# Quantum Information

Nonclassical states of light cannot be described by a (classical) probability distribution of coherent states. In other words: in nonclassical states of light the photons are not independent from each other but rather show quantum correlations. This property can be used in quantum information protocols in order to outperform any classical protocol. A general problem in quantum information is decoherence, i.e. uncontrollable information loss to the environment.

The Quantum Interferometry Group generates distributed entanglement. Characterization of entanglement is done via quantum state tomography.

In collaboration with the Australian National University entangled laser beams involving different colours were produced.

The QI-group has researched the effect of phase noise on squeezed states and has been able to distil a less decohered ensemble from a strong decohered ensemble of phase diffused squeezed states.

Distillation and purification of entangled states have been realized in order to counteract decoherence in quantum communication [Hage *et al.*, Nat. Phys. (2008)]. This ongoing work is in collaboration with the Palacky University (Czech Republic).

Strong squeezing in combination with high purity, high bandwidth and high spatial mode quality is desirable in the field of quantum information, e.g. for teleportation, quantum cryptography, and distribution of entanglement in quantum computation networks. In 2010 we beat our own world record of 10 dB squeezing and observed 11.5 dB, together with unprecedented high state purity corresponding to a vacuum contribution of less than 5%, and a squeezing bandwidth of about 170 MHz. The analysis of our squeezed states reveals a significant production of higher-order pairs of quantum-correlated photons, and the existence of strong photon number oscillations.

[M. Mehmet *et al.*, Phys. Rev. A 81, 013814 (2010)]

Document Actions