A Clock that Will Last Forever

Imagine a clock that will keep perfect time forever, even after the heat-death of the universe. This is the “wow” factor behind a device known as a “space-time crystal,” a four-dimensional crystal that has periodic structure in time as well as space.

Bold prediction.

Let me say at the outset that I don’t implicitly trust any press release, especially one that gets quantum entanglement wrong or explains it way too vaguely (“an action on one particle impacts another particle” No!), as this one does, so it’s possible this wasn’t fully vetted by the scientists involved.

But there are other reasons to think they are overselling the experiment here. Let me say at the outset that I find the proposal intriguing; it’s not the physics that is in question, and the claims in the press release are not present in the paper. It’s those promises, of what we’ll be able to do with the experiment, that give me pause. Namely:

Imagine a clock that will keep perfect time forever, even after the heat-death of the universe.

Ok, yeah, about that. It might be fair to claim an atom, or possibly a molecule, will survive the heat death of the universe, but a macroscopic device? The device forms a quantum-mechanical oscillator with an ion trap, requiring a certain configuration of electric and magnetic fields, i.e. this space-time crystal is not a physical crystal. Somehow I doubt that the equipment running it will last forever.

If we lose the expectation that this will last a super long time, we still have the idea that it will be a perfect clock, right? Why is this supposed to be perfect?

The persistent rotation of trapped ions produces temporal order, leading to the formation of a space-time crystal at the lowest quantum energy state.

Because the space-time crystal is already at its lowest quantum energy state, its temporal order – or timekeeping – will theoretically persist [for a long time]

It’s true that a quantum mechanical ground state can persist without violating any laws of thermodynamics, and the ground state of a system has a frequency that is infinitely narrow — excited states have a width that is dictated by the uncertainty relation \(Delta{E}Delta{t}>hbar/2 \)    but a ground state has an infinite lifetime. Thus, no time uncertainty.

However… (you knew this was coming)

The paper shows that the rotation frequency of the ions in the crystal depends on the magnetic field you apply to it. That magnetic field will not have a perfectly precise value — it will have fluctuations in it, which means that the oscillation frequency is not going to be a delta function — there will be uncertainty.

Not only that, but how do you count the oscillations and discern the phase? That introduces error into any clock — the perturbation of measurement. In most atomic clocks you have a transition at some frequency, and the excited state does have some width to it, which is why long-lived transitions are used whenever possible — it means the transition will be narrow — but the proposal for this clock is to measure where a particular ion is by shining a spatially narrow laser on it. So they aren’t leveraging the infinitely narrow state; I don’t think they can. The mental picture I have is that it would be like counting a wheel’s rotation by painting a spot on its rim and counting how many rotations you have. The problem is that any ion is going to have an inherent location uncertainty, and the laser will add to that because the spot will likewise have a spatial extent. So even if that’s small, it won’t vanish — there will be a measurement uncertainty introduced, on top of the frequency uncertainty from the magnetic field. Not perfect.

Go ahead and blame me for being the reason we can’t have nice things that are perfect and last beyond the heat death of the universe.

Microwave laser fulfills 60 years of promise

Because of this [low power] impediment, most in the field gave up on masers and moved on to lasers, which use the same principles of physics, but work with optical light instead of microwaves. Lasers are now used in applications ranging from eye surgery to CD players.

The poor maser lived on in obscurity. It found only a few niche uses, such as boosting radio signals from distant spacecraft — including NASA’s Curiosity Mars rover. Those masers work only when cooled to less than ten degrees above absolute zero, and even then they are not nearly as powerful as lasers.

To paraphrase Ray “Bones” Barboni, this is the exact frikkin’ thing I needed. A little pique after a blogcation to get the blood going again. And to quote Jules Winnfield, “Well, allow me to retort.”

First of all, “microwave laser” is just … wrong. The maser came first, so popularity aside, you don’t just ignore the history. That’s like touting a cover song while ignoring the songwriter who first recorded it. Blasphemy.

Second, and more importantly, the “first practical maser”? The mind boggles. Well, my mind does, anyway. Hydrogen masers have been the best atomic clocks at time scales out to a day or so for quite a while, and even with the advent of laser-cooled atomic clocks in the past decade, they only surpass masers after about a day of integration. (This is why the even more advanced optical clocks you read about every few months cannot be called better, in some sense — they don’t yet run long enough to make a significant contribution to timekeeping). You can make the argument that the world’s timekeeping, backbone for GPS and other timing-dependent technologies is living in obscurity, but I can’t see how that isn’t practical.

MINOS reports new measurement of neutrino velocity

[T]he new MINOS study significantly reduces the systematic errors of its earlier work with detailed measurements of the behavior of the experiment’s GPS timing system, improved understanding of the delays of electronic components at every stage of the MINOS detectors and the use of upgraded timing equipment, designed and implemented with the assistance of the National Institute of Science and Technology and the United States Naval Observatory.

Applying these improved understandings, the MINOS collaboration measures a neutrino arrival time for travel between Fermilab and Soudan, Minn., that is consistent with the expected travel time at the speed of light. The difference between the measured and calculated times is -15 ± 31 nanoseconds, indicating no observable effect.

Gotta include the plug for the home team.

I saw this paper last week and I might have gotten around to doing a blog post, but for reading Chad’s book, so he beat me to it. (OK: no, not really; I have to many other unfinished posts in the queue) Clock Synchronization Done Right: “A 920-Kilometer Optical Fiber Link for Frequency Metrology at the 19th Decimal Place”

OK, I admit, that’s a lot of zeroes. But why does that matter? Isn’t the whole point of ultra-precise atomic clocks that they all work exactly the same way? Why do you need to compare them? All atomic clocks using a given type of atom have the same basic frequency, but not all clocks are equally well made. The only way to determine the performance of a new one is to compare it to one you know works, but they’re also not very portable, so you need to be able to do the comparison remotely.

A nit: I wouldn’t necessarily characterize this as an issue of not being “equally well made”. One problem is that they are not identical — they are not the ideal clock used in thought experiments: there is noise, and it’s different for each clock. Each component in the clock can impact the clock in slightly different ways. But even if they were identical in all respects, you still have natural processes present in your oscillator, and these will give you white noise in the frequency of the clock. The integral of frequency give you the time, and the integral of white frequency noise is a random walk in phase, i.e. in the time. What does that mean? It means two clocks at identical frequencies will still undergo a random walk away from each other — they will lose synchronization. So you will still need to synchronize clocks, even if you could get them to the exact same frequency and remove all other kinds of noise. (Which you can’t).

Currently we are in a regime where clocks are better than time transfer techniques, at least over interesting distances. One can compare co-located clocks pretty well, but it doesn’t do anyone else any good if the precise time cannot be disseminated to them.