5 ms·

Physicists ‘squeeze’ light to cool microscopic drum below quantum limit

88 points by upen 10y ago

jessriedel 10y ago
arXiv: https://arxiv.org/pdf/1606.08795.pdf https://arxiv.org/pdf/1606.08795.pdf By the way, this group achieved ground-state cooling of a very similar mechanical oscillator back in 2011. "Sideband cooling of micromechanical motion to the quantum ground state" http://www.nature.com/nature/journal/v475/n7356/abs/nature10261.html http://www.nature.com/nature/journal/v475/n7356/abs/nature10... arXiv: https://arxiv.org/abs/1103.2144 https://arxiv.org/abs/1103.2144 Both oscillators were of roughly the same frequency (10 MHz) and size (~15 nm diameter aluminum disc). In terms of phonon occupation number, things improved from 0.34 to 0.19. They emphasize in this new paper that they were able to get colder by using a squeezed vacuum state of laser light. I'm no expert, but I think the use of this technique is much more important than the improvement in the mechanical state they were able to achieve.
- robinduckett 10y ago
  What does this mean to the layman? Is physics broken now?
  - rubidium 10y ago
    No, it's a pretty traditional technique to "beat" quantum mechanics by using squeezed light states. Short version: you don't really beat quantum mechanics, you just shift where the uncertainty resides.
    - basicplus2 10y ago
      So where does the uncertainty move to?
      - deepnotderp 10y ago
        To a different subset of the Hilbert space. See here for beginners: http://www.squeezed-light.de http://www.squeezed-light.de
  - correlator 10y ago
    Physics is not broken. --not 100% accurate, but a close approximation in lay terms-- If you want to measure the position of something precisely, you can bounce a laser off of it and time how long it takes the light to return. If you want take this experiment to it's extreme in sensitivity, you eventually end up measuring tiny changes in the phase of the light bouncing off of the object. Your ability to measure position is limited by your ability to measure phase. Quantum mechanics however requires there to be noise both in the amplitude and phase of the light. This is heisenberg's uncertainty principle. So measuring at the "quantum limit" means measuring fluctuations in phase equal to half of the uncertainty limit (half noise in amplitude and half in phase). There is no law that said half the noise has to be in phase, just that the product of amplitude and phase noise is greater than or equal to the heisenberg limit. You can take extra noise out of phase and put it into amplitude to measure "beyond the quantum limit" without breaking any rules. This is called a "squeezed state." That is exactly what they did. They created "squeezed light" and used it to measure a degree of freedom (like phase) "beyond the quantum limit" This was a very challenging and impressive measurement. John Teufel has been doing work leading up to this for over 7 years (2 post doctorates and a professorship). He worked on quantum computers as an undergraduate too so you may even be able to add a few more years onto that.
    - jacobush 10y ago
      Wow, he's a devil of a scientist!
      - deleted 10y ago
        [deleted]
daveguy 10y ago
From the article: "squeezed light removes the generally accepted cooling limit". Generally removing a limit in physics is a huge deal. What was this limit based on? Has some fundamental quantum physical property been disproven? Is this a breakthrough more significant than setting a coldness record? Was the generally accepted limit based on what would be "really tough" or something else? Anyone know?
- jessriedel 10y ago
  > What was this limit based on? It's base on an assumption that the cooling apparatus isn't using squeezed light. If you can squeeze light, then the limit no longer applies. > Was the generally accepted limit based on what would be "really tough" or something else? Squeezed light is experimentally difficult to produce. But the definition of "squeezed" is defined as a particular, precise mathematical property of the state of light generated. So it was not some arbitrarily level of "toughness". An analogy: Historically, there weren't many practical ways to cool things besides putting it along side things that were already cold and allowing them to transfer heat. (This is why ice harvesting existed: https://en.wikipedia.org/wiki/Ice_trade https://en.wikipedia.org/wiki/Ice_trade ). Therefore, there was a "limit" to how cold you could easily make something, namely, the temperature of the coldest natural stuff you had access to. But once you can build cheap refrigerators, the limit disappeared, and you could easily cool things much colder than ice. > Has some fundamental quantum physical property been disproven? No. Squeezing light doesn't violate any laws of physics. > Is this a breakthrough more significant than setting a coldness record? Coldness records themselves are almost never intrinsically important. They just make good popular press releases. So the technique is more important than the coldness record, but it's not necessarily that important.
