10 ms·
For precision, the sapphire clock outshines even the best atomic clocks
- jjoonathan 5y agoHah, the editor must have banned the term "phase noise"! Impressive accomplishment.
- a9h74j 5y agoNo way to infer a Q, either.
- spot5010 5y agoIndeed. No mention of Allan deviation, and no links to references. I found this old reference https://ieeexplore.ieee.org/abstract/document/918181/ https://ieeexplore.ieee.org/abstract/document/918181/ Which claims a short term Allan deviation of 5.4e-16/sqrt(tau) for 1-4s. This is pretty impressive and is comparable to Rb or Cs clocks for short time scales. The best optical clocks achieve something like 1e-18/sqrt(tau). Of course, since the sapphire oscillator is not locked to an absolute reference they drift after a few tens of seconds.
- jjoonathan 5y agoYeah, let's throw some dBc/Hz @ 10kHz offset too for good measure, just to confound the editor :} Aren't stability units grand? Hey, I'm just glad we heard from them before they got body snatched. (Context: since you can drop a low-phase-noise clock into a radar and improve the performance, this area of research is one that the spooky defense types pay active attention to. Many physics departments have legends about projects like this suddenly vanishing into classified status.)
- spot5010 5y agoInteresting. I’m guessing for radar applications it’s really the phase noise above a certain frequency that matters.
- nextaccountic 5y agoWhat do you mean?
- segfaultbuserr 5y agoThe article used simplified explanations and analogies, such as > Tick, tick, tick. The rhythm of clocks is universal. But how exactly alike are those ticks? > [...] Precision has to do not with delineating the perfect second but rather with creating extremely regular ticks, or oscillations. Imagine a game of darts. Atomic clocks are able to land all their darts, or oscillations, broadly around the bull's-eye so that the average position is right on target, even though any given dart might be a centimeter or two away from dead center. Luiten's device doesn't aim for the bull's-eye: instead, it is able to land all its darts at exactly the same point on the dartboard. In other words, each tick is really, really, really just like another. But didn't even once mention the technical term for that in the entire article, making it more difficult to see what the author is talking about for people with an engineering background. This is simply known as jitter, or phase noise. It's especially funny given that the IEEE Spectrum's audience is engineering-minded readers.
- nextaccountic 5y agoOk thank you! Jitter is a term I'm familiar with, but I've never heard phase noise before (but it makes sense)
- PoignardAzur 5y agoThe headline is the perfect blend of sci-fi and fantasy.
- fjfaase 5y agoI wonder if this could also be used to more accurately time pulsar signals and be used to detect gravitational waves?
- Anon854 5y agoYes, it can. Andre's PhD was using this on a bar detector.
- elihu 5y agoSomething kind of similar I've wondered about is whether, with sufficiently accurate time and distance measurements and a dense-enough network of satellites scattered around Earth and the solar system in general, could one detect the perturbations in the paths of the satellites caused by the transit of massive objects like large-ish asteroids? (One could think of it in science-fiction terms as a detector to counter a cloaking device.)
- BurningFrog 5y agoYou should be able to detect deviations from the paths your sensors should travel based on all known objects. Those deviations must then be caused by unknown objects. Deducing where a large enough such object is seems entirely possible.
- parksy 5y agoPerhaps we could "shotgun" a swarm of tiny cubic reflectors, literally just use a controlled explosion to scatter them through the solar system, then track their movements to spot hidden objects. Some may even be drawn in and attach to otherwise dark objects by gravity, allowing them to be more easily tracked. I'm guessing the issue with that other than the technical challenge would be filling space with too much noise?
- adrianN 5y agoSpace is really big. You'd need a very large number of tiny reflectors to have a decent chance of any passing close enough to unknown objects to be measurably affected by their gravity.
- mbertschler 5y ago> But for some applications, accuracy is less important than precision. Precision has to do not with delineating the perfect second but rather with creating extremely regular ticks, or oscillations. I don’t follow here, doesn’t accuracy follow if you have a precise oscillator? What is the difference between an accurate or a precise clock?
- nightfly 5y agoIt implies that the "tick" intervals in atomic clocks aren't always the same length in time, they just average out very well. While the new sapphire clocks have a more consistent tick interval.
- tehbeard 5y agoSo.... Cesium clock is "guaranteed" 10 ticks in X timespan, but the distribution is not perfectly equidistant? Do I understand that correct?
- jjoonathan 5y agoRight. Actually, from what I understand, the situation is even more dire and Cesium clocks are (would be) complete rubbish at timing consecutive ticks, so when people say cesium clock they actually mean a quartz oscillator (ultrapure, double ovenized, binned, aged, etc -- but still quartz) to create the output and then a control loop to push/pull the quartz oscillator and frequency lock it to the cesium absorption line. A similar trick is more commonly used to lock, say, a 5.8GHz on-chip oscillator to a 10MHz quartz oscillator, except in this case the quartz oscillator plays the role of long-term reference and the on-chip VCO plays the role of short-term reference, whereas in the case of an atomic clock it's the other way around. The overall game is that different filters/oscillators are better at different timescales, so you use control loops to synthesize the best parts of each of them into a clock that is good at all timescales.
