
Physicists Unveil World's Most Precise Clock - Libertatea
http://www.technologyreview.com/view/515456/physicists-unveil-worlds-most-precise-clock-and-a-twin-to-compare-it-against/
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niels_olson
The NIST Time and Frequency Division is one of the unsung heros of physics.
Their senior guy, Wineland, got the 2012 Nobel Prize.

I remember when the current Primary Standard, NIST-F1, was meeting the press
in the late 90s. I was in undergrad and one of their junior guys came through
interviewing for a faculty job. His lecture remains one of the most memorable
hours of my life. Probably helped by the fact that I had just heard about the
F1 in Scientific American a few weeks prior.

It is unclear that this particular clock will take over as the Primary
Standard, whose data is compared to a competing project in France and
coordinated with the astronomical measurements at the US Naval Observatory to
construct the Universal Coordinated Time standard.

That is then what the USNO transmits to coordinate the GPS satellites, radio
services, etc.

<http://www.nist.gov/pml/div688/>

~~~
breadbox
"one of the unsung heros of physics ... got the 2012 Nobel Prize." I can't say
that I would complain about being forgotten like that.

~~~
adestefan
When one thinks of "power house" national labs NIST is the first thing that
springs into people's mind.

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yread
>These clocks are so sensitive that they can easily measure the gravitational
redshift, in which clocks tick more slowly in more powerful gravitational
fields. In other words they can sense changes in height....The new clock
should be able to discern changes of around 1 cm at the Earth’s surface

This seems like a pretty big deal! Perhaps not for measuring height, but
measuring gravity field with such a precision can have more applications

~~~
dcminter
I'm kind of curious as to how close we are to being able to use them for
Gravitational-Wave astronomy giving LIGO[1] a run for its money. Anyone know
if this is even possible?

[1] <http://en.wikipedia.org/wiki/LIGO>

~~~
ISL
Short answer - not anytime soon.

Longer answer: there might be a clever way to use clocks to pick off
gravitational waves, particularly at low frequency, but I doubt that they'll
be competitive with pulsar timing/LISA.

There are proposals to use atom interferometers for gravitational wave
detection. The community is quite divided on the chances for success (I'm in
the pessimistic camp at the moment). Barring any surprises, an atom
interferometer is much more likely than a timing-based measurement to see any
gravitational wave signal. On the LIGO front, so long as the source estimates
for gravitational wave signals are correct and as long as Advanced LIGO hits
its sensitivity goals, a LIGO GW detection is imminent.

There's always room for cleverness.

~~~
jessriedel
Could you explain why you're pessimistic on GW atom interferometers, or point
me to an argument elsewhere? I'm especially interested in the AGIS proposal by
the group at Stanford ( <http://arxiv.org/abs/0806.2125> ).

Also, is it obvious how one could use an ultra precise clock to see evidence
for GWs? Such clock can reveal spatial gradients in the rate of ticks (and so
reveal things like local mass densities), but GWs should be spatially
homogeneous, no?

Thanks for any help!

~~~
ISL
There are some rebuttals by Peter Bender (his abstract for the 2013 April APS
meeting may point in useful directions). For the space-based experiment,
barring concerns about the technique itself, the right question may be, "Even
if the atom interferometer might work, is it a better bet than the existing
plans for LISA?"

I haven't studied the atom interferometer GW stuff in great detail, but the
measurements look challenging from a feasibility and systematic perspective.
Any one objection can be addressed, but there are a lot of them, making it
challenging in the aggregate. Continued work on the ground in the atom
interferometer field over longer baselines will explore both. If that work can
show that it works as proposed, then pessimism will turn to optimism;
experiment is the arbiter of truth.

As for the second point, regarding clocks, my initial response of "no" came
from exactly your argument. I hesitated when considering a global array of
clocks. For wavelengths comparable to the size of Earth and smaller, there's
enough phase difference that perhaps a sufficiently precise clock, read out
sufficiently fast, might pick off a signal. Additionally, if there are signals
in the micro-to-nanohertz regimes, it might appear as an unexplained fit
residual to an overall model of the gravitational potential at any one clock.
My GR-fu is insufficient to make reliable estimates of the sizes of these
effects (I bet they're small!).

