Quantum MeasurementsQuantum random number generatorQuantum 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 [1] measures the noise of the electric field variables of a vacuum state. This can be realized with a simple homodyne setup. By appropriately postprocessing the measured date, truly random numbers which are solely based on quantum noise can be extracted. Samples of our truly random numbers can be downloaded here: 50MB, 200MB Involved in this project is: [1]
Christian Gabriel, Christoffer Wittmann, Denis Sych, Ruifang Dong, Wolfgang Mauerer, Ulrik L. Andersen, Christoph Marquardt, Gerd Leuchs
Nature Photonics. 4, 711715 (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 quasicontinuous and flat space.
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 [1]. A visual perspective of how we perform this reconstruction can be obtained from the video abstract shown on the right.
Video abstract: Quantum polarization tomography of bright squeezed light
Thereby, we could show that the ML technique allows us to get _{Involved in this project are:Christian Mueller }_{}_{, Christoph Marquardt, Gerd Leuchs} _{[1] C. R. Müller, B. Stoklasa, C. Peuntinger, C. Gabriel, J. Řeháček, Z. Hradil, A. B. Klimov, G. Leuchs, Ch. Marquardt, and L. L. SánchezSoto,New Journal of Physics 14, 085002 (2012)} _{[preprint]} Novel quantum receiversQuantum 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 nearminimal 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 phaseshift keyed quantum signals (QPSK) which outperforms the standard scheme – heterodyne detection – for any signal power [5]. 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. _{Involved in this project are:Christian Müller} _{, Christoffer Wittmann, Christoph Marquardt, Gerd Leuchs} _{ [1] C. Wittmann, M. Takeoka, Katiuscia N. Cassemiro, M. Sasaki, G. Leuchs, and U. L. Andersen, Phys. Rev. Lett 101, 2105014 (2008). [preprint] } _{ [2] C. Wittmann, U. L. Andersen, and G. Leuchs, J. Mod. Opt. 57, 213 (2010). [preprint] } _{ [3] C. Wittmann, U. L. Andersen, M. Takeoka, D. Sych, and G. Leuchs, Phys. Rev. Lett 104, 100505 (2010). [preprint] } _{ [4] C. Wittmann, U. L. Andersen, M. Takeoka, D. Sych, and G. Leuchs, Phys. Rev. A 81, 062338 (2010). [preprint] } _{ [5] C. R. Müller, M. A. Usuga, C. Wittmann, M. Takeoka, Ch. Marquardt, }_{ U. L. }_{Andersen,}_{ and, G. Leuchs, New Journal of Physics 14, 083009 (2012). [preprint] }