  - lisivka 10y ago
    > Coldness records themselves are almost never intrinsically important. Temperatures below 0K (i.e. you need to heat a system to have 0K back) are very important to physics. They will allow to understand vacuum much better than today.
deleted 10y ago
[deleted]
smaili 10y ago
For those curious of possible applications: Sensors would become more sensitive. You can store information longer. If you were using it in a quantum computer, then you would compute without distortion, and you would actually get the answer you want.
yummybear 10y ago
It's a beautiful image, I hope it's not an illustration.
pmuk 10y ago
How can something have a fraction of a quantum of energy? I thought energy levels had to be discreet integer multiples of Planck's constant? The more I read about Quantum Mechanics, the less I understand!
- witty_username 10y ago
  (I don't know a lot about QM) I think that only applies for particular bounded situations (like electrons). There's nothing inherent in the wave function that says energy is discrete multiples; it's only due to the boundary conditions in the electron (i.e. the energy at r=0 and r=inf IIRC) that forces particular solutions. The analogy (it's actually quite similar in the differential equations) is a string. If you hold both ends only certain solutions are possible; standing waves where the length of the string is a multiple of half the wavelength. But if you don't, anything is possible.
sleavey 10y ago
Fun fact: LIGO's next big upgrade will be to install a squeezed light source to reduce quantum noise at higher frequencies.
- Already__Taken 10y ago
  This all sounds very delicate and refined but doesn't LIGO use a 1MW laser? Will this be equally as powerful?
  - sleavey 10y ago
    Squeezing is independent of the laser power used, so that's not really relevant here. Squeezed light is injected at the output port, where the photodetectors are. It is injected in the opposite direction to the classical light leaving the interferometer, where it mixes with the light field in the interferometer and has the effect of reducing quantum shot noise of the classical light that leaves. In any case, I think you've confused the circulating light power with the input laser power. The circulating light power will eventually reach something like 1MW (currently it's lower) thanks to optical cavities, but the input laser power is actually only 50W (currently), eventually 125W.
blauditore 10y ago
I've often read that absolute zero (0 K) is impossible to obtain, but never really understood whether it's just for practical reasons or if there's actually a theoretical background. As far as I understand, there's no reason why a particle having 0 kinetic energy can't exist. Also, achieving it seems theoretically possible when a particle has kinetic energy equal to one photon, then emits a photon. Obviously it's hard to experimentally produce such a situation, but not theoretically impossible. So, would 0 K theoretically be possible or am I missing something?
- ben_w 10y ago
  Well, Heisenburg uncertainty becomes more relevant the colder you get. In this case, [uncertainty in position] * [uncertainty in momentum] >= ħ/2, so something with mass but no kinetic energy would be everywhere in an infinite universe with equal probability.
  - blauditore 10y ago
    Ah, this definitely makes sense, so it's at the least not observable. Another thing crossing my mind is whether Heisenberg uncertainity becomes relevant for these kind of experiments. I think we're talking about ~10^-7 K, thus ~10^-30 J mean energy. For a hydrogen atom (1 u), that would be roughly p = ~10^28 Ns. Hence, uncertainity of location would be h/p ~= 10^-6 m, i.e. in micrometer range, which seems quite large. Does this make sense? Edit: It's aluminium in the experiment (27 u), but the numbers should be in the same ballpark.
    - ben_w 10y ago
      Sounds about right to me. Wikipedia's list of Orders of magnitude (temperature)* shows atomic waves being coherent over centimetres at 10^-15 K, and a 10^8 difference in temperature is a 10^4 difference in momentum — which fits μm to cm. * https://en.wikipedia.org/wiki/Orders_of_magnitude_(temperature https://en.wikipedia.org/wiki/Orders_of_magnitude_(temperatu...
- contravariant 10y ago

