Testing Quantum Foundations With Atomic Clocks

Funky clock at Aachen University.

Nobel laureate Steven Weinberg has recently drawn attention by disliking quantum mechanics. Besides an article for The New York Review of Books and a public lecture to bemoan how unsatisfactory the current situation is, he has, however, also written a technical paper:

Lindblad Decoherence in Atomic Clocks
Steven Weinberg
Phys. Rev. A 94, 042117 (2016)
arXiv:1610.02537 [quant-ph]

In this paper, Weinberg studies the use of atomic clocks for precision tests of quantum mechanics. Specifically, to search for an unexpected, omnipresent, decoherence .

Decoherence is the process that destroys quantum-ness. It happens constantly and everywhere. Each time a quantum state interacts with an environment – air, light, neutrinos, what have you – it becomes a little less quantum.

This type of decoherence explains why, in every-day life, we don’t see quantum-typical behavior, like cats being both dead and alive and similar nonsense. Trouble is, decoherence takes place only if you consider the environment a source of noise whose exact behavior is unknown. If you look at the combined system of the quantum state plus environment, that still doesn’t decohere. So how come on large scales our world is distinctly un-quantum?

It seems that besides this usual decoherence, quantum mechanics must do something else, that is explaining the measurement process. Decoherence merely converts a quantum state into a probabilistic (“mixed”) state. But upon measurement, this probabilistic state must suddenly change to reflect that, after observation, the state is in the measured configuration with 100% certainty. This update is also sometimes referred to as the “collapse” of the wave-function.

Whether or not decoherence solves the measurement problem then depends on your favorite interpretation of quantum mechanics. If you don’t think the wave-function, which describes the quantum state, is real but merely encodes information, then decoherence does the trick. If you do, in contrast, think the wave-function is real, then decoherence doesn’t help you understand what happens in a measurement because you still have to update probabilities.

That is so unless you are a fan of the the many-worlds interpretation which simply declares the problem nonexistent by postulating all possible measurement outcomes are equally real. It just so happens that we find ourselves in only one of these realities. I’m not a fan of many worlds because defining problems away rarely leads to progress. Weinberg finds all the many worlds “distasteful,” which also rarely leads to progress.

What would really solve the problem, however, is some type of fundamental decoherence, an actual collapse prescription basically. It’s not a particularly popular idea, but at least it is an idea, and it’s one that’s worth testing.

What has any of that to do with atomic clocks? Well, atomic clocks work thanks to quantum mechanics, and they work extremely precisely. And so, Weinberg’s idea is to use atomic clocks to look for evidence of fundamental decoherence.

An atomic clock trades off the precise measurement of time for the precise measurement of a wavelength, or frequency respectively, which counts oscillations per time. And that is where quantum mechanics comes in handy. A hundred years or so ago, physicist found that the energies of electrons which surround the atomic nucleus can take on only discrete values. This also means they can absorb and emit light only of energies that corresponds to the difference in the discrete levels.

Now, as Einstein demonstrated with the photoelectric effect, the energy of light is proportional to its frequency. So, if you find light of a frequency that the atom can absorb, you must have hit one of the differences in energy levels. These differences in energy levels are (at moderate temperatures) properties of the atom and almost insensitive to external disturbances. That’s what makes atomic clocks tick so regularly.

So, it comes down to measuring atomic transition frequencies. Such measurements works by tuning a laser until a cloud of atoms (usually Cesium or Rubidium) absorbs most of the light. The absorbtion indicates you have hit the transition frequency.

In modern atomic clocks, one employs a two-pulse scheme, known as the Ramsey method. A cloud of atoms is exposed to a first pulse, then left to drift for a second or so, and then comes a second pulse. After that, you measure how many atoms were affected by the pulses, and use a feedback loop to tune the frequency of the light to maximize the number of atoms. (Further reading: “Real Clock Tutorial” by Chad Orzel.)

If, however, between the two pulses some unexpected decoherence happens, then the frequency tuning doesn’t work as well as it does in normal quantum mechanics. And this, so Weinberg’s argument, would have been noticed already if decoherence were relevant for atomic masses on the timescale of seconds. This way, he obtains constraints on fundamental decoherence. And, as bonus, proposes a new way of testing the foundations of quantum mechanics by use of the Ramsey method.

It’s a neat idea. It strikes me as the kind of paper that comes about as spin-off when thinking about a problem. I find this an interesting work because my biggest frustration with quantum foundations is all the talk about what is or isn’t distasteful about this or that interpretation. For me, the real question is whether quantum mechanics – in whatever interpretation – is fundamental, or whether there is an underlying theory. And if so, how to test that.

As a phenomenologist, you won’t be surprised to hear that I think research on the foundations of quantum mechanics would benefit from more phenomenology. Or, in summary: A little less talk, a little more action please.