# Quantum networks

### Research objectives

- quantum walk
- modeling topological insulators
- measurement back-action induced complex chaos

Schematic representation of one iteration step of the entanglement purification process. The diamond shaped elements denote acceptance of the pair of qubits only when both measurements yield 0. The depicted procedure is used to prepare from an ensemble *E′* made up of pairs of qubits all in a state |*ψ*⟩, an ensemble *E* made made up of pairs of qubits in the state |*ψ*′⟩. During the procedure, half of the pairs in *E* are completely used up, while a portion of the other half is retained depending on the measurement outcome. *H* is the Hadamard gate.

Convergence of given initial states to the two limiting cycles as a function of the complex parameter *ζ*. The blue color denotes convergence to a maximally entangled state, and the green color the convergence to the cycle of the two separable states.

### Participants

Tamás Kiss

(senior scientist)

János Asbóth

(postdoc)

Zoltán Darázs

(PhD student)

Bálint Kollár

(PhD student)

### Collaborators

- Prof. Igor Jex, Czech Technical University in Prague
- Prof. Oliver Mülken and Prof. Alexander Blumen, University of Freiburg

### References

- T. Kiss, S. Vymetal, L. D. Toth, A. Gabris, I. Jex, G. Alber: Measurement-Induced Chaos with Entangled States, Phys. Rev. Lett.
**107**, 100501 (2011)