
Building the World’s Smallest Atomic Clock: A Nitrogen Atom in a Carbon Cage - sohkamyung
https://spectrum.ieee.org/semiconductors/materials/to-build-the-worlds-smallest-atomic-clock-trap-a-nitrogen-atom-in-a-carbon-cage
======
LeoPanthera
They're shockingly ugly (and expensive) beasts, but you can now get atomic
wristwatches. (Actual atomic watches, not the "atomic" kind that simply
receives the radio time signal.)

[https://www.hoptroff.com/collections/atomic-
timepieces](https://www.hoptroff.com/collections/atomic-timepieces)

~~~
baddox
Could atomic clocks provide more accurate GPS positioning for GPS receivers?
It’s my understanding that GPS works by measuring tiny differences in the time
it takes equivalent signals to reach the receiver from different satellites.
It may be the case that other factors (like noise and refraction) dwarf any
inaccuracies in the quartz clocks in our everyday GPS receivers.

~~~
azernik
We don't need super accurate clocks in the GPS receivers; instead, the GPS
satellites carry atomic clocks with them, and include timestamps in their
broadcasts. So you get two signals at the same time, compare the time the
satellite said it sent each signal, and you've got the difference in travel
time.

~~~
baddox
Oh, interesting. What’s the time granularity transmitted by the satellites? I
assumed that the satellites broadcasted a course stream of times and the
receiver had to finely measure its local time difference between receiving the
same time from multiple satellites.

~~~
azernik
So did some poking around; apparently, there are two timestamps in GPS
messages, and the more precise one is packaged in a custom hash function and
called the "ranging code". The timestep (called a "chip") is a bit under a
microsecond.

But yes, the receiver does have to measure small time _differences_ between
received signals; but that just requires a local clock with high resolution,
not one with low drift or low absolute error.

For example, if your local clock is a second off, but still measures time
intervals to pretty high precision (say, 0.01% - losing a second every three
years) that's probably not going to mess up your location fix very much.
Especially when the problem is overconstrained as it usually is intentionally.
Compare the theoretical minimum requirement of three visible satellites to
make a measurement with the 4-12 satellites that the system guarantees will be
in sight at any time.

------
akavel
Hm; tried to revive some primary school-level physics/chemistry knowledge to
calculate price-per-molecule or #-of-molecules-per-£. Is my calculation
correct, or have I botched it? :)

Given atomic mass of C is 12.011, of N is 14.007, the molar mass [1] of the
fullerene should be:

_M_ (N@C60) = 14.007 + 60×12.011 [g/mol] = ~734.667 [g/mol]

Then, given:

N_A = 6.022140857e23 [1/mol] —
[https://en.wikipedia.org/wiki/Avogadro_constant](https://en.wikipedia.org/wiki/Avogadro_constant)
(number of molecules in 1 mol)

p_g = 200e6 [£/g] — price per gram given in article

n = ? [1/£] — number of molecules one would get for £1

I devised:

n = N_A / (p_g × _M_ (N@C60)) = [1/mol] / ([£/g] × [g/mol]) = [1/mol] /
[£/mol] = [1/£] = 6.022140857e23 / (200e6 × 734.667) [1/£] = ~ 4e12 [1/£] ?

In other words, it seems you could buy ~4'000'000'000'000 molecules for £1 ?

—

[1]
[https://en.wikipedia.org/wiki/Molar_mass](https://en.wikipedia.org/wiki/Molar_mass)

------
derefr
The description of their "high-pressure liquid chromatography" technique
sounds an awful lot like how one would enrich uranium (you know, as one does.)
Did they accidentally invent a better uranium enrichment process (or
independently reinvent a classified one?) Or is there some difference between
{C60, N@C60} and {U235, U238} that makes using HPLC to enrich it impossible?

~~~
mscholz
HPLC separates molecules based on some chemical property. In this case, the
N@C60 fullerenes are probably very slightly larger, distorted from pure
icosahedral symmetry, or have different electronic structure to the plain C60.
Chemically speaking, U235 is probably very much like U238 and the chemical
compounds of the isotopes are probably chemically almost identical (since
isotopic chemical reactivity depends basically on the so-called kinetic
isotope effect, which is larger if there is a large difference in mass between
isotopes). HPLC is basically a standard synthetic laboratory procedure and has
no relevance (that I know of) to uranium separation aside from purifying bulk
chemicals.

~~~
derefr
My understanding was that HPLC was just a way to do gaseous-diffusion
enrichment (i.e. "standard chromatography") without the really long tubes.
From the article:

> In standard chromatography, substances having different chemical
> characteristics are separated by making them run a kind of gauntlet—an
> obstacle course that blocks the passage of one thing more than the other.
> HPLC works by using a pumped solvent (hence the term “high pressure”) to
> strip off the laid-down film of carbon fullerenes in such a way that the
> desired molecules—the fullerenes encasing nitrogen—are carried away
> preferentially.

My interpretation of that statement is that HPLC uses a solvent that manages
to detach the slightly _lighter_ (and/or less electrostatically-attracted-to-
the-film) molecule first, giving it a head start down the diffusion tube,
letting the tubes be really short, and thus giving you a tabletop device
instead of capital equipment in a dedicated facility.

Maybe HPLC of N@C60 really leans on that differential-electrostatic-attraction
property, but it seems like the process would still _work_ to separate
isotopes that differ only by molecular weight.

I'm not a chemist of any sort, though, so I'd be glad to "get schooled" here.

------
amelius
So how precise would this make our location?

Could I replace my 3D-mouse by a cellphone using this technique?

~~~
simpsond
I’m not sure that is the goal. Having cheap and small atomic clocks simplifies
time sensitive applications that can’t always rely on satellites or ntp. Clock
synchronization is currently not a solved problem for the masses.

