have three or more clocks the number of independent equations equals n*(n-1)/2 and you are off to the races. You have an overdetermined set of more equations than unknowns.
25 Or 6 To 4
Chicago
Songwriters: LAMM, ROBERT
Waiting for the break of day
Searching for something to say
Flashing lights against the sky
Giving up I close my eyes
Sitting cross-legged on the floor
25 or 6 to 4
Staring blindly into space
Getting up to splash my face
Wanting just to stay awake
Wondering how much I can take
Should I try to do some more
25 or 6 to 4
Feeling like I ought to sleep
Spinning room is sinking deep
Searching for something to say
Waiting for the break of day
25 or 6 to 4
25 or 6 to 4
Read this,
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
See pp. 22-26, “An Illustrative Timekeeping Example”, for a clearer and fuller explanation. It’s really pretty staightforward.
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