# Physicists: what does this article mean by "squeezed vacuum"? **URL:** \ **Category:** Factual Questions\ **Created:** [April 15, 2014, 4:37pm UTC](https://boards.straightdope.com/t/physicists-what-does-this-article-mean-by-squeezed-vacuum/686188 "2014-04-15T16:37:10Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [April 15, 2014, 4:37pm UTC](https://boards.straightdope.com/t/physicists-what-does-this-article-mean-by-squeezed-vacuum/686188/1 "2014-04-15T16:37:10Z")
It can be a fun game trying to reconstruct what is really happening when a reporter explains physics in an article, but I can’t work out this one. From [an article](http://www.newscientist.com/article/mg22229640.600-einsteins-silence-listening-for-spacetime-ripples.html?full=true#.U0vJuPldVrM) in _New Scientist_ about LIGO: > [@](#): > > The physicist Carlton Caves first understood this bizarre quantum noise in the early 1980s, and showed how you might eliminate some of it. Uncertainty does not limit how much we can know about single quantities, but pairs of them in tandem, so it implies a certain elasticity. If you need to measure a photon’s time of arrival, for example, you might constrain its uncertainty by agreeing to know less about its energy. > > That is perfectly doable: starting with one photon you can squeeze it to make two whose uncertainty is concentrated in one of a pair of properties, allowing the other to be measured almost noise-free. Even odder, in theory at least you can do the same thing not to a photon of light, but to an absence of one. Gradually reduce the stream of light entering a squeezer to nothing, and a squeezed field of nothing will emerge from the other end, a vacuum whose nothingness has been manipulated in just the way you wanted. “It’s a pretty remarkable thing,” says David McClelland of the Australian National University in Canberra. “It’s almost like magic; it’s vacuum but it’s not.” A few years back, his team actually built a nothing squeezer. By shining squeezed vacuum onto a photodetector, they reduced the detector’s intrinsic noise to below its natural floor. > > “This is beyond spooky, right?” says Mavalvala. Together with her MIT colleague Sheila Dwyer, she has since sent squeezed vacuum into the LIGO interferometer, and seen its quantum noise drop, too. The detector becomes quieter than nothing. What specifically are they talking about when they use the term “squeezed vacuum”? Can someone de-metaphor this for me, and explain it in more technical language? Also, when I first read this, I imagined squeezing a photon into two to mean using a non-linear material to convert a high-energy photon into two entangled lower-energy photons, allowing them to get highly precise position information. But possibly I’m wrong, and they’re just referring to sending a photon through a partially silvered mirror. Or maybe something else entirely. So what do they mean there, as well? ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [April 15, 2014, 5:33pm UTC](https://boards.straightdope.com/t/physicists-what-does-this-article-mean-by-squeezed-vacuum/686188/2 "2014-04-15T17:33:23Z")
Generally, a squeezed state is a state that saturates the uncertainty principle, i.e. for which ∆x∆p = ħ/2, and in such a way that ∆x doesn’t equal ∆p. The name derives from the fact that you can express the uncertainty in a state by an area in a diagram; for a state in which position and momentum uncertainty agree, that area is a circle, and if both don’t agree, it’s an ellipse—i.e. the uncertainty region has been ‘squeezed’. A squeezed vacuum is a squeezed state such that the average amplitude of the light is zero (but not, in general, the average photon number). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 15, 2014, 8:57pm UTC](https://boards.straightdope.com/t/physicists-what-does-this-article-mean-by-squeezed-vacuum/686188/3 "2014-04-15T20:57:39Z")
> [@](#): > > …and in such a way that ∆x doesn’t equal ∆p. Which is, of course, a meaningless criterion, given that they have different units. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [April 15, 2014, 10:19pm UTC](https://boards.straightdope.com/t/physicists-what-does-this-article-mean-by-squeezed-vacuum/686188/4 "2014-04-15T22:19:57Z")
How do they ensure a squeezed state starting with one photon and converting it to two? What are they doing when the squeeze a vacuum? ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [April 16, 2014, 2:11am UTC](https://boards.straightdope.com/t/physicists-what-does-this-article-mean-by-squeezed-vacuum/686188/5 "2014-04-16T02:11:24Z")
It is easiest to think about this in terms of a harmonic oscillator. The generalization from harmonic oscillators to the modes of oscillation in an optical resonator is straightforward (for a physicist). The “vacuum state” of a harmonic oscillator (and the modes of an optical resonator) is the lowest energy state. For the oscillator, it does NOT represent a state of no motion, it is represented by a gaussian wave packet with a characteristic width in which delta x and delta p are equal in the natural dimensionless units for the system. In the oscillator, we can excite the system to higher energy states by adding energy in units of twice the vacuum state energy. The same is true in the optical case, but we have a special name for these extra boosts of energy. We call them photons. It is just terminology, which can lead to confusion. Now, it is possible to imagine infinitely many other excited states, which are not in general energy states (states with a well-defined energy). For example, in the oscillator, imagine that you squeeze the ground state gaussian wave function so delta x is smaller. In this case delta p will be bigger, preserving the product of delta x and delta p. We call this a squeezed state of the vacuum. You can squeeze any state in this fashion. If you squeeze the vacuum (ground state) or any other energy state, it will no longer be a stationary state, it will evolve in time. For a squeezed vacuum state, if it is very narrow in position at time zero (and broad in momentum), it will be very narrow in momentum (and broad in position) a quarter cycle later. I’ve probably only confused you further, but the important thing is not to get confused by terms like “vacuum” and “photon” that tend to obscure the simple picture of what is going on here. In atoms, the energy states of the electron are of prime importance, because electrons are electrically charged and will radiate their energy away until they decay into an energy state, which is stationary. For light, no such thing occurs, so optical states are usually far away from being energy states characterized by a fixed number of photons. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [April 16, 2014, 4:46am UTC](https://boards.straightdope.com/t/physicists-what-does-this-article-mean-by-squeezed-vacuum/686188/6 "2014-04-16T04:46:43Z")
> [@Chronos](#): > > Which is, of course, a meaningless criterion, given that they have different units. OK, yes, that was too much of a gloss. Effectively, you can make both dimensionless, by measuring position in units of √(ћ/(ωm)), and momentum in units of √(ћωm) (I’ve heard this called ‘natural oscillator units’). In terms of those, the uncertainty relation for a squeezed state should then be ∆x∆p = 1/2, of course. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [April 16, 2014, 11:29am UTC](https://boards.straightdope.com/t/physicists-what-does-this-article-mean-by-squeezed-vacuum/686188/7 "2014-04-16T11:29:06Z")
You start out with nothing, and you end up with less than nothing. ☹