# Did NIST fudge this news story about absolute zero?

This press release by NIST, titled "NIST Physicists ‘Squeeze’ Light to Cool Microscopic Drum Below Quantum Limit", makes the following claim:

The new technique theoretically could be used to cool objects to absolute zero, the temperature at which matter is devoid of nearly all energy and motion, NIST scientists said.

I'm not sure that's exactly what the NIST scientists said, nor what they meant. I'm very suspicious of anyone who claims to be even theoretically capable of reducing a mechanical system to absolute zero.

The full publication itself is at

Sideband cooling beyond the quantum backaction limit with squeezed light. J.B. Clark et al. Nature 541, 191 (2017), arXiv:1606.08795.

Can someone with access to the actual article in Nature clarify whether the news article at NIST was accurately reporting on the content of the Nature article regarding achieving absolute zero? The abstract instead references cooling "arbitrarily close to the motional ground state".

I'm not looking for a debate about whether or not reaching absolute zero is possible, nor a discussion about the discrepancies in definitions of absolute zero. I just want someone to clear up the probable discrepancy between what NIST wrote and what Nature published.

• My understanding is that they managed to go below a theoretical limit by modifying the waves in a way that it didn't account for. They approach closer to absolute zero, but I don't think it's actually reached, we just get exponentially closer, so if you round it's essentially absolute zero. – JMac Jan 18 '17 at 22:13
• I can offer a lot of information on this but I'm not entirely certain what you're asking. The popsci article does not say they got to absolute zero. It says that the technique they used could, in theory, be used to get a thing to absolute zero. That's sort of true, but only if you have a zero-entropy coherent source, which is impractical. Would an expansion of that thought be the answer you're looking for? – DanielSank Jan 18 '17 at 22:20
• FWIW, I sent NIST an email, asking was the quote correct, I will let you know if I ever receive a reply. You may have done the same, but no harm to see if a reply emerges. – user140606 Jan 18 '17 at 22:30
• @DanielSank That was what I was looking for- a bridge between what they said was theoretically possible and what they had actually accomplished. – Sasha Jan 18 '17 at 22:43
• @JMac: ... so if you round it's essentially absolute zero. I think you're lost, JMac. Here you go: engineering.stackexchange.com – loneboat Jan 19 '17 at 16:58

Let's go through the article's abstract (emphasis added by me):

Quantum fluctuations of the electromagnetic vacuum produce measurable physical effects such as Casimir forces and the Lamb shift. They also impose an observable limit—known as the quantum backaction limi on the lowest temperatures that can be reached using conventional laser cooling techniques. As laser cooling experiments continue to bring massive mechanical systems to unprecedentedly low temperatures, this seemingly fundamental limit is increasingly important in the laboratory.

Right, conventional laser cooling cannot get a system below a certain minimum temperature. This is essentially because lasers (or microwave sources, or whatever coherent field generator you care about) output a so-called coherent state, which has a finite width in both of its quadratures. This limit is called, later in the abstract, the "quantum backaction limit", as we'll see in just a moment.

Fortunately, vacuum fluctuations are not immutable and can be ‘squeezed’, reducing amplitude fluctuations at the expense of phase fluctuations.

Right, coherent states aren't the only possible states of the electromagnetic field (or any other harmonic oscillator)! It is possible to generate so-called "squeezed states" where one of the quadratures is more narrow than the other. These squeezed states do not violate the Heisenberg uncertainty relation: you get squeezing in one direction at the expense of broadening in the other. This is directly related to what the authors refer to as reducing amplitude fluctuations at the expense of phase fluctuations. I'm not getting in the details on this because it's out of bounds for what OP is asking.

Here we propose and experimentally demonstrate that squeezed light can be used to cool the motion of a macroscopic mechanical object below the quantum backaction limit.

Ok fine. They get beyond the "quantum backaction limit" because they're not using a normal coherent state. They use a squeezed state.

We first cool a microwave cavity optomechanical system using a coherent state of light to within 15 per cent of this limit. We then cool the system to more than two decibels below the quantum backaction limit using a squeezed microwave field generated by a Josephson parametric amplifier.

Yep, as we just said, using a squeezed state lets you get past the limit you have with normal coherent states.

From heterodyne spectroscopy of the mechanical sidebands, we measure a minimum thermal occupancy of 0.19 ± 0.01 phonons. With our technique, even low-frequency mechanical oscillators can in principle be cooled arbitrarily close to the motional ground state, enabling the exploration of quantum physics in larger, more massive systems.

Ok so there we clearly see that they did not get to absolute zero. They still had about 20% of a phonon (one quantum unit of vibrational excitation) in their oscillator, whereas absolute zero would be zero phonons. They say that in-principle you can used squeezed states to get to arbitrarily low temperatures (i.e. arbitrarily low phonons). That may technically be true, but to get arbitrarly low temperature you need arbitrarily much squeezing, which is very, very hard to actually do in the lab. They're making an "in-principle" statement where the conditions for the in-principle thing to be achieved are totally unrealistic, and what's more, it's not even known whether the theory accurately describes the physical system in the parameter range you would need to get, say, $10^{-10^6}$ phonons (see comments under Emilio's answer for more on this).

Statements like that are still useful, because they tell the reader that there's no known hard limit to how far you can go with the squeezed state technique, i.e. the limits are entirely practical. This is important because other protocols could have in-principle limitations. For example, I might have some protocol which cools an oscillator but cannot possibly get below 0.3 phonons because of something baked into the physics. In that case, you know that if you need a phonon number lower than 0.3, don't even consider using my protocol.

