QIBEC for beginners

QIBEC for beginners

The aim of the QIBEC project is the investigation and development of ultra-precise measurement devices exploiting the resources available in the quantum world. In these devices, called quantum interferometers, very small forces or accelerations can be precisely inferred from the effect they produce on an exotic quantum state of matter called Bose-Einstein condensate (BEC). This is obtained by cooling a dilute gas of a suitable atomic species down to temperatures very close to the absolute zero.
While the atoms in a gas at higher temperature bump chaotically into each other at very large speeds, the atoms in a BEC are considerably slower and move in a much more orderly fashion. Using a simple analogy, we could think of the atoms in a “hot” gas as people performing a wild pogo dance. a wild pogo dance
airline staff flash-mob dance As temperature is lowered, and the BEC state is entered, the atoms synchronize in a sort of choreographed action, like people in a flash-mob dance act. An even better-suited analogy is the “Mexican wave” performed by the spectators in a packed stadium. Indeed, the movement of the atoms in a BEC can be thought in terms of waves, and it does exhibit wave phenomena such as interference.
A wave is an oscillation accompanied by a transfer of energy that travels through space. An neat example of wave in everyday life is provided by the concentric ripples traveling on the surface of water that we can generate by throwing a pebble into a pool. If we throw two stones some distance apart we can see that the simple pattern of crests and troughs on the surface of water becomes somewhat more complex in the region where the two trains of ripples meet. interference of two trains of ripples on the
        surface of water
A snapshot of the water surface reveals that at some points in space the crests (or the throughs) of two overlapping ripples have summed, i.e. the displacement of the water surface is larger than on a crest of the original ripples.
constructive and
  destructive interference This is called constructive interference. At some other points, a crest “filled” a through, so that the level of the water is the same as when there are no ripples at all. This is called destructive interference. In the figure here on the left, the crests and throughs are represented by thick and thin lines, respectively. Since the waves travel, the interference pattern evolves with time, but the constructive and destructive interference points remain aligned, and lines of constructive and destructive interference lines are alternated.
Now let us assume that the the right side of the image above is the edge of the pool. If we carefully look at the average height of the waves hitting that edge, we can observe it not uniform, but it is large at constructive-interference points and small or null at destructive-interference points, also in an alternating pattern. The details of such pattern depends on the distance between the points where the stones plunged. Also it depends on the distance between the line joining these points and the edge of the pool.
interference of the light
  emerging from two holes pierced in a screen Somewhat strikingly, the very same phenomenon can be observed in the intensity pattern formed on a screen by the light emerging from two small holes drilled in an opaque plate. The intensity pattern is not the one expected from the sum of two uniform light cones, but consists in an alternation of dark and bright fringes. These correspond to destructive and constructive interference of light, respectively.
Again, the details of the pattern on the screen depend on the on the distance between the plate and the screen as well as on the distance between the holes. Also, it depends on the color of the light.
As you probably know, white light "contains" different colors, which can be separated when refracted by a prism, or by the rain drops when a rainbow is formed. Colors can be separated also as a result of interference. Indeed, each color contained in the white light produce bright fringes at slightly different positions.
The image on the left shows the interference patter formed by monochromatic (green) light, and the one formed by white light.
You have certainly seen this phenomenon in everyday life. The iridescence on the surface of a soap bubble, a DVD disc or on mother of pearl is caused by interference of white light.
double-slit
        interference patterns
soap bubbles a DVD mother-of-pearl
The analogy between water ripples and light intensity, noticed by Thomas Young at the dawn of the 1800’s, played a crucial role in establishing the wave nature of light. In fact, the above described double slit apparatus is a simple example of a measurement device exploiting interference, i.e. an interferometer. Young used it to measure the wavelength, i.e. the distance between two subsequent crests, in ligth of different colors.
optical interferometer Conceptually, the interferometers employed in modern “classical” interferometry are not much more complex than Young's simple tool. In fact, their basic design dates back to more than 100 years ago. They are usually based on the interference of electromagnetic waves, such as light or radio waves. Basically, the value of the measured quantity is inferred from the features of the fringes produced by the interference of two electromagnetic waves.
One of these waves, or beams, acts as a reference, while the other acts as a probe for the quantity to be measured (e.g. some optical property of the green object in the above figure). Interferometry is a key technique for precision measurements in many fields of science and technology (astronomy, aerodynamics, plasma physics, quantum mechanics, materials engineering etc).
In quantum interferometry, some often puzzling properties of quantum systems are used as resources to further improve the precision of the measurements. The paradigm of these resources is quantum entanglement, which was described as “spooky action at a distance” by Albert Einstein. As you probably know, an electromagnetic wave, such as for instance a ray of light, can be thought as consisting of many individual particles called photons. This “particle theory of light” is crucial when one has to take quantum effects into account. It turns out that the precision of the measurement obtained with an interferometer can be greatly enhanced if the photons in the reference and probe beams are entangled. artist's impression of the entanglement between two particles
quantum interference of two BECs As we mentioned earlier, a BEC is obtained by cooling a dilute gas of a suitable atomic species down to temperatures very close to the absolute zero. Despite being formed by a comparatively small number of atoms, a BEC can be thought of as a wave, and exhibits wave phenomena such as interference. Thus a beam of light and a BEC are very similar in many respects. In fact, it is possible to perform interferometry measurements using two BECs in place of two electromagnetic beams. Again, the precision of the measurements can be greatly enhanced if the atoms in the probe and reference BECs of the quantum interferometer are entangled.
It is worth stressing that quantum interferometers based on BECs are not a mere alternative to those based on electromagnetic waves. Indeed, unlike photons, the atoms in a BEC have a nonzero mass. Therefore, quantum interferometers based on BECs are particularly suited to measure gravitational or inertial forces very precisely.

All the partner in the QIBEC international collaboration are bringing their extensive theoretical and/or experimental expertise on BECs and interferometry to the attainment of the goals of the project.

Concept by P. Buonsante.