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jessriedel
Thanks so much for the thoughtful reply. I think the argument for these atom
interferometers as GW detectors is that they are sensitive to a different
frequency range and so would compliment rather than compete with LISA (or
LIGO). (See <http://meetings.aps.org/Meeting/DAMOP12/Event/172056>) But I
suppose the funding is pretty limited, so in that sense they compete. I'm
actually rooting for them because the cool quantum superpositions they would
need to produce (and possible sensitivity to low-mass dark matter!) rather
than gravitational waves.

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peeters
It's the combination of accuracy and precision that makes this remarkable.
It's either the world's most accurate clock with a certain precision, or the
world's most precise clock with a certain accuracy. I'm not sure which they
actually claim.

>They say their new clocks can keep time with an unprecedented precision of
one part in 10^18.

That seems to a measure of accuracy, not precision.

I can make a clock that gives the time in trillions of years since the Big
Bang, and it will be perfectly accurate for the distant future as long as it
always returns "0". But that's not a great precision.

I can also make a clock that gives the time in one 10^256th of a second, but
it won't be accurate.

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ISL
This measurement states that it's possible to build two clocks that differ by
less than two parts in 10^18.

Accuracy: getting as close to the target as you can. Precision: hitting the
same spot repeatedly.

The paper is correct - these measurements hit the same spot repeatedly.
Accuracy in this context is a tricky thing. The SI second is defined by the
Cesium standard, but the best Cesium clocks are >100 times less precise. These
two clocks are hitting the same tiny spot over and over again somewhere within
the fuzzy bullseye that is the Cesium target. The SI standard is not defined
as well as the new generation of clocks can measure.

Once the world settles in on an optimal optical clock design (the previous
leader has been an Aluminum clock, this one uses Ytterbium), the SI standard
will move to the new technology.

The impressive success of these clocks as a measure of gravitational redshift
is also an Achilles heel. I'm not sure that anyone really knows how to do
reliable time transfer with them yet, as one's depth in Earth's gravitational
_potential_ relative to another clock must be known at the centimeter scale.
Another factor of 10-100 in precision will require that such metrology will
improve by the same factor. It's a challenge.

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jessriedel
I think "Achilles heel" is the wrong term. These clocks do not have an unusual
sensitivity to something (e.g. vibrations) which could make them poor compared
to some other hypothetical clock. Rather, _all_ clocks of this accuracy must
necessarily be sensitive to these gravitational effects because the
gravitational field effects the passage of time itself.

~~~
ISL
Agreed - as an experimentalist specializing in precision gravitational
measurements, their lives just got worlds harder. Now they're not fighting
challenges within their instruments, but they have to ensure that they _know_
each clock's depth in Earth's gravitational potential. Atomic clocks are small
and elegant things with a pristine heart. They're now irreducibly sensitive to
a very messy world.

Earth tides (dirt, not water) are larger than centimeters. With further
improved precision, they'll be sensitive to small changes in the locations of
local gravitational sources, even big ones at long range. In our gravity lab
in Seattle, we see the effects of annual variation in water table. All of
these things must be properly characterized in order to compare two clocks
that are not co-located.

These are good problems to have, and are inherent to measuring time at this
precision, but they're very daunting. Not insurmountable, but often
frustrating.

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ctdonath
_The new clock should be able to discern changes of around 1 cm at the Earth’s
surface_

Another reminder the amount of information hitting us every moment is
absolutely astounding. That we're building devices which can collect & present
that information is equally astounding.

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pkfrank
Can anyone provide more background to this point?

>And physicists rely on clocks to test the fundamental laws of the universe to
ever deeper levels.

What will this instrument allow us to further discover, confirm, or
understand?

This project also reminds me of the 10,000 Year Clock--
<http://longnow.org/clock/>

~~~
breadbox
Presumably a more accurate clock will make it much easier to detect small
relativistic effects. This could be useful for e.g. measuring gravity waves.
Or it may just make it possible to do various kinds of experiments on a
smaller scale. An early direct test of relativity involved putting an atomic
clock on a jet to measure time dilation due to relative motion. A more precise
clock might allow the same test to be done with an automobile.

In the end, the real answer to your question might well be something that we
currently know nothing about. In other words: "If we already knew the answer
to that question, we wouldn't need the clock."

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guimarin
awesome. when can I buy one to put in my HFT datacenter...

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tenpoundhammer
It's about time.