• +1 I learnt an (embarrassing large) amount from your answer, ta for posting it. – user140606 Jan 18 '17 at 23:26
• And note that conventional laser cooling (Doppler, Sisyphus, etc) does have such a hard limit set by the atomic transition in question, so having a cooling technique which is not fundamentally limited is a significant advance. – zeldredge Jan 19 '17 at 8:00
• Also, it's important to note that "arbitrarily close to absolute zero" is very different to actually reaching absolute zero. – Emilio Pisanty Jan 19 '17 at 9:37
• @DanielSank uh... one of them is consistent with the third law of thermodynamics and the other one isn't? $T=0$ and $T>0$ look very different to me, no matter how small a positive temperature you've got. – Emilio Pisanty Jan 19 '17 at 20:53
• Of course, this is all just in-theory, but then so is the third law of thermodynamics ('can never reach absolute zero in a finite number of steps', but it doesn't say anything about procedures that take $10↑↑↑↑↑↑↑↑10$ steps, which I think you'll be equally uncomfortable with). The question is at heart a theoretical question about the third law, so I don't see how those arguments are irrelevant. – Emilio Pisanty Jan 19 '17 at 21:29

Daniel Sank has done a very good job at explaining the detailed physics as explained in the original paper, but he's not talked much about the NIST press release and I think it's worth mentioning some things about it.

More specifically, the press release makes two claims which misrepresent the physics of the paper, as explained by Daniel, and indeed those claims are inconsistent with the laws of thermodynamics.

• The new technique theoretically could be used to cool objects to absolute zero, the temperature at which matter is devoid of nearly all energy and motion, NIST scientists said.

This is incorrect. The new technique gives us a method that could, as far as we know with the hard information we know now, be used to cool objects to arbitrarily cold temperatures $T$, but it does not allow us to actually reach absolute zero.

The valuable part of the new method is that all previous methods to cool stuff down have some sort of fundamental limit: for instance, even quite sophisticated 'sub-Doppler' laser cooling methods will be unable to cool down atoms to temperatures where the atom's momentum is smaller than one photon momentum, because that's the currency of the laser beam.

In the new method, if you have enough resources (i.e. if you can build a device that can produce light that's 'squeezed' to an sufficiently large extent) then you'd be in principle be able to cool down to any positive temperature $T>0$ you cared to name, down to a zeptokelvin if you so desired, as far as we know. That said, the technique almost certainly has some form of fundamental lower bound on what temperatures it can reach, but we just don't know what that limit might be as of yet.

Where the press release strays is in confusing 'arbitrarily close to zero' with 'actually equal to zero', and indeed the third law of thermodynamics implies that we can never actually reach absolute zero. Even if the new technique did turn out to have no hard lower limit, and it could indeed get to arbitrarily low temperatures (itself unlikely), as far as the third law goes, that would still be a positive temperature and, however small, it is still infinite steps away from actual absolute zero.

Moreover, while we're here, the press release makes a second similar incorrect statement:

• Again, this is not true, and for the same reasons (though maybe this one can simply be chalked up to just being internally inconsistent). In the previous experiment being described, the system was cooled down so that it was in the ground state 70% of the time, and it had an excitation energy of just a single quantum the other 30%. This means that it's pretty cold, but it does not mean that it is actually in the ground state - it's still at a finite temperature.

• "In the new method, if you have enough resources..." I think that statement could use some clarification. "Enough resources" makes it sound like if I had enough money or enough graduate students, then I can get as low a temperature as I can dream up. This is not known to be true. The theory of squeezed state cooling doesn't seem to have a lower temperature limit, but that theory is absolutely not a complete representation of the actual physical system. – DanielSank Jan 19 '17 at 21:11
• Pretending that our systems are well described by approximate theory is responsible for a lot of confusion in my field alone. The resonator does not have infinitely high internal $Q$. The cooler you want to get, the more power you have to drive with, and before you know it the rotating wave approximation breaks down. All kinds of things go wrong when you try go to the next level of performance with a given protocol and I think it's important to make that more explicit in these discussions. – DanielSank Jan 19 '17 at 21:12
• So, I would say that this answer is focused on the wrong issue. The distinction between "arbitrarily low" and "zero" is, in my opinion, unimportant. What's far more important is that the statement that we can go arbitrarily low is itself unsubstantiated. Look, this experiment didn't even get that low; they got to 0.2 phonons. To go from that to saying we can get arbitrarily low with their technique, we have to swallow an approximate theory tested only in the 0.2 phonon range and pretend that it will keep working when you want to go down to $10^{-10^{10000}}$ phonons. – DanielSank Jan 19 '17 at 21:17
• I disagree with your third comment: on the contrary, I think your answer sort of misses the point of the OP (while still remaining more valuable than mine, I should say). The way I read it, OP is saying:" there's this text, it claims they've reached T=0, we know that that's not possible, so what gives?" - and what gives is precisely that the press-release people have failed to distinguish between arbitrarily low and zero, which does matter from a theory perspective. The rest of your comments just make my argument stronger. – Emilio Pisanty Jan 19 '17 at 21:28
• Yes, but "resources" fails to capture the idea of "our theory is actually wrong" or at least definitely not known to work in the limit of higher power, or phonon numbers lower than what we've actually measured, etc. – DanielSank Jan 19 '17 at 22:37