Quantum random number generators have the potential to deliver truly random numbers, an important attribute for many applications ranging from simulations over gambling to cryptography. Our random number generator  measures the noise of the electric field variables of a vacuum state. This can be realized with a simple homodyne setup. By appropriately post-processing the measured date, truly random numbers which are solely based on quantum noise can be extracted. For more information, visit www.mpl.mpg.de/quantumbits.
Christian Gabriel, Christoffer Wittmann, Denis Sych, Ruifang Dong, Wolfgang Mauerer, Ulrik L. Andersen, Christoph Marquardt, Gerd Leuchs
Nature Photonics. 4, 711-715 (2010).
Artists' illustration of the continuous variable quantum random number generator. Design by Michael Förtsch.
Tomography of polarization squeezed states
Polarization states of the light field are conveniently described in the Stokes space, which is spanned by the Stokes vectors. In the quantum mechanical treatment, the Stokes space is not continuous, but is composed of a set of nested spheres. A polarization state is generally distributed over several of these spheres. However, in the case of a bright and localized state, the state can be approximated to lie in a quasi-continuous and flat space.
Video abstract: Quantum polarization tomography of bright squeezed light
We could show explicitly that the state reconstruction is then equivalent to the inverse Radon transformation from projections of the state along all directions in Stokes space. This technique, however, is statistically unstable and is likely to produce unphysical features in the reconstructed state. As an alternative, we have proposed a novel maximum likelihood reconstruction (ML) and compared the two techniques for the reconstruction of a bright polarization squeezed state . A visual perspective of how we perform this reconstruction can be obtained from the video abstract shown on the right.
Novel quantum receivers
Quantum state discrimination is one of the fundamental issues in optical communication and quantum signal detection. An important figure of merit here is the error rate. For a pair of coherent states, homodyne detection represents an effective way to achieve near-minimal error discrimination. A receiver based on photon counting would lead to even smaller error rates, provided if signal intensities are sufficiently large. In order to outperform the homodyne scheme also for smaller photon numbers, we have succesfully implemented a novel detection schemes [1–4].
In our latest work on quantum state discrimination, we experimentally demonstrated a hybrid discrimination scheme for quadrature phase-shift keyed quantum signals (QPSK) which outperforms the standard scheme – heterodyne detection – for any signal power . The QPSK alphabet comprises four states with identical amplitude but separated by a relative phase shift of π/2 and is used for instance by the HSDPA protocol in UMTS networks for mobile phones, in digital satellite communication and in backbone fiber networks.