Georg Harder

An optimized photon pair source for quantum circuits

We implement an ultrafast pulsed type-II parametric down conversion source in a periodically poled KTP waveguide at telecommunication wavelengths with almost identical properties between signal and idler. As such, our source resembles closely a pure, genuine single mode photon pair source with indistinguishable modes. We measure the joint spectral intensity distribution and second order correlation functions of the marginal beams and find with both methods very low effective mode numbers corresponding to a Schmidt number below 1.16. We further demonstrate the indistinguishability as well as the purity of signal and idler photons by Hong-Ou-Mandel interferences between signal and idler and between signal/idler and a coherent field, respectively. Without using narrowband spectral filtering, we achieve a visibility for the interference between signal and idler of 94.8% and determine a purity of more than 80% for the heralded single photon states. Moreover, we measure raw heralding efficiencies of 20.5% and 15.5% for the signal and idler beams corresponding to detector-loss corrected values of 80% and 70%.

Single-Mode Parametric-Down-Conversion States with 50 Photons as a Source for Mesoscopic Quantum Optics.

We generate pulsed, two-mode squeezed states in a single spatiotemporal mode with mean photon numbers up to 20. We directly measure photon-number correlations between the two modes with transition edge sensors up to 80 photons per mode. This corresponds roughly to a state dimensionality of 6400. We achieve detection efficiencies of 64% in the technologically crucial telecom regime and demonstrate the high quality of our measurements by heralded nonclassical distributions up to 50 photons per pulse and calculated correlation functions up to 40th order.

Uncovering Quantum Correlations with Time-Multiplexed Click Detection

We report on the implementation of a time-multiplexed click detection scheme to probe quantum correlations between different spatial optical modes. We demonstrate that such measurement setups can uncover nonclassical correlations in multimode light fields even if the single mode reductions are purely classical. The nonclassical character of correlated photon pairs, generated by a parametric down-conversion, is immediately measurable employing the theory of click counting instead of low-intensity approximations with photoelectric detection models. The analysis is based on second- and higher-order moments, which are directly retrieved from the measured click statistics, for relatively high mean photon numbers. No data postprocessing is required to demonstrate the effects of interest with high significance, despite low efficiencies and experimental imperfections. Our approach shows that such novel detection schemes are a reliable and robust way to characterize quantum-correlated light fields for practical applications in quantum communications.

Highly efficient frequency conversion with bandwidth compression of quantum light

We demonstrate an engineered sum-frequency-conversion process in lithium nio-bate that provides a bandwidth compression factor of 7.47 at a high efficiency of 61.5 %, thus outperforming spectral filtering. The process preserves non-classical photon-number statistics.

Temporal-mode measurement tomography of a quantum pulse gate

Encoding quantum information in the photon temporal mode (TM) offers a robust platform for high-dimensional quantum protocols. The main practical challenge, however, is to design a device that operates on single photons in specific TMs and all coherent superpositions. The quantum pulse gate (QPG) is a mode-selective sum-frequency generation designed for this task. Here, we perform a full modal characterization of a QPG using weak coherent states in well-defined TMs. We reconstruct a full set of measurement operators, which show an average fidelity of 0.85 to a theoretically ideal device when operating on a seven-dimensional space. Then we use these characterized measurement operators of the QPG to calibrate the device. Using the calibrated device and a tomographically complete set of measurements, we show that the QPG can perform high-dimensional TM state tomography with 0.99 fidelity.

Local Sampling of the Wigner Function at Telecom Wavelength with Loss-Tolerant Detection of Photon Statistics.

We report the experimental point-by-point sampling of the Wigner function for nonclassical states created in an ultrafast pulsed type-II parametric down-conversion source. We use a loss-tolerant time-multiplexed detector based on a fiber-optical setup and a pair of photon-number-resolving avalanche photodiodes. By capitalizing on an expedient data-pattern tomography, we assess the properties of the light states with outstanding accuracy. The method allows us to reliably infer the squeezing of genuine two-mode states without any phase reference.

Tomography by noise.

We present an efficient and robust method for the reconstruction of photon number distributions by using solely thermal noise as a probe. The method uses a minimal number of precalibrated quantum devices; only one on-off single-photon detector is sufficient. The feasibility of the method is demonstrated by the experimental inference of single-photon, thermal. and two-photon states. The method is stable to experimental imperfections and provides a direct, user-friendly quantum diagnostics tool.

Time-multiplexed measurements of nonclassical light at telecom wavelengths

We report the experimental reconstruction of the statistical properties of an ultrafast pulsed type II parametric down-conversion source in a periodically poled potassium titanyl phosphate waveguide at telecom wavelengths, with almost perfect photon-number correlations. We use a photon-number-resolving time-multiplexed detector based on a fiber-optical setup and a pair of avalanche photodiodes. By resorting to a germane data-pattern tomography, we assess the properties of the nonclassical light states with unprecedented precision.

Quantum state and mode profile tomography by the overlap

VS acknowledge support from the National Council for Scientific and Technological Development (CNPq) of Brazil, grant 304129/2015-1, and by the Sao Paulo Research Foundation (FAPESP), grant 2015/23296-8. DM acknowledge support from the EUproject Horizon-2020 SUPERTWIN id.686731, the National Academy of Sciences of Belarus program ‘Convergence’ and FAPESP grant 2014/21188-0. NK acknowledges the support from the Scottish Universities Physics Alliance (SUPA) and from the International Max Planck Partnership (IMPP) with Scottish Universities. JT and CS acknowledge support from European Union Grant No. 665148 (QCUMbER). TB acknowledges support from theDFG under TRR 142.

Discorrelated quantum states

Using the building blocks of quantum optics - single photons, coherent states, beam splitters and projective measurement - we construct a two-mode quantum state for which coincident photon number terms in each mode are removed.