- mikewarot 5y agoCesium beam atomic clocks are actually just a very selective band pass filter / detector. There is a DC output in microamperes which is proportional to how well centered the input frequency is in relationship to the resonance of the Cesium atoms in the ambient conditions as they drift across the tube. To use this filter as a clock, it is slaved to a quartz oscillator which is them multiplied and phase locked, with a small bit of FM at 137 hz prior to being fed into the tube. If the frequency is too low, the output will be in phase with the FM, if the frequency is too high, the phase will be opposite, and if it is centered, there will be a 274 hz "2x" output, which is used to indicate lock. Thus, the cesium isn't actually sampled at ~9.192 Ghz, but rather a much slower rate. The actual loop maintaining phase is as slow as possible to keep phase noise down, which is part of why they take a while to lock on startup. Further complicating things is the need for a small persistent magnetic field in the tube, as getting zero in all 3 dimensions is a much harder problem. This bias keeps things stable, but also changes the frequency slightly, but is offset out in the divider chain.
- ticktockkasdf 5y agoThe distinction between precision and accuracy in the article leaves a lot to be desired. Couldn't we just redefine the second to be X number of ticks of this clock?
- rtkwe 5y agoThere could be issues with creating accurate copies of this. You could still get very precisely spaced ticks but not get an accurate measure that could be replicated around the world. A standard doesn’t work well if there’s one extant copy you can compare against. That’s why they’re working so hard to create a new standard kilogram where it could in theory be replicated without reference to the original.
- zinekeller 5y ago> That’s why they’re working so hard to create a new standard kilogram That's done, for now: https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...
- jimmySixDOF 5y agoIronically in terms of this conversation, the new definition of a Kg requires a time measurement. >The kilogram is the mass of a body at rest whose equivalent energy equals the energy of a collection of photons whose frequencies sum to [1.356392489652×1050] hertz.
- adrian_b 5y agoNow the units for almost all physical quantities (the few exceptions are those that are not dependent on time, length or force, e.g. angles and amount of substance) are derived from the unit of frequency. The frequency of an atomic clock with cesium has a conventional value, and the units for the other physical quantities, e.g. length, force, voltage, electrical current, temperature and so on are derived from the unit of frequency by conversion factors that are functions of various universal constants, all of which have conventional values chosen so that the units derived in this way are approximately equal with the units that have been used in the past.
- mackman 5y agoIsn’t Doppler shift a frequency shift, not a phase shift? Or is it both?
- kingkawn 5y agoRelated side note: It has always bugged me that the defintion of 1 second, a unit of time, is defined by an atomic oscillation frequency, which is itself a unit x/t. A unit of time refers back to itself in its own definition. My lay understanding is imprecise and I'm sure there's an expert in the wings who could fix it. Then again most belief systems have a root paradox somewhere within them. Perhaps this is ours.
- beecafe 5y agoIt's more like, 1/time (i.e, frequency), is what has been defined by the observed atomic oscillator frequency, and the second is derived from that. A real weird unit is km/l or miles per gallon, which cancels out to a unit of area.
- kingkawn 5y agoWhat’s to say any two oscillations of the atom are the same length?
- CamperBob2 5y agoIt has to do with the assumed constancy of the so-called "fine structure constant." Google that for more info. TL,DR: the only way these atomic resonant frequencies can possibly change is if the fine structure constant varies from place to place, or over time. If that turns out to be the case, the discovery will invalidate, or at least upset, the last 100 years worth of physics and cosmology. It's all but unthinkable.
- kingkawn 5y agoSounds exciting
- taejo 5y agoYou might be interested in the book Inventing Temperature by Hasok Chang which looks at similar questions but for temperature instead of time, both historically and philosophically. For example, you can define a temperature scale using the boiling point of water, but how do you know water always boils at the same temperature without a temperature scale? (And of course, it doesn't -- even at constant pressure, liquid water can exceed 100°C without boiling)
- throwaway81523 5y agoThe thing about the AU military using this to replace a quartz frequency standard seems lacking in detail. They can get a matchbox sized rubidium standard for a couple of K$ which has to be a fraction of the cost and headache of this cryogenic thing. Maybe the new thing is even better, but is it by enough to matter? Out of stock at Sparkfun, but this is the rubidium thing I'm referring to: https://www.sparkfun.com/products/14830 https://www.sparkfun.com/products/14830
- mikewarot 5y agoIt appears the resonance has a small, but predictable temperature dependence, you could therefor phase lock this to an atomic clock, if you have a slow enough control loop to keep the phase noise very low, and have the best of both worlds.
- spot5010 5y agoThere’s a difference in phase locking and frequency locking. It’s hard to phase lock to an atomic reference for long periods of time. Usually atomic clocks offer correction of the frequency drift of the oscillator. So the phase of the oscillator can still wander over large time scales.
- mikewarot 5y agoAtomic clocks have no phase to lock onto, they are narrowband RF detectors, built of a stream of atoms (or a cloud in the newer Cesium Cell clocks) unlike Quartz oscillators which are always vibrating.