What are quantum fluctuations

"We squeezed the vacuum"

Even in a perfect vacuum, quantum fluctuations ensure that particles arise spontaneously and immediately decay again - thus creating a background noise in the electromagnetic field. Scientists at the University of Konstanz first measured this vacuum noise two years ago. In the meantime, they have succeeded in influencing this phenomenon in a targeted manner. You have implemented a so-called squeezed vacuum, in which the noise floor of the quantum fluctuations is reduced compared to the absolute vacuum - an area that is so to speak, emptier than empty. Welt der Physik spoke to Alfred Leitenstorfer from the University of Konstanz about their results, which the team published in the specialist magazine "Nature".

World of Physics: What are Quantum Fluctuations?

Alfred Leitenstorfer: Quantum fluctuations are fluctuations that occur in the ground state of a field. In our case it is the electromagnetic field. If you look at a component of the electric field at one point in space and in time and the magnetic field amplitude perpendicular to it, then quantum electrodynamics says that you cannot measure these two quantities at the same time with arbitrary precision. This inevitably results in the finding that they cannot both be exactly zero at the same time. That means: Even if the light intensity is zero and you are in absolute darkness, there are still finite fluctuations of the electric or magnetic field, a kind of background noise of the vacuum. We first succeeded in studying these vacuum fields directly two years ago.

Alfred Leitenstorfer and his colleagues

How can one measure these quantum fluctuations directly?

Werner Heisenberg had the basic understanding many decades ago, but it was only a few months ago that I began to dig up this story in detail. After a lecture, a colleague motivated me to read through Heisenberg's old lectures that he had given at the University of Chicago in 1929. There he derives a generalized uncertainty relationship for wave fields. It seems that the approach has been largely forgotten - or at least not pursued further because no one has been able to carry out such experiments.

But Heisenberg has already described what we have been thinking about in parallel to our work: If the spatial area and the time interval over which these fields are measured are kept as small as possible, then the uncertainty principle means that the quantum fluctuations become very large . So you have to detect an electric field in a small area and over a very short time interval. We have developed a special measurement technology that allows us to do this. In the meantime we have managed to influence and change these quantum fluctuations directly in space and time.

How can one influence quantum fluctuations?

We created what is known as a squeezed vacuum. This is already known from quantum optics, but so far it has not been possible to generate it in the mid-infrared spectral range that we are investigating, nor to analyze it in such a direct way - namely with a time resolution of less than half a light oscillation and directly in the electric field. We have now achieved all of this. A squeezed vacuum works as follows: These vacuum fluctuations, the quantum fluctuations of the electromagnetic field, also spread in space and time as described by Maxwell's equations.

The quantum fluctuations are measured using a laser experiment

They also behave like normal electromagnetic fields in media and at interfaces, and they can be manipulated accordingly, for example by phase modulation. Phase modulation occurs when an electromagnetic wave experiences a refractive index that changes over time - this corresponds to a change in the speed of light. If one works with a short impulse, this is done locally in space-time. So we can imagine that for a short time the reference system in which the vacuum fluctuations spread is accelerated and then decelerated again. As a result, the fluctuations are thinned out in a certain area of ​​space and accumulate in another space-time segment.

Can you imagine that figuratively?

It's similar to a traffic jam. Imagine a random distribution of cars on the freeway, all of which are more or less the same speed. Now a group of cars are starting to slow down. Then the traffic will build up behind it and in front of it there will be a thinning of the traffic. As a first approximation, the same thing happens when squeezing a vacuum state.

How do you do this in the laboratory?

We use an optically non-linear crystal through which we send what is known as pump light. This is a very short light pulse in the near infrared range, which is shorter than half a light oscillation in the spectral range in which you want to squeeze the vacuum. In the non-linear crystal, the refractive index changes locally with the amplitude of the light pulse that traverses the crystal. The short infrared light pulse will pass through the crystal and wherever it is present, the speed of light will be different than in the surroundings.

What happens to the vacuum fluctuations at these points?

Scheme of the quantum fluctuations

The vacuum fluctuations that happen to propagate with this pump light are phase modulated, which means that their amplitudes are redistributed in space-time. At the end of the non-linear crystal, it goes back out into free space and this non-linear interaction ceases. Then we can filter out the pump light in such a way that only the squeezed vacuum remains in the mid-infrared spectral range. With the help of a second non-linear crystal and another light pulse, we can then analyze it.

What did the analysis show?

We have found that this electromagnetic field noises less than the naked vacuum noise for certain times. Of course this is only a small effect, but we can see it directly in the experiment. That's astonishing.

So this tiny region in spacetime is even emptier than empty?

Exactly, although physically the absolute void is of course never empty. But we are able to create even more emptiness locally than absolute nothing. It only has the price - and here we come back to Heisenberg - that more noise then has to occur at a different point in time. And here, at the latest, the simplified picture of traffic jams is no longer sufficient: The uncertainty principle contains the product of two conjugate quantities in every form, which must always remain greater than or equal to a certain value. In our case of the electric field, this means that close to a space-time region with very little noise there is an adjacent region in which the noise increases disproportionately.