88 points by upen 10y ago

arXiv: https://arxiv.org/pdf/1606.08795.pdf https://arxiv.org/pdf/1606.08795.pdf By the way, this group achieved ground-state cooling of a very similar mechanical oscillator back in 2011. "Sideband cooling of micromechanical motion to the quantum ground state" http://www.nature.com/nature/journal/v475/n7356/abs/nature10261.html http://www.nature.com/nature/journal/v475/n7356/abs/nature10... arXiv: https://arxiv.org/abs/1103.2144 https://arxiv.org/abs/1103.2144 Both oscillators were of roughly the same frequency (10 MHz) and size (~15 nm diameter aluminum disc). In terms of phonon occupation number, things improved from 0.34 to 0.19. They emphasize in this new paper that they were able to get colder by using a squeezed vacuum state of laser light. I'm no expert, but I think the use of this technique is much more important than the improvement in the mechanical state they were able to achieve.

robinduckett 10y ago
What does this mean to the layman? Is physics broken now?
- rubidium 10y ago
  No, it's a pretty traditional technique to "beat" quantum mechanics by using squeezed light states. Short version: you don't really beat quantum mechanics, you just shift where the uncertainty resides.
  - basicplus2 10y ago
    So where does the uncertainty move to?
    - deepnotderp 10y ago
      To a different subset of the Hilbert space. See here for beginners: http://www.squeezed-light.de http://www.squeezed-light.de
- correlator 10y ago
  Physics is not broken. --not 100% accurate, but a close approximation in lay terms-- If you want to measure the position of something precisely, you can bounce a laser off of it and time how long it takes the light to return. If you want take this experiment to it's extreme in sensitivity, you eventually end up measuring tiny changes in the phase of the light bouncing off of the object. Your ability to measure position is limited by your ability to measure phase. Quantum mechanics however requires there to be noise both in the amplitude and phase of the light. This is heisenberg's uncertainty principle. So measuring at the "quantum limit" means measuring fluctuations in phase equal to half of the uncertainty limit (half noise in amplitude and half in phase). There is no law that said half the noise has to be in phase, just that the product of amplitude and phase noise is greater than or equal to the heisenberg limit. You can take extra noise out of phase and put it into amplitude to measure "beyond the quantum limit" without breaking any rules. This is called a "squeezed state." That is exactly what they did. They created "squeezed light" and used it to measure a degree of freedom (like phase) "beyond the quantum limit" This was a very challenging and impressive measurement. John Teufel has been doing work leading up to this for over 7 years (2 post doctorates and a professorship). He worked on quantum computers as an undergraduate too so you may even be able to add a few more years onto that.
  - jacobush 10y ago
    Wow, he's a devil of a scientist!
    - deleted 10y ago
      [deleted]

daveguy 10y ago

From the article: "squeezed light removes the generally accepted cooling limit". Generally removing a limit in physics is a huge deal. What was this limit based on? Has some fundamental quantum physical property been disproven? Is this a breakthrough more significant than setting a coldness record? Was the generally accepted limit based on what would be "really tough" or something else? Anyone know?

jessriedel 10y ago
> What was this limit based on? It's base on an assumption that the cooling apparatus isn't using squeezed light. If you can squeeze light, then the limit no longer applies. > Was the generally accepted limit based on what would be "really tough" or something else? Squeezed light is experimentally difficult to produce. But the definition of "squeezed" is defined as a particular, precise mathematical property of the state of light generated. So it was not some arbitrarily level of "toughness". An analogy: Historically, there weren't many practical ways to cool things besides putting it along side things that were already cold and allowing them to transfer heat. (This is why ice harvesting existed: https://en.wikipedia.org/wiki/Ice_trade https://en.wikipedia.org/wiki/Ice_trade ). Therefore, there was a "limit" to how cold you could easily make something, namely, the temperature of the coldest natural stuff you had access to. But once you can build cheap refrigerators, the limit disappeared, and you could easily cool things much colder than ice. > Has some fundamental quantum physical property been disproven? No. Squeezing light doesn't violate any laws of physics. > Is this a breakthrough more significant than setting a coldness record? Coldness records themselves are almost never intrinsically important. They just make good popular press releases. So the technique is more important than the coldness record, but it's not necessarily that important.
- lisivka 10y ago
  > Coldness records themselves are almost never intrinsically important. Temperatures below 0K (i.e. you need to heat a system to have 0K back) are very important to physics. They will allow to understand vacuum much better than today.

deleted 10y ago

[deleted]

smaili 10y ago

For those curious of possible applications: Sensors would become more sensitive. You can store information longer. If you were using it in a quantum computer, then you would compute without distortion, and you would actually get the answer you want.

yummybear 10y ago

It's a beautiful image, I hope it's not an illustration.

pmuk 10y ago

How can something have a fraction of a quantum of energy? I thought energy levels had to be discreet integer multiples of Planck's constant? The more I read about Quantum Mechanics, the less I understand!

witty_username 10y ago
(I don't know a lot about QM) I think that only applies for particular bounded situations (like electrons). There's nothing inherent in the wave function that says energy is discrete multiples; it's only due to the boundary conditions in the electron (i.e. the energy at r=0 and r=inf IIRC) that forces particular solutions. The analogy (it's actually quite similar in the differential equations) is a string. If you hold both ends only certain solutions are possible; standing waves where the length of the string is a multiple of half the wavelength. But if you don't, anything is possible.

sleavey 10y ago

Fun fact: LIGO's next big upgrade will be to install a squeezed light source to reduce quantum noise at higher frequencies.

Already__Taken 10y ago
This all sounds very delicate and refined but doesn't LIGO use a 1MW laser? Will this be equally as powerful?
- sleavey 10y ago
  Squeezing is independent of the laser power used, so that's not really relevant here. Squeezed light is injected at the output port, where the photodetectors are. It is injected in the opposite direction to the classical light leaving the interferometer, where it mixes with the light field in the interferometer and has the effect of reducing quantum shot noise of the classical light that leaves. In any case, I think you've confused the circulating light power with the input laser power. The circulating light power will eventually reach something like 1MW (currently it's lower) thanks to optical cavities, but the input laser power is actually only 50W (currently), eventually 125W.

blauditore 10y ago

I've often read that absolute zero (0 K) is impossible to obtain, but never really understood whether it's just for practical reasons or if there's actually a theoretical background. As far as I understand, there's no reason why a particle having 0 kinetic energy can't exist. Also, achieving it seems theoretically possible when a particle has kinetic energy equal to one photon, then emits a photon. Obviously it's hard to experimentally produce such a situation, but not theoretically impossible. So, would 0 K theoretically be possible or am I missing something?

ben_w 10y ago
Well, Heisenburg uncertainty becomes more relevant the colder you get. In this case, [uncertainty in position] * [uncertainty in momentum] >= ħ/2, so something with mass but no kinetic energy would be everywhere in an infinite universe with equal probability.
- blauditore 10y ago
  Ah, this definitely makes sense, so it's at the least not observable. Another thing crossing my mind is whether Heisenberg uncertainity becomes relevant for these kind of experiments. I think we're talking about ~10^-7 K, thus ~10^-30 J mean energy. For a hydrogen atom (1 u), that would be roughly p = ~10^28 Ns. Hence, uncertainity of location would be h/p ~= 10^-6 m, i.e. in micrometer range, which seems quite large. Does this make sense? Edit: It's aluminium in the experiment (27 u), but the numbers should be in the same ballpark.
  - ben_w 10y ago
    Sounds about right to me. Wikipedia's list of Orders of magnitude (temperature)* shows atomic waves being coherent over centimetres at 10^-15 K, and a 10^8 difference in temperature is a 10^4 difference in momentum — which fits μm to cm. * https://en.wikipedia.org/wiki/Orders_of_magnitude_(temperature https://en.wikipedia.org/wiki/Orders_of_magnitude_(temperatu...
contravariant 10y ago