Cooperativity in Highly Excited Rydberg
Ensembles — Control and Entanglement

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Science behind COHERENCE

In these pages, we aim to explain the science behind COHERENCE to a general audience. To this end, some of the most exciting results obtained by the young researches involved in the network will be presented in an accessible form. In the meantime, below you can find a couple of questions and answers explaining in simple terms what COHERENCE is all about. If you have further questions, we will be happy to answer them.

Q: In a nutshell, what kind of science does COHERENCE do?
A: COHERENCE investigates what happens when atoms are excited to so-called Rydberg states in which they interact very strongly with one another. In particular, we would like to understand what kind of collective and cooperative effects occur in such systems – that is, how they behave in large numbers – and to what extent it is possible to actively and coherently control such systems.

Q: What are Rydberg atoms?
A: An atom consists of a nucleus made up of neutrons and positively charged protons, around which a number of negatively charged electrons orbit. Quantum mechanics dictates that only certain well–defined orbits are possible, and there exists an orbit for which the total energy (that of the motion and the electrostatic energy) is at a minimum – this is called the ground state. From this lowest energy state, we can now excite the electrons to higher–lying orbits, for example by shining laser light on the atoms. These orbits are characterized by a number called the ”principal quantum number”; very high–lying orbits have large principal quantum numbers, and for values above 15–20 one speaks of ”Rydberg atoms”.

Q: Why are Rydberg atoms so interesting?
A: The fact that Rydberg atoms are highly excited has several consequences. For one thing, in a Rydberg atom the outermost electron orbits the nucleus at a much larger distance than a ground&8211;#state atom. To give you a real–life example: If the orbit of the electron in a “normal“ atom corresponds to the altitude at which commercial jet planes fly, then the corresponding orbit in a Rydberg atom can be as far away as the moon.

These abnormally large orbits, in turn, make Rydberg atoms extremely sensitive to electric fields because they feel the electric charge of the nucleus only very weakly. Finally, if one puts two such atoms close together, their orbiting electrons can influence each other and lead to an attractive or repulsive force between Rydberg atoms that can be billions of times larger than the forces acting between ground–state atoms.

Q: What is meant by “coherent“ control?
A: In the quantum world, the behaviour of a physical object is determined by its wavefunction. You may have heard, for example, that particles behave like waves and that those waves can reinforce and cancel each other, leading to a phenomenon called interference. This phenomenon is at the heart of quantum mechanics and allows us to do things with quantum objects that cannot be done in classical physics, which dominates our everyday experience.

However, interference can only occur if the wavefunctions of the particles involved are synchronized. Unfortunately, any small disturbance will easily destroy that synchronization, such as collisions between atoms or the effect of stray fields (which might be caused by something as banal as the lead of an electric appliance being in the wrong place in the laboratory). “ Coherent control“ means that we have to be able to shield our experiments from such harmful effects and devise clever schemes which make sure that our Rydberg atoms don’t lose their synchronization. Not an easy thing to do!

Q: Are there any real-life applications of the research done in COHERENCE
A: There are, indeed. For one thing, studying large numbers of strongly interacting particles allows us to learn more about similar systems that occur in nature. In the laboratory, we have a great degree of control over how many atoms are involved, how strongly they interact, etc. In this way, we can tailor-make quantum systems that mimic the behaviour of real-life systems and allow us to study their properties under controlled conditions. Also, the coherent control of Rydberg states is a possible avenue towards to realization of quantum gates (that do logical operations such as NOT or AND) and, ultimately, a quantum computer.

Oliver Morsch, COHERENCE

Popular science accounts of the young researchers’ topics

Making single photons interact using giant atoms (Hannes Busche)

Magnetic–film atom chip with 10 μm period lattices of microtraps for quantum information science with Rydberg atoms (Julian Naber)

Antiferromagnetic long–range order in dissipative Rydberg lattices (Wildan Abdussalam)

Cluster Luttinger Liquids of Rydberg–Dressed Atoms in Optical Lattices (Marco Mattioli)

Dipole–mediated energy transport in cold Rydberg clouds (Vladislav Gavryusev)

Cluster Surface science with Rydberg atoms (Mike Kohlhoff)

Damping of a classical nano-mechanical oscillator using electromagnetically induced Transparency (Adrian Sanz Mora)

Engineering correlations on demand using Rydberg atoms (Maria Martínez Valado)

COHERENCE young researchers’ presentation during the heavily frequented “ Long Night of Sciences“ in Dresden (July 2013) which was targetted at the general public (in German language):