A microscopic view on quantum fluctuations

October 14, 2011
Schematic view of the atom distribution in the optical lattice. Quantum fluctuations (white) are directly visible as neighbouring dark spots.

(PhysOrg.com) -- Scientists at the Max Planck Institute of Quantum Optics achieve direct imaging of quantum fluctuations at absolute zero temperature.

Fluctuations are fundamental to many in our everyday life, such as the from a liquid into a gas or from a solid into a liquid. But even at absolute zero temperature, where all motion in the classical world is frozen out, special mechanical fluctuations prevail that can drive the transition between two quantum phases. Now a team around Immanuel Bloch and Stefan Kuhr at the Max Planck Institute of (MPQ) has succeeded in directly observing such (Science, 14 October 2011).

Using a high resolution microscope, they were able to image quantum-correlated particle-hole pairs in a gas of . This allowed the physicists to unravel a hidden order in the crystal and to characterize the different phases of the quantum gas. The work was performed together with scientists from the Theory Division at the MPQ and ETH Zurich. These measurements open new ways to characterize novel quantum phases of matter.

The scientists start by cooling a small cloud of down to a temperature near absolute zero, about minus 273 degree Celsius. The ensemble is then subjected to a light field that severely restricts the motion of the particles along one-dimensional tubes of light aligned in parallel. An additional standing laser wave along the tubes creates a one-dimensional optical lattice that holds the atoms in a periodic array of bright and dark regions of light.

The atoms move in the periodic light field like electrons in solids. As these can be electric conductors or insulators, also the one-dimensional quantum gases can behave like a superfluid or like an insulator at low temperatures. In particular, the height of the potential plays an important role: it determines whether the atom is fixed on a particular lattice site or whether it is able to move to a neighbouring site. At very large lattice depths, each lattice site is occupied by exactly one atom. This highly ordered state is called a “Mott insulator”, after the British physicist and Nobel laureate Sir Neville Mott. When the lattice depth is decreased slightly, the atoms have enough energy to reach a neighbouring site by quantum mechanical tunneling. In this way, pairs of empty and doubly occupied sites emerge, so-called particle-hole pairs. Intriguingly, these quantum fluctuations also occur at absolute zero temperature, when all movement in the classical world is frozen out. The position of the quantum-correlated particle-hole pairs in the crystal is completely undetermined and is fixed only by the measurement process.

In recent experiments, the physicists around Stefan Kuhr and Immanuel Bloch had already developed a method, which allowed to image single atoms lattice site by lattice site. The atoms are cooled using laser beams, and the fluorescence photons emitted in this process are used to observe the atoms with a high resolution microscope. Holes naturally show up as dark spots, but so do doubly occupied sites as the two particles kick each other out of the lattice in the experiment. Therefore particle-hole pairs appear as two neighbouring dark lattice sites (see figure below). “With our technique, we can directly observe this fundamental quantum phenomenon for the first time”, describes doctoral student Manuel Endres enthusiastically.

The physicists measure the number of neighbouring particle-hole pairs through a correlation function. With increasing kinetic energy, more and more particles tunnel to neighbouring sites and the pair correlations increase. However, when the number of particle-hole pairs is very large, it becomes difficult to unambiguously identify them. Hence the correlation function takes on smaller values. Finally, the ordered state of a Mott insulator vanishes completely und the becomes a superfluid. Here fluctuations of holes and particles occur independently. The correlation function measured in the experiment is very well reproduced by model calculations, which were performed by scientists from the Theory Division at the MPQ and the ETH Zurich. Interestingly, the same investigations on two-dimensional quantum-gases clearly showed that quantum fluctuations are not as prominent as in one-dimensional systems.

The scientists extended their analysis to correlations between several sites along a string. Such non-local correlation functions contain important information about the underlying many-body system and can be used as an order parameter to characterize different quantum phases. In the experiment described here, such non-local order parameters have been measured for the first time. In the future, the scientists plan to use these measurements for the detection of topological quantum phases. These can be useful for robust quantum computers and could help to understand superconductivity at high temperatures.

Explore further: Quantum gas microscope offers glimpse of quirky ultracold atoms

More information: M. Endres, M. Cheneau, T. Fukuhara, C. Weitenberg, P. Schauß, C. Groß, L. Mazza, M.C. Banuls, L. Pollet, I. Bloch, and S. Kuhr, Observation of Correlated Particle-Hole Pairs and String Order in Low-Dimensional Mott Insulators, Science, 14 October 2011, DOI: 10.1126/science.1209284

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1 / 5 (2) Oct 14, 2011
Is it me or does an ultra cold atom gas sound a bit contradictory? It is probably correct, but I am used to gas states being present at higher temperatures than liquid or solid states.
5 / 5 (1) Oct 14, 2011
The state of matter is not only dependent on the temperature. Pressure also plays a role (and even the presence/absence of an electric and/or magnetic field can change the state of matter).

So it's quite possible to have low temperature gases (or even low tempeature plasmas).
not rated yet Oct 14, 2011
I fully agree with you, though reading on they talk about quantum gas and atom lattice site. All a bit confusing for me, especially when they mention -273C where I assume all matter is a solid?
not rated yet Oct 14, 2011
Better check your assumptions.

not rated yet Oct 14, 2011
Isn't this just a BSE (Bose Einstein Condensate)?
not rated yet Oct 14, 2011
No its not a bose einstein condensate, -> (rubidium atoms)

the article is also misleading beyond the fact that it really wasn't directly observed at 0 K.
not rated yet Oct 15, 2011
Isn't this just a BSE (Bose Einstein Condensate)?
It isn't neither gas, neither BEC, because the atoms are hold at their places with external fields of optical lattice like the eggs in their cartoon. It's solely an artificial system, which probably doesn't exist anywhere in the Nature.
not rated yet Oct 15, 2011
so do doubly occupied sites as the two particles kick each other out of the lattice in the experiment. Therefore particle-hole pairs appear as two neighbouring dark lattice sites
This is very vague description of what actually happened there. The double dark spots occurs, when two atoms are held together in single cell of optical lattice at proximity. Due their mutual collisions and interactions the fluorescence of excited atoms is darkened due the thermal vibrations of the resulting pair. So that the place with two atoms in one cell of optical lattice remains as dark, as the neighboring place with atom missing.

In infrared light we would see instead, one of the dark sites from each pair is glowing brightly, because of presence of pair of atoms, which are colliding and bouncing furiously there. It would serve as an undeniable evidence, these atoms weren't both kicked from optical lattice at distance, but they're actually remain hidden inside of one of the dark sites pair.
not rated yet Oct 15, 2011
hello. I need information about XPS if someone know please help me. I'm student of ULA in venezuela. Thank
not rated yet Oct 15, 2011
hello. I need information about XPS if someone know please help me. I'm student of ULA in venezuela. Thank

I believe that is a file format...
not rated yet Oct 15, 2011
It could be a Bose-Einstein condensate on top of a one-dimensional gas held together with light... Well, what ever it is in actuality, it sure made a pretty picture and that is what they liked so much about it.
not rated yet Oct 16, 2011
@ Ed, thanks
@ hard2grep, agreed, it is prety

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