Friday, February 03, 2017

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.


  1. I agree, being a distasteful idea is not a good criticism.
    The many world's is a good explanation just as GR is better than Newtons. Neither are correct I hope.

  2. It can't be better than this. Thanks Sabine.

  3. "It’s a neat idea. It strikes me as the kind of paper that comes about as spin-off when thinking about a problem."

    And as something which not a run-of-the-mill physicist, but rather someone like Weinberg, would come up with.

  4. "A little less talk, a little more action please."

    Or a little less conversation. :-)

    Just dig those groovy threads and cool dance moves!

  5. bee: sorry for being off-topic but i would like to suggest that you reprint your recent Nautilus article on quantum gravity ( here (if copyright permits) because it is excellent. As you point out in the article, theoretical physics needs to use the methodology known as abductive reasoning which utilizes a 'facts-before-theory' sequence (as opposed to a hypothetico-deductive sequence) to escape from the current deadend that quantum gravity theory finds itself in as a result of the mathematical masturbation methodology commonly being practiced by theoreticians in the field today.

  6. When you use the phrase "fundamental decoherence" is that bascially synonymous with the process described by "objective collapse theories" like that of Roger Penrose?

  7. Phys. Rev. Lett. 103(2) 023202 (2009)
    ..."Hund’s Paradox and the Collisional Stabilization of Chiral Molecules"
    ..."Chiral Symmetry Breaking In Molecules And Solids"

    One molecule in absolute vacuum (no collisional decoherence) has an arbitrary chiral configuration via the superposition principle. An alpha-amino acid with one acyclic chiral center might succumb.

    Current Organic Chemistry 16(22) 2632 (2012), DOI: 10.2174/138527212804004508 An 11-carbon, rigid, point group D_3, pentacyclic hydrocarbon sits at the bottom of a deep thermodynamic well. It has eight homochiral centers - all at once or no structure. I empirically doubt it could be turned inside-out by a footnote. Look.

  8. naivetheorist,

    Copyright does not permit, sorry.

  9. Alex,

    Yes, but the paper does not specify a particular model. It works with any model whose decoherence can be brought into a particular form which (if you trust Weinberg) is any such process that is physically viable. Best,


  10. Bert,

    No, you have it backwards. If one would observe such an unexpected decoherence, that would require an explanation. I've never heard of a modification of quantum mechanics that becomes *less* noisy at some scale. Interesting thought. I'll have to think about whether that makes sense. Best,


    ...A novel method for cooling trapped ions could boost the accuracy of atomic clocks.

    Inertia against looking in new places is disappointing. Frivolity of publishing more theory is status quo. Euclid reigned for 2000 years because nobody had the big brass clangers to look at a globe...and report it. The solar system does not work using God's perfect circles. All the fun is in the footnotes.

  12. Steven Weinberg does not grasp quantum mechanics, and I can prove that statement: read his (silly!) paper, "Precision test of Quantum Mechanics," PRL 62, 485 (1989), and then read Polchinski's demolition of that Weinberg's nonsense: PRL 66,397 (1991). QM is not mysterious: if observations have the character of numbers (and they do), and if simple symmetries are present in the universe (and they are, at least locally), then you get QM automatically:;

  13. Speaking as a long-time fan of the many worlds interpretation, the criticism "rarely leads to progress" hits where it hurts. While "defining the problem away" is a bit too dismissive for my tastes, it seems to be true that very little actual progress has come from adapting the many worlds interpretation. Early on there were hopes for progress in understanding Born's law, but that didn't work out. In hindsight, saying "it's all there in the QM mathematical formalism" does not add much to the physical insight. Of course that doesn't mean it's wrong, but...

    I wonder if a case can be made that Zurek et al discovered decoherence by thinking in terms of many worlds, but that is a much more indirect effect.

  14. Hi Sabine,

    I agree with Steve Bryson that "defining problems away" is an unfair description of the Many Worlds approach.
    The Many Worlds interpretation does not "postulate" all possible measurement outcomes are equally real.
    The Many Worlds interpretation uses the well-established formalism of quantum mechanics and this formalism
    in its minimal version predicts these alternative measurement outcomes/Many Worlds/Everett branches.

    Moreover I disagree, that it is puzzling that "on large scales our world is distinctly un-quantum".
    While I agree that decoherence "takes place only if you consider the environment a source of noise whose exact behavior is unknown" it is absolutely clear why this is the situation we encounter: It is a consequence of our perspective onto the Universe which simply doesn't feed us all possible information about "combined system of the quantum state plus environment" aka as the entire Universe. I believe this perspectival element is the most interesting property of quantum mechanics and it is this phenomenon we should try to test: Is there a possibility to adopt a less local perspective onto the Universe? Nobody knows whether that's possible, but I would try to look into altered states of consciousness to explore this.
    I wrote this idea up here:

    Can the Many-Worlds-Interpretation be probed in Psychology?
    Heinrich Päs
    International Journal of Quantum Foundations, in the press

    Even if it sounds crazy I believe this approach is more interesting and promising than Weinberg's idea.

    Finally, also @Steve Bryson: Decoherence was discovered by H.D. Zeh in 1970, not by Zurek et al who started working on decoherence only in the 1980ies. I belive Zeh derserves more credit for this important discovery.

    Cheers, Heinrich


  15. I have always though that objective collapse theories were poorly motivated. It seems to be an attempt to restore some aspects of classical physics. Maybe it could be made to work but I see no reason to think so.

    Without some form of objective collapse we are stuck with the irreducibly subjective nature of wave collapse. But what is the problem with that? Qm describes what an observer will see and sure enough when an observer looks that is exactly what they see. What more do you want?

    And without objective collapse there can be no difference between wave collapse and decoherence. They are just two different perspectives on the same thing. I don't think the quantum measurement problem is a problem at all. It is just our classical minds struggling with quantum weirdness.

    While many worlds may be a way to visualize things I'm not sure what it means to insist on the reality of other branches of the universe that you can never go to. What does real even mean in this context?

  16. bee:

    FYI: what you are suggesting here (and in your excellent article in Nautilus ( is known in philosophy of science as abductive reasoning, the development of theories/models based on experimental findings, in contrast to the hypothetico-deductive method of Popper which does does not offer any basis for the creation of a theory/model. Abductive reasoning is the predominant methodology of theoretical physics with few successful exemptions (there can be debate as to whether Einstein was employing abductive reasoning via his use of 'thought experiments').

    best regards,


  17. There is another Nobel Prize winner, who has problems with the conventional view on QM:
    -> Watch here <-

  18. akidbelle,

    Looked at it, couldn't make sense of it. Doesn't look like it's worth more time. Modifying gravity is hard, really hard, and the paper is below the current quality standard.


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