Light can behave as an electromagnetic wave or a shower of particles that have no mass, called photons, depending on the conditions under which it is studied or used. Matter, on the other hand, is composed of particles, but it can actually exhibit wave-like properties, giving rise to many astonishing phenomena in the microcosm.

 

At our institute we explore the interaction of light and quantum systems, exploiting the two extreme regimes of the wave-particle duality of light and matter. On the one hand we handle light at the single photon level where wave-interference phenomena differ from those of intense light beams. On the other hand, when cooling ensembles of massive particles down to extremely low temperatures we suddenly observe phenomena that go back to their wave-like nature. Furthermore, when dealing with ultrashort and highly intense light pulses comprising trillions of photons we can completely neglect the particle properties of light. We take advantage of the large force that the rapidly oscillating electromagnetic field exerts on electrons to steer their motion within molecules or accelerate them to relativistic energies.

 

Research at MPQ currently focuses on four different areas:

 

• High-precision spectroscopy of hydrogen

As hydrogen has a very simple atomic structure it is the ideal subject upon which to test the theory of quantum electrodynamics which describes the interaction between light and matter. The development of the frequency comb for which Prof. Theodor W. Hänsch received the Nobel Prize in Physics 2005 led to a precision in the measurement of the spectral lines of hydrogen of 1 in 10¹⁴, so far in agreement with theory. With the aim of further testing fundamental laws of physics, the division of Prof. Hänsch is extending such investigations to anti-hydrogen, myonic hydrogen, and other elements that resemble hydrogen.

 

• Single photons and individual atoms

A cubic centimetre of normal air contains about 10²⁰ atoms, a light bulb emits about 10²⁰ photons per second. Scientists at MPQ, however, are able to store and manipulate single atoms and photons and let them interact in a controlled way. Experiments of this kind are an important step towards building a quantum computer or a quantum network, in which quantum particles (such as atoms, molecules or photons) serve as quantum bits. Theorists on the other hand are devising new ways of building quantum computers and communication devices which could process and transmit information in an extremely efficient and secure way. They are also participating very actively in the development of a new theory of information based on quantum mechanics, which should form the basis for the operation of the new quantum information-processing and communication devices.

 

• Matter at very low temperatures

At extremely low temperatures (less than one millionth of a Kelvin above absolute zero) systems composed of many quantum particles can take on bizarre properties which go back to their wave properties. For example, an ensemble of a million particles can suddenly behave like one large super atom, with amazing properties like superfluidity and macroscopic interference. These and other exotic properties are of fundamental interest for the understanding of quantum physics. Scientists at MPQ regard these systems as storage systems for quantum information, or as model systems for many-body systems appearing in other fields of science, for example solid-state physics. Besides that, they are developing new theoretical tools to describe those complex systems.

 

• Experiments at extremely short time scales

Several attoseconds – billionths of a billionth of a second – is the time it takes for electrons to interact with each other and with light in matter. The inconceivably short period of an attosecond is to a second what a second is to the age of the universe! At MPQ scientists have pioneered the generation and measurement of intense laser light with attosecond control of its electric-field waveform and extreme ultraviolet light flashes of attosecond duration. They use these novel tools for control and real-time observation of the atomic-scale motion of electrons in all forms of matter: inside atoms, molecules, clusters, as well as in solids and plasmas. At ultrahigh laser intensities, MPQ researchers accelerate electrons and ions to velocities approaching the speed of light and pursue the development of compact brilliant particle sources for applications in physics, biology and medicine.

